post-tensioned transfertensioned … convention/s2-3...compression applied by the pre-stressing...

POST-TENSIONED TRANSFERPOST-TENSIONED TRANSFER GIRDER DESIGN FOR BUILDINGS

Overload, Restraint and Construction Staging Issues, g g

Almıla Uzel and Neb Erakovic

2008 PTI TECHNICAL CONFERENCE SESSION 2 - POST-TENSIONED TRANSFER GIRDER DESIGN FOR BUILDINGS ALMILA UZEL AND NEB ERAKOVIC

(C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHTS RESERVED of 25

POST-TENSIONED TRANSFER GIRDERS

• Fewer columns at lower stories due to architectural demands

• High-rise tower columns that do not extend to lower stories




• Effective Solution• Control of creep

deflections• Control of cracking• Control of cracking• Increased load-carrying

capacity• Reduced member depth

CHALLENGES???



CHALLENGES

• Stage Pre-stressing and Construction Issues

• Anchorage and Bursting Zone Reinforcement

• Shear Design of Transfer Girders



STAGE PRE-STRESSING

• Large concentrated loads require high pre-stressing forces to be appliedforces to be applied

• Stressing must be completed in several stages• In order to keep stresses under allowable code

limitslimits

Stage1 Stage2 Stage3Peff1 Peff2 Peff3



LOADS DURING INTERMEDIATE STAGES

• Balance about 80-120% of the existing dead load

Stage1 - At StressingP=Pi=1.15-1.20Peff1

Stage1 - FinalP=Peff1

StagePre-stressing Force

0.75(D@stressing)fci’

1.25((D+L)@final)fc’

Loads on StructureConcrete Strength



LOADS AT FINAL STAGE

Final Stage - StressingP=Pi=1.15-1.20Peff3

0.75(D@stressing)

Final Stage - FinalP=Peff3

((D+L)@final)

StagePre-stressing ForceLoads on Structure ( @ g) (( )@ )



STAGE PRE-STRESSING AND CONSTRUCTION ISSUES

• Curtain wall installation starts while tower constructionwhile tower construction progresses– Pocket region may be left

openopenOR

– Finish stressing earlier and dd t ild i f tadd top mild reinforcement

to control cracking

Lower stories may be occupied• Lower stories may be occupied– Finish stressing earlier and

add top mild reinforcement to control crackingto control cracking



ANCHORAGE AND BURSTING ZONE

• Transverse stresses occur due to high longitudinaldue to high longitudinal compression applied by the pre-stressing force

• Disturbed region extends about beam depth away from the face of the beamthe face of the beam

• Vertical mesh of mild reinforcement should bereinforcement should be provided within this disturbed region Adapted from

Prestressed ConcretePrestressed Concrete Structures by Collins &Mitchell




• For typical anchor arrangements tensilearrangements tensile stresses in the bursting zone are about 15-20% of the compressive stress pdue to pre-stressing force

• Congestion of mildCongestion of mild reinforcement

• Concrete placement• Concrete placement becomes challenging




• Use of pins for easy mild reinforcement placementreinforcement placement

• Self compacting concrete can be used to ensurecan be used to ensure concrete quality in casting dense reinforcement

• Research by others on Steel Fiber Reinforced Concrete to reduce theConcrete to reduce the amount of required mild reinforcement




Veer Towers – MGM City Center Project

• Three PT transfer girders intersecting at• Three PT transfer girders intersecting at one point

Single leg pins are used for easy rebar• Single leg pins are used for easy rebar placement



SHEAR DESIGN OF POST-TENSIONED TRANSFER GIRDERS

• High rise tower columns are transferred at lower stories• Transfer girders are subjected to large shear forces and

bending moments due to large concentrated loadsbending moments due to large concentrated loads• Shear design of PT transfer girders becomes important




• Response-2000 (Bentz, 1999) – Non-linear Sectional Analysis Program based on Modified Compression Field Theory (Vecchio&Collins, 1986)

• R2K considers the adverse effect of bending moment on the shear capacity of a reinforced concrete section

• Slope of tendon at the analyzed section and the pre-S ope o te do at t e a a y ed sect o a d t e p estressing force can easily be defined




M/V= 12.5 ft. Peff=5330 kips




b=h=8’0”Peff=5330 kipsMu =37500 ft-kipsVu=3000 kipsd=8.24 ft.M/V@d=37500/3000=12.5 ft

Vc=(0.6 fc’+700Vud/Mu)bwdACI318-99 φVcACI=4300 kips




M/V=12.5 ft φVnR2K= 0.85*3196 = 2717 kips < Vu= 3000 kipsφVnACI= 0.85*5068 = 4310 kips > Vu= 3000 kips




M/V=12.5 ft 3-#5@8 φVnR2K=0.85*5506=4680 kips>Vu=3000 kips




• Offset Columns as a result of architectural design• Perpendicular beams introduced to avoid torsional• Perpendicular beams introduced to avoid torsional

rotation




Perpendicular beams are introduced toare introduced to avoid torsional rotation of the transfer girderg



NLFE ANALYSIS OF INTERSECTION OF TWO BEAMS

• The intersection of these two beams is a disturbed area• Analyzed using VecTor (NLFEA) based on MCFT• Analyzed using VecTor (NLFEA) based on MCFT

3PT3

3PT5



NLFE ANALYSIS OF INTERSECTION OF TWO BEAMS

• Push-out failure of the bottom of supporting beam with no hanger reinforcement



NLFE ANALYSIS RESULTS

• Additional transverse reinforcement should be placed to avoid push-out failure of supporting beamto avoid push out failure of supporting beam



ADDITIONAL TRANSVERSE REINFORCEMENT

Adapted from

CSA-A23.3-04

• CSA-A23.3-04 requires additional reinforcement capable of transmitting a tensile force of

T (1 h /h )*VTf=(1-hb/h1)*Vf

• However, ACI318 does not specify any requirements regarding the shear design of such regions g g g g



CONCLUSIONS

• The Use of Post-tensioned T f Gi d E bl thTransfer Girders Enables the Engineer to Provide Economical Solutions

• Stage Post-tensioning –Stage Post tensioning Assessment of Loads on Structure at Different Construction Stages

• Anchorage and Bursting Zone -Reinforcement Congestion

• Shear Design of Transfer Girders That are Subjected to LargeThat are Subjected to Large Concentrated Loads

• Beam-to-beam Intersection Zone Reinforcement



post-tensioned transfertensioned … convention/s2-3...compression applied by the pre-stressing...

Documents