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Roy L. Eriksson, P.E. - Eriksson Technologies, Inc.AASHTO T-4 Committee - June 28, 2016, Minneapolis, MN
Stability of Precast/Prestressed Concrete Bridge Girders
Copyright © 2016 Eriksson Technologies, Inc.
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Roy Eriksson, PE
President of Eriksson Technologies, Inc. 40-person full-service structural engineering firm Offices Tampa, FL & Denver, CO Special expertise in the precast/prestressed
concrete markets:- Bridges & marine- Buildings
Software development & automation
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Stability Overview Girder Stability: Who is Responsible? Stages in the Life of a Girder Types of Stability Brief Overview of Theory Lifting Hardware Ways to Improve Stability Stakeholder Roles & Responsibilities
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Stability: Who’s Responsible? Owner? Engineer of Record? Fabricator? Trucking Company? Erector? Contractor?
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Stages in the Life of a Girder Release of Prestress Lifting Girder from Bed Moving Girder to Storage Storing Girder Hauling Girder Erection Erected Girder
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Stages in the Life of a Girder
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Stages in the Life of a Girder
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Stages in the Life of a Girder
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Stages in the Life of a Girder
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Stages in the Life of a Girder
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Stages in the Life of a Girder
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Stages in the Life of a Girder
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Lateral Stability Two basic cases:- Hanging beams- Supported beams
Lateral Stability of Long Prestressed Concrete Beams – Part 1 (Mast 1989)- Lateral bending stability of beams- Hanging beams
Lateral Stability of Long Prestressed Concrete Beams – Part 2 (Mast 1993)- Beams supported from below- Extends and updates Part 1
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Lateral Stability Torsional stiffness of prestressed concrete beams
>> steel beams Therefore, assume P/S beams are torsionally rigid Lateral bending stability of beams For P/S beams, we are mainly concerned with:- Statical equilibrium of the system- Ability of the beam to resist lateral bending
• Cracking• Flexural strength
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Theory of Stability Roll Axis Roll Equilibrium Factors of Safety
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Roll Axis Between Lift Points CG Under Roll Axis
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Roll Equilibrium
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Factors of Safety
Where:- cr = resisting moment arm- ca = applied overturning moment arm
Min. Factors of Safety (FS):- Against cracking: 1.0- Against failure: 1.5
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Factors Influencing Stability - Lifting
Type of girder Girder length Lifting points Amount of prestress Material Properties
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Factors Influencing Stability - Transport
Type of girder Girder length Amount of prestress Material Properties Truck type, properties Roadway geometry
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Lifting Hardware Strand lifters Commercially available lifters Special hardware
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Strand LiftersPros Inexpensive Readily available- Typically made in-
plant Variable sizing:- Variable stand dia.- # strands/lifter- # lifters/end
Cons Deterioration Fabrication tolerance Placement tolerance Load distribution to
each lifter
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Strand Lifters
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Commercial LiftersPros Reliable Corrosion
resistant
Cons Higher cost Somewhat limited
capacity
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Special HardwarePros Corrosion resistant High capacity Quick connect
Cons Requires coordination
with designer Engineered system
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Special Hardware
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Improving Stability Adjust lift point location Raise roll axis Add top strands Brace top flange Increase f’c Increase Iyy (i.e., change beam)
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Move Lift Points Reduces distance between roll axis and CG Easiest, most effective Need to check stresses!
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Raise Roll Axis Raise roll axis above the top of the girder Requires special hardware
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Add Top Strands Add 2 to 6 strands to top flange Can be pretensioned to post-tensioned Strands can be fully or partially stressed Add mid-span debonding if necessary to
pretensioned strands Coordinate with EOR!
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Add Top Strands
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Add Top Strands
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Add Top Strands
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Allowable Stresses Allowable tension Allowable compression What are appropriate limits? EOR vs. fabricator Impact factors
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Impact Factors Handling in yard- Under fabricator’s control- Range: 1.00 – 1.20
During transport- Dependent upon many variables
• Higher speed• Variable road conditions
- Range: 1.20 – 1.50• Truck• Barge• Rail
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Brace Top Flange Bolt a transversely-oriented steel truss to top
flange of girder Increases lateral stiffness Labor-intensive Last resort
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Rigging Single-crane pick Two-crane pick
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PCI Resources PCI Girder Stability Subcommittee- Glenn Myers, P.E., Chair
Lateral Stability Guidelines for Prestressed Concrete Girders
Excel Spreadsheet Tool Upcoming Training William N. Nickas, P.E.- Managing Director, Transportation Systems
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Questions?Eriksson@ LRFD.com