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REPAIR OF PRECAST PRE-TENSIONED GIRDERS
USING EXTERNAL POST-TENSIONING
ASIT N. BAXI – BAXI ENGINEERING MIGUEL FORERO – SUNCOAST POST-TENSION
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
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KEY POINTS
Project background Existing structure Analysis and design Structural details Retrofit pictures
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PROJECT BACKGROUND
LAKETOWN WHARF RETROFIT
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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
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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
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EXISTING STRUCTURE
LAKETOWN WHARF RETROFIT
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EXISTING STRUCTURE
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GIRDER 7-8 GIRDER 6-7 GIRDER 5-6
EXISTING STRUCTURE
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ANALYSIS AND DESIGN
LAKETOWN WHARF RETROFIT
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ANALYSIS AND DESIGN
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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
ANALYSIS AND DESIGN
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εt
REFERENCE ELEVATION
εci
εb
εck
εcj
fci Ai
fpk
fsj Apk
Asj
MODEL STRAIN STRESS
CONCRETE
PRESTRESSING STEEL
CONVENTIONAL REBAR
SECTIONAL ANALYSIS (M-φ) USING LAYERS NET AXIAL
FORCE
ANALYSIS AND DESIGN
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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
ANALYSIS AND DESIGN
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-4000
-3000
-2000
-1000
0
1000
2000
3000
4000
5000
6000
7000
-80 -60 -40 -20 0 20 40 60 80
Mom
ent (
ft*k
ip)
Curvature (10-6 rad/in)
Girders 5-6 & 6-7: Moment-Curvature ResponseLong-Term Behavior
Original Section Section with external post-tensioning (P = 168 kips)
MMAX 5-6 = 4300 ft*kip
MMAX 6-7 = 4130 ft*kip
ANALYSIS AND DESIGN
Summary of Stress Analysis – Girder 5-6
Loading Condition Location Moment (ft*kip)
Concrete Stress (psi) Stress Limit (psi) Status / Comment
Top Bottom Top Bottom
Original section prior to external post-tensioning Joist 5 4298 -2534 - -3600 581 Concrete cracked on bottom face
Immediately after external post-tensioning Joist 5 4298 -1063 253 -3600 581 OK
Long-term response after external post-tensioning Joist 5 4298 -1007 286 -3600 581 OK
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ANALYSIS AND DESIGN
Summary of Stress Analysis – Girder 6-7
Loading Condition Location Moment (ft*kip)
Concrete Stress (psi) Stress Limit (psi) Status / Comment
Top Bottom Top Bottom
Original section prior to external post-tensioning Joist 5 4134 -2111 - -3600 581 Concrete cracked on bottom face
Immediately after external post-tensioning Joist 5 4134 -877 132 -3600 581 OK
Long-term response after external post-tensioning Joist 5 4134 -874 194 -3600 581 OK
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ANALYSIS AND DESIGN
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-4000
-3000
-2000
-1000
0
1000
2000
3000
4000
5000
6000
7000
-80 -60 -40 -20 0 20 40 60 80
Mom
ent (
ft*k
ip)
Curvature (10-6 rad/in)
Girder 7-8: Moment-Curvature ResponseLong-Term Behavior
Original Section Section with external post-tensioning (P = 81 kips)
MMAX 7-8 = 2560 ft*kip
ANALYSIS AND DESIGN
Summary of Stress Analysis – Girder 7-8
Loading Condition Location Moment (ft*kip)
Concrete Stress (psi) Stress Limit (psi) Status / Comment
Top Bottom Top Bottom
Original section prior to external post-tensioning Joist 4 2557 -630 133 -3600 581 Bottom face in tension
Immediately after external post-tensioning Joist 4 2557 -537 -115 -3600 -3600 Bottom face in compression
Long-term response after external post-tensioning Joist 4 2557 -537 -53 -3600 -3600 Bottom face in compression
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DESIGN DETAILS
LAKETOWN WHARF RETROFIT
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DESIGN DETAILS
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DESIGN DETAILS
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DESIGN DETAILS
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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
DESIGN DETAILS
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DESIGN DETAILS
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RETROFIT PICTURES
LAKETOWN WHARF RETROFIT
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RETROFIT PICTURES
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RETROFIT PICTURES
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RETROFIT PICTURES
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RETROFIT PICTURES
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RETROFIT PICTURES
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RETROFIT PICTURES
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RETROFIT PICTURES
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THANK YOU
ASIT N. BAXI – BAXI ENGINEERING MIGUEL FORERO – SUNCOAST POST-TENSION
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