Major Building Codes for DesignMajor Building Codes for Designof Postof Post--Tensioned BuildingsTensioned Buildingsof Postof Post Tensioned BuildingsTensioned Buildings

D Bij O A l iDr Bijan O AalamiProfessor Emeritus,

San Francisco State UniversityPrincipal, ADAPT Corporation

www.adaptsoft.comwww.adaptsoft.com

Major Building Codes and ReportsCovered

International Building Code(IBC 2009)( )

ACI-318 2011ASCE -07

European Code (EC2 – EN2002)

Numerous Post-TensioningInstitute Reports (PTI USA)

Concrete Society Report (TR43)

Major Building Codes for Design of Post-Tensioned Floors

1 – Materials

Characteristic strengthDesign strengthModulus of Elasticity

Building CodesSupplemental Literature

IBCACI 318-11 Requirements for Design

of Concrete Floor SystemsyEC2

European Coe Requirements for Design of Concrete Floors IncludingDesign of Concrete Floors, Including Post-TensioningServiceability of Post-Tensioned

M b B d E C dMembers Based on European Code

Common Requirements Common Requirements PostPost--Tensioning DesignTensioning Design

Design or evaluate the gadequacy of the structure for the following conditions

Service ConditionStrength ConditionInitial (transfer of prestressing)

POSTPOST--TENSIONING IN BUILDING TENSIONING IN BUILDING CONSTRUCTIONCONSTRUCTION

SERVICE CONDITIONSERVICE CONDITION

Crack control Limitation on “representative” phypothetical tensile stressesMinimum bonded reinforcementCrack mitigation schemesCrack mitigation schemesdetailing for restraint of supportsTendon and rebar arrangement

Deflection controlSpan to deflection ratioLimitation on concrete compressive stressAllowance for creepAllowance for creep

DurabilityCover to tendonUse of special hardwareUse of special hardware

Fire resistivityCover to tendon

Vibrations Limitation on span to depth ratio

POSTPOST--TENSIONING IN BUILDING TENSIONING IN BUILDING CONSTRUCTIONCONSTRUCTION

STRENGTH (SAFETY) CONDITION

OVERALL

Design capacity greater than designDesign capacity greater than designmoment (Capacity to be greater than demand)

Minimum bonded reinforcement for ductilityDesign capacity greater than CrackingMoment (applicable to most cases) Safe transfer of column moment to slabFollow code prescription for tendon andrebar detailing arrangement (not columnstrip/middle strip)

LOCAL

Force transfer at anchorage (bursting steel)Force transfer at anchorage (bursting steel)

Detailing to avoid blow out of concrete

POSITION OF REINFORCEMENTTO RESIST COLUMN MOMENT

SLABd

REBAR STRIP

DROP 1.5d 1.5dh

REBARSTRIP

(a) SLAB WITH DROP

DROP 5d 5d

(b) FLAT PLATE

1.5h 1.5hCOLUMN

REBAR STRIPFRAME

SLAB

COLUMN

DIRECTIONFRAME

( ) VIEW OF A SLAB JOINT

DESIGNSTRIP

(c) VIEW OF A SLAB JOINT

Position the reinforcement within the Position the reinforcement within the narrow band identified as rebar stripnarrow band identified as rebar strippp

POSTPOST--TENSIONING IN BUILDING TENSIONING IN BUILDING CONSTRUCTIONCONSTRUCTION

INITIAL (TRANSFER) CONDITION

At time of stressing,

Tendon has its maximum force;

concrete is at its weakest strength; and

li l d t t t t i i b tlive load to counteract prestressing is absent

Hence the member is likely to experience stresses more severe than when in service

Code requirement for crack and creepcontrol at initial conditioncontrol at initial condition

Add rebar when “representative” (hypothetical) tension stresses exceeda threshold

Do not exceed “representative”hypothetical compressive stresses

Load CombinationsLoad Combinations

Service Condition

Total Load (frequent)

Typically 1.0D + 1.0L + 1.0 PTTypically 1.0D 1.0L 1.0 PTUsed to check tension and compressionstresses in concrete

Sustained Load (quasi permanent)

1.0D + K*L + 1.0 PTK is less than 1

Primarily to check compression stresses in concrete D fl ti i i ditiDeflection in service condition

Load CombinationsLoad Combinations

Strength (safety) condition

Factored combinations of:Factored combinations of:Dead (1.2 to 1.4)Live (1.3 to 1.6)

(S )Hyperstatic (Secondary) due toprestressing with factor “1”

Initial (transfer) condition

Factored combinations of:Factored combinations of:Selfweight (1.0)Prestressing (1.15)

POSTPOST--TENSIONING IN BUILDING TENSIONING IN BUILDING CONSTRUCTIONCONSTRUCTION

Safety factor for strength

Different code specified applied loadsp pp

EitherReliability in the properties of theReliability in the properties of the material used (most building codes)concrete (typically 0.65)nonprestressed steel (0.85 – 0.95)prestressing steel (0.85)

ORReliability in the analysis procedure (US building codes)bending 0.90shear 0 75shear 0.75axial 0.70

Design for StrengthDesign for StrengthDuctility Requirements of All CodesDuctility Requirements of All Codes

PRESTRESSING REBAR TOMAX

aMAX

REBARCOMPRESSION

REBARTENSILEREQUIRED

(1) (2)

COMPRESSIONFORCE OFBALANCE

TENSILEREBAR

MAX

(3)

REBAR

TENSION REBARPRESTRESSING PLUSPRESTRESSING

ADEQUATE

ADEQUATEREBAR NOT

AND TENSIONPRESTRESSING

REQUIRED COMPRESSION

MAXa

MAXa

REBARREBAR TO BALANCE

ADDED TENSILE

MAX MAXa REBAR

STRESSING

EXCESSIVEPRE-

(4) (5)

REBARCOMPRESSION

MAX

(6)

AND COMPRESSIONPRESTRESSING

COMPRESSION ZONEDESIGN BASED ONOVERREINFORCED,

REBAR REBAR NOTADEQUATE

AND COMPRESSIONPRESTRESSING

Limit depth of compression zoneLimit depth of compression zone

PostPost--Tensioned Members Tensioned Members Serviceability Serviceability

Check According to EC2Check According to EC2--EN 2002EN 2002

Frequentload combination

One-way & Two-waySystems

DL+0.5LL+PT

Calculate design values

ConcreteCompression0.60 fck (7.2)

Tension,fct,eff (7.3.2(4))

Select allowable stresses Steel

0.8fyk (7.2)

PT

0.75fyk (7.2)

Yes No

Compare with hypothetical values

Serviceability fails

Modify Calculate hypothetical

stresses

Is PT stressOK ?

No

Yes

Is steel stressOK ?

NoIs conc.

compression stress OK ?

Yes

No Go to( A )Yes

Is conc.tensionstress OK ?

* EN 1992-1-1:2004(E). section 7.2

Provide Min. As( 9.2.1.1,9.3.1.1)

Go to( B )

(A)(A) EC2EC2--EN 2002EN 2002

Coming from flow-chart

EC2-EN-2002

System?

Calculate Min. overallAs (Asmin)

( 9.2.1.1,9.3.1.1)

Calculate Max. overallAs (Asmin)

( 9.2.1.1,9.3.1.1)

Calculate min.crack control reinforcement

based on system.

y

Unbonded7.3.2(2)Ascrack

Bonded7.3.2(3)Ascrack

Select allowable crack-widthTable 7.1N

Determine "Computed" crack width (EC2-7.3.4)

Yes Exceedsallowable? No

Limit crack width by adding rebar(Eqn. 7.8 & 7.9)

or, limit max.spacing or diameter (table 7.2N or 7.3N)

Ascrack >Asmin ?

Yes

ProvideAsmin No

Provide As,crack Return

(B)(B)Ffrom

flow chart

EC2EC2--EN 2002EN 2002

flow-chartEC2-EN-2002

Quasiload combination

DL+0 3LL+PT

Select allowable stressesConcrete

Compression0 45 fck (7 2)

Tension,fct eff (7 3 2(4))

Calculate hypothetical

stresses

DL+0.3LL+PT

Compare with hypothetical values

0.45 fck (7.2) fct,eff (7.3.2(4))

Is conc.compressionstress OK ?

NoYesModify computed

deflection fornon-linear creep

YesIs conc.tension

stressOK ?

Provide Min. As( 9.2.1.1,9.3.1.1)

No Go to ( A)

Serviceabilty OKExit

IBC 2009; ACI 318-11Service Condition

Design is based on theclassification of members into thefollowing classesg

Class U (uncracked) ft <= 7 5 √ f’c ; (0 625 √ f’c)ft < 7.5 √ f c ; (0.625 √ f c)

Class T (transition)7 5 √ f’c < f <=12 √ f’c7.5 √ f c < ft <=12 √ f c (0.625 √ f’c < ft <=1.0 √ f’c )

Class C (cracked)Class C (cracked)ft > 12 √ f’c(ft > 1.0 √ f’c )

ft = tensile stress under “sustained service loadcondition”

Values in parenthesis refer to SI units (N mm)Values in parenthesis refer to SI units (N, mm)

Service Condition

Deflection calculationFor Class U, use gross-sectionFor Class T use bilinear momentFor Class T, use bilinear momentproperties, or IeFor class C, use cracked sectionproperties or bilinear moment or Iproperties, or bilinear moment, or Ie

Two-way systems shall bedesigned as Class Ug

Stresses are calculated usinguncracked section

One-way systems may be designedOne-way systems may be designedas Class U, T or C

Stress calculation For class U and T, calculate stresses using uncracked section (gross cross-sectional properties)For Class C use cracked section for stressFor Class C, use cracked section for stress calculation; control crack width and check for skin rebar

IBC 2009; ACIIBC 2009; ACI--318 Load 318 Load Combinations for Strength CheckCombinations for Strength Check

D = Dead LoadsL = Live LoadsE = Earthquake EffectsW = Wind EffectsHyp = Hyperstatic Effects Due to

Prestressing

Dead and LiveU = 1.4D + 1.0 Hyp (9-1)U = 1.2D + 1.6L + 1.0 Hyp (9-2)

D d Li d Wi dDead, Live and WindU = 1.2D + 1.6Lr+ 0.8W + 1.0 Hyp (9-3)U = 1.2D + 1.0L + 1.6W + 1.0 Hyp (9-4)U = 0.9D + 1.6W + 1.0 Hyp (9-6)U 0.9D 1.6W 1.0 Hyp (9 6)

Dead, Live and EarthquakeU = 1.2D + 1.0L+ 1.0E + 1.0Hyp (9-5)U 0 9D 1 0E 1 0H (9 7)U = 0.9D + 1.0E + 1.0Hyp (9-7)

IBC and ACI 318’s Other Considerations

Crack Width Control and Rebar Spacing Based on Detailing notSpacing Based on Detailing not

Crack Width Calculation

For Class C flexural members rebar requirement and spacing shall meet the crack width control provisions ofnonprestressed members ( section 10.6.4) modified for prestressing (section 18.4.4.1)

Strength Reduction Factor for Bending and Compression

c = depth of compression zonedt = distance of extreme compressiont p

fiber to the extreme tensionreinforcement

φR

ON

FAC

TOR

0.9 dtc

∈

RED

UC

TIO

0.65

TREN

GTH

0 0.375c/d t

0.600

ST

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