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Building Code Requirements forStructural Concrete (ACI 318M-11)

Overview of ACI 318M

Design of Prestressed ConcreteEvaluation of Existing Structures

David Darwin

Vietnam Institute for Building Science andTechnology (IBST)

Hanoi and Ho Chi Minh City

December 12-16, 2011

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This morning

Overview of ACI 318M-11

Design of Prestressed Concrete(Chapter 18)

Strength Evaluation of ExistingStructures (Chapter 20)

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This afternoon

Analysis and design of

Flexure

Shear

Torsion

Axial load

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Tomorrow morning

Design of slender columns

Design of wall structures

High-strength concrete

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Overview of ACI 318M-11

Legal standing

Scope

Approach to DesignLoads and Load Cases

Strength Reduction Factors

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Legal standing

Serves as the legal structural concretebuilding code in the U.S. because it isadopted by the general building code (IBC).

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Scope

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

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Chapters

1. GENERAL REQUIREMENTSScope, Contract Documents, Inspection,

Approval of Special Systems

2. NOTATION AND DEFINITIONS

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Chapters

3. MATERIALSCementitious Materials, Water, Aggregates,

Admixtures, Reinforcing Materials

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4. DURABILITY REQUIREMENTSFreezing and Thawing, Sulfates, Permeability,

Corrosion

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5. CONCRETE QUALITY, MIXING, AND PLACING

6. FORMWORK, EMBEDMENTS,AND CONSTRUCTION JOINTS

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7. DETAILS OF REINFORCEMENT

Hooks and Bends, Surface Condition, Tolerances,Spacing, Concrete Cover, Columns, Flexural Members,Shrinkage and Temperature Steel, Structural Integrity
http://upload.wikimedia.org/wikipedia/en/5/55/Ronan_Point_-_Daily_Telegraph.jpg

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8. ANALYSIS AND DESIGN GENERALCONSIDERATIONS

Design Methods; Loading, including Arrangement ofLoad; Methods of Analysis; Redistribution of Moments;Selected Concrete Properties; Requirements forModeling Structures (Spans, T-beams, Joists...)

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9. STRENGTH AND SERVICEABILITYREQUIREMENTS

Load Combinations, Strength Reduction Factors,Deflection Control

10. FLEXURE AND AXIAL LOADSBeams and One-way Slabs, Columns, Deep Beams,

Bearing

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11. SHEAR AND TORSION

12. DEVELOPMENT

AND SPLICES OF REINFORCEMENT

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13. TWO-WAY SLAB SYSTEMS

14. WALLS

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15. FOOTINGS

16. PRECAST

CONCRETE

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17. COMPOSITE CONCRETE FLEXURALMEMBERS

18. PRESTRESSED CONCRETE

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19. SHELLS AND FOLDED PLATE MEMBERS

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20. STRENGTH EVALUATION OF EXISTING

STRUCTURES

21. EARTHQUAKE-

RESISTANT

STRUCTURES

22. STRUCTURAL PLAIN CONCRETE

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Appendices

A. STRUT-AND-TIE MODELS*

B. ALTERNATIVEPROVISIONS FOR REINFORCED ANDPRESTRESSED CONCRETE FLEXURAL ANDCOMPRESSION MEMBERS

C. ALTERNATIVE LOAD AND STRENGTHREDUCTION FACTORS

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D. ANCHORING TO CONCRETE*

E. STEEL REINFORCEMENT INFORMATION

F. EQUIVALENCE BETWEEN SI-METRIC, MKS-

METRIC, AND U.S. CUSTOMARY UNITS OFNONHOMOGENOUS EQUATIONS IN THE CODE

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Approach to design

Qd= design loads

Sn= nominal strengthSd=design strength

M =safety margin

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Design Strength Required Strength

Sd=SnQd

Sd = design strength =Sn

= strength reduction factor

= load factors

Qd = design loads

and in Chapter 9 of ACI 318M

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Loads Qd

specified in ASCE 7, Minimum Design Loadsfor Buildings and Other Structures

American Society of Civil Engineers (ASCE)

Reston, Virginia, USA

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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

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Loads

Impact include in LSelf-straining effects (temperature, creep,shrinkage, differential settlement, andshrinkage compensating concrete) (T)

Fluid loads (F)

Lateral soil pressure (H)

Factored Load = U= Qd

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Load cases and load factorsby ASCE 7 and ACI 318M

U= 1.4D

U= 1.2D+ 1.6L + + 0.5(Lror Sor R)U= 1.2D+ 1.6(Lror Sor R) + (1.0Lor 0.5W)

U= 1.2D+ 1.0W+ 1.0L + 0.5(Lror Sor R)

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

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U= 0.9D+ 1.0W

U= 0.9D+ 1.0E

Load cases and load factors

by ASCE 7 and ACI 318M

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If Wbased on service-level forces, use 1.6Wplace of1.0W

If Ebased on service-level forces, use 1.4Ein placeof 1.0E

Details of other cases covered in the Code

Load factors by ACI 318M

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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

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Tension-controlled and compression-controlled sections

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T-beam

d

h

b

hf

bw

As

dt

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Strain through depth of beam

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Design Strength (x nominal strength) must

exceed the Required Strength (factored load)

Bending MnMu

Axial load PnPu

Shear VnVu

Torsion TnTu

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Load distributions and modelingrequirements

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Structure may be analyzed as elastic

using properties of gross sections

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

Beams: Ib= IgIweb =

Columns: Ic= Ig=

wb h3

12

bh3

12

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2. The arrangement of load may be limited tocombinations of

(a) factored dead loadon all spans with full

factored live load on alternate spans, and(b) factored deadload on all spans with full

factored live load on two adjacent spans

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(a)

(b)

(c)

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Moment and shear envelopes

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Columns designed to resist

(a) axial forces from factored loads on all floorsor roof and maximum moment from factoredlive loads on a single adjacent spanof thefloor or roof under consideration

(b) loading condition giving maximum ratio ofmoment to axial load

More on columns

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For frames or continuous construction, considereffect of unbalanced floor or roof loads on both

exterior and interior columns and of eccentricloading due to other causes

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

At any floor or roof level, distribute the moment

between columns immediately above and belowthat floor in proportion to the relative columnstiffness

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Simplified loading criteria

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Beams, twoor more spans

Beams, twospans only

Slabs,spans 3 m

Beams, col stiffnesses 8 beam stiffnesses

u nM w l 2factor

ln

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Composite

Maxve right

Maxve leftMax +ve

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Allowable adjustment in maximummoments for t 0.0075

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Design of prestressed concrete(Chapter 18)

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Behavior of reinforced concrete

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Reinforced concrete under service loads

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Theory of prestressed concrete

Stresses

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57

Methods of prestressing concrete members

Post-Tensioning

Pretensioning

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Prestressing steels

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Strength of prestressing steels available inU.S.

Seven-wire strand: fpu 1725, 1860 MPa

fpy(stress at 1% extension) 85% (for stress-

relieved strand) or 90% (for low-relaxationstrand) of fpu

fpu= ultimate strengthfpy= yield strength

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

fpy(at 1% extension) 85% of fpu

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High-strength steel bars: fpu

1035 MPa

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

fpybased on either 0.2% offset or 0.7% strain

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Maximum permissible stresses inprestressing steel

Due to prestressing steel jacking force:0.94fpy0.80fpu

manufacturers recommendation

Post-tensioning tendons, at anchorage devicesand couplers, immediately after force transfer:

0.70fpu

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Prestressed concrete members aredesigned based on both

Elastic flexural analysis

Strength

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Elastic flexural analysis

Considers stresses under both theInitial prestress force Piand the

Effective prestress force Pe

Note: = concrete compressive strength

= initial concrete compressivestrength (value at prestress transfer)

cf

cif

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Classes of members

U uncracked calculated tensile stress in

precompressed tensile zone at serviceloads = ft

T transition between uncracked andcracked < ft

C cracked ft>

. cf0 62

. cf0 62 . cf10

.c

f10

cf in MPa

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Concrete section properties

e =tendon eccentricityk1= upper kern point

k2= lower kern point

Ic= moment of inertia

Ac= area

radius of gyration:

r2 = Ic/Ac

section moduli:S1 = Ic/c1S2 = Ic/c2

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Bending moments

Mo= self-weight moment

Md= superimposed dead load moment

Ml= live load moment

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Concrete stresses under Pi

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S bl k

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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

S i i l l i

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Stress in prestressing steel at ultimate

Members with bonded tendons:

p=Aps/bdp = reinforcement ratiob = width of compression facedp=d(effective depth) of prestressing steel

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Members with bonded tendons and non-prestressed bars:

p pu ps pu pc p

f df f

f d

11

and y c y c f / f f / f

and refer to compression reinforcement, sA

shall be takenpup pc p

f d . , d . d f d

017 015

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Members with unbonded tendons with span/depth

ratios > 35:

but not greater thanfpy or greater thanfpe + 210 MPa

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Li it i f t i fl l

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Limits on reinforcement in flexuralmembers

Classify as tension-controlled, transition, orcompression-controlled to determine

Total amount of prestressed and nonprestressedreinforcement in members with bondedreinforcement must be able to carry 1.2

cracking load

Mi i b d d i f t A i

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Minimum bonded reinforcement As inmembers with unbonded tendons

Except in two-way slabs, As= 0.004ActAct=area of that part of cross sectionbetween the flexural tension face and

center of gravity of gross section

Distribute Asuniformly over precompressedtension zone as close as possible to

extreme tensile fiber

Two way slabs:

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Two-way slabs:

Positive moment regions:

Bonded reinforcement not required where tensilestress ft

Otherwise, use As=

Nc= resultant tensile force acting on portion ofconcrete cross section in tension under effectiveprestress and service loads

Distribute Asuniformly over precompressedtension zone as close as possible to extremetensile fiber

c. f0 17

c

y

N

. f0 5

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Two-way slabs:

Negative moment areas at column supports:

As= 0.00075AcfAcf= larger gross cross-sectional area of slab-beam strips in two orthogonal equivalent

frames intersecting at the columns

Distribute Asbetween lines 1.5hon outside

opposite edges of the column support

Code includes spacing and length requirements

T o a slabs

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Two-way slabsUse Equivalent Frame Design Method

(Section 13.7)

Banded tendon distribution

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Banded tendon distribution

Photo courtesy of Portland Cement Association

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Post tensioned tendon anchorage zone

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Post-tensioned tendon anchorage zonedesign

Load factor = 1.2Ppu= 1.2Pj

Pj= maximum jacking force

= 0.85

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Strength evaluation of existing structures

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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

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A strength evaluation is required

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

If the effect of the strength deficiency is wellunderstood and if it is feasible to measure thedimensions and material properties required foranalysis, analytical evaluations of strength

based on those measurements can be used

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If the effect of the strength deficiency is not wellunderstood or if it is not feasible to establish therequired dimensions and material properties bymeasurement, a load test is required if the

structure is to remain in service

Establishing dimensions and material

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Establishing dimensions and materialproperties

1. Dimensions established at critical sections

2. Reinforcement locations established by

measurement (can use drawings if spotchecks confirm information in drawings)

3. Use cylinder and core tests to estimate cf

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Load intensity

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Load intensity

Total test load = larger of

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

(b) 1.15D + 0.9L + 1.5(Lror 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/m

2

L may be reduced as permitted by general

building code

Age at time of loading 56 days

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Age at time of loading 56 days

Loading criteria

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Loading criteria

Obtain initial measurements (deflection,rotation, strain, slip, crack widths) not morethan 1 hour before application of the firstload increment

Take readings where maximum response isexpected

Use at least four load increments

Ensure uniform load is uniform no arching

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Take measurements after each load

increment and after the total load has beenapplied for at least 24 hours

Remove total test load immediately after allresponse measurements are made

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

Acceptance criteria

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Acceptance criteria

No signs of failure no crushing or spalling

of concrete

No cracks indicating a shear failure isimminent

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

Evaluate cracks along the line of

reinforcement in regions of anchorage andlap splices

Acceptance criteria

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Acceptance criteria

Measured deflections

At maximum load:

24 hours after load removed:

,

2

120 000

t

h

14

r

MIN(distance between supports, clear span + )

2 x span for cantilever

t h

Acceptance criteria

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Acceptance criteria

If deflection criteria not met, may repeat thetest (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

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Provision for lower loading

If the structure does not satisfy conditions orcriteria based on analysis, deflection, or shear,it may be permitted for use at a lower loadrating based on the results of the load test or

analysis, if approved by the building official

Case study

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Case study

1905 buildingChicago, Illinois

USA

Cinder concrete

floors

Load capacity OK for use

as an office building?

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Safety shoring

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Deflectionmeasurement

devices

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Load through

window

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Moving lead ingots through the window

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Load stage 14

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Findings

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g

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

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y

Overview

Prestressed concrete

Strength evaluation of existing structures

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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.

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The University of KansasDavid Darwin, Ph.D., P.E.Deane E. Ackers Distinguished Professor

Director, Structural Engineering & Materials Laboratory

Dept. of Civil, Environmental & Architectural Engineering

2142 Learned Hall

Lawrence, Kansas, 66045-7609

(785) 864-3827 Fax: (785) 864-5631

[email protected]