Strain Limits for Concrete Filled Steel Tubes in AASHTO...

Strain Limits for Concrete Filled Steel Tubes in AASHTO Seismic

Provisions

PIs: Mervyn Kowalsky and James NauRA: Nicole King

Project Overview

• Seismic behavior of reinforced concrete filled steel pipe piles

Concrete Filled Steel Tube Uses

1) Driven as a pile and filled with reinforced concrete.

2) Used as a permanent casing for a drilled shaft and filled with reinforced concrete.

Omalley Bridge (Courtesy, AKDOT)

3) Serves as a pile below the ground surface, and the above‐ground columns to support the cap beam.

Goals

1) Study the effect of the diameter to thickness ratio on the pile‐columns’ performance limit states (strain limit states).

2) Determine the below ground plastic hinge length for the pile‐columns.

3) Confirm or improve current analysis and modeling assumptions in the design of the pile‐columns.

Methods

1) Large scale tests on 10‐12 reinforced concrete filled steel tubes subjected to reverse cyclic bending.

Test Setup at Constructed Facilities Lab, NCSU

2) Finite Element Analysis of reinforced concrete filled steel tubes

‐ABAQUS Finite Element Analysis Program

1) Design expressions relating D/t to strains at various limit states.

2) Design expressions for plastic hinge lengths for reinforced concrete filled steel pipes.

3) Modifications to moment‐curvature analysis tools (if necessary) to predict the force‐deformation response to cyclic loading.

4) The research findings will be summarized in a concise design manual appropriate for AKDOT use.

Outcomes

Impact of Diameter‐thickness ratio

• Study the effect of D/t on:– Local pipe buckling– Deformation capacity– Strain limits associated with elastic, repairable and ultimate response

• D/t ratios from 24 to 180 will be tested.


• Past Research:

– There have been multiple studies on concrete filled pipes without internal reinforcement.

– The majority of past studies were small scale tests (diameters ranging from 1.75 inches to 12.75 inches).

• Elchalakani (2008) noticed a trend between D/t and ductility on a small scale but needed large scale tests to verify the findings.

– Large scale tests on reinforced concrete filled steel tubes were conducted at NCSU with a constant D/t = 48.


• Past Research:


• Methods:– Large scale tests on 10‐12 pipe piles in two phases

• Phase 1: 5 piles– All have 24” diameter, same rebar configuration, 30 foot span, 6 foot constant moment length

– D/t ratio ranging from 48 to 179

Material Sources: Consolidated Pipe Co and Naylor Spiral Weld Pipe

Option

Geometric Properties Longitudinal Rebars Spiral Rebars

Outer Diameter

[in]

Wall Thickness

[in]Span [ft]

Constant Moment Length [ft]

D/t L/DLong. Pipe

Ratio[%]

Pipe Vol. Ratio [%] Number Size

[#]Ratio [%]

Size [#] Pitch (in) Volume

Ratio [%]

5 24 0.500 30 6 48.00 15 8.16 8.16 0 0 0.00 0 0 0.0006 24 0.375 30 6 64.00 15 6.15 6.15 12 7 1.60 3 12 0.1538 24 0.281 30 6 85.41 15 4.63 4.63 12 7 1.60 3 12 0.153

10 24 0.188 30 6 128.00 15 3.10 3.10 12 7 1.60 3 12 0.153

11 24 0.134 30 6 179.10 15 2.22 2.22 12 7 1.60 3 12 0.153

– Phase 2: exact pile selection will depend on the results from phase 1

– Will have a pile with a D/t ratio = 24


Option

Geometric Properties Longitudinal Rebars Spiral RebarsOuter

Diameter [in]

Wall Thickness

[in]

Span [ft]

Constant Moment Length [ft]

D/t L/D Long. Pipe Ratio [%]

Pipe Vol. Ratio [%] Number Size [#]

Ratio [%] Size [#] Pitch

(in)

Volume Ratio [%]

1e 18 0.75 24 6 24.00 16 15.97 15.97 9 6 1.56 3 12 0.2052 18 0.625 24 6 28.80 16 13.41 13.41 9 6 1.56 3 12 0.2053 18 0.500 24 6 36.00 16 10.80 10.80 9 6 1.56 3 12 0.2057 18 0.250 24 6 72.00 16 5.48 5.48 9 6 1.56 3 12 0.20512 24 0.134 30 6 179.10 15 2.22 2.22 12 10 3.26 3 12 0.15313 24 0.134 30 6 179.10 15 2.22 2.22 12 7 1.60 3 2 0.92014 24 0.134 30 6 179.10 15 2.22 2.22 0 0 0.00 0 0 0.00018 24 0.134 18 6 179.10 9 2.22 2.22 12 7 1.60 3 12 0.153

Pipe Suppliers

Cannot manufacture wall thicknesses• US Steel• Brampton Plate & Structural Steel

Rolling• Southland Pipe• Millerbend Company

• Minimum thickness = ¼”• Pipe Industries

• Minimum thickness at an OD = 24” is 0.212”

• Alpha Pipe Company• “Could not fit it in to make it

cost efficient on such a low quantity”

• Skyline Steel• Minimum thickness = ¼”

Cannot manufacture outer diameter• Chicago Tube and Iron

• Maximum Diameter of 20 inches

• Atlas Tube, JMC Steel Group• Maximum Diameter of 20

inches• MST Seamless Tube and Pipe

• Maximum Outer Diameter 4.5”

• Northwest Pipe Company• Maximum Outer Diameter

16”

Can manufacture at least one of the piles• Hodgson Custom Rolling

• 24” OD x 0.1875” thick x 420” long: $9,022 US each

• 24” OD x 0.125” thick x 420” long: $7,992 US each

• Maximum roll width is 10 feet: must make in 4 sections with 3 girth welds

• Arntzen Corporation Custom Rolling• 24” OD x 0.1875” thick x 30’ long • Ships loose as (3) 96” long and

(1) 72” long: $2,842 US each• Dixie Southern Custom Steel Fabrication

• 3/16” thickness with welded spool piece

Manufacturers (Options 10,11,12, 13,14,18)

Distributor (Options 10,11,12, 13,14,18)• Texas Pipe

• Thinnest wall at 24” outer diameter is ¼”

Spiral Pipe (Options 10,11,12, 13,14,18)• Naylor Pipe Company (ASTM A139)

• Option 10 (24”OD x 0.188”t): $840 US each• Option 11 (24” OD x 0.134”t): $1,160 US each

Distributor (Options 1e,2,3,5,6,7,8)• Consolidated Pipe and Supply, Inc.

• ASTM A500 Grade B• Prices: Option 2 (18”OD x 0.625”t) = $2,407; Option 3 (18”OD x 0.5”t) = $1,914; Option 6 (24”OD x 0.375”t) = $2,566;

(Option 7) 18”OD x 0.25”t = $1,102; (Option 8) 24”OD x 0.281”t = $1,840 US each


• Methods (continued):– ABAQUS model

• Include multiaxial stresses• Run multiple models in order to decide which piles to test in phase two

• Analyze other configurations that will not be physically tested in the lab

Plastic Hinge Lengths

• The below ground plastic hinge length varies depending on the stiffness of the soil.

– Soils with lower stiffness have larger plastic hinge lengths.

– The constant moment region will mimic the soil stiffness.

Plastic hinges

• The plastic hinge length correlates the material strains to the member deformations.

– Without knowing what the below ground plastic hinge length is, a fiber or finite element analysis is necessary in predicting the deformation capacity.

Effect of Soil Stiffness on In‐ground Plastic Hinge Length.

Locations of plastic hinges on a double column bent.

Plastic Hinge‐1 and Plastic Hinge‐2 :Lp=0.08L+ 0.022fydbl ≥ 0.044fydbl

Plastic Hinge‐3 and Plastic Hinge‐4 :Lp is a function of the steel tube gap and strain penetration.

Above Ground Plastic Hinge Length

Below Ground Plastic Hinge Length

Plastic Hinge‐5:Lp=0.08L+ 0.022fydbl ≥ 0.044fydbl

Plastic Hinge‐6:The length and location of the plastic hinge is a function of the soil properties (Chai).

Plastic Hinge‐7:Lp is a function of the steel tube gap and strain penetration.

Plastic Hinge‐8:(currently assumed to be the same as plastic hinge 6)The plastic hinge length is unknown, possible factors that affect it are:• Moment‐curvature response• Stress‐Strain response• Soil effects on the steel tube

Locations of plastic hinges on a double column bent.

Plastic Hinge Lengths

• Methods (continued):– Use existing model for reinforced concrete (without steel casing) on test specimens to compare the actual to predicted displacements.

– If necessary, include the stress strain interaction to calculate the plastic hinge length.

– Correctly model the plastic hinge length in ABAQUS finite element analysis (would likely include soil springs).

Analysis & Modeling Assumptions

• Objectives:– Determine the shape of the steel and concrete stress‐strain curves for concrete filled steel tubes.

– Determine the appropriate level of fl (currently assumed to be 50% of fy).

– Determine accuracy of strain compatibility assumption along the pipe‐pile cross sections.

– Include the impact of reinforcing steel and steel casing in an analysis method.

Analysis & Modeling AssumptionsSteel Stress‐Strain Curve

• Section A has lateral stress only (fl).

• Section B has both axial stress (fa) and lateral stress (fl) creating a multi‐axial stress state.

Ɛ

σ

Analysis & Modeling AssumptionsSteel Stress‐Strain Curve

• What?• How do multi‐axial stresses in the tube alter the

steel stress strain curve?

• Why?• To obtain an accurate stress‐strain relationship of

concrete confined by a steel tube.• To be able to determine the limit states, material

response and the plastic hinge length.

• How? • Analyze experimental stress strain curves

• Past work shows that the multi‐axial stresses have small effects on the stress‐strain curve.


Concrete Stress‐Strain Curve

Ɛ

σ • What?• What is the confinement effectiveness of the steel

tube on the concrete?• What is the level of longitudinal stress in the hinge?

• MANDER Model assumes 100% fy, but when this is used to calculate the ultimate concrete strength it is unrealistically large. Model was not defined account for the large steel ratio.

• Why?• To obtain an accurate stress‐strain relationship of

concrete confined by a steel tube.• To be able to determine the limit states, material

response and the plastic hinge length.

• How?• Monitor the transverse strains in the pipe during

testing.

Analysis & Modeling AssumptionsStrain Compatibility

Ɛ

• What?• Is strain compatibility applicable?

• Why?• Needed for section analysis.

• How?• Use Optotrack to obtain strain distribution along the section.

• Past Research:• Previous work at NCSU showed the strain was relatively linear.

Strain Distribution along the pipe‐pile cross section.


• Previous Research:– Impact of confinement on axial behavior impacts flexural strength and deformation capacity.

– The hoop stress has been inferred from analysis results and has not been measured directly from the experiment.

– No research on the complex state of stress in the pipe and its effect on confinement.


• Methods:– Measure fl on full scale tests and incorporate it into the concrete model.

– Measure the longitudinal stresses and incorporate them into the steel model.

– Create a comprehensive model in ABAQUS finite element analysis.

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Strain Limits for Concrete Filled Steel Tubes in AASHTO...

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