STRENGTHENING SLENDER S-SECTION STEEL COLUMNS USING …

STRENGTHENING SLENDER S-SECTION STEEL COLUMNS

USING CFRP PLATES OF VARIOUS MODULI

by

Allison Christine Ritchie

A thesis submitted to the Department of Civil Engineering

In conformity with the requirements for

the degree of Master of Applied Science

Queen’s University

Kingston, Ontario, Canada

(June, 2014)

Copyright ©Allison Christine Ritchie, 2014

ii

Abstract

This thesis investigates strengthening slender steel columns with carbon fibre

reinforced polymer (CFRP) plates of various moduli. Three different types of CFRP

were used in the study: Ultra-high modulus (430GPa), High modulus (212GPa) and

Normal modulus (168GPa). In this study, specimens were grouped according to

measured initial out-of-straightness values. The first section examines the effect of

adding CFRP plates to the column flanges when buckling about the weak axis. Twelve

columns, with a slenderness ratio of 197, were tested, of which nine were strengthened

with CFRP. The main parameters tested were the level of initial out-of-straightness

(length (L)/8387 to L/1020), CFRP modulus (168 to 430 GPa), CFRP reinforcement ratio

(13% to 34%) and the length of CFRP plate (33% to 95% of L). The gain in axial

strength due to CFRP retrofitting ranged from 11% to 29%, depending on the various

parameters. The gain generally increased as CFRP modulus, initial out-of-straightness, or

CFRP reinforcement ratio increased. Global buckling consistently governed the

maximum load. In the case of the 430 GPa CFRP, buckling was followed by CFRP

crushing in compression, then rupture in tension.

The second section of the thesis examines the effect of CFRP plates added to the

flanges and tested for buckling in the strong axis. Eight columns, with a slenderness ratio

of 83, were tested of which five were strengthened with CFRP. The main parameters

examined were the level of initial out-of-straightness (L/28889 to L/1635), CFRP

modulus (168 to 430 GPa), CFRP reinforcement ratio (13% to 34%) and the axis of

bending. The gain in axial strength due to CFRP retrofitting ranged from 0% to 25%,

depending on the various parameters. The gain generally increased as initial out-of-

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straightness, or CFRP reinforcement ratio increased. The higher modulus did not

perform as expected, showing no gain in strength, because the compressive strains were

too large and the CFRP crushed before the specimen experienced any gain. Specimens

compared with the weak axis, strengthened with normal modulus CFRP, had similar

percentage gains in strength.

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Acknowledgements

I would like to take the time to thank all of those who have helped me complete

my Master’s degree and have supported me over the last two years. First, I would like to

thank my supervisors, Dr. Amir Fam and Dr. Colin MacDougall, for their guidance,

reassurance and constant support.

I would also like to thank all of the technical staff, in particular, Paul Thrasher

and Neil Porter, for their expert knowledge and assistance with my test setup and

instrumentation. In addition, the office staff, Maxine Wilson, Debbie Ritchie and Cathy

Wagar, have been extremely helpful.

I would like to than Sika Canada for the CFRP strips, epoxy and guidance. I

would like to thank Gus at Sydenham Welding and Stefan at Nybom Welding for their

work done on preparing my test setup.

Lastly, my fellow graduate students also deserve special thanks. To Kenneth Mak,

Ciaran McSwiggan, James St Onge, Nik Wooton, Doug Tomilson, Stefano Arcovio and

all the other grad students who have lent me some of their time, I thank you for helping

me in the lab, in the office and with my sanity over the past two years.

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Table of Contents

Abstract ............................................................................................................................................ ii

Acknowledgements ......................................................................................................................... iv

List of Figures ................................................................................................................................ vii

List of Tables ................................................................................................................................... x

Chapter 1 Introduction ..................................................................................................................... 1

1.1 General ................................................................................................................................... 1

1.2 Objectives .............................................................................................................................. 3

1.3 Scope ...................................................................................................................................... 3

1.4 Thesis Outline ........................................................................................................................ 4

Chapter 2 Literature Review ............................................................................................................ 5

2.1 Introduction ............................................................................................................................ 5

2.2 General ................................................................................................................................... 5

2.3 Strengthening of Steel Beams with FRP ................................................................................ 5

2.4 Strengthening of Steel Columns with FRP ............................................................................ 7

2.5 Failure Modes of FRP on Steel Structures ........................................................................... 11

Chapter 3 Strengthening Long Steel Columns of S-Sections against Global Buckling around

Weak Axis using CFRP Plates of Various Moduli ........................................................................ 16

3.1 Introduction .......................................................................................................................... 16

3.2 Experimental Program ......................................................................................................... 18

3.2.1 Test Specimens and Parameters .................................................................................... 18

3.2.2 Materials ....................................................................................................................... 20

3.2.3 Fabrication of Test Specimens ...................................................................................... 21

3.2.4 Test Setup and Instrumentation..................................................................................... 22

3.3 Experimental Results ........................................................................................................... 23

3.3.1 Effect of Column Out-of-Straightness .......................................................................... 24

3.3.2 Effect of CFRP Modulus .............................................................................................. 25

3.3.3 Effect of CFRP Reinforcement Ratio ........................................................................... 25

3.3.4 Effect of CFRP Length ................................................................................................. 26

3.3.5 Failure Modes ............................................................................................................... 27

3.4 Summary .............................................................................................................................. 28

Chapter 4 Strengthening Long Steel Columns of S-Sections against Global buckling around

Strong Axis using CFRP Plates of various Moduli ........................................................................ 41

vi

4.1 Introduction .......................................................................................................................... 41

4.2 Experimental Program ......................................................................................................... 42

4.2.1 Test Specimens and Parameters .................................................................................... 42

4.2.2 Materials ....................................................................................................................... 44

4.2.3 Fabrication of Test Specimens ...................................................................................... 45

4.2.4 Instrumentation and Test Setup..................................................................................... 46

4.3 Experimental Results ........................................................................................................... 47

4.3.1 Load –Deflection Behaviour ......................................................................................... 47

4.3.2 Load-Strain Behaviour .................................................................................................. 48

4.3.3 Effect of Out-of-Straightness ........................................................................................ 48

4.3.4 Effect of the Reinforcement Ratio and Young’s Modulus ............................................ 49

4.3.5 Effect of Axis of Bending During Buckling ................................................................. 51

4.3.6 Failure Modes ............................................................................................................... 53

4.4 Summary .............................................................................................................................. 55

Chapter 5 Conclusions ................................................................................................................... 67

5.1 Summary .............................................................................................................................. 67

5.2 Performance of Strengthening Long Steel Columns of S-Sections against Global Buckling

around Weak Axis using CFRP Plates of various Moduli ......................................................... 67

5.3 Performance of Strengthening Long Steel Columns of S-Sections against Global Buckling

around Strong Axis using CFRP Plates of various Moduli ....................................................... 69

5.4 Future Research ................................................................................................................... 70

References ...................................................................................................................................... 72

Appendix A Procedure Appendix .................................................................................................. 76

A.1 Additional Testing Details .................................................................................................. 76

A.2 Bracing Design .................................................................................................................... 78

Appendix B Results Appendix ....................................................................................................... 83

B.2 PIV Analysis ....................................................................................................................... 83

B.2 Supplementary Test Result Figures ..................................................................................... 84

B.2.1 Weak Axis – Top Lateral LP ........................................................................................ 85

B.2.2 Weak Axis – Bottom Lateral LP .................................................................................. 87

B.2.3 Strong Axis – Top Lateral LP ...................................................................................... 89

B.2.4 Strong Axis – Bottom Lateral LP ................................................................................. 90

B.2.5 Strain Gauge Data ........................................................................................................ 91

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List of Figures

Figure 2-1: Common failure modes (Buyukozturk et al., 2004) .................................................... 15

Figure 2-2: Forms of debonding (Buyukozturk et al., 2004) ......................................................... 15

Figure 3-1: Test specimens ............................................................................................................ 32

Figure 3-2: Steel columns out-of-straightness at mid-height before retrofitting ........................... 32

Figure 3-3: Tensile stress-strain responses of different CFRP plates and steel ............................. 33

Figure 3-4: CFRP installation ........................................................................................................ 33

Figure 3-5: Test setup .................................................................................................................... 34

Figure 3-6: The effect of out-of-straightness on columns strengthened using 430GPa CFRP ...... 35

Figure 3-7: The effect of CFRP modulus on columns of comparable out-of-straightness ............ 36

Figure 3-8: The effect of CFRP reinforcement ratio ...................................................................... 37

Figure 3-9: The effect of the 430 GPa CFRP length ratio ............................................................. 38

Figure 3-10: Effect of out-of-straightness on ultimate loads ......................................................... 38

Figure 3-11: Effect of CFRP modulus on ultimate loads ............................................................... 39

Figure 3-12: Effect of CFRP reinforcement ratio on ultimate loads .............................................. 39

Figure 3-13: Effect of CFRP length on ultimate loads .................................................................. 40

Figure 3-14: Failure modes ............................................................................................................ 40

Figure 4-1: Test specimens ............................................................................................................ 58

Figure 4-2: Steel columns out-of-straightness at mid-height before retrofitting ........................... 58

Figure 4-3: Tensile stress-strain responses of different CFRP plates and steel ............................. 59

Figure 4-4: CFRP installation ........................................................................................................ 59

Figure 4-5: Test setup .................................................................................................................... 60

Figure 4-6: The effect of out-of-straightness on columns strengthened with 168GPa CFRP ........ 61

Figure 4-7: The effect of CFRP modulus on columns with comparable out-of-straightness (δ/L =

0.00004 to 0.00061) ....................................................................................................................... 62

Figure 4-8: The effect of Young’s modulus and CFRP reinforcement ratio ................................. 63

Figure 4-9: Effect of out-of-straightness on ultimate loads ........................................................... 64

Figure 4-10: Effect on CFRP modulus and reinforcement ratio on ultimate loads........................ 64

Figure 4-11: Effect of CFRP modulus on ultimate loads ............................................................... 65

Figure 4-12: Effect on axis of bending on ultimate loads .............................................................. 65

Figure 4-13: Failure modes ............................................................................................................ 66

Figure A-1: First control test .......................................................................................................... 79

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Figure A-2: SAP system ................................................................................................................ 80

Figure A-3: Second attempt ........................................................................................................... 82

Figure B-4: PIV analysis ................................................................................................................ 84

Figure B-5: PIV and LP data for B4 .............................................................................................. 84

Figure B-6: The effect of out-of-straightness on columns strengthened using 430 GPa CFRP .... 85

Figure B-7: The effect of CFRP modulus on columns of comparable out-of-straightness ............ 85

Figure B-8: The effect of CFRP reinforcement ratio ..................................................................... 86

Figure B-9: The effect of the 430 GPa CFRP length ratio ............................................................. 86

Figure B-10: The effect of out-of-straightness on columns strengthened using 430 GPa CFRP .. 87

Figure B-11: The effect of CFRP modulus on columns of comparable out-of-straightness .......... 87

Figure B-12: The effect of CFRP reinforcement ratio ................................................................... 88

Figure B-13: The effect of the 430 GPa CFRP length ratio ........................................................... 88

Figure B-14: The effect of out-of-straightness on columns strengthened with 168GPa CFRP ..... 89

Figure B-15: The effect of CFRP modulus on columns with comparable out-of-straightness (δ/L

= 0.00004 to 0.00061) .................................................................................................................... 89

Figure B-16: The effect of Young’s modulus and CFRP reinforcement ratio ............................... 90

Figure B-17: The effect of out-of-straightness on columns strengthened with 168GPa CFRP ..... 90

Figure B-18: The effect of CFRP modulus on columns with comparable out-of-straightness (δ/L

= 0.00004 to 0.00061) .................................................................................................................... 91

Figure B-19: The effect of Young’s modulus and CFRP reinforcement ratio ............................... 91

Figure B-20: Specimen B1 gauges in top quarter .......................................................................... 92

Figure B-21: Specimen B1 gauges in bottom quarter .................................................................... 92











Figure B-32: Specimen CX4 gauges in top quarter ....................................................................... 98

Figure B-33: Specimen CX4 gauges in bottom quarter ................................................................. 98

ix

Figure B-34: Specimen CX5 gauges in top quarter ....................................................................... 99

Figure B-35: Specimen CX5 gauges in bottom quarter ................................................................. 99

x

List of Tables

Table 3-1: Test matrix .................................................................................................................... 30

Table 3-2: Summary of column test results ................................................................................... 31

Table 4-1: Test matrix .................................................................................................................... 56

Table 4-2: Summary of column test results ................................................................................... 57

Table A-1: Out-of-straightness values for columns tested in weak axis ........................................ 77

Table A-2: Out-of-straightness values for specimens tested in the strong axis ............................. 77

Table A-3: FRP material properties based on coupon tests ........................................................... 78

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Chapter 1

Introduction

1.1 General

A large part of infrastructure today consists of steel structures, which may need to

be strengthened for increased loading. In other cases, structures can deteriorate due to

corrosion, fatigue, design errors and lack of maintenance. For most cases, retrofitting a

structure costs less than replacing it entirely and takes less time to implement, therefore,

reducing the service interruption time.

Bolting or welding additional plates is the conventional method for retrofitting or

strengthening a steel member. The strengthened member of course will continue to be

susceptible to corrosion and fatigue. Adding plates also increases the area of the column

as well as the dead weight, which could cause issues for some projects. Naturally, steel

has high strength and stiffness, which makes it harder to strengthen compared to

materials such as concrete and wood. When steel is strengthened with a material whose

Young’s modulus is lower, the strengthening should only be effective after the steel

yields.

To solve the issues of conventional methods, there is a need for accepting a long-

lasting cost effective material. A possible solution is the use of Fibre Reinforced

Polymers (FRPs). Though they originally may have a larger material cost than steel, the

material cost alone is normally small compared to the total cost of the project. This is

demonstrated by Moy et al. (2001) for strengthening the London underground railway.

The option of using steel and FRPs were both examined. The FRP strengthening scheme

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gave a comparable overall cost with the steel strengthening and was used because of the

time restraints of the project (Moy et al. 2001). FRPs provide many advantages such as

being lightweight, corrosive resistant and having high-strength capabilities (Karbhari &

Shulley, 1995). There are many different types of FRPs with the main two using carbon

or glass fibres. Due to wanting a higher modulus, carbon fibres are generally used when

strengthening steel. They are available in strips or flexible sheets allowing for easy

application to any project, even when the member is already in use.

Most researchers have investigated the properties of steel beams strengthened

with plates or sheets (Hai et al., 2010; Dawood et al., 2006; Peiris, 2011; Narmashiri et

al., 2011). These researchers studied optimum fibre to resin ratios and different modulus

types on I-beams or wide flange beams. The increase in ultimate loads were measured

and studied along with the failure modes experienced. This research has shown that

strengthening beams is promising and does get a significant increase in strength of the

beam without sacrificing too much space or adding more weight.

The next step was to apply the method to steel columns and see if the axial

capacity can also experience a significant increase. Research has been done by several

people on various sections of steel varying from T-sections to hollow sections (Harries et

al., 2009; Kalavagunta et al., 2014; Silvestre et al., 2008; Bambach et al., 2009; Bambach

& Elchalakani, 2007; Shaat & Fam, 2006; Shaat & Fam, 2009). This research has shown

that applying Carbon Fibre Reinforced Polymers (CFRP) in sheets or plates has helped to

increase the capacity of the columns tested, whether short or slender.

Little research has been done on slender S-section columns, which may be used in

a building where a small section is needed due to space requirements. The study

3

presented will fill that gap and investigate the advantage of strengthening with CFRP

strips tested in both axes of bending.

1.2 Objectives

The experimental research carried out in this thesis explores the strengthening of

steel S-section (S75x8) columns. CFRP plates are applied to the flanges of the section to

improve the buckling capacity of the section in the weak and strong axis.

The main objectives addressed by this study are to:

1. Examine and compare the change in peak load and displacement of columns

strengthened with ultra-high modulus, high modulus and normal modulus

CFRP in the weak and strong axes


strengthened with different lengths (2/3, 1/3 and full) of ultra-high modulus

CFRP in the weak axis


strengthened with various reinforcement ratios of ultra-high modulus, high

modulus and normal modulus CFRP in the weak and strong axes

4. Observe various failure mechanisms in all columns tested

1.3 Scope

The scope of this thesis comprises of experimental investigations and

comparisons with control specimens. The experimental data covers the performance of

slender steel S-sections strengthened with ultra-high, high or normal modulus CFRP

plates.

4

The experimental phase includes two parts. The first part involves testing

specimens for bending about the weak axis. The second part includes columns tested in

the strong axis. For both parts, control specimens were tested along with specimens

strengthened with one of the three types of CFRP plates used.

1.4 Thesis Outline

The following is a brief description of the contents of this thesis:

Chapter 2: reviews the literature and research on steel beams and columns strengthened

with externally bonded CFRP plates. A summary of the performance is included.

Chapter 3: includes the study of the performance of slender steel s-section columns

strengthened in the weak axis with three different types and number of layers of

externally bonded CFRP plates. Also addresses the difference in applying the ultra-high

modulus CFRP over 33%, 67% or 95% of the full length of the column.

Chapter 4: includes the study of the performance of slender steel s-section columns

strengthened in the strong axis with different modulus types and number of layers of

externally bonded CFRP plates. This data was compared to the weak axis specimens in

Chapter 3.

Chapter 5: summarizes the conclusions found from the studies and outlines any areas of

future research.

5

Chapter 2

Literature Review

2.1 Introduction

This section presents the background and describes applications of externally

bonded Fibre Reinforced Polymer (FRP) plates or sheets used as a method to strengthen

or rehabilitate structural steel.

2.2 General

The conventional strengthening method for a steel member includes bolting or

welding additional steel plates onto the member or cutting out the deteriorated section

and replacing it with a steel plate. The plates require heavy equipment or possibly

shoring to lift them in place during installation. The plates can add to the dead load of the

structure. The plates are susceptible to corrosion. Fastening plates to a steel structure

produces stress concentrations and welding can generate thermally induced stresses or

heat affected areas (Grabovac et al., 1991). On the other hand, FRP materials are

lightweight, non-corrosive, have minimal impact visually, and have minimal effect on

clearance. They are versatile as they are available in rigid plates or flexible sheets.

2.3 Strengthening of Steel Beams with FRP

FRP systems have a wide range of uses including rehabilitating existing structural

elements, retrofitting or strengthening a structurally sound member or correcting

construction errors. Retrofitting steel structures using FRPs has been somewhat limited

compared to concrete structures, but has shown great success. Shaat et al. (2004)

performed an overall review of research done in the field of strengthening and repairing

6

steel structures with FRPs. Strengthening or retrofitting steel beams has been the subject

of most research with FRP systems and steel. Hai et al. focused on strengthening I-

beams by using a hybrid FRP with carbon/glass fibres and a vinyl-ester resin. They

conducted four-point bending tests and found an optimal carbon volume content of 25-

30% (Hai et al., 2010). Dawood et al. (2006) used carbon fibre reinforced polymers

(CFRP) in the form of strips with different moduli applied to steel-concrete composite

beams located in bridges. They concluded that high and intermediate modulus of CFRP

increased the elastic stiffness, yield load and ultimate capacity of the beams. When

testing the beams for fatigue resistance, comparable results to the unstrengthened beams

were obtained (Dawood et al., 2006). Peiris (2011) looked at the bond characteristics and

flexural behaviour of normal and ultra-high modulus CFRP applied to wide flange steel

beams. Peiris noted that the normal modulus had a load carrying capacity of 22% higher

than the ultra-high modulus (Peiris, 2011). Narmashiri et al. (2011) performed flexural

strengthening experiments for steel I-beams using CFRP strips and studied the effects of

varying the thickness and type of CFRP. The authors noted that a thicker plate increased

the load bearing capacity but the specimens experienced a brittle failure and premature

debonding at the ends (Narmashiri et al., 2011).

Gillespie et al. (1996) strengthened a steel girder on a bridge in the field with

CFRP plates and monitored the effect. Load tests in the field observed 11% reduction in

the tension flange strains (Gillespie et al. 1996). Tavakkolizadeh, and Saadatmanesh

(2001) repaired steel-concrete composite girders with CFRP sheets and showed that the

ultimate load capacities and stiffness could be regained. Three large tests were done with

one, three or five layers of CFRP applied to repair specimens with 25, 50 or 100% loss in

7

cross-sectional area (Tavakkolizadeh & Saadatmanesh, 2001). Shaat and Fam (2008)

performed similar tests on artificially damaged steel-concrete composite beams. Varied

lengths and number of layers were applied on the cracked flange and strengths of 46-

116% of the original undamaged specimens were observed (Shaat & Fam, 2008).

These studies, among many others, validate the use of strengthening steel beams

with CFRP. The focus of this thesis is on strengthening steel columns with CFRP strips

and the research done so far on this subject is discussed in the following section.

2.4 Strengthening of Steel Columns with FRP

FRP systems have also had success when used to strengthen steel columns. The

FRP has proven to increase the axial capacity and stiffness of the columns. Research has

been done on a variety of steel sections ranging from T-sections, C channels, square

hollow sections, steel tubes and S-sections. Each study has also used a range of FRP

types. This section presents a summary of their findings.

Harries et al. tried to enhance web or flange capacity of T-sections against local

buckling by using high strength CFRP strips or ultra-high modulus GFRP strips. The

FRP was applied on both sides of the web with either one or two layers. Testing the

specimens in loading cycles showed that the axial capacities did not improve

significantly. The GFRP specimen capacity increased 6 and 9% and the CFRP specimens

showed a minor decrease in capacity. The specimens did exhibit greater resistance to

weak-axis lateral displacement. It was shown that decreasing the slenderness of a

member increases the cyclic loading lifespan and capacity (Harries et al., 2009).

Kalavagunta et al. (2014) studied the effect of using CFRP to control the local

buckling of cold formed lipped steel channels. These tests showed an increase in the

8

load-carrying capacity of up to 16.75% (Kalavagunta et al., 2014). Silvestre et al. (2008)

also investigated CFRP strengthened cold formed lipped steel channel columns. The

parameters involved were seven different strengthening configurations, a constant CFRP

modulus of 235GPa, and two different lengths of columns, 600mm and 2200mm. The

largest increase in ultimate strength of 19.8% was obtained for a column strengthened

with CFRP wrapped around the entire column. Just strengthening the web and flange

increased the load by 15.7-18.4%. The columns that demonstrated higher ultimate loads

also had brittle failures (Silvestre et al., 2008).

Bambach et al. (Bambach et al., 2009) tested short square hollow sections with

CFRP applied using the wet layup method. The sections had FRP applied in two

different fibre layouts and the steel sections varied in slenderness (

) ratio. The axial

capacities of the FRP reinforced sections generally increased up to two times the capacity

of the plain steel. The increase could have arisen from the CFRP confining the short

column, therefore allowing fewer deformations. Bambach et al. (2009) noted that there

was a plateau in the gain beyond plate slenderness ratios of 2.5. Bambach and

Elchalakani (2007) extended the research to examine the effect of slenderness ratios and

the number of layers applied. Increasing the number of layers proved to provide a larger

increase in strength with the largest increase occurring in the slender columns. Since

slender columns experience larger elastic deformations before reaching the ultimate load,

a greater increase in strength was observed due to the presence of the CFRP. Bambach

and Elchalakani also measured energy absorption increases. More layers increased the

absorption, with the highest increase occurring in short columns (Bambach &

Elchalakani, 2007).

9

Shaat and Fam (2006) performed multiple studies on hollow steel square columns

of many different slenderness ratios, ranging from 5 to 93. The first study focussed on

short columns, with a few long columns as well, strengthened with CFRP sheets oriented

in the longitudinal and transverse directions for the short columns but only longitudinal

for the long columns. A layer of GFRP was installed on the steel surface between the

carbon fibres and the steel as a measure to prevent galvanic corrosion (West, 2001). The

short columns had a slenderness ratio of five with varying number of layers, fibre

orientation and CFRP types. The maximum gain in capacity for the short columns, 18%,

came from two transverse layers of the lower modulus fibre. The long columns had a

slenderness ratio of 68 with the number of layers varying and the CFRP applied to

opposite sides or on all four sides. The long columns strengthened with three layers on

all sides produced the maximum 23% gain in ultimate capacity. It was noticed that the

increase in capacity did not correlate with the number of layers added to the long

columns. After examining the strain data, the existence of imperfections of various

magnitudes were discovered, possibly from out-of-straightness in the specimens or

misalignment within the setup. The specimens that had similar imperfections were

compared and shown to have higher strengths with CFRP added and enhanced stability

against lateral deflections (Shaat & Fam, 2006). Shaat and Fam (2009) also performed

tests on slender sections strengthened with CFRP strips and saw an increase in ultimate

load ranging from 6 to 71% and an increase in stiffness ranging from 10 to 17%. The

CFRP plates were less effective at low slenderness ratios, changing the failure mode of

the plates to debonding prior to buckling rather than crushing afterwards. The focus of

the slender column study was on using high modulus CFRP plates with the reinforcement

10

ratio held constant and the length of the columns varied. The columns with similar out-

of-straightness values were grouped together. Shaat and Fam also mentioned that these

values may differ once the epoxy and CFRP are applied and that in real life applications

the out-of-straightness would not be an issue due to the fact that two columns that are

compared would actually be the exact same column. For the application of the plates,

they were cut 25mm shorter on each end to simulate the case where it would not be

feasible to access the ends. When applying more than one layer the second layer was

also cut 25mm shorter to allow for a gradual termination of CFRP, then two layers of

GFRP were wrapped around the ends. The data indicated that the effectiveness of the

CFRP is larger at higher slenderness ratios (Shaat & Fam, 2009). Shaat and Fam (2009)

also developed an analytical model that predicted the axial load capacity of hollow square

steel sections.

Teng and Hu (2007) studied the effect of adding GFRP to steel tubes assuming

that due to GFRP possessing a larger ultimate tensile strain it would increase the ductility

of the tubes. The GFRP strengthened load-axial curve showed a slow lengthy ascending

branch before finally reaching the peak, indicating ductility. The ultimate load increased

5-10% proving that the number of layers of GFRP applied had a minimal impact on the

ultimate load (Teng & Hu, 2007).

Haedir and Zhao (2011) studied the effect of adding CFRP sheets to circular steel

tubular short columns to investigate the effects of the yield strength, modulus of elasticity

of the hoop fibre and the amount and direction of the fibre reinforcement. The columns,

with slenderness values ranging from 67-142, were tested in a concentric compression

test. The axial load-shortening of each of the columns were observed with an increase of

11

15-36% in the peak load. With this information, Haedir and Zhao proposed a set of

design curves to be used for CFRP-reinforced steel short columns (Haedir & Zhao,

2011).

Liu et al. (2005) looked at retrofitting steel-notched S-sections with an open

GFRP jacket and then filling the jacket with expansive lightweight concrete to provide

active confinement. Of the seven specimens, there were three different lengths of jackets

tested in the Y-Y axis. Buckling in these specimens occurred at the end of the GFRP

jacket as opposed to in the middle like the bare steel column. It was noticed that the

longer retrofit lengths reached a higher ultimate load capacity. Liu et al. used their data

to develop a model to predict the capacities and design guidelines (Liu et al., 2005).

2.5 Failure Modes of FRP on Steel Structures

Four main failure modes have been observed when testing steel beams

strengthened with FRP. These modes are the flange buckling in compression, the web

buckling in shear, the FRP strips rupturing or the FRP strips debonding, and are shown

below in Figure 2-1 (Buyukozturk et al., 2004).

When a debonding failure happens, it commonly occurs in areas where high stress

concentrations occur, which are generally where there are material discontinuities or

cracks. Debonding can take place at different boundaries such as the steel and FRP

interface, the adhesive and FRP interface, in the adhesive itself or a delamination of the

FRP (Buyukozturk et al., 2004). These areas of debonding are shown in Figure 2-2.

Narmashiri et al. (2011) found that for different thicknesses of CFRP there were various

failure modes observed. For a thickness of 1.2mm there was splitting and debonding

below the load points, as well as delamination and debonding in the tips. For thickness

12

values of 1.4 and 2mm, again debonding happened below the point load with debonding

and delamination at the ends. For the largest thickness of 4mm, there was no

delamination at the ends (Narmashiri et al., 2011). Peiris noted that the ultra-high

modulus laminates used in their study debonded before the steel plate yielded while the

normal modulus laminates debonded afterwards (Peiris, 2011).

The failure modes normally observed for unstrengthened S-section columns

include lateral torsional buckling, global buckling and local buckling. The purpose of

many of the studies discussed in the previous section was to delay the onset of these

failure modes. Most of the experiments performed by previous researchers have noticed

that less displacement was observed at each load with the addition of FRP. Harries et al.

(2009) found that for the WT sections, the control sample failures occurred with large

lateral translations of the stem tip and twisting about the centroid and nominal strong

axis. For slender stems, plastic “kinking” was seen with increases in axial displacement.

Adding FRP to these columns helped diminish the post-buckling crippling and resulted in

a more ductile failure, controlling the plastic buckling. It was noted that debonding was

always observed for these tests after the peak load was reached and was driven based on

the degree of curvature in the stem (Harries et al., 2009).

Cold formed lipped steel channel columns commonly fail due to local-plate or

distortional buckling with collapse always taking place in a distortional mechanism.

When Silvestre et al. attached carbon fibres to these columns, the fibres debonded after

reaching the maximum load, similar to the Harries et al. observations. Cracking sounds

were heard as the CFRP sheets detached from the steel surface near the yield line zone

(Silvestre et al., 2008).

13

For square hollow sections, Bambach and Elchalakani (2007) observed no

delamination before reaching the ultimate load. Once large deformations occurred, the

CFRP typically delaminated from the ends and ruptured at the corners and exterior folds.

The CFRP delaminated across some folds rather than rupturing under large deformations

for the slender specimens. As the width over thickness (b/t) ratio increased, the folding

mechanisms changed. The folds become more noticeable for higher b/t ratios and

Bambach and Elchalakani found that the CFRP would delaminate across the fold rather

than maintain the bond and rupture as for lower b/t ratios (Bambach & Elchalakani,

2007).

Shaat and Fam (2006) also looked at the failure modes in hollow square sections.

For the short specimens strengthened with longitudinal and transverse sheets,

delamination occurred in all specimens. Specimens with a higher modulus of CFRP in

the transverse direction underwent rupture of the fibres near the corners. It was noted

that none of the specimens failed at the CFRP joint overlap. The long specimens failed

mostly due to overall buckling with subsequent local buckling in the compression side,

near mid-length. This local buckling caused delamination and crushing of the FRP

sheets. For the column with CFRP on all four faces, the CFRP fractured due to the local

bending from the buckling (Shaat & Fam, 2006). For the tests done using CFRP strips,

Shaat and Fam (2009) found that there were two different failure modes, along with

overall buckling. For lower slenderness ratios, the CFRP debonded on the inner side and

the GFRP transverse wraps located at the ends partially ruptured. At the lowest

slenderness (46) value, debonding happened before overall buckling, exhibited as a load

drop followed by an increase in load up to the peak value. The middle slenderness (70)

14

specimens underwent debonding on one side, increasing the lateral deflection because of

the eccentricity from the now asymmetric column. The load suddenly dropped and then

did not increase again. At the highest slenderness ratio (93), the CFRP layers on the

inner side crushed at mid height, after buckling, without debonding. This occurred a long

time after reaching the peak load. The strains occurring at this time were 58% of the

tensile rupture strain. The outer side of the column did not observe any debonding or

CFRP rupture (Shaat & Fam, 2009).

Based on the research presented, it is clear that there are still gaps of knowledge

for steel columns strengthened with FRP. A variety of sections including, WT, circular

and HSS, have been strengthened with GFRP and various moduli of CFRP and their new

properties have been observed. During these programs, the S-section has not been used

and the length of the FRP has consistently been the whole length of the column. This

study will examine the effect of using three different moduli of CFRP to strengthen S-

sections of a constant length. The length and number of layers of CFRP applied to each

column will be varied between full, two-third and one-third of the column length and one,

two and three layers.

15

Figure 2-1: Common failure modes (Buyukozturk et al., 2004)

Figure 2-2: Forms of debonding (Buyukozturk et al., 2004)

16

Chapter 3

Strengthening Long Steel Columns of S-Sections against Global

Buckling around Weak Axis using CFRP Plates of Various Moduli1

3.1 Introduction

There are several conventional methods to strengthen a steel member. These

include bolting or welding additional steel plates onto the member. Heavy equipment or

shoring is generally needed to lift plates into place for installation and the plates add to

the dead load of the structure and are also susceptible to corrosion. Welding can generate

thermally induced stresses and brittleness that may impact fatigue life while fastening

plates to the structure produces stress concentrations (Grabovac et al., 1991).

Adhesively bonded Fibre Reinforced Polymer (FRP) composites have shown

great success and wide spread use in retrofitting concrete structures due to their light-

weight, non-corrosive nature and thin profiles. Retrofitting steel structures using FRPs

has been somewhat limited compared to concrete structures, but successful and promising

nonetheless. Shaat et al (2004) provided a review of these applications, including repair

of naturally deteriorated girders (Gillespie et al, 1999), repair of artificially notched

girders to simulate fatigue cracks or section loss due to corrosion (Tavakkolizadeh and

Saadatmanesh, 2001, and Shaat and Fam, 2008), strengthening of intact sections to

increase their flexural strength and stiffness (Edberg et al, 1996 and Fam et al, 2009), and

retrofit of steel girders in composite action with a concrete deck (Sen et al, 2001).

1 This chapter has been submitted for publication as the following journal paper:

Ritchie, A. Fam, A. and MacDougall, C. (2014) “Strengthening Long Steel Columns of S-Sections against

Global Buckling around Weak Axis using CFRP Plates of various Moduli", Journal of Composites for

Construction, Under review.

17

Durability of CFRP retrofitted steel structures has also been studied (Dawood and

Rizkalla, 2010).

A pioneering study was conducted by Shaat and Fam (2006) on strengthening

slender Hollow Square Section (HSS) steel columns against global buckling using

longitudinally oriented carbon-FRP laminates, and also short HSS columns against local

buckling, using combined transverse and longitudinal CFRP sheets. The study

demonstrated the strong potential of the concept but also pointed out the important effect

of the initial out-of-straightness on performance. The work was then extended to include

high modulus CFRP of 313 GPa and showed that effectiveness of the concept increases

as slenderness ratio of long columns increase (Shaat and Fam, 2009), where gains in axial

capacity of up to 71% were achieved for columns with 93 slenderness ratio, which is the

maximum ratio studied. Bambach and Elchalakani (2007) and Bambach et al (2009)

strengthened short HSS sections with different fibre layouts and were able to achieve

axial capacities up to two times that of control columns. However, there was a plateau in

the gain beyond a certain plate slenderness ratio. Recently, Kalavagunta et al (2014)

observed that bonding CFRP to cold-formed lipped steel channels can increase their axial

capacity by 17%. Generally speaking, research on FRP-retrofitting of steel columns has

been quite limited.

This chapter investigates CFRP strengthening of a different type of slender steel

column, namely I-shaped standard (S-) sections, with a slenderness ratio of 197. This

slenderness ratio is close to the upper limit of 200 typically permitted by code (e.g.

Clause 10.4.2.1 of CAN/CSA-S16-09 (2009)). The CRFP was bonded to the flanges, and

no bracing was provided, so buckling occurred about the weak (y-) axis. The columns

were grouped according to their initial out-of-straightness values, as this has a

18

considerable effect on column strength and CFRP effectiveness. The classical Euler

buckling theory indicates that critical buckling load of a column is function of its flexural

rigidity (EI) and not the material strength. Given that CFRP plates are available with a

wide range of Young’s moduli, including some with ultra-high modulus well in excess of

the 200 GPa of steel, they would be much more efficient than bolted or welded steel

plates for strengthening long steel columns. In this study, the effect of varying CFRP

modulus is studied. Also, previous studies on slender HSS columns have always used

CFRP plates of the full length of the column, which is not necessary. In this study the

plate length was varied and its effect on strengthening columns, which have not been

deteriorated, was investigated. Finally, the number of layers of CFRP was also varied.

3.2 Experimental Program

The following sections provide details of test specimens and parameters, materials,

fabrication of specimens, and test setup and instrumentation.

3.2.1 Test Specimens and Parameters

A total of 12 steel columns were tested in this study, including three control

specimens A1-A3 and nine CFRP-strengthened columns B1-B9. The columns were all

2.6 m long standard S75x8 steel sections (Fig. 3-1) with pin-ended conditions, providing

a slenderness ratio of 197. Table 3-1 provides a summary of test specimens and

parameters. These parameters are:

(a) The inherent out-of-straightness of the steel column: This is considered the most

critical geometric imperfection in slender columns as it has some effect on the column’s

strength due to the induced small initial bending. As such, to assess other parameters, it is

important that both the control and strengthened specimens have comparable out-of-

19

straightness (δ) values. Table 3-1 provides the values of (δ) at mid-height of each column,

measured before applying the CFRP, along with the (δ/L) ratios, where (L) is the length

of the column. It should be noted that this study is focused on the column buckling about

the weak axis. As such, the (δ) values indicated here are those with respect to this

buckling direction. The values for the out-of-straightness in the strong axis for these

specimens are listed in Table A-1 in Appendix A. As can be seen in Fig. 3-2, the

specimens were grouped according to their (δ) values, in three categories as follows:

‘small’ with average δ = 0.31 mm, ‘medium’ with average δ = 1.68 mm and ‘large’ with

average δ = 2.55 mm. It is important to realize that steel sections are received with this

inherent imperfection and usually one has little control over this parameter, other than

attempting to group specimens of comparable values. It is also important to note that the

largest (δ) value of 2.91 mm (specimen B8), still satisfy the permissible limit of L/500 =

5.2 mm, which is given by CAN/CSA-G40.20-13 (2013) as an ‘accept-reject’ criterion of

steel sections. The effect of (δ) in this study can be assessed by comparing control

specimens A1, A2 and A3 of small, medium and large (δ) and retrofitted specimens B1,

B2 and B3 of (δ) values comparable to those of A1 to A3.

(b) CFRP modulus: The availability of high-and ultra-high modulus CFRP plates with

moduli well in excess of that of steel makes them a preferred alternative to retrofitting

slender columns using steel plates. CFRP plates with 168, 212 and 430 GPa moduli were

used in this study (Table 3-1). Their effect can be analyzed using specimens B3, B6 and

B8 of comparable (δ) and similar CFRP reinforcement ratio.

(c) CFRP reinforcement ratio: The CFRP reinforcement ratio (ρ) is defined as the ratio

of cross-sectional area of CFRP and the steel section, and ranged from 11 to 34%. It was

20

studied by comparing specimens B6 and B7 strengthened with the same 212 GPa CFRP,

similar (δ), but with one and two layers, respectively (Table 3-1). It is also possible to

compare B8 and B9 of the same 168 GPa CFRP but with one and three CFRP layers,

respectively. However, their (δ) values are not quite comparable.

(d) CFRP plate length: Given the buckling mode of pin-ended slender columns, the

maximum moment is generally at mid-height and is zero at both ends. As such, it may not

be necessary to provide full length CFRP plates. In this study CFRP plates covering the

middle third (33%), the middle two thirds (67%) and almost full length (95%) of the

column, were examined by comparing specimens B1 (or B2), B4 and B5, all with one

layer of the same 430 GPa CFRP (Table 3-1).

3.2.2 Materials

S-section steel columns: Standard S75x8 steel sections (Fig. 3-1) were used. Stub

column tests were conducted on two specimens of length 300 mm as per (Galambos,

1998). Figure 3-3 shows the resulting stress-strain curve in compression. The average

yield strength was 386 MPa.

CFRP plates: Three different types of Sika®CarboDur® unidirectional pultruded CFRP

plates were used in this study, namely: (a) UH514 plates (ultra-high modulus of 430

GPa), 50 mm wide and 1.4 mm thick, (b) M514 (high modulus of 212 GPa), 50 mm wide

and 1.4 mm thick, and (c) S512 (standard modulus of 168 GPa), 50 mm wide and 1.2 mm

thick. Tension tests were performed on coupons in accordance with ASTM

D3039/D3039M-08. Six coupons were performed for UH514 and S512 and four

coupons were performed for M514. Figure 3-3 shows the stress-strain curves of the three

CFRP types. The average ultimate tensile strengths of the UH514, M514, and S512

21

plates were 1273, 3270 and 2935 MPa, respectively. These values are very close to the

manufacturer reported values marked in Fig. 3-3. Table A1 in Appendix A gives a

summary of the coupon material properties.

Epoxy resin: Sikadur®30 resin was used. It is a two-component, solid, moisture-tolerant

epoxy adhesive. The mixing ratio was three parts component A to one part component B

by volume. The tensile strength and modulus of elasticity reported by the manufacturer

were 24.8 MPa and 4.48 GPa, respectively, while the shear strength of the epoxy was

24.8 MPa.

3.2.3 Fabrication of Test Specimens

The strengthening system was designed such that the CFRP plates were adhered

to the outer surface of the two flanges of the S-section in a symmetric manner about both

axes (Fig. 3-1). The full length CFRP plate was installed such that it was 60 mm short at

each end, from the end of the steel column, to accommodate the end bearing steel caps

and avoid direct bearing on CFRP. Also, in practice it may not be possible to apply CFRP

along the whole length of existing columns because of end connections to other members.

For columns with more than one CFRP layer, subsequent layers were 25 mm shorter than

previous layers, at each end. Miller (2000) stated that a minimum bond length of 100mm

was needed in order to fully transfer the load to the CFRP plates. This minimum length

is met for all specimens.

Steel sections were first sandblasted before application of the CFRP to enhance

bonding. The CFRP strips were cut to the appropriate length with a tile saw. The strips

and the steel sections were cleaned with acetone to remove any remaining dust particles

(Fig. 3-4(a)). The epoxy was mixed following the manufacturer’s instructions and a thin

22

layer was applied on the steel specimen (Fig. 3-4(b)). Glass beads (0.87 mm) were

spread along the column to ensure an even thickness of epoxy. The strips were pushed

through a special jig to ensure an even epoxy thickness of 1.5 mm (Fig. 3-4(c)). The

CFRP was laid on top of the steel and rolled down to release any excess epoxy (Fig. 3-

4(d)). The specimens were left to cure for a minimum of seven days before being tested.

3.2.4 Test Setup and Instrumentation

As shown in Fig. 3-5, the columns were tested under hinged end conditions with

respect to the direction of buckling, which in this study was about the weak axis of the S-

section. Each hinge was essentially a 12 mm diameter steel cylinder underneath a 12 mm

thick steel cap that is fitted snug to the end of the column by means of lips welded to the

steel plate. The cylinder was free to rotate within a lubricated space between two

restraining steel guides within the setup. The tests were performing using a two-way

hydraulic ram placed at the lower end of the column below the hinged end. A flat load

cell was placed below the hydraulic ram. The load cell, the hydraulic ram and the hinge

assembly were aligned in a concentric manner with proper attachments and bracings.

The hinged end at the top was similar to the bottom one but was attached to the heavy

steel beam of the reaction frame (Fig. 3-5).

Each specimen strengthened with CFRP was instrumented with electrical

resistance strain gauges attached to the CFRP plates on both sides (Fig. 3-5). Two linear

potentiometers (LPs) were used to measure axial displacement at the loading end of the

specimen (Fig. 3-5). Additional transverse LPs were used at various points along the

length, including mid-height, to measure lateral deflections (Fig. 3-5).

23

3.3 Experimental Results

Table 3-2 provides a summary of test results in terms of the maximum loads

reached for each specimen and the percentage gain in strength due to CFRP

strengthening, relative to the control counterparts (which are identified in Table 3-2 for

specimens B1 to B9). The table also provides the axial displacement at peak load, the

longitudinal CFRP strain at peak load at the outermost and innermost surfaces of the

specimen, as well as the failure modes. The increases in axial strength generally ranged

from 11 to 29%, depending on the various parameters studied. This gain in axial strength

is due to the fact that the CFRP delays the onset of global buckling by contributing to the

cross-sectional flexural rigidity (EI).

The load-axial displacement responses are provided in Figs. 3-6(a) to 3-9(a). The

axial displacement values are taken as the average of the two LPs located on two sides at

the loading end. The curves generally show a steep initial response before the peak load

is reached. The peak load is generally associated with the occurrence of global buckling.

Beyond the peak load, the axial displacement increases at a faster rate as the load

gradually descends. The figures generally show that although CFRP strengthening

increases the peak load, it has little effect on the initial axial stiffness, as evident by the

only slight increase in slope of the curves, relative to control specimens.

The load-lateral mid-height displacement responses are provided in Figs. 3-6(b) to

3-9(b). The figures show that the responses are quite steep initially before global

buckling occurs, but then excessive lateral deflection takes place. The CFRP

strengthening clearly reduces the lateral deflection at a given axial load.

24

The load-CFRP longitudinal strain responses are provided in Figs. 3-6(c) to 3-

9(c). The strains are given at both the innermost and outermost CFRP fibers. It can be

seen that initially both sides experience small compressive strains. As the peak load is

approached and global buckling takes place, the innermost surface strains increase

rapidly in compression while the outermost surface strains reduce in compression and

switch to tension. This is a result of the moment increase due to the excessive lateral

deflection.

The following sections provide a summary of the effect of various parameters on column

behavior.

3.3.1 Effect of Column Out-of-Straightness

Figure 3-10 shows the variation of ultimate load with out-of-straightness (δ) for

control specimens A1 to A3 and CFRP-strengthened specimens B1 to B3, which have

very close (δ) values to their control counterparts. All other CFRP parameters were kept

constant. Figure 3-10 also shows the variation of the percentage gain in strength with (δ).

It can be seen that the strength of both the control and retrofitted columns reduces as (δ)

increases. This is because of the increased moment associated with the increased lateral

deflection, which can be seen in Fig. 3-6(b) where the lateral deflection of specimens A1

and B1 is less than those of A3 and B3. On the other hand, Fig. 3-10 shows that the

percentage gain in axial strength due to CFRP retrofitting increased from 11 to 29% as

(δ) increased from 0.38 to 2.5 mm. It is clear the CFRP strengthening becomes more

effective in columns that originally have a larger geometric imperfection. This finding is

consistent with the observations of Shaat and Fam (2007) for Hollow Square Section

(HSS) columns.

25

It is noted that the maximum out-of-straightness value used is still approximately

half of the permitted standard. Therefore, the results can be considered conservative with

respect to the gain that could be experienced.

3.3.2 Effect of CFRP Modulus

Figure 3-11 shows the variation of ultimate loads of specimens B8, B6 and B3

with CFRP of Young’s moduli of 168, 212 and 430 GPa, respectively, as well as the

percentage gain in strength, compared to control specimen A3. All specimens have

comparable values of ‘large’ (δ) ranging from 2.4 to 2.91 mm and almost similar amounts

of CFRP (the very small difference in CFRP plate thickness between the 168 GPa plate

(1.2 mm) and the other CFRP plates (1.4 mm) results in an insignificant difference

(2.3%) in the transformed section flexural stiffness (EsIt), where Es is the Young’s

modulus of steel (200 GPa) and It is the transformed section moment of inertia). Figure

3-11 shows that CFRP Young’s modulus has a considerable effect on both the ultimate

load and the percentage gain in strength. As the modulus increased from 168 to 430 GPa,

the percentage gain in strength increased from 12 to 29%. This is an important finding

and is the first time to be demonstrated experimentally for columns. It is clear that the

variation of strength gain with Young’s modulus is highly nonlinear where it appears to

approach a flat plateau at higher moduli. It is also worth observing in Fig. 3-7(b) how the

lateral deflection of the column reduces as the CFRP modulus increase.

3.3.3 Effect of CFRP Reinforcement Ratio

Figure 3-12 shows the variation of ultimate loads and percentage gain in strength

with CFRP reinforcement ratio (ρ). The variation is given for specimens B6 and B7 of

comparable out-of-straightness values and same CFRP modulus, compared to their

26

control counterpart A3. The comparison is also given for specimens B8 and B9 with the

same CFRP modulus but different out-of-straightness values, compared to their control

counterparts A3 and A2. It can be seen that the percentage gain in strength increases

with (ρ), almost at the same rate for both CFRP types, with the higher gain occurring for

the larger CFRP modulus as discussed earlier. As (ρ) increased from 11 to 34%, the gain

in strength increased from 12 to 23% for the 168 GPa CFRP and as (ρ) increased from 13

to 26%, the gain in strength increased from 21 to 26% for the 212 GPa CFRP. It is also

noted that the rate of gain is relatively low. For example doubling the amount of the 212

GPa CFRP led to increasing the percent gain in strength by only 23%.

3.3.4 Effect of CFRP Length

Figure 3-13 shows the variation of ultimate load and percentage gain in strength

with the length of CFRP plates. Specimens B1 and B4 had the same CFRP plate

modulus and reinforcement ratio and comparable out-of-straightness, but the lengths of

the plates were 67% and 95% of the column length, respectively. Figure 3-13 shows that

the ultimate strength and percentage gain in strength relative to the control counterpart

A1 are the same for both specimens, suggesting that a CFRP plate length of two thirds of

the column length is sufficient. Figure 3-13 also compares specimens B2 and B5, which

only differ in plate lengths, being 33% and 95%, respectively, of column length. It can

be seen that the ultimate load reduced by only 4% as the plate length reduced to only one

third of the column length. From those two cases, it can be seen that it is not necessary to

have the CFRP plates extending the full length of the column. A short CFRP plate of a

length between one third and two thirds of the column, centered about the column mid-

27

height, is sufficient to achieve the full strength of a column strengthened with a full-

length plate.

3.3.5 Failure Modes

All three control specimens, with various levels of out-of-straightness, failed

similarly due to the classical global buckling of the pin-ended slender column. All CFRP-

strengthened specimens also experienced similar global buckling (Fig. 3-14(a)).

However, after the peak load was reached, this was also followed by local failure of

CFRP plates, with cracking sounds that were heard, in specimens with 430 GPa CFRP

plates. Specimens B1 to B4 experienced CFRP crushing on the compression side (Fig. 3-

14(b)), followed by CFRP rupture on the tension side (Fig. 3-14(c)). Specimens B5 (with

the shortest length of 430 GPa CFRP plate) and B6 to B9 (with the 212 GPa and 168 GPa

CFRP full length plates) all experienced just global buckling with no observed CFRP

material failure. It is interesting to note the deformed shape of specimen B5 with the short

CFRP plate (Fig. 3-14(a) right), where a distinct change in curvature (‘kink’) appears in

the buckled column, right where the CFRP plate terminates, due to the sudden change in

stiffness of the column. In this specimen, cracking sounds were noticed after the peak

load but no CFRP material failure was observed.

The average strains on the CFRP extreme compression side of specimens B1 to

B4 at CFRP crushing failure are 0.00205 and 0.00246 for the full length and 2/3 CFRP

length, respectively (Fig. 3-9(c)). These compressive strains are approximately 68% and

81%, respectively, of the tensile rupture strain of this CFRP from the coupon tests (Fig.

3-3). Since the specimens that have CFRP crushing on the compression side also had

CFRP rupture on the tension side, the tensile average strain values, at rupture, were also

28

examined. The average strains on the extreme tension side were 0.004% and 0.0045%

for the full length and 2/3 CFRP length, respectively. These values are 32% and 50%,

respectively, greater than the rupture strains measured from the tension coupon tests (Fig.

3-3). It should be noted, however, that the CFRP plate in the column configuration tested

in this study was placed through an in-plane bending, where one edge of the CFRP plate

is in compression and the other edge is in tension. As such, a steep strain gradient occurs.

On the other hand, the tension coupons were under uniform direct tension. It is also

possible that the CFRP is actually higher in strength than what the coupon results and the

manufacturer data show.

3.4 Summary

The traditional Euler’s buckling theory of slender columns indicates that column

capacity depends on flexural rigidity (EI), rather than material strength. As such, the

availability of ultra-high modulus CFRP plates, which could be much stiffer than steel,

can offer a unique alternative for strengthening slender steel columns, in lieu of welding

or bolting steel plates. In this study, twelve 2.6 m long S75x8 steel columns of 197

slenderness ratio, that represents the upper limit permitted by code, were tested under

concentric axial loading using pin-ended conditions. The columns were allowed to buckle

around their weak axes. CFRP plates were adhesively bonded to the flanges of the steel

I-shape sections in nine of the columns. The main parameters studied were the level of

initial out-of-straightness (length (L)/8387 to L/1020), CFRP modulus (168 to 430 GPa),

CFRP reinforcement ratio (13% to 34%) and the length of CFRP plate (33% to 95% of

L). The gain in axial strength due to CFRP retrofitting ranged from 11% to 29%,

depending on the various parameters. The gain generally increased as CFRP modulus,

29

initial out-of-straightness, or CFRP reinforcement ratio increased. Global buckling

consistently governed the maximum load. In the case of the 430 GPa CFRP, buckling

was followed by CFRP crushing in compression, then rupture in tension.

30

Table 3-1: Test matrix

Specimen I.D

Out-of-straightness

δ (mm)

Out-of-straightness

/ Length (δ/L) ratio

CFRP Young's

Modulus ECFRP (GPa)

No. of CFRP

Layers

CFRP Reinforc.

ratio ρ (%)

CFRP Length LCFRP (m) and

% age of Lsteel

A1 0.31 0.00012

N/A (Control) A2 1.59 0.00061

A3 2.48 0.00095

B1 0.38 0.00015

430 1 13.1

2.48 (95%) B2 1.83 0.00070

B3 2.50 0.00096

B4 0.21 0.00008 1.73 (67%)

B5 1.70 0.00065 0.87 (33%)

B6 2.40 0.00092 212

2.48 (95%) B7 2.46 0.00095 2 26.2

B8 2.91 0.00112 168

1 11.2

B9 1.60 0.00062 3 33.6

31

Table 3-2: Summary of column test results

Spec. I.D Control

Counter-part

Peak Load (kN)

Avg. Peak Load (kN)

% gain in strength

Avg. % gain in strength

Lateral Disp. @ Peak Load (mm)

Axial Strain @ Peak Load (με) Failure Mode

Outermost Innermost

A1 N/A

(Control)

38.1

33.77 N/A (Control)

12.7

Global Buckling (GB) A2 32.4 41.4

A3 30.8 42.2

B1 A1 42.4

40.9

11.4

21.1

36.2 1250 -1762 GB then CFRP crushing, then CFRP tensile

rupture

B2 A2 40.4 24.8 36.8 1314 -1761

B3 A3 39.9 29.2 43.4 1306 -1696

B4 A1 42.5

11.6

32.8 1297 -1913

B5 A2 38.9 20.1 47.9 1485 -2019

Global Buckling (GB)

B6

A3

37.2 20.5 38.02 1196 -1616

B7 38.7 25.5 34.3 1148 -1510

B8 34.5 11.9 46.7 1527 -1916

B9 A2 39.9 23.2 51.34 1822 -2280

32

Figure 3-1: Test specimens

Figure 3-2: Steel columns out-of-straightness at mid-height before retrofitting

(a) S75x8 steel as received

(b) Cross-section configurations

1 CFRP layer 2 CFRP layers 3 CFRP layers

6.6

77

16

60

5

0

0.5

1

1.5

2

2.5

3

3.5

A1 B1 B4 A2 B2 B5 B9 A3 B3 B6 B7 B8

Ou

t o

f st

raig

htn

ess

(mm

)

Specimen I.D

1.68 mm Avg.

0.31 mm Avg.

2.55 mm Avg.

33

Figure 3-3: Tensile stress-strain responses of different CFRP plates and steel

Figure 3-4: CFRP installation

0

500

1000

1500

2000

2500

3000

3500

4000

0 5000 10000 15000 20000

Stre

ss (

MP

a)

Strain (με)

430 GPa

212 GPa

168 GPa

Manufacturer strength

Steel

(a) Cleaning of sandblasted steel surface and CFRP strips

(b) Applying epoxy to steel

(c) Applying epoxy to CFRP (d) Rolling the installed CFRP strip

34

Figure 3-5: Test setup

Hinged end

Hinged end

LP

LP

LP

LP LP

Specimen

CFRP plate

30mm deep

sleeve

Fixed reference

Hydraulic ram Load cell

LP

CFRP plates

Roller

Roller

Front View Side View

18.5 mm

11.5mm

5

mm

Strain gauges

Roller

Strain gauges

Hinged end

Hydraulic ram

Load cell

LP’s

LP

LP

LP

Hinged end

Specimen

35

Figure 3-6: The effect of out-of-straightness on columns strengthened using 430GPa CFRP

0

5

10

15

20

25

30

35

40

45

-5 5 15 25 35 45 55 65 75 85 95

Load

(kN

)

Lateral Displacement (mm)

0

5

10

15

20

25

30

35

40

45

-5000 -4000 -3000 -2000 -1000 0

Load

(kN

)

Innermost Strain (με)

0

5

10

15

20

25

30

35

40

45

-500 500 1500 2500 3500 4500

Load

(kN

)

Outermost Strain (με)

a) Load-axial displacement response b) Load-lateral displacement response

c) Load-longitudinal CFRP strains response

B3

B2

A1 A3

A2

B1

B1 B1

B2 B3

B2

B3

0

5

10

15

20

25

30

35

40

45

0 2 4 6 8 10

Load

(kN

)

Axial Displacement (mm)

B1 (δ/L = 0.00015) B3 (δ/L =0.00096)

A1 (Control, δ/L =0.00012)

A2 (Control, δ/L =0.00061)

A3 (Control, δ/L =0.00095)

B2 (δ/L =

0.00070)

36

Figure 3-7: The effect of CFRP modulus on columns of comparable out-of-straightness

0

5

10

15

20

25

30

35

40

45

0 2 4 6 8 10

Load

(kN

)


B8 (168 GPa)

B6 (212 GPa)

A3 (Control)

B3 (430 GPa)

0

5

10

15

20

25

30

35

40

45

-5 5 15 25 35 45 55 65 75 85 95

Load

(kN

)


0

5

10

15

20

25

30

35

40

45

-5000 -4000 -3000 -2000 -1000 0

Load

(kN

)

Strain (με)

0

5

10

15

20

25

30

35

40

45

-500 500 1500 2500 3500 4500

Load

(kN

)

Strain (με)



B3

A3

B6

B8

B8

B8 B6 B3

B6

B3

37

Figure 3-8: The effect of CFRP reinforcement ratio

0

5

10

15

20

25

30

35

40

45

-500 500 1500 2500 3500 4500

Load

(kN

)

Strain (με)

0

5

10

15

20

25

30

35

40

45

-5000 -4000 -3000 -2000 -1000 0

Load

(kN

)

Strain (με)

0

5

10

15

20

25

30

35

40

45

-5 5 15 25 35 45 55 65 75 85 95

Load

(kN

)


0

5

10

15

20

25

30

35

40

45

0 2 4 6 8 10

Load

(kN

)




B8 (ρf=11%, δ/L=0.00112)

A2 (Control, δ/L=0.00061)


B8

B7

B6

A3

A2

B9

B8

B9

B6

B7

B6

B7

B7 (ρf=26%, δ/L=0.00095)

B6 (ρf=13%, δ/L=0.00092)

B9 (ρf=34%, δ/L=0.00062)

B8

B9

38

Figure 3-9: The effect of the 430 GPa CFRP length ratio

Figure 3-10: Effect of out-of-straightness on ultimate loads

0

5

10

15

20

25

30

35

40

45

-5000 -4000 -3000 -2000 -1000 0

Load

(kN

)

Strain (με)

0

5

10

15

20

25

30

35

40

45

-500 500 1500 2500 3500 4500

Load

(kN

)

Strain (με)

0

5

10

15

20

25

30

35

40

45

-5 5 15 25 35 45 55 65 75 85 95

Load

(kN

)


0

5

10

15

20

25

30

35

40

45

0 2 4 6 8 10

Load

(kN

)






B4 (LCFRP

/Lst=67%, δ/L=0.00008)

B4

A1

B2

A2

B1

B1 B1

B2

B4

B2

B5

B5 (LCFRP

/Lst= 33%, δ/L =0.00065)

B1 (LCFRP

/Lst=95%, δ/L=0.00015)

B2 (LCFRP

/Lst=95%, δ/L=0.0007)

B5

B5

B4

0

5

10

15

20

25

30

35

40

45

28

30

32

34

36

38

40

42

44

0 0.5 1 1.5 2 2.5 3

% g

ain

in P

ult

Ult

imat

e ax

ial l

oad

(kN

) P u

lt

Out-of-straightness (mm)

ρCFRP

= 13% ECFRP

= 430 GPa LCFRP

/Lst

= 95%

A1

A2 A3

B1

B2

B3

39

Figure 3-11: Effect of CFRP modulus on ultimate loads

Figure 3-12: Effect of CFRP reinforcement ratio on ultimate loads

0

5

10

15

20

25

30

35

40

45

28

30

32

34

36

38

40

42

44

150 200 250 300 350 400 450

% g

ain

in P

ult

Ult

imat

e Lo

ad (

kN)

Pu

lt

CFRP Elastic Modulus (GPa)

Pult

of unstrengthened

ρCFRP

=13% δ/L=0.00092-0.00112 LCFRP

/Lst

=95%

B8

B6

B3

A3 A3 A3

0

5

10

15

20

25

30

35

40

45

28

30

32

34

36

38

40

42

44

10 15 20 25 30 35%

gai

n in

Pu

lt

Ult

imat

e Lo

ad (

kN)

Pu

lt

Reinforcement Ratio ρ (%)

B9

A3

B7

B8

B6

A2

A3 A3

ECFRP

= 212GPa

40

Figure 3-13: Effect of CFRP length on ultimate loads

Figure 3-14: Failure modes

0

5

10

15

20

25

30

35

40

45

28

30

32

34

36

38

40

42

44

30 50 70 90

% g

ain

in P

ult

Ult

imat

e Lo

ad (

kN)

Pu

lt

LCFRP/Lst (%)

Pult

of strengthened B5

% gain in Pult

Pult

of unstrengthened

B1 B4

B2

A2

A1 A1

A2

ρCFRP

=13% ECFRP

= 430 GPa

a) Global buckling b) CFRP crushing in compression at innermost face

c) CFRP tensile rupture at outermost surface

41

Chapter 4

Strengthening Long Steel Columns of S-Sections against Global

buckling around Strong Axis using CFRP Plates of various Moduli

4.1 Introduction

Conventional methods to strengthen a steel member include bolting or welding

additional plates onto the member. Heavy equipment or shoring is generally needed to

lift plates into place for installation and the plates add to the dead load of the structure.

Replacement plates are susceptible to corrosion, and welding can generate thermally

induced stresses or heat affected areas leading to stress concentrations (Grabovac et al.,

1991) and potential fatigue cracking. As a result, there is interest in investigating Fibre

Reinforced Polymers (FRP) as a means of strengthening steel members. FRPs are

lightweight, non-corrosive, have minimal impact visually, have minimal effect on

clearance and are available in rigid laminate plates or flexible sheets that provide

flexibility for installation. FRP systems have a large range of uses including but not

limited to rehabilitating existing structural elements, retrofitting or strengthening

structurally sound members or correcting construction mistakes.

Research on steel columns retrofitted or strengthened with FRPs is limited.

Kalavagunta et al. (2014) observed that adding carbon fibre reinforced polymer (CFRP)

to cold formed lipped steel channels can increase capacity by 16.75%. Silvestre et al.

(2008) strengthened C-channels by 19.8% by wrapping CFRP around the entire column,

although in some cases brittle failures were observed.

42

Bambach & Elchalakani (2007) and Bambach et al.(2009) strengthened square

hollow sections with different fibre layouts and were able to observe axial capacities up

to two times the capacity of the plain steel columns. However, there was a plateau in the

gain beyond a plate slenderness ratio of 2.5 (Bambach et al., 2009). Shaat and Fam

(2006) and Shaat and Fam (2009) strengthened square hollow sections with CFRP sheets,

observing axial capacity gains up to 71% and compression failure strains of 58% of the

tensile rupture strain. Rather than using bonded CFRP, Liu et al. (2005) retrofitted steel-

notched S-sections with an open GFRP jacket filled with expansive lightweight concrete.

Previous research of steel columns strengthened using bonded FRP has mainly

focused on symmetrical hollow sections. Many steel structures employ bi-symmetric I-

shaped columns.

This chapter hopes to prove that strengthening steel S-sections with CFRP strips

along both flanges will have a significant gain in axial capacity in the strong axis. The

columns are braced to ensure buckling about the strong axis. Increases in capacity are

compared to strengthened columns dominated by weak axis buckling. The purpose of the

investigation is to determine the type, and number of layers of CFRP that will produce

the greatest increase in capacity.

4.2 Experimental Program

The following sections provide details about the test specimens and parameters,

materials, fabrication of test specimens, instrumentation and test setup.

4.2.1 Test Specimens and Parameters

A total of eight steel columns were tested to failure. This included three control

specimens, CX1-CX3 and five strengthened specimens by bonding CFRP plates to the

43

flanges, CX4-CX8. The specimens chosen for this experiment were 2.6m long standard

steel S75x8 sections, with a slenderness ratio of 83 (Figure 4-1(a)). The parameters

examined in this study are provided in Table 4-1. The parameters are:

(a) The axis of bending: The specimens tested in this study were all tested for buckling

about the strong axis. Five sets of bracing were used to ensure that the column buckled in

the correct direction, details are found in Appendix A. These columns were compared to

specimens tested in the weak axis with similar parameters presented in the previous

chapter.

(b) The inherent out-of-straightness of the steel column: This is considered the most

critical geometric imperfection in slender columns as it has some effect on the column’s

strength due to the induced small initial bending. To assess other parameters, it is

important that both the control and strengthened specimens have comparable out-of-

straightness (δ) values. Table 4-1 provides the values of (δ) at mid-height of each column,

measured before applying the CFRP, as well as the (δ/L) ratios, where (L) is the length of

the column. The (δ) values indicated here are those with respect to the buckling direction.

The (δ) values were also measured for the weak axis of these specimens and the values

are shown in Table A-2 in Appendix A. As can be seen in Fig. 4-2, the specimens were

grouped according to their (δ) values, in two categories as follows: ‘small’ with average δ

= 0.55 mm, and ‘large’ with average δ = 1.47 mm. It is important to realize that steel

sections are received with this natural imperfection and usually one has little control over

this parameter, other than attempting to group specimens of comparable values. Also

note that the largest (δ) value of 1.59 mm (specimen CX3), still satisfies the permissible

limit of L/500 = 5.2 mm, which is given by CAN/CSA-G40.20-13 (2013) as an ‘accept-

44

reject’ criterion of steel sections. The effect of (δ) in this study can be evaluated by

comparing control specimens CX1 and CX3 of small and large (δ) with retrofitted

specimens CX7 and CX8 of (δ) values comparable to those of CX1 and CX3.

(c) CFRP modulus: The availability of high-and ultra-high modulus CFRP plates with

moduli much larger than that of steel makes them a preferred alternative to retrofitting

slender columns using steel plates. CFRP plates with 168, 212 and 430 GPa moduli were

used in this study (Table 4-1). The effect between the ultra-high modulus and normal

modulus can be examined using specimens CX4 and CX6 of comparable (δ) and similar

CFRP reinforcement ratio. The effect between the high modulus and normal modulus can

be assessed using specimens CX5 and CX7 of comparable (δ) and transforming CX5 to a

comparable CFRP reinforcement ratio.

(d) CFRP reinforcement ratio: The CFRP reinforcement ratio (ρ) is defined as the ratio

of cross-sectional area of CFRP and the steel section (Equation 1), and ranged from 11 to

34%. It was studied by comparing specimens CX6 and CX7 strengthened with the same

168 GPa CFRP, similar (δ), but with one and three layers, respectively (Table 4-1).

(

) (1)

4.2.2 Materials

S-Section Steel Columns: Standard steel S75x8 sections (Figure 4-1) were tested. Stub

column tests were completed on two specimens of length 300 mm as per (Galambos,

1998) (Figure 4-3). The average yield strength is 386MPa.

CFRP Plates: Three different types of Sika®CarboDur® unidirectional pultruded carbon

fibre plates were used in this study: 1) Sika® CarboDur® UH514 Plates (ultra-high

45

modulus – 430GPa) 50mm x 1.4mm plates; 2) Sika® CarboDur® M514: (high modulus

– 212GPa) 50 mm x 1.4 mm plates; 3) Sika® CarboDur® S512: (normal modulus –

168GPa) 50 mm x 1.2 mm plates. Tension tests were performed on coupons in

accordance 1with ASTM D3039/D3039M-08(ASTM, 2000) (Figure 4-3). Six coupon

tests were performed for UH514 and S512 and four coupons were performed for M514.

Table A4, in the Appendix A, summarizes the average measured properties and standard

deviation for each CFRP.

Epoxy Resin: Sikadur®30 was the epoxy resin used. It is a two-component, solid,

moisture-tolerant epoxy adhesive. The mixing ratio was three parts component A and

one part component B by volume. The tensile strength and modulus of elasticity reported

by the manufacturer were 24.8MPa and 4.48GPa, respectively. The shear strength of the

epoxy was listed as 24.8MPa.

4.2.3 Fabrication of Test Specimens

Steel sections were sandblasted to enhance bonding of the CFRP. The CFRP

strips were cut to the appropriate lengths with a tile saw. The CFRP strips were centred

on each flange before bonding. With the exception of CX5, 60 mm was left at each

column end since it may not be possible to access the ends of an existing column, and to

ensure that there would not be any bearing on the strips from the support plates, which

reach up 30mm on each end of the column. The columns with more than one layer

applied, had a distance of 25mm from the end of one layer to the next, with the exception

of CX5.

46

The strips and the steel sections were cleaned with acetone to remove any

remaining dust particles (Figure 4-4 (a)). The epoxy was mixed following the

manufacturer’s instructions and a thin layer was applied on the steel specimen (Figure 4-

4 (b)). Glass beads (0.87mm) were spread along the column to help promote an even

thickness of the epoxy. The strips were pushed through a jig to ensure an even epoxy

thickness of 1.5mm (Figure 4-4 (c)). The CFRP was laid on top of the steel and rolled

down to release any excess epoxy (Figure 4-4 (d)). The specimens were cured for a

minimum of 7 days before being tested.

4.2.4 Instrumentation and Test Setup

Each specimen was instrumented with electrical resistance strain gauges. The

first few tests had 12 strain gauges, two on each flange at the quarter points, to measure

the strain in the CFRP or steel during the test. The strain responses for these specimens

are included in Appendix B. The later tests only had four gauges, two on each opposite

flanges located at the centre of the column (Figure 4-5a). During testing, linear

potentiometers (LPs) were located along column quarter points to record the lateral

displacement of the column. To measure the axial displacement and the rotation of the

column ends, two more LP’s were placed at the bottom of the column. Another LP was

near the bottom of the column in between two braces to measure any out of plane

movement that could have occurred. The LP measurements were also used to ensure that

the specimen loading was axial and uniform.

Figure 4-5 shows a schematic of the setup and a photo of the test setup during

testing. The end supports were pinned. The column was braced approximately every

430mm to ensure that buckling would occur about the strong axis.

47

4.3 Experimental Results

A summary of all the tests performed is included in Table 4-2. Data for the

columns tested in the weak axis is included from the previous chapter. The ultimate peak

load (kN), the maximum lateral displacement at the peak load, the axial strain at peak

load and the failure modes are included in the table. The percentage increase or decrease

of the strength of the CFRP strengthened specimens compared to their counterpart control

specimens (similar out-of-straightness values) is also included in Table 4-2. As well, the

table includes the average values for specimens that had the same parameters. As seen

from the table, changes in axial strength ranged from a decrease of 1% to an increase of

25%. The gain in axial strength is due to the CFRP delaying the onset of global

buckling.

4.3.1 Load –Deflection Behaviour

The load-axial displacement response for each column tested is presented in

Figures 4-6 to 4-8 (a). The axial displacement values are taken as an average of the two

LPs located near the bottom of the columns. Each plot includes the data for control and

CFRP-strengthened specimens. The results show that after the peak load was reached, a

sudden drop in the load occurred along with a relatively small increase in the axial

displacement.

The load versus mid-height lateral displacement response for each column tested

is presented in Figures 4-6 to 4-8 (b). In general, there is negligible lateral displacement

until very close to the peak load. After reaching the peak load, the lateral displacements

increase rapidly due to the global buckling of the column.

48

4.3.2 Load-Strain Behaviour

The load versus mid-height longitudinal strain response for each test is presented

in Figures 4-6 to 4-8 (c). The strains for each specimen are in compression on both sides

of the column until after the peak load was reached. The ‘innermost fibre’ develops

flexural compressive strain after buckling, while the ‘outermost fibre’ develops flexural

tensile strain after buckling.

In cases where delamination of the CFRP occurred at the peak load, (Figures 4-6

and 4-8 (c), specimens CX7, CX8 and CX5) the slope of the ‘innermost’ strain curve

reversed (i.e. strains became smaller) but then quickly reversed again to continue

increasing in compression. Examining the strains of the control specimens at the peak

load, in Table 4-2, on the ‘innermost’ side of the column shows that the yield strain of the

steel was exceeded and plastic deformations did occur. The strengthened specimens,

with the exception of CX6, showed strains at peak lower than the yield strain of steel,

therefore, the buckling experienced at peak was elastic. All strains experienced at the

peak load are lower than the tensile rupture strains of the coupons performed. The

specimen that failed due to the CFRP crushing at the peak load experienced a strain on

the ‘innermost’ side that was 54% of the tensile rupture strain. It is also noted that

compared to the control specimens, the strengthened specimens experienced less bending,

with the exception of CX6 and CX4.

4.3.3 Effect of Out-of-Straightness

In the following sections, various parameters that affect the strength gain of CFRP

strengthened columns will be discussed. In most cases, the discussion is based on just two

test values and linear trends are assumed. It should be noted that nominally identical

49

specimens (e.g. the controls CX1, CX2, CX3; and, strengthened specimens CX7 and

CX8) had failure loads within a 10% range, indicating that the test apparatus provides

repeatable results, and the scatter in results is small. However, without additional data,

the reader is reminded that the assumption of a linear trend should be treated with

caution.

To assess the effect of different initial out-of-straightness values, CX7 and CX8

were compared. These specimens were strengthened with the same amount

(reinforcement ratio of 33.6%) of 168GPa CFRP and so are nominally identical except

for their initial (before application of CFRP) out-of-straightness. Figure 4-9 compares

the ultimate loads of the strengthened specimens and their counterpart controls

(unstrengthened columns). Percentage strength gains are calculated by finding the

percent difference between the strengthened and control specimens with similar out-of-

straightness values.

It is well-known for plain steel columns that an increased out-of-straightness

reduces the capacity of a column. The results indicate that the strengthened column with

large out-of-straightness had a higher capacity than the similarly strengthened column

with a low out-of-straightness. In other words, the percent gain in capacity as a result of

strengthening the columns increases as the initial out-of-straightness increases.

4.3.4 Effect of the Reinforcement Ratio and Young’s Modulus

Figure 4-10 examines the effect of increasing the reinforcement ratio, i.e the area

of CFRP relative to the area of the steel as well as the effect of increasing the CFRP

modulus. To determine the effect of different reinforcement ratios, two different

modulus types had to be used, 212GPa and 168GPa. Specimens CX6 and CX7 had

50

reinforcement ratios of 11.2% and 33.6% and were both strengthened with 168GPa.

Their ultimate loads of strengthened and unstrengthened specimens are shown as solid

lines in the figure and the percentage gain in capacity is shown in the figure by the dotted

line. Specimen CX5 was strengthened using 212GPa and had a reinforcement ratio of

26.2%. The strengthened load and percentage gain are shown as single dots in the figure.

Since specimen CX5 used a different modulus, the reinforcement ratio was transformed

by multiplying by the ratio of the modulus (212GPa/168GPa) to get a reinforcement ratio

of 33.1%. These are also shown on the figure as points and are now able to be compared

to the percentage gain experienced by CX7. All specimens were grouped together due to

similar small initial out-of-straightness values.

The highest percentage increase of 18% is seen in CX5 with the transformed ratio

of 33.1% reinforcement ratio. The gains observed for specimen CX7 and CX5 were 15%

and 18% respectively. This difference could be due to the difference in modulus,

meaning that strengthening with a higher modulus with similar reinforcement ratios does

show an increase in percentage gain in strength of 20%. This increase can only be seen if

the compressive failure strain of the CFRP is high enough to ensure the CFRP does not

fail at the peak load. When comparing specimens of the same modulus (168GPa) but

different ratios of 11.2% and 33.6%, a difference of 5.4% and 15.1% respectively is

observed. This means that there is more of a gain observed when increasing the

reinforcement ratio rather than increasing the modulus, but both are effective in

increasing the strength of a column.

Figure 4-11 examines the effect of CFRP modulus on column strength gains.

Two modulus values are compared: 430GPa, (CX4) and 168GPa, (CX6). The

51

reinforcement ratio of these specimens is similar and the initial out-of-straightness values

are considered to be small. The ultimate loads of the unstrengthened and strengthened

specimens are shown in the figure as well as the percentage gains for each specimen.

The lower modulus, CX6, gives a larger percentage gain in the load, 5.35% compared to

a decrease for specimen CX4 of -1.0%. Comparatively the increase seen for CX6 is still

small compared to the increases seen above with Figure 4-10.

Generally, an increase in percentage gain would be expected when strengthening

a specimen with a higher modulus material but for these specimens that was not the case

due to different failure modes. Specimen CX6 failed due to global buckling, and a

secondary failure of CFRP crushing occurred sometime after the peak load was reached.

This specimen experienced a larger amount of bending at the peak load. Comparatively,

specimen CX4 did not undergo as much bending at ultimate and the CFRP crushing

caused failure. The 430GPa CFRP provides more stiffness but because of its low rupture

strain the column failed at the same load as the control specimen, meaning it essentially

does not strengthen the column. This means that there is a threshold at some point past

212GPa where an increase in modulus no longer shows an increase in strength due to the

rupture strain of the material being so much lower.

4.3.5 Effect of Axis of Bending During Buckling

The columns tested in the current chapter were braced to force buckling to occur

by bending about the strong axis (x-axis). Previous tests performed in Chapter 3 involved

similar sections that were not braced and hence at buckling underwent bending about the

weak axis (y-axis). Figure 4-12 shows the peak load and percentage increase in

specimens buckling about the strong axis (CX4 and CX8) and comparable specimens

52

buckling about the weak axis (CY1, CY2 and CY3). CX8 and CY3 had similar initial

out-of-straightness values, and were strengthened with the same type and amount of

CFRP, and thus differed only in whether they were braced (CX8) or not (CY3). CX4,

CY1, and CY2 were strengthened with the same type and amount of CFRP. However,

CY1 had a slightly lower initial out-of-straightness than CX4, while CY2 had a slightly

larger initial out-of-straightness than CX4. Although not directly comparable to CX4, it is

expected that the behaviour of CY1 and CY2 should ‘bracket’ that of a comparable

unbraced column for CX4.

The ultimate loads for CX4 and CX8 are shown in Figure 4-12 as points and are

compared to their counterpart control specimens to produce the percentage gain shown by

the hollow points. The ultimate loads for CY1, CY2 and CY3 are shown by three points,

to show the comparison with the small initial out-of-straightness, and the large initial out-

of-straightness. The percentage gains for all three points are again shown with by hollow

points. The points are compared based on the type of CFRP that was applied.

Specimens CX8 and CY3 have the same reinforcement ratio and are strengthened

with the same type of CFRP (168GPa). Their percentage gains are shown to be very

similar 24.7% and 23.2%, respectively. Each specimen experienced different failure

modes. Specimen CY3, buckling about the weak axis, failed due to global buckling and

no visible sign of failure in the CFRP, whereas the specimen CX8, buckling about the

strong axis, exhibited debonding of the CFRP at the top of the column.

Observing specimens CX4, CY1 and CY2 shows an increase in the weak axis of

11.4% and 24.8%, for CY1 and CY2, respectively, and a decrease in the strong axis of

1%. This is not consistent with what was observed for specimen CX8 and CY3.

53

Specimens tested in the weak axis experienced global buckling, and then crushing and

rupture of the CFRP after the peak load was reached. Bending about the strong axis

experienced global buckling and CFRP crushing occurring at the peak load. The

percentage gain in strength would be expected to be similar, whether buckling about the

strong or weak axis, as long as the compressive failure strain of the CFRP is large enough

to ensure it does not fail at the peak load. In the case of CX4, the CFRP crushed at 54%

of the tensile rupture strain where specimen CX8 did not reach anywhere close to its

tensile rupture strain at the peak load.

4.3.6 Failure Modes

All of the unstrengthened specimens failed by overall buckling (Figure 4-13a).

All of the CFRP strengthened specimens also experienced buckling but in some cases

rupturing or debonding of the CFRP played a role in the failure. The column that failed

due to crushing of the CFRP was the column strengthened using 430GPa CFRP, CX4

(Figure 4-13b). During testing of specimen CX4, popping noises were heard before the

peak load was reached (around 180kN). As the column buckled a crunching noise was

heard indicating CFRP crushing (Figure 4-13b). As the column was loaded past the peak,

cracking of the CFRP spread across the width of the strip.

Along with overall buckling the columns experienced secondary failure modes

occurring after the peak load was reached. For specimen CX6 this involved CFRP

crushing, at approximately 100kN, after the peak load was reached (Figure 4-13 (c)).

Another secondary failure mode observed occurred in the specimens strengthened with

212GPa and 168GPa, which both experienced debonding after the peak load was

reached. Debonding was only seen with specimens that had two or three layers of CFRP.

54

For CX5, after the peak load was reached, (approximately 175kN) the CFRP on the

compression side delaminated at the top (Figure 4-13d). It is believed it was pinched by

the end supports due to the fact that there was not enough room left between the

termination of the CFRP and the support. Specimens CX7 and CX8 failed in an identical

manner, both delaminating at the top on the compression side, similar to CX5. During

the tests, the load stabilized at the peak and then eventually started to decrease. Slowly

the column started to buckle and the CFRP started to delaminate along with it (Figure 4-

13e).

Carefully examining the strain on the ‘innermost fibre’ side specimen CX4 in

Figure 4-7 (c) it is seen that at 183kN before peak load a maximum average of the two

strain gauges on this side was 0.121%, then one of the gauges popped off. The second

gauge continued to read up until the peak load where the maximum value was 0.162%

(Table 4-2). These strains are 40% and 54% of the coupon tensile rupture strains. The

strain observed at peak in specimen CX6, shown in Figure 4-7 (c), are 0.207%. The

gauges measured a drop in the strain at 100kN, when the crushing occurred.

Strain data for specimens strengthened with 212GPa and 168GPa showed

evidence the secondary failure mode of debonding just after the peak load. For CX5

(Figure 4-8 (c)), the strain measured at the peak load on the ‘innermost fibre’ side was

0.142%. These correspond to 9% of the tensile rupture strain of 212GPa. The strains

suddenly decreased after the peak, when the debonding occurred, at 175kN. The

maximum tensile strains observed on the ‘outermost fibre’ (Figure 4-8c) side were

minimal compared to the rupture strain of the material. The strains experienced with

55

specimen CX7 and CX8 (Figure 4-6(c)) on the ‘innermost fibre’ side at peak were

0.135% and 0.129%, respectively.

4.4 Summary

The traditional Euler’s buckling theory of slender columns indicates that column

capacity depends on flexural rigidity (EI), rather than material strength. As such, the

availability of ultra-high modulus CFRP plates, which could be much stiffer than steel,

can offer a unique alternative for strengthening slender steel columns, in lieu of welding

or bolting steel plates. In this study, eight 2.6 m long S75x8 steel columns, with a

slenderness ratio of 83, were tested under concentric axial loading using pin-ended

conditions. The columns were allowed to buckle around their strong axes. CFRP plates

were adhesively bonded to the flanges of the steel S-shape sections in five of the

columns. The main parameters studied were the axis of bending, the level of initial out-

of-straightness (L/28889 to L/1635), CFRP modulus (168 to 430 GPa), and CFRP

reinforcement ratio (13% to 34%). The gain in axial strength due to CFRP strengthening

ranged from -1% to 25%, depending on the different parameters. The gain generally

increased as initial out-of-straightness, or CFRP reinforcement ratio increased. Global

buckling consistently governed the maximum load. The higher modulus did not perform

as expected, showing no gain in strength, because the compressive strains were too large

and the CFRP crushed before the specimen experienced any gain. Specimens compared

with the weak axis, strengthened with normal modulus CFRP, had similar percentage

gains in strength.

56

Table 4-1: Test matrix

Specimen I.D Axis of

Bending

Out-of-straightness δ

(mm)

Out-of-straightness / Length (δ/L)

ratio

CFRP Young's Modulus ECFRP

(GPa)

No. of CFRP

Layers

CFRP Reinforc.

Ratio ρ (%)

CX1 Strong (X-X)

0.52 0.00020

N/A (Control) CX2 1.26 0.00048

CX3 1.59 0.00061

CX4 0.80 0.00031 430 1 13.1

CX5 0.73 0.00028 212 2 26.2

CX6 0.09 0.00004

168

1 11.2

CX7 0.60 0.00023 3 33.6

CX8 1.57 0.00060

CY1* Weak (Y-Y)

0.38 0.00015 430 1 13.1

CY2* 1.83 0.00070

CY3* 1.60 0.00062 168 3 33.6

* Chapter 3

X X

Y

Y

57

Table 4-2: Summary of column test results

Spec. I.D Control

Counter-part

Peak Load (kN)

Avg. Peak Load (kN)

% gain in strength

Avg. % gain in strength

Lateral Disp. @ Peak Load

(mm)

Axial Strain @ Peak Load (με) Failure Mode

Outermost Innermost

CX1 N/A

(Control)

199.2

189.7 N/A (Control)

3.19 -1358 -1684 Global

Buckling (GB) CX2 177.3 9.26 -835 -1911

CX3 192.5 6.14 -1065 -1798

CX4

CX1

197.2

N/A

-1.02

N/A

2.68 -1071 -1624 GB and CFRP

crushing

CX5 235.4 18.16

-0.13 -1434 -1421 GB and

delamination at top

CX6 209.9

5.4

6.92 -981 -2071 GB then CFRP

crushing

CX7 229.4 234.7

15.1 19.9

0.25 -1432 -1346 GB and delamination

at top CX8 CX3 240.1 24.7 -0.73 -1482 -1293

CY1*

CX4

42.4

41.4

11.4

N/A

36.2 1250 -1762 GB then CFRP crushing, then CFRP tensile

rupture CY2* 40.4 24.8 36.8 1314 -1761

CY3* CX8 39.9 N/A 23.2 51.34 1822 -2280 GB

58

Figure 4-1: Test specimens

Figure 4-2: Steel columns out-of-straightness at mid-height before retrofitting

(a) S75x8 steel as received

(b) Cross-section configurations

1 CFRP layer 2 CFRP layers 3 CFRP layers 6.6

77

16

60

5

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

A4 C1 C2 C3 C5 A5 A6 C4

Ou

t o

f st

raig

htn

ess

(mm

)

Specimen I.D

1.47 mm Avg.

0.55 mm Avg.

59

Figure 4-3: Tensile stress-strain responses of different CFRP plates and steel

Figure 4-4: CFRP installation

0

500

1000

1500

2000

2500

3000

3500

4000

0 5000 10000 15000 20000

Stre

ss (

MP

a)

Strain (με)

430 GPa

212 GPa

168 GPa

Manufacturer strength

Steel

(a) Cleaning of sandblasted steel surface and CFRP strips

(b) Applying epoxy to steel

(c) Applying epoxy to CFRP (d) Rolling the installed CFRP strip

60

Figure 4-5: Test setup

Hinged end

Hydraulic ram

Load cell

LP’s

LP

LP

LP

Hinged end

Specimen

Hinged end

Hinged end

LP

Specimen

CFRP plate

30mm deep

sleeve

Fixed Reference

Hydraulic ram

Load cell

LP

CFRP plates

Roller

Roller

Side View

LP LP

LP

LP

Front View

Bracing

Roller

18.5 mm

11.5mm

5 mm

Strain gauges

Strain gauges

2600 mm

61

Figure 4-6: The effect of out-of-straightness on columns strengthened with 168GPa CFRP

0

50

100

150

200

250

-2000 -1000 0 1000 2000


0

50

100

150

200

250

0 2 4 6 8 10

Load

(kN

)


0

50

100

150

200

250

-5 5 15 25 35 45 55

Load

(kN

)


0

50

100

150

200

250

-6000 -5000 -4000 -3000 -2000 -1000 0

Load

(kN

)




CX1 (Control, δ/L =0.00020)

CX3 (Control, δ/L =0.00061)

CX7 (δ/L =0.00023)

CX1

CX7

CX3

CX8 (δ/L =0.00060) CX8

CX1

CX7

CX3

CX8

CX1

CX7

CX3

CX8

62

Figure 4-7: The effect of CFRP modulus on columns with comparable out-of-straightness

(δ/L = 0.00004 to 0.00061)

0

50

100

150

200

250

-6000 -5000 -4000 -3000 -2000 -1000 0

Load

(kN

)


0

50

100

150

200

250

0 2 4 6 8 10

Load

(kN

)


0

50

100

150

200

250

-5 5 15 25 35 45 55

Load

(kN

)


0

50

100

150

200

250

-2000 -1000 0 1000 2000

Load

(kN

)




CX1 (Control)

CX4 (430 GPa)

CX6

CX4

CX6 (168 GPa)

CX1

CX6 CX1

CX4 CX6

CX1

CX4

63

Figure 4-8: The effect of Young’s modulus and CFRP reinforcement ratio

0

50

100

150

200

250

-2000 -1000 0 1000 2000


0

50

100

150

200

250

0 2 4 6 8 10

Load

(kN

)


0

50

100

150

200

250

-5 5 15 25 35 45 55

Load

(kN

)


0

50

100

150

200

250

-6000 -5000 -4000 -3000 -2000 -1000 0

Load

(kN

)




CX1 (Control)

CX5 (212 GPa, ρf= 26.2%)

CX1

CX7

CX5

CX6

CX1

CX7

CX5

CX6

CX1

CX7 CX5

CX6

CX6 (168 GPa, ρf= 11.2%)

CX7 (168 GPa, ρf= 33.6%)

64

Figure 4-9: Effect of out-of-straightness on ultimate loads

Figure 4-10: Effect on CFRP modulus and reinforcement ratio on ultimate loads

0

5

10

15

20

25

30

170

180

190

200

210

220

230

240

250

0.4 0.9 1.4

% g

ain

in P

ult

Ult

imat

e A

xial

Lo

ad P

ult (

kN)

Out-of-straightness (mm)

ρCFRP

= 33.6% ECFRP

= 168GPa

0

2

4

6

8

10

12

14

16

18

20

180

190

200

210

220

230

240

10 15 20 25 30 35

% g

ain

in P

ult

Ult

imat

e A

xial

Lo

ad P

ult (

kN)

Reinforcement ratio ρ (%)

Pult

of unstrengthened

δ/L = 0.00004 to 0.00028

P

ult strengthened and % gain

of CX5 (ECFRP

=212GPa)

65

Figure 4-11: Effect of CFRP modulus on ultimate loads

Figure 4-12: Effect on axis of bending on ultimate loads

-3

-2

-1

0

1

2

3

4

5

6

180

185

190

195

200

205

210

215

150 250 350 450

% g

ain

in P

ult

Ult

imat

e Lo

ad P

ult (

kN)

CFRP modulus (GPa)

Pult

of strengthened

Pult

of unstrengthened

% gain (CX6)

ρCFRP

=11.2%-13.1% δ/L=0.00004-0.00061

% gain (CX4)

-5

0

5

10

15

20

25

30

0

50

100

150

200

250

150 250 350 450%

gai

n in

Pu

lt

Ult

imat

e Lo

ad P

ult (

kN)

CFRP modulus (GPa)

Pult

of weak axis

Pult

of strong axis

% gain (weak axis)

% gain (strong axis)

CX4

CY2

CY3

CX8

CX4

CY1 CY2

CY1

CY3

66

Figure 4-13: Failure modes

a) Control - Buckling

b) CX4 – CFRP crushing

c) CX6 – CFRP crushing

d) CX5 – CFRP debonding

e) CX7 & CX8 – CFRP debonding

67

Chapter 5

Conclusions

5.1 Summary

The objective this thesis was to examine the effectiveness of strengthening

slender steel S-sections on the flanges with CFRP plates. The study demonstrated that

adding CFRP strips to steel columns does increase the strength of the column. Three

different types of CFRP were tested: Ultra-High modulus (430GPa), High modulus

(212GPa) and Normal modulus (168GPa). Two different studies were performed, the

first was to examine specimens tested about the weak axis of buckling and the second

was in the strong axis of buckling. In the weak axis the type of modulus, number of

layers and the length of CFRP were studied. The strong axis also studied similar

parameters as weak axis bending, but did not look at the length of CFRP and instead

studied the effect of the axis of buckling. The effect of initial out-of-straightness was

examined for both studies. The conclusions of the experimental investigation performed

are summarized in the following sections.

5.2 Performance of Strengthening Long Steel Columns of S-Sections against Global

Buckling around Weak Axis using CFRP Plates of various Moduli

In this study, slender S-section steel columns with a slenderness ratio of 197, and

various levels of initial out-of-straightness, were tested. The columns were axially loaded

concentrically to examine buckling about the weak axis and to examine the effectiveness

of CFRP plates bonded to the flanges in enhancing their axial strength. CFRP plates of

68

168 to 430 GPa modulus, ranging from one to three layers were used. The plate length

relative to column length was also studied. The following conclusions are drawn:

1. The gains in axial strength of the steel columns due to CFRP strengthening ranged

from 11% to 29% in this study, depending on the various parameters studied.

2. The axial load capacity of both control and retrofitted columns reduces as the

initial out-of-straightness (δ) increases, but then, CFRP strengthening becomes

more effective as (δ) increases. The percent gain in strength increased from 11%

to 29% as (δ) of the 2600 mm long columns increased from 0.38 to 2.5 mm.

3. The axial load capacity of retrofitted columns increases and the lateral deflection

reduces as the CFRP Young’s modulus increases. As a result, the percentage gain

in strength increased from 12% to 29% as the modulus increased from 168 to 430

GPa.

4. The axial load capacity of retrofitted columns increases, but at a very small rate,

as CFRP reinforcement ratio (ρ) increases. As (ρ) increased from 13% to 26%

(i.e. doubled), the gain in strength increased from 21 to 26% only.

5. The optimal length of a CFRP plate being used to strengthen a pin-ended steel

column is between one third and two thirds of column length, centered about mid-

height. This length will provide a similar strength to a column strengthened by a

full length plate.

6. The peak loads of the columns are consistently associated with global buckling

failure. In the case of ultra-high modulus (430 GPa) CFRP, this was followed by

CFRP crushing in compression then rupture in tension. Other CFRP types

remained intact.

69

5.3 Performance of Strengthening Long Steel Columns of S-Sections against Global

Buckling around Strong Axis using CFRP Plates of various Moduli

For this study, slender steel S75x8 columns with a length of 2.6m were

strengthened by adhesively bonding CFRP to the flanges. The effect of CFRP elastic

modulus and area of CFRP on the ultimate strength of the columns was examined

experimentally. The following conclusions are drawn:

1. The effectiveness of the CFRP in increasing the axial strength of the columns

increases as the out-of-straightness of the original specimen increases. The

increase in strength ranged from 15% for the smallest out-of-straightness to 25%

for the largest.

2. Strengthening with 168GPa and a reinforcement ratio of 33.6% improves the

capacity of the column by 10% compared to when the column is strengthened

with a reinforcement ratio of 11.2%.

3. Specimens with similar initial out-of-straightness values but are strengthened

with 212GPa rather than 168GPa show an increase of in strength of 18% from

15%.

4. Specimens strengthened with one layer of the normal modulus CFRP (168GPa)

gave a higher increase in capacity, 5.35%, than the ultra-high modulus (430GPa)

specimen, -1.0%. This is because the ultra-high modulus CFRP crushed before

any strengthened could occur.

5. Similar increases in capacity, approximately 24%, are demonstrated when

strengthening with a reinforcement ratio of 33.6% of 168GPa and tested in both

the strong and weak axis. When strengthening with a reinforcement ratio of

70

13.1% of 430GPa, the weak axis specimens experienced much larger gains, 11%

and 25%, than the strong axis, -1.0%.

6. Strengthening specimens with one layer of CFRP causes the column to fail

because the CFRP crushes either at the ultimate load or has a secondary failure

after the peak load on the innermost side. For specimens strengthened with more

than one layer of CFRP the columns experience debonding as a secondary failure

mode.

5.4 Future Research

This study has drawn many conclusions, as stated above, but there are also many areas

that could still be researched. Some areas that could be researched further are as follows:

1. Determine the behaviour when various lengths of steel are used. This study was

only concerned with slender columns, but most structural projects will use

intermediate columns.

2. Determine the behaviour of strengthening with CFRP that have a modulus in

between the range that was tested, specifically in between 212GPa and 430GPa or

lower than 168GPa.

3. To determine the effect of increasing the reinforcement ratio using the Ultra-High

modulus CFRP (430GPa) for the weak axis.

4. To determine the effect on changing the length of CFRP in the strong axis for all

modulus types and to explore further with different CFRP types in the weak axis.

5. To design a model that could predict the capacity of all columns strengthened

with different reinforcement ratios, types of CFRP and length of CFRP.

71

6. Effect of fatigue loading on the strengthening performance.

7. The focus of this research was on strengthening columns that had not

experienced any sort of deterioration. Depending on the amount and location of

deterioration, some of the conclusions from this study may not be applicable.

For example, if a column experienced loss of cross-section due to corrosion at

its ends, strengthening at the middle third may not increase the capacity of

the specimen. Research should be undertaken to investigate the effect of

adding CFRP to columns that experience corrosion or other forms of

deterioration.

8. In practice, most columns that need to be retrofitted would be subjected to dead

loads at the time of FRP application. For steel structures, dead loads tend to be

small compared to live loads. However, there may be difference strength gains

than those observed in the current research. Future research should address the

strengthening of steel columns subjected to dead loads at the time of FRP

application.

72

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Teng, J.G. and Hu, Y.M. (2007) "Behaviour of FRP-Jacketed Circular Steel Tubes and

Cylindrical Shells Under Axial Compression." Construction and Building Materials,

21(4): 827-838.

West, T.D. (2001) ”Enhancements to the Bond between Advanced Composite Materials

and Steel for Bridge Rehabilitation.” Master’s Thesis. University of Delaware,

Newark, 207pp

76

Appendix A

Procedure Appendix

A.1 Additional Testing Details

The slenderness ratios are found using the equation

, where k is the effective

length factor of 1 for pin-pin connections, L is the unbraced length and r is the radius of

gyration. Based on the geometric properties of this section, the slenderness ratio in the

weak axis is 197 and in the strong axis, it is 83.

Due to the importance of measuring the out-of-straightness, found by Shaat and

Fam (2006), an approximate out-of-straightness was measured for all columns in order to

group them accordingly. To find an approximation for each specimen’s out-of-

straightness values, measurements from a string strung taught along the top of the column

were obtained with the use of calipers. Taking the measurements on all four sides at

three locations along the column, allowed for a calculation of an average value for the

out-of-straightness in the strong and weak axes. The maximum out-of-straightness for

both studies was

. These out-of-straightness values for both directions were

calculated for each specimen and are shown below in Table A-1 and A-2.

77

Table A-1: Out-of-straightness values for columns tested in weak axis

Specimen I.D

Out-of-straight δ

(mm) (weak axis)

Out-of-straight δ

(mm) (strong axis)

A1 0.31 1.34

A2 1.59 0.31

A3 2.48 1.79

B1 0.38 1.30

B2 1.83 0.32

B3 2.50 0.04

B4 0.21 1.97

B5 1.70 1.48

B6 2.40 1.23

B7 2.46 0.08

B8 2.91 1.25

B9 1.60 0.07

Table A-2: Out-of-straightness values for specimens tested in the strong axis

Specimen I.D

Out-of-straight δ

(mm) (strong axis)

Out-of-straight δ

(mm) (weak axis)

CX1 0.52 1.16

CX2 1.26 1.08

CX3 1.59 0.30

CX4 0.80 0.44

CX5 0.73 0.53

CX6 0.09 0.86

CX7 0.60 1.91

CX8 1.57 1.12

The column was aligned before testing using a plumb bob hung from the top of

the column to ensure it was as straight as possible. A system 7000 data acquisition was

used to acquire the data from the tests. The tests were performed using a two way

78

hydraulic hand pump and the column was moved by the hydraulic ram. Prior to testing,

the load cell was calibrated in the Riehle machine.

Table A-3 shows the average material properties of the coupon tests performed.

Table A-3: FRP material properties based on coupon tests

CFRP Type

Average & Standard Deviation

Width (mm)

Thickness (mm)

Elastic Modulus

(GPa)

Ultimate Strength

(MPa)

Ultimate Strain x10

-3

(mm/mm)

UH514 Average 24.12 1.40 430.15 1272.92 3.01

St. Dev. 1.03 0.02 34.66 200.60 0.49

M514 Average 24.27 1.36 212.05 3270.33 15.57

St. Dev. 0.70 0.02 21.51 159.20 1.46

S512 Average 23.98 1.29 168.47 2934.89 17.49

St. Dev. 0.61 0.02 6.41 113.70 1.18

A.2 Bracing Design

In order to force the column to bend about the strong axis (X-X), bracings needed

to be added. Beginning calculations showed that two bracings would be needed (Ly =

867mm).

As shown in the calculations above the slenderness ratio in the weak axis (Y-Y)

was lower than the strong axis meaning that it should want to buckle about the strong axis

first. Unfortunately, that was not the case. In the first test performed, the end support

plates were yielding when the load was applied. Due to original restraints in the height of

the system, the thickness of the plates was under designed, which was why they bent

79

under the load. The setup was rearranged allowing for the design of thicker plates, as

well as using a stronger steel to ensure no yielding of the plates.

Once the end plates were redesigned, the bracings, which were originally

designed as two separate pieces were proven to move too much in the first test and not

give the proper resistance to the load experienced at that point. The bending and bracings

can be seen in Figure A3.

Figure A-1: First control test

The bracings were thought to work together, due to the rods seen in the picture,

but that was not the case. More calculations were done to fully design the bracings. The

maximum deflection allowed is equal to the initial out-of-straightness tolerance (Δo).

Rods

80

Using Clause 9.2.6 of S16-2009, the load on each bracing was calculated. The

approximation of Pb = 0.02Pcr was used as a conservative assumption. Therefore, Pb was

8kN. Examining the deflection for a cantilever with an 8kN load pushing on it 500mm

away from the base gives a deflection of approximately 22mm, which is way above the

stated maximum allowance. This is why the bracings originally did not provide enough

support. SAP2000 was used to run a model with the design shown in Figure A4, the

distances shown are centre to centre of the HSS sections used. An 8kN load was pushed

on each side of the system and the deflection of the bracings were measured. The system

was tweaked until the deflection was less than 2.6mm.

Figure A-2: SAP system

Calculations were done to figure out how many bolts would be needed to attach

the bracings to the red frame. Clause 13.12.2.2 in S16-2009 deals with shear connections

and the number of bolts was calculated using the equations below.

111 mm 115 mm 162 mm

280 mm

81

Therefore, only one bolt was needed but four were used for symmetry and to

carry the weight of the bracings. Clause 13.12.2.3 was also checked because of the

possibility of the bracings experiencing torsion. Vs was recalculated with four bolts to be

44.7kN and the torsional force was calculated to be 14.29kN. The equation below was

used to verify that the connections were adequate in combined shear and torsion.

The first test was run again with the new bracing system but still buckled about

the weak axis as seen in Figure A5.

82

Figure A-3: Second attempt

This proved that more braces were needed to ensure that the column did not buckle in the

weak axis. Three more bracings were made to ensure that the strengthened columns did

not buckle in the weak axis. After these bracings were implemented in the setup the

control specimens buckled in the proper direction.

83

Appendix B

Results Appendix

B.2 PIV Analysis

For all of the tests, cameras were used to capture the movements of the column

over time. Using the pictures, a digital image correlation (DIC) along with running the

program particle image velocimetry (PIV) were used to detect the axial displacement and

lateral displacements of the columns. For column B3, the LP’s that measured the axial

displacement were not working and in order to get the axial displacement PIV was used.

The program was run on both B4 and B3 so that it could be validated with the LP’s used

for B4. The PIV axial displacement graphs are shown in Figure A1. As would be

expected, both have similar slopes because both were strengthened with the same type of

CFRP. Figure A2 shows the difference between the PIV analysis and the LP data for B4.

It was found that the PIV data was 33% less than the LP values. When originally

examining the LP data it was assumed that the red frame at the top of the column was

rigid and unable to move, but based on the PIV data, this is untrue. The LP’s captured

the movement of the whole system where the PIV analysis only captured the true

movement of the specimen. The red frame did not move a large amount, as seen in

Figure A2, at peak the largest difference was approximately 0.5mm. If one were to need

the true axial displacement of the column PIV would have to be run for every test.

84

Figure B-4: PIV analysis

Figure B-5: PIV and LP data for B4

B.2 Supplementary Test Result Figures

The following figures are data from the LP’s that were located at the top quarter and the

bottom quarter of the columns.

0

5

10

15

20

25

30

35

40

45

0 1 2 3 4

Load

(kN

)


B3 B4

0

5

10

15

20

25

30

35

40

45

0 1 2 3 4

Load

(kN

)


LP PIV B4 x1.33

0.5mm

85

B.2.1 Weak Axis – Top Lateral LP

Figure B-6: The effect of out-of-straightness on columns strengthened using 430 GPa CFRP

Figure B-7: The effect of CFRP modulus on columns of comparable out-of-straightness

0

5

10

15

20

25

30

35

40

45

0 10 20 30 40 50

Load

(kN

)


A1 A2 A3 B1 B2

0

5

10

15

20

25

30

35

40

0 10 20 30 40 50

Load

(kN

)


A3 B6 B8

86

Figure B-8: The effect of CFRP reinforcement ratio

Figure B-9: The effect of the 430 GPa CFRP length ratio

0

5

10

15

20

25

30

35

40

45

0 10 20 30 40 50

Load

(kN

)


A2 A3 B6 B7 B8 B9

0

5

10

15

20

25

30

35

40

45

0 10 20 30 40 50

Load

(kN

)


A1 A2 B1 B2 B4 B5

87

B.2.2 Weak Axis – Bottom Lateral LP

Figure B-10: The effect of out-of-straightness on columns strengthened using 430 GPa

CFRP

Figure B-11: The effect of CFRP modulus on columns of comparable out-of-straightness

0

5

10

15

20

25

30

35

40

45

0 10 20 30 40 50

Load

(kN

)

Lateral Displacement (mm) A1 A2 A3 B1 B2

0

5

10

15

20

25

30

35

40

0 10 20 30 40 50

Load

(kN

)


A3 B6 B8

88

Figure B-12: The effect of CFRP reinforcement ratio

Figure B-13: The effect of the 430 GPa CFRP length ratio

0

5

10

15

20

25

30

35

40

45

0 10 20 30 40 50

Load

(kN

)

Lateral Displacement (mm) A2 A3 B6 B7 B8 B9

0

5

10

15

20

25

30

35

40

45

0 10 20 30 40 50

Load

(kN

)


A1 A2 B1 B2 B4 B5

89

B.2.3 Strong Axis – Top Lateral LP

Figure B-14: The effect of out-of-straightness on columns strengthened with 168GPa CFRP

Figure B-15: The effect of CFRP modulus on columns with comparable out-of-straightness

(δ/L = 0.00004 to 0.00061)

0

50

100

150

200

250

-5 5 15 25 35

Load

(kN

)


A4 A6 C3 C4

0

50

100

150

200

250

-5 5 15 25 35

Load

(kN

)


A4 C1 C5

90

Figure B-16: The effect of Young’s modulus and CFRP reinforcement ratio

B.2.4 Strong Axis – Bottom Lateral LP

Figure B-17: The effect of out-of-straightness on columns strengthened with 168GPa CFRP

0

50

100

150

200

250

-5 5 15 25 35

Load

(kN

)

Lateral Displacement (mm) A4 C2 C3 C5

0

50

100

150

200

250

-5 5 15 25 35

Load

(kN

)


A4 A6 C3 C4

91

Figure B-18: The effect of CFRP modulus on columns with comparable out-of-straightness

(δ/L = 0.00004 to 0.00061)

Figure B-19: The effect of Young’s modulus and CFRP reinforcement ratio

B.2.5 Strain Gauge Data

Below is strain gauge data from the columns that had extra gauges at the quarter points.

The values are not extrapolated.

0

50

100

150

200

250

-5 5 15 25 35

Load

(kN

)


A4 C1 C5

0

50

100

150

200

250

-5 5 15 25 35

Load

(kN

)


A4 C2 C3 C5

92

Figure B-20: Specimen B1 gauges in top quarter

Figure B-21: Specimen B1 gauges in bottom quarter

0

5

10

15

20

25

30

35

40

45

-600 -400 -200 0 200

Load

(kN

)

Strain (με)

N S

0

5

10

15

20

25

30

35

40

45

-400 -300 -200 -100 0 100

Load

(kN

)

Strain (με) N S

93



0

5

10

15

20

25

30

35

40

45

-300 -250 -200 -150 -100 -50 0

Load

(kN

)

Strain (με)

N S

0

5

10

15

20

25

30

35

40

45

-400 -300 -200 -100 0

Load

(kN

)

Strain (με) N S

94



0

5

10

15

20

25

30

35

40

45

-1500 -1000 -500 0 500 1000

Load

(kN

)

Strain (με)

NW NE SE SW

0

5

10

15

20

25

30

35

40

45

-1000 -500 0 500 1000

Load

(kN

)

Strain (με) NW SW NE SE

95



0

5

10

15

20

25

30

35

40

45

-1500 -1000 -500 0 500 1000

Load

(kN

)

Strain (με)

NW NE SE

0

5

10

15

20

25

30

35

40

45

-1500 -1000 -500 0 500 1000

Load

(kN

)


96



0

5

10

15

20

25

30

35

40

45

-2000 -1000 0 1000 2000

Load

(kN

)

Strain (με)

NW NE SE SW

0

5

10

15

20

25

30

35

40

45

-2000 -1000 0 1000 2000

Load

(kN

)


97



0

5

10

15

20

25

30

35

40

-1500 -1000 -500 0 500 1000 1500

Load

(kN

)

Strain (με)

NW NE SE SW

0

5

10

15

20

25

30

35

40

-2000 -1000 0 1000 2000

Load

(kN

)


98

Figure B-32: Specimen CX4 gauges in top quarter

Figure B-33: Specimen CX4 gauges in bottom quarter

0

50

100

150

200

250

-2000 -1000 0 1000 2000 3000

Load

(kN

)

Strain (με)

NW NE SE SW

0

50

100

150

200

250

-1500 -1000 -500 0 500

Load

(kN

)

Strain (με) NW SW SE

99

Figure B-34: Specimen CX5 gauges in top quarter

Figure B-35: Specimen CX5 gauges in bottom quarter

0

50

100

150

200

250

-3000 -2000 -1000 0 1000 2000

Load

(kN

)

Strain (με)

NW NE SE SW

0

50

100

150

200

250

-2000 -1500 -1000 -500 0 500

Load

(kN

)

Strain (με) NW SW SE NE

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