Building Code Requirements for Structural Concrete (ACI 318M-11)

Overview of ACI 318MDesign of Prestressed Concrete

Evaluation of Existing Structures

David Darwin

Vietnam Institute for Building Science and Technology (IBST)

Hanoi and Ho Chi Minh City

December 12-16, 2011

This morning

Overview of ACI 318M-11

Design of Prestressed Concrete (Chapter 18)

Strength Evaluation of Existing Structures (Chapter 20)

This afternoon

Analysis and design of

Flexure

Shear

Torsion

Axial load

Tomorrow morning

Design of slender columns

Design of wall structures

High-strength concrete

Overview of ACI 318M-11

Legal standing

Scope

Approach to Design

Loads and Load Cases

Strength Reduction Factors

Legal standing

Serves as the legal structural concrete building code in the U.S. because it is adopted by the general building code (IBC).

Scope

ACI 318M consists of 22 chapters and 6 appendices that cover all aspects of building design

Chapters

1. GENERAL REQUIREMENTS Scope, Contract Documents, Inspection,

Approval of Special Systems

2. NOTATION AND DEFINITIONS

Chapters

3. MATERIALS

Cementitious Materials, Water, Aggregates, Admixtures, Reinforcing Materials

4. DURABILITY REQUIREMENTSFreezing and Thawing, Sulfates, Permeability, Corrosion

5. CONCRETE QUALITY, MIXING, AND PLACING

6. FORMWORK, EMBEDMENTS, AND CONSTRUCTION JOINTS

7. DETAILS OF REINFORCEMENTHooks and Bends, Surface Condition, Tolerances, Spacing, Concrete Cover, Columns, Flexural Members, Shrinkage and Temperature Steel, Structural Integrity

8. ANALYSIS AND DESIGN — GENERAL CONSIDERATIONSDesign Methods; Loading, including Arrangement of Load; Methods of Analysis; Redistribution of Moments; Selected Concrete Properties; Requirements for Modeling Structures (Spans, T-beams, Joists...)

9. STRENGTH AND SERVICEABILITY REQUIREMENTSLoad Combinations, Strength Reduction Factors, Deflection Control

10. FLEXURE AND AXIAL LOADS

Beams and One-way Slabs, Columns, Deep Beams, Bearing

11. SHEAR AND TORSION

12. DEVELOPMENT

AND SPLICES OF REINFORCEMENT

13. TWO-WAY SLAB SYSTEMS

14. WALLS

15. FOOTINGS

16. PRECAST

CONCRETE

17. COMPOSITE CONCRETE FLEXURAL MEMBERS

18. PRESTRESSED CONCRETE

19. SHELLS AND FOLDED PLATE MEMBERS

20. STRENGTH EVALUATION OF EXISTING STRUCTURES

21. EARTHQUAKE-

RESISTANT

STRUCTURES

22. STRUCTURAL PLAIN CONCRETE

AppendicesA. STRUT-AND-TIE MODELS*

B. ALTERNATIVE PROVISIONS FOR REINFORCED AND PRESTRESSED CONCRETE FLEXURAL AND COMPRESSION MEMBERS

C. ALTERNATIVE LOAD AND STRENGTH REDUCTION FACTORS

D. ANCHORING TO CONCRETE*

E. STEEL REINFORCEMENT INFORMATION

F. EQUIVALENCE BETWEEN SI-METRIC, MKS-METRIC, AND U.S. CUSTOMARY UNITS OF NONHOMOGENOUS EQUATIONS IN THE CODE

Approach to design

Qd = design loads

Sn = nominal strength

Sd = design strength

M = safety margin

Design Strength Required Strength

Sd = Sn Qd

Sd = design strength = Sn

= strength reduction factor

= load factors

Qd = design loads

and in Chapter 9 of ACI 318M

Loads Qd

specified in ASCE 7, Minimum Design Loads for Buildings and Other Structures

American Society of Civil Engineers (ASCE)

Reston, Virginia, USA

Loads

Dead loads (D)*

Live loads (L)*

Roof live loads (Lr)*

Wind loads (W) full load

Earthquake loads (E) full load

Rain loads (R)*

Snow loads (S)*

* Service-level loads

Loads

Impact – include in L

Self-straining effects (temperature, creep, shrinkage, differential settlement, and shrinkage compensating concrete) (T)

Fluid loads (F)

Lateral soil pressure (H)

Factored Load = U = Qd

Load cases and load factors by ASCE 7 and ACI 318M

U = 1.4D

U = 1.2D + 1.6L + + 0.5(Lr or S or R)

U = 1.2D + 1.6(Lr or S or R) + (1.0L or 0.5W)

U = 1.2D + 1.0W + 1.0L + 0.5(Lr or S or R)

U = 1.2D + 1.0E + 1.0L + 0.2S

U = 0.9D + 1.0W

U = 0.9D + 1.0E

Load cases and load factors by ASCE 7 and ACI 318M

If W based on service-level forces, use 1.6W place of 1.0W

If E based on service-level forces, use 1.4E in place of 1.0E

Details of other cases covered in the Code

Load factors by ACI 318M

Strength reduction () factors

Tension-controlled sections 0.90

Compression-controlled sections

Members with spiral reinforcement 0.75

Other members 0.65

Shear and torsion 0.75

Bearing 0.65

Post-tensioning anchorages 0.85

Other cases 0.60 – 0.90

Tension-controlled and compression-controlled sections

T-beam

dh

b

hf

bw

As

dt

Strain through depth of beam

Design Strength ( x nominal strength) must exceed the Required Strength (factored load)

Bending Mn Mu

Axial load Pn Pu

Shear Vn Vu

Torsion Tn Tu

Load distributions and modeling requirements

Structure may be analyzed as elastic

using properties of gross sections

Ig = moment of inertia of gross (uncracked) cross section

Beams: Ib = ½ Ig Iweb =

Columns: Ic = Ig =

wb h3

12

bh3

12

Analysis by subframes

1. The live load applied only to the floor or roof under consideration, and the far ends of columns built integrally with the structure considered fixed

2. The arrangement of load may be limited to combinations of

(a)factored dead load on all spans with full factored live load on alternate spans, and

(b)factored dead load on all spans with full factored live load on two adjacent spans

(a)

(b)

(c)

Moment and shear envelopes

Columns designed to resist

(a) axial forces from factored loads on all floors or roof and maximum moment from factored live loads on a single adjacent span of the floor or roof under consideration

(b) loading condition giving maximum ratio of moment to axial load

More on columns

For frames or continuous construction, consider effect of unbalanced floor or roof loads on both exterior and interior columns and of eccentric loading due to other causes

For gravity load, far ends of columns built integrally with the structure may be considered fixed

At any floor or roof level, distribute the moment between columns immediately above and below that floor in proportion to the relative column stiffness

Simplified loading criteria

Beams, two or more spans

Beams, two spans only

Slabs, spans ≤ 3 m

Beams, col stiffnesses ≥ 8 beam stiffnesses

u nM w l 2factorln

Composite

Max –ve right

Max –ve leftMax +ve

Allowable adjustment in maximum moments for t 0.0075

Design of prestressed concrete(Chapter 18)

Behavior of reinforced concrete

Reinforced concrete under service loads

Theory of prestressed concrete

Stresses

57

Methods of prestressing concrete members

• Post-Tensioning

• Pretensioning

Prestressing steels

Strength of prestressing steels available in U.S.

Seven-wire strand: fpu 1725, 1860 MPa

fpy (stress at 1% extension) 85% (for stress-relieved strand) or 90% (for low-relaxation strand) of fpu

fpu = ultimate strength

fpy = yield strength

Strength of prestressing steels available in U.S.

Prestressing wire: fpu 1620 to 1725 MPa (function of size)

fpy (at 1% extension) 85% of fpu

Strength of prestressing steels available in U.S.

High-strength steel bars: fpu 1035 MPa

fpy 85% (for plain bars) and 80% (for deformed bars) of fpu

fpy based on either 0.2% offset or 0.7% strain

Maximum permissible stresses in prestressing steel

Due to prestressing steel jacking force:0.94fpy

0.80fpu

manufacturers recommendation

Post-tensioning tendons, at anchorage devices and couplers, immediately after force transfer:0.70fpu

Prestressed concrete members are designed based on both

Elastic flexural analysis

Strength

Elastic flexural analysis

Considers stresses under both the

Initial prestress force Pi and the

Effective prestress force Pe

Note: = concrete compressive strength

= initial concrete compressive strength (value at prestress transfer)

cfcif

Classes of membersU – uncracked – calculated tensile stress in precompressed tensile zone at service loads = ft

T – transition between uncracked and cracked < ft

C – cracked ft >

. cf0 62

. cf0 62 . cf10

. cf10

cf in MPa

Concrete section properties

e = tendon eccentricity

k1= upper kern point

k2= lower kern point

Ic = moment of inertia

Ac = area

radius of gyration:

r2 = Ic/Ac

section moduli:

S1 = Ic/c1

S2 = Ic/c2

Bending moments

Mo = self-weight moment

Md = superimposed dead load moment

Ml = live load moment

Concrete stresses under Pi

Concrete stresses under Pi + Mo

Concrete stresses under Pe + Mo + Md + Ml

Maximum permissible stresses in concrete at transfer(a) Extreme fiber stress in compression, except as in

(b),

(b) Extreme fiber stress in compression at ends of simply supported members

(c) Extreme fiber stress in tension at ends of simply supported members *

(d) Extreme fiber stress in tension at other locations

*

* Add tensile reinforcement if exceeded

. 0 60 cif

. 070 cif

. cif0 25

. cif050

Maximum permissible compressive stresses in concrete at service loadsClass U and T members

(a) Extreme fiber stress in compression due to prestress plus sustained load

(b) Extreme fiber stress in compression due to prestress plus total load

. 0 45 cf

. 0 60 cf

Flexural strength

ApsT = Apsfps

ps

Stress-block parameter 1

1

1

1

0.85 for 17 MPa 28 MPa

For between 28 and 56 MPa,

decreases by 0.05 for each 7 MPa

increase in

0.65 for 56 MPa

c

c

c

c

f

f

f

f

Stress in prestressing steel at ultimateMembers with bonded tendons:

p = Aps/bdp = reinforcement ratio

b = width of compression face

dp = d (effective depth) of prestressing steel

Members with bonded tendons and non-prestressed bars:

p pups pu p

c p

f df f

f d

1

1

and y c y cf / f f / f

and refer to compression reinforcement, sA

shall be taken pup p

c p

f d. , d . d

f d

017 015

Members with unbonded tendons with span/depth ratios 35:

but not greater than fpy or greater than fpe + 420 MPa

fpe = stress in Aps at Pe = e

ps

P

A

Members with unbonded tendons with span/depth ratios > 35:

but not greater than fpy or greater than fpe + 210 MPa

Loss of prestress

(a) Prestessing steel seating at transfer

(b) Elastic shortening of concrete

(c) Creep of concrete

(d) Shrinkage of concrete

(e) Relaxation of prestressing steel

(f) Friction loss due to intended or unintended curvature of post-tensioning tendons

Limits on reinforcement in flexural members

Classify as tension-controlled, transition, or compression-controlled to determine

Total amount of prestressed and nonprestressed reinforcement in members with bonded reinforcement must be able to carry 1.2 cracking load

Minimum bonded reinforcement As in members with unbonded tendons

Except in two-way slabs, As = 0.004Act

Act = area of that part of cross section between the flexural tension face and center of gravity of gross section

Distribute As uniformly over precompressed tension zone as close as possible to extreme tensile fiber

Two-way slabs:

Positive moment regions:

Bonded reinforcement not required where tensile stress ft

Otherwise, use As =

Nc = resultant tensile force acting on portion of concrete cross section in tension under effective prestress and service loads

Distribute As uniformly over precompressed tension zone as close as possible to extreme tensile fiber

c. f0 17

c

y

N

. f0 5

Two-way slabs:

Negative moment areas at column supports:

As = 0.00075Acf

Acf = larger gross cross-sectional area of slab-beam strips in two orthogonal equivalent frames intersecting at the columns

Distribute As between lines 1.5h on outside opposite edges of the column support

Code includes spacing and length requirements

Two-way slabsUse Equivalent Frame Design Method (Section 13.7)

Banded tendon distribution

Photo courtesy of Portland Cement Association

Development of prestressing strand

development length

= transfer length

ese pe

ps

Pf f

A

Shear for prestressed concrete members is similar to that for reinforced concrete members, but it takes advantage of presence of prestressing force

Post-tensioned tendon anchorage zone design

Load factor = 1.2 Ppu = 1.2Pj

Pj = maximum jacking force

= 0.85

Strength evaluation of existing structures (Chapter 20)

Strength evaluation of existing structures (Chapter 20)

When it is required

When we use analysis and when perform a load test

When core testing is sufficient

Load testing

A strength evaluation is required

when there is a doubt if a part or all of a structure meets safety requirements of the Code

If the effect of the strength deficiency is well understood and if it is feasible to measure the dimensions and material properties required for analysis, analytical evaluations of strength based on those measurements can be used

If the effect of the strength deficiency is not well understood or if it is not feasible to establish the required dimensions and material properties by measurement, a load test is required if the structure is to remain in service

Establishing dimensions and material properties

1. Dimensions established at critical sections

2. Reinforcement locations established by measurement (can use drawings if spot checks confirm information in drawings)

3. Use cylinder and core tests to estimate cf

Core testing

If the deficiency involves only the compressive strength of the concrete based on cylinder tests

Strength is considered satisfactory if:

1.Three cores are taken for each low-strength test

2.The average of the three cores 3.No individual core has a strength <

. cf085

. cf075

Steel

Reinforcing and prestressing steel may be evaluated based on representative material

If analysis is used, values of may be increased

Tension-controlled 0.90 1.0

Compression controlled 0.75 and 0.65 0.90 and 0.80

Shear and torsion 0.75 0.80

Bearing 0.65 0.80

Load test procedure

Load arrangement:Select number and arrangement of spans or panels loaded to maximize the deflection and stresses in the critical regions

Use more than one arrangement if needed (deflection, rotation, stress)

Load intensityTotal test load = larger of

(a) 1.15D + 1.5L + 0.4(Lr or S or R)

(b) 1.15D + 0.9L + 1.5(Lr or S or R)

(c) 1.3D

In (b), load factor for L may be reduced to 0.45, except for garages, places of assembly, and where L > 4.8 kN/m2

L may be reduced as permitted by general building code

Age at time of loading 56 days

Loading criteria

Obtain initial measurements (deflection, rotation, strain, slip, crack widths) not more than 1 hour before application of the first load increment

Take readings where maximum response is expected

Use at least four load increments

Ensure uniform load is uniform – no arching

Take measurements after each load increment and after the total load has been applied for at least 24 hours

Remove total test load immediately after all response measurements are made

Take a set of final measurements 24 hours after the test load is removed

Acceptance criteria

No signs of failure – no crushing or spalling of concrete

No cracks indicating a shear failure is imminent

In regions without transverse reinforcement, evaluate any inclined cracks with horizontal projection > depth of member

Evaluate cracks along the line of reinforcement in regions of anchorage and lap splices

Acceptance criteria

Measured deflections

At maximum load:

24 hours after load removed:

,

2

1 20 000t

h

1

4r

MIN(distance between supports, clear span + )

2 x span for cantilevert h

Acceptance criteria

If deflection criteria not met, may repeat the test (at least 72 hours after first test)

Satisfactory if:

2

5r

2 maximum deflection of second test relative to

postion of structure at beginning of second test

Provision for lower loading

If the structure does not satisfy conditions or criteria based on analysis, deflection, or shear, it may be permitted for use at a lower load rating based on the results of the load test or analysis, if approved by the building official

Case study

1905 building

Chicago, Illinois

USA

Cinder concrete

floors

Load capacity OK for use

as an office building?

Safety shoring

Deflection

measurement

devices

Load through

window

Moving lead ingots through the window

Load stage 14

Findings

Floor could carry uniform load of

2.4 kN/m2

Building satisfactory for both apartments (1.9 kN/m2) and offices (2.4 kN/m2)

Summary

Overview

Prestressed concrete

Strength evaluation of existing structures

118

Figures copyright 2010 by

McGraw-Hill Companies, Inc.

1221 Avenue of the America

New York, NY 10020 USA

Figures copyright 2011 by

American Concrete Institute

38800 Country Club Drive

Farmington Hills, MI 48331 USA

Duplication authorized or use with this presentation only.

The University of

Kansas

David Darwin, Ph.D., P.E.Deane E. Ackers Distinguished Professor Director, Structural Engineering & Materials Laboratory

Dept. of Civil, Environmental & Architectural Engineering2142 Learned HallLawrence, Kansas, 66045-7609(785) 864-3827 Fax: (785) 864-5631

daved@ku.edu