Effectiveness of Bent Plate Connection for End Cross Frames in Skewed Steel Bridges

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Effectiveness of Bent Plate Connection for End
Cross Frames in Skewed Steel Bridges

• Anthony Battistini
• Craig Quadrato
• Dr. Michael Engelhardt
• Dr. Karl Frank
• Dr. Todd Helwig

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Acknowledgements

• Texas Department of Transportation (TxDOT)
• Ferguson Structural Engineering Lab at The
  University of Texas
• Hirschfeld Industries
• Jeremiah Fasl and Brian Petruzzi

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Research Purpose

• Investigate cross frame bracing layout and
  connection details for skewed steel girder
  bridges to improve stability and fatigue
  performance

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Outline
The project encompasses field instrumentation,
laboratory tests, and finite element analysis

• Bracing Provisions
• Current Details
• Field Investigation
• Bent Plate Fabrication
• Small Scale Lab Tests
• Proposed Detail
• Future Plan

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Background

• Girder stability provided solely by cross frames
  during construction
• Stability depends on brace strength and stiffness
• Current AASHTO provisions require end cross
  frames parallel to skew angle

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Cross Frame Construction

• Skew Angle
• Requires angled brace to girder connection

Many states use bent plate to make the skewed
connection
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Skewed Bridge Issues

• Differential deflections cause fatigue issues
• Lean-on bracing
• Support geometry causes twist
• Reduction in end frame stiffness
• Longer length
• Angled connection
• Bent plate connection impacts brace system
  stiffness

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Brace System Stiffness

• Total stiffness calculated as a series of springs
• Maximum brace system stiffness can only be as
  large as least stiff component

From Yura (1992) and Wang (2006)
Therefore, flexible bent plate connection could
compromise cross frame systems effectiveness
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Field Investigation

• End cross frame
• Only end frames are skewed
• Skew angle 60
• Concrete deck placement
• Live load test

End Cross Frame Location
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Field Test Results
Total forces in any cross frame did not exceed 4
k (0.6 ksi)
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Field Test Bearing Pads

• With low cross frame forces measured, large
  deformations would be expected
• Rigid bearing pad provides tipping restraint

Extremely stiff Fabreeka bearing pads prevent
rotation of girder
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Field Test Conclusions

• Helped to determine what range of forces bent
  plates are likely to experience
• Data will be used to validate FEA bridge model
• Stiffness Issue- low forces exist because the
  flexible bent plates do not attract forces

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Small Scale Lab Tests

• Focus on structural behavior of bent plate
• Determine important connection details
• Improved control of testing
• Examine different geometries
• Skew Angle 15, 30, 45, 60
• Bend Radius 0.6?, 0.9?, 2.4?
• Compare to tests of proposed connection

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Small Scale Test Specimens
15 Skew Angle
60 Skew Angle
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Bent Plate Fabrication

• Flat Plate

• Bent Plate

Typical bent plates would need to have a bend
radius 1.5 times its thickness (0.75? for 0.50?
plate)
As required by AASHTO/NSBA Steel Bridge
Collaboration (2002)
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Typical Procedure

• Plates were bent by Hirschfeld Steel
• Plates marked at bend location, placed in press
  break
• Accuracy of 1

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Bend Radius

• In-house fabrication allows desired radius of
  curvature
• Similar procedure and accuracy as Hirschfeld

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Small Scale Test Measurements

• Measurement Locations
• Load
• Load Cell
• Lateral Deflection
• Angle
• Top of Plate
• Bottom of Plate
• Stiffener
• Web
• Vertical Deflection
• Top of Plate
• Web
• Rotation
• Top of Plate
• Bottom of Plate

Load Cell
Top of Plate
Angle
Top of Plate
Top of Plate
Bottom of Plate
Bottom of Plate
Stiffener
Web
Web
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Skew Angle Comparison
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Bend Radius Comparison
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FEA Connection Model 30 Skew
Temperature change applied to ram to simulate
contraction
Overlapping plates connected by link elements
ANSYS 11.0 used to create model and perform
non-linear geometric analysis
No translation at corners provides fixed
connection
All components built from 8-node shell elements
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FEA Bent Plate Connection Validation
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FEA Bent Plate Connection Validation
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Proposed SolutionBearing Stiffener Replaced by
Two Half Pipes

• Larger stiffness than bent plate
• Fewer connections for fewer fatigue issues
• Less fabrication
• Serves as bearing stiffener and cross frame
  stiffener
• Offers warping restraint
• May allow fewer cross frames during construction

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Twin Girder FEA Results

• Pipe stiffener increases buckling capacity by 50

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Fatigue Test

• Compare stiffener details
• Bent plate
• Skewed stiffener
• Pipe stiffener

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Future Plans

• Small scale pipe stiffener connection tests
• Fatigue testing program underway
• Three girder buckling tests
• Parametric studies using validated FEA model
• Design guidance creation

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Three Girder Test

• Large scale buckling tests
• Load applied with gravity load simulator

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Three Girder Test

• Examine effect of different connection details
• Consider different bracing layout schemes

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Questions?