GIRDER DESIGN FOR BUILDINGS

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1 POST-TENSIONED TRANSFER GIRDER DESIGN FOR BUILDINGS Overload, Restraint and Construction Staging g Issues Almıla Uzel and Neb Erakovic (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHTS RESERVED Page 1 of 25

2 POST-TENSIONED TRANSFER GIRDERS Fewer columns at lower stories due to architectural demands High-rise tower columns that do not extend to lower stories (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHTS RESERVED Page 2 of 25

3 POST-TENSIONED TRANSFER GIRDERS Effective Solution Control of creep deflections Control of cracking Increased load-carrying capacity Reduced member depth CHALLENGES??? (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHTS RESERVED Page 3 of 25

4 CHALLENGES Stage Pre-stressing and Construction Issues Anchorage and Bursting Zone Reinforcement Shear Design of Transfer Girders (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHTS RESERVED Page 4 of 25

5 STAGE PRE-STRESSING Large concentrated loads require high pre-stressing forces to be applied Stressing must be completed in several stages In order to keep stresses under allowable code limits Stage1 Stage2 Stage3 P eff1 P eff2 P eff3 (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHTS RESERVED Page 5 of 25

LOADS DURING INTERMEDIATE STAGES Balance about 80-120% of the existing dead load

Stressing P=P i =1.15-1.20P eff1 0.75(D@stressing) f ci Stage1 - Final P=P eff1 1.

6 LOADS DURING INTERMEDIATE STAGES Balance about % of the existing dead load Stage Pre-stressing Force Loads on Structure Concrete Strength Stage1 - At Stressing P=P i = P eff1 0.75(D@stressing) f ci Stage1 - Final P=P eff1 1.25((D+L)@final) f c (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHTS RESERVED Page 6 of 25

7 LOADS AT FINAL STAGE Stage Final Stage - Stressing Pre-stressing Force P=P i = P eff3 Final Stage - Final P=P eff3 Loads on Structure 0.75(D@stressing) ((D+L)@final) ) (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHTS RESERVED Page 7 of 25

STAGE PRE-STRESSING AND CONSTRUCTION ISSUES Curtain wall

region may be left open OR Finish stressing earlier and add top

occupied Finish stressing earlier and add top mild reinforcement

8 STAGE PRE-STRESSING AND CONSTRUCTION ISSUES Curtain wall installation starts while tower construction progresses Pocket region may be left open OR Finish stressing earlier and add top mild reinforcement to control cracking Lower stories may be occupied Finish stressing earlier and add top mild reinforcement to control cracking (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHTS RESERVED Page 8 of 25

9 ANCHORAGE AND BURSTING ZONE Transverse stresses occur due to high longitudinal compression applied by the pre-stressing force Disturbed region extends about beam depth away from the face of the beam Vertical mesh of mild reinforcement should be provided within this disturbed region Adapted from Prestressed Concrete Structures by Collins &Mitchell (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHTS RESERVED Page 9 of 25

pre-stressing force Congestion of mild reinforcement Concrete placement

10 ANCHORAGE AND BURSTING ZONE For typical anchor arrangements tensile stresses in the bursting zone are about 15-20% of the compressive stress due to pre-stressing force Congestion of mild reinforcement Concrete placement becomes challenging (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHTS RESERVED Page 10 of 25

ANCHORAGE AND BURSTING ZONE Use of pins for easy mild

used to ensure concrete quality in casting dense

Concrete to reduce the amount of required mild

11 ANCHORAGE AND BURSTING ZONE Use of pins for easy mild reinforcement placement Self compacting concrete can be used to ensure concrete quality in casting dense reinforcement Research by others on Steel Fiber Reinforced Concrete to reduce the amount of required mild reinforcement (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHTS RESERVED Page 11 of 25

12 ANCHORAGE AND BURSTING ZONE Veer Towers MGM City Center Project Three PT transfer girders intersecting at one point Single leg pins are used for easy rebar placement (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHTS RESERVED Page 12 of 25

13 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 loads Shear design of PT transfer girders becomes important (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHTS RESERVED Page 13 of 25

SHEAR DESIGN OF POST-TENSIONED TRANSFER GIRDERS Response-2000 (Bentz, 1999) Non-linear Sectional Analysis Program based on Modified Compression Field Theory (Vecchio&Collins, 1986) R2K considers the

14 SHEAR DESIGN OF POST-TENSIONED TRANSFER GIRDERS 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 a section and the pre- stressing force can easily be defined (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHTS RESERVED Page 14 of 25

16 SHEAR DESIGN OF POST-TENSIONED TRANSFER GIRDERS b=h=8 0 P eff =5330 kips M u =37500 ft-kips V u =3000 kips d=8.24 ft. M/V@d=37500/3000=12.5 ft V c =(0.6 f c +700V u d/m u )b w d ACI φvcaci=4300 kips (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHTS RESERVED Page 16 of 25

17 SHEAR DESIGN OF POST-TENSIONED TRANSFER GIRDERS M/V=12.5 ft φv nr2k = 0.85*3196 = 2717 kips < V u = 3000 kips φv naci = 0.85*5068 = 4310 kips > V u = 3000 kips (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHTS RESERVED Page 17 of 25

19 POST-TENSIONED TRANSFER GIRDERS Offset Columns as a result of architectural design Perpendicular beams introduced to avoid torsional rotation (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHTS RESERVED Page 19 of 25

Analyzed using VecTor (NLFEA) based on MCFT 3PT3 3PT5

21 NLFE ANALYSIS OF INTERSECTION OF TWO BEAMS The intersection of these two beams is a disturbed area Analyzed using VecTor (NLFEA) based on MCFT 3PT3 3PT5 (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHTS RESERVED Page 21 of 25

ADDITIONAL TRANSVERSE REINFORCEMENT CSA-A23.

24 ADDITIONAL TRANSVERSE REINFORCEMENT CSA-A requires additional reinforcement capable of transmitting a tensile force of Adapted from CSA-A T f =(1-h b /h 1 )*V f However, ACI318 does not specify any requirements regarding g the shear design of such regions (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHTS RESERVED Page 24 of 25

25 CONCLUSIONS The Use of Post-tensioned Transfer Girders Enables the Engineer to Provide Economical Solutions Stage Post-tensioning 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 Large Concentrated Loads Beam-to-beam Intersection Zone Reinforcement (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHTS RESERVED Page 25 of 25

Appendix A Proposed LRFD Specifications and Commentary

NCHRP Project 12-71 Design Specifications and Commentary for Horizontally Curved Concrete Box-Girder Highway Bridges Appendix A Proposed LRFD Specifications and Commentary A-1 A-2 4.2 DEFINITIONS (Additional)