REPAIR OF PRECAST PRE-TENSIONED GIRDERS USING EXTERNAL POST-TENSIONING ASIT N. BAXI BAXI ENGINEERING MIGUEL FORERO SUNCOAST POST-TENSION

Transcription

1 REPAIR OF PRECAST PRE-TENSIONED GIRDERS USING EXTERNAL POST-TENSIONING ASIT N. BAXI BAXI ENGINEERING MIGUEL FORERO SUNCOAST POST-TENSION

2 PROJECT INFORMATION Project name: Laketown Wharf Retrofit Location: Panama City Beach, Florida Owner: Laketown Wharf Marketing, LLC Retrofit designer & contractor: Suncoast Post-Tension, Ltd. Grouting subcontractor: Florida Lemark Corporation 2

3 KEY POINTS Project background Existing structure Analysis and design Structural details Retrofit pictures 3

4 PROJECT BACKGROUND LAKETOWN WHARF RETROFIT 4

5 PROJECT BACKGROUND Owner observed cracks in the bottom face of two precast pretensioned girders supporting the pool deck The owner s structural engineer reviewed the original design and found that the girders were not adequately reinforced to prevent cracking under full service loads Owner requested a retrofit proposal with a minimum effect on the building s architectural design 5

6 PROJECT BACKGROUND Suncoast Post-Tension (SCPT) submitted a retrofit proposal using external post-tensioning with ½-in. strands SCPT designed and executed the retrofit The design and the executed work were reviewed by the owner s structural engineer 6

7 EXISTING STRUCTURE LAKETOWN WHARF RETROFIT 7

8 EXISTING STRUCTURE GIRDER 7-8 GIRDER 6-7 GIRDER 5-6 8

9 EXISTING STRUCTURE 9

10 ANALYSIS AND DESIGN LAKETOWN WHARF RETROFIT 10

11 ANALYSIS AND DESIGN A load analysis was performed to determine the flexural moment and the shear along each girder A sectional analysis (M-φ) program was used to determine the strain and stress profile at critical locations along the girders (before and after the external post-tensioning) The program used was based on a layered model and accounted for the force due to the external post-tensioning 11

12 ANALYSIS AND DESIGN MODEL STRAIN STRESS ε t CONCRETE A i ε ci f ci PRESTRESSING STEEL A pk ε ck f pk CONVENTIONAL REBAR A sj REFERENCE ELEVATION ε cj ε b f sj NET AXIAL FORCE SECTIONAL ANALYSIS (M-φ) USING LAYERS 12

13 ANALYSIS AND DESIGN Based on the analysis results, six external tendons were used for girders 5-6 and 6-7, while three external tendons were used for girder 7-8 The external tendons were placed below the girders, at a constant elevation, instead of the side faces to minimize the impact on the architectural design The tendons were placed continuously between two bracket assemblies instead of staggering the tendons and using additional bracket assemblies 13

14 ANALYSIS AND DESIGN 7000 Girders 5-6 & 6-7: Moment-Curvature Response Long-Term Behavior Original Section Section with external post-tensioning (P = 168 kips) M MAX 5-6 = 4300 ft*kip Moment (ft*kip) M MAX 6-7 = 4130 ft*kip Curvature (10-6 rad/in) 14

15 ANALYSIS AND DESIGN Summary of Stress Analysis Girder 5-6 Loading Condition Location Moment (ft*kip) Concrete Stress (psi) Stress Limit (psi) Top Bottom Top Bottom Status / Comment Original section prior to external post-tensioning Immediately after external post-tensioning Long-term response after external post-tensioning Joist Concrete cracked on bottom face Joist OK Joist OK 15

16 ANALYSIS AND DESIGN Summary of Stress Analysis Girder 6-7 Loading Condition Location Moment (ft*kip) Concrete Stress (psi) Stress Limit (psi) Top Bottom Top Bottom Status / Comment Original section prior to external post-tensioning Immediately after external post-tensioning Long-term response after external post-tensioning Joist Concrete cracked on bottom face Joist OK Joist OK 16

17 ANALYSIS AND DESIGN Girder 7-8: Moment-Curvature Response Long-Term Behavior 7000 Original Section Section with external post-tensioning (P = 81 kips) Moment (ft*kip) M MAX 7-8 = 2560 ft*kip Curvature (10-6 rad/in) 17

18 ANALYSIS AND DESIGN Summary of Stress Analysis Girder 7-8 Loading Condition Location Moment (ft*kip) Concrete Stress (psi) Stress Limit (psi) Top Bottom Top Bottom Status / Comment Original section prior to external post-tensioning Immediately after external post-tensioning Long-term response after external post-tensioning Joist Bottom face in tension Joist Bottom face in compression Joist Bottom face in compression 18

19 DESIGN DETAILS LAKETOWN WHARF RETROFIT 19

20 DESIGN DETAILS 20

21 DESIGN DETAILS 21

22 DESIGN DETAILS The bracket assemblies were located away from the end of the girders to avoid disturbing the transfer zone of the pretensioned strands The bolts used in the brackets were located to avoid interference with the conventional reinforcement and the pretensioned strands Barrel anchors were used and located as close as possible to the girder soffit to minimize the eccentricity 22

23 DESIGN DETAILS 23

24 DESIGN DETAILS 24

25 RETROFIT PICTURES LAKETOWN WHARF RETROFIT 25

26 RETROFIT PICTURES 26

27 RETROFIT PICTURES 27

28 RETROFIT PICTURES 28

29 RETROFIT PICTURES 29

30 RETROFIT PICTURES 30

31 RETROFIT PICTURES 31

32 RETROFIT PICTURES 32

33 THANK YOU ASIT N. BAXI BAXI ENGINEERING MIGUEL FORERO SUNCOAST POST-TENSION 33