ACI VN 2011 Topic 1 (David Darwin)

7/25/2019 ACI VN 2011 Topic 1 (David Darwin)

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

Overview of ACI 318MDesign 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

This morning

Overview of ACI 318M-11

Design of Prestressed Concrete(Chapter 18)

Strength Evaluation of Existing

Structures (Chapter 20)


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This afternoon

Analysis and design of

Flexure

Shear

Torsion

Axial load

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 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).


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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


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Chapters

3. MATERIALS

Cementitious Materials, Water, Aggregates,

Admixtures, Reinforcing Materials

4. DURABILITY REQUIREMENTS

Freezing and Thawing, Sulfates, Permeability,

Corrosion


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5. CONCRETE QUALITY, MIXING, AND PLACING

6. FORMWORK, EMBEDMENTS,

AND CONSTRUCTION JOINTS

7. DETAILS OF REINFORCEMENT

Hooks and Bends, Surface Condition, Tolerances,

Spacing, Concrete Cover, Columns, Flexural Members,

Shrinkage and Temperature Steel, Structural Integrity


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8. ANALYSIS AND DESIGN GENERAL

CONSIDERATIONS

Design 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

REQUIREMENTS

Load Combinations, Strength Reduction Factors,Deflection Control

10. FLEXURE AND AXIAL LOADS

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

Bearing


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11. SHEAR AND TORSION

12. DEVELOPMENT

AND SPLICES OF REINFORCEMENT

13. TWO-WAY SLAB SYSTEMS

14. WALLS


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19. SHELLS AND FOLDED PLATE MEMBERS

20. STRENGTH EVALUATION OF EXISTING

STRUCTURES

21. EARTHQUAKE-

RESISTANTSTRUCTURES

22. STRUCTURAL PLAIN CONCRETE


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Appendices

A. STRUT-AND-TIE MODELS*

B. ALTERNATIVE

PROVISIONS FOR REINFORCED AND

PRESTRESSED CONCRETE FLEXURAL AND

COMPRESSION MEMBERS

C. ALTERNATIVE LOAD AND STRENGTHREDUCTION 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


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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


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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 loadEarthquake loads (E) full load

Rain loads (R)*

Snow loads (S)*

* Service-level loads


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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(Lror S or R)

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

U= 1.2D + 1.0W+ 1.0L + 0.5(Lror S or 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 factorsby ASCE 7 and ACI 318M

If Wbased on service-level forces, use 1.6Wplace of

1.0W

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

Tension-controlled and compression-

controlled sections


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T-beam

d

h

b

hf

bw

As

dt

Strain through depth of beam


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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


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Load distributions and modeling

requirements


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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 = Ig Iweb =

Columns: Ic= Ig=

wb h3

12

bh3

12


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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


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(a)

(b)

(c)

Moment and shear envelopes


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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 integrallywith 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


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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 l2factor

ln


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Composite

Max ve right

Max ve leftMax +ve

Allowable adjustment in maximum

moments for t 0.0075


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Design of prestressed concrete

(Chapter 18)

Behavior of reinforced concrete


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Reinforced concrete under service loads

Theory of prestressed concrete

Stresses


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57

Methods of prestressing concrete members

Post-Tensioning

Pretensioning

Prestressing steels


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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 strengthfpy= yield strength


U.S.

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

fpy(at 1% extension) 85% of fpu


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U.S.

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

Maximum permissible stresses in

prestressing steel

Due to prestressing steel jacking force:

0.94fpy0.80fpumanufacturers recommendation

Post-tensioning tendons, at anchorage devices

and couplers, immediately after force transfer:

0.70fpu


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Prestressed concrete members are

designed based on both

Elastic flexural analysis

Strength

Elastic flexural analysis

Considers stresses under both the

Initial prestress force Piand the

Effective prestress force Pe

Note: = concrete compressive strength

= initial concrete compressive

strength (value at prestress transfer)

cf

cif


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Classes of members

U 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


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Bending moments

Mo = self-weight moment

Md= superimposed dead load moment

Ml= live load moment

Concrete stresses under Pi


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Concrete stresses under Pi + Mo

Concrete stresses under Pe + Mo + Md+ Ml


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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

. cif025

. cif050

Maximum permissible compressive

stresses in concrete at service loads

Class U and T members

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

(b) Extreme fiber stress in compression due to

prestress plus total load

. 0 45 cf

. 0 60 cf


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Flexural strength

Aps T = 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


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Stress in prestressing steel at ultimate

Members 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 pc p

f df f

f d

11

andc y cf / 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 + 420 MPa

fpe = stress inAps atPe=e

ps

P

Members with unbonded tendons with span/depth

ratios > 35:

but not greater thanfpy or greater thanfpe + 210 MPa


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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


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Minimum bonded reinforcementAs 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

DistributeAs 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, useAs =

Nc= resultant tensile force acting on portion of

concrete cross section in tension under effective

prestress and service loads

DistributeAs uniformly over precompressed

tension zone as close as possible to extreme

tensile fiber

c. f0 17

c

y

N

. f0 5


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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

DistributeAs 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)


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Banded tendon distribution

Photo courtesy of Portland Cement Association

Development of prestressing strand

development length

= transfer length

ese peps

Pf f

A


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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


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Strength evaluation of existing structures

(Chapter 20)


(Chapter 20)

When it is required

When we use analysis and when perform a load test

When core testing is sufficient

Load testing


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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 thestructure is to remain in service


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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


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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-strengthtest

2.The average of the three cores

3.No individual core has a strength 4.8 kN/m2

L may be reduced as permitted by general

building code


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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


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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 hoursafter 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


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Acceptance criteria

Measured deflections

At maximum load:

24 hours after load removed:

,

2

120 000

t

h

1

4r

MIN(distance between supports, clear span + )

2 x span for cantilever

t 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


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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?


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Safety shoring

Deflection

measurement

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

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)


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Summary

Overview

Prestressed concrete


118


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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.

The Un iversity of Kansas

David 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]