Design of Kinked Steel Girders For I-91 over the Whetstone Brook, Brattleboro, Vermont Joe Krajewski, Senior Technical Advisor - Structures, HNTB Transportation, Inc. Date : June 11, 2012 International Bridge Conference Pittsburgh, PA, June 10-13, 2012
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Design of Kinked Steel GirdersFor I-91 over the Whetstone Brook, Brattleboro, VermontJoe Krajewski, Senior Technical Advisor - Structures, HNTB Transportation, Inc.
Date : June 11, 2012
International Bridge ConferencePittsburgh, PA, June 10-13, 2012
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Design of Kinked Steel Girders – Outline
• Background Information• Design Build Concept Plans – Curved Girders• AASHTO Curved Girder Requirements• Curved Girder Other Requirements• Kinked Girder Design Concept• AASHTO Kinked Girder Requirements• Kinked Girder Details• Project Status• Summary
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Design of Kinked Steel Girders – Background Information
• Bridges 6N & 6S carry I-91 over the Whetstone Brook in Brattleboro, Vermont in the SE corner of the state.
• I-91 Alignment @ the Bridges is on a 1980’ radius.
• The existing bridges were 5 chorded simple spans (70’,84’,84’,84’,83’ = 405’)
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Design of Kinked Steel Girders – Design Build Concept Design
• In 2010 the Vermont Agency of Transportation (Vtrans) let out a Design Build Contract to replace both bridges.
• The DB Concept plans called for 3 span continuous curved steel girders (105’, 115’, 105’ = 325’).
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Design of Kinked Steel Girders – Design Build Concept Design
• 5 Girders spaced 9’• 9” Exposed Concrete Deck• 7.8% Super-elevation• 42” F Barriers
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Design of Kinked Steel Girders – Curved Girder AASHTO Requirements
• Deflections Limits applied to individual girders (2.5.2.6.2)
• Optimal Depth deeper than straight girders (2.5.2.6.3)
• Small Deflection Theory may not recognize lateral deflection (4.6.1.2)
• Large Radial Force may develop @ abutment bearings (4.6.1.2.4a)
• Stability Effects shall be investigated (4.6.1.2.4b)
• X-Frames & Diaphragms are Primary Members (6.7.4.1)
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Design of Kinked Steel Girders – Curved Girder Other Requirements
• 3D Analysis to ensure lateral and torsional effects have been captured.− Several approximation methods such as V-Load and M/R methods
− Use with caution. For ex: V-Load does not directly account for lateral twisting and displacements nor will it necessarily detect uplift at bearings. Skewed supports only make the analysis more complicated.
• FABRICATION: Curved girders require extra fabrication− Horizontal curving either by cutting or heat curving.− Heat curving is most used method.− Heat and Cut curving must meet AASHTO requirements.
• Girders laterally deflect outwards during concrete deck pouring.− The orientation of the girders & cross bracing must be carefully fabricated to
ensure that the girders end up plumb after all dead loads are applied.
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Design of Kinked Steel Girders – Kinked Girder Design Concept
• DB RFP Allowed for Innovation & Alternative Design.
• Large Radius presented an opportunity to use Kinked Girder to eliminate a pier & use semi-integral stub abutments.
• Proposed Design - 2 span continuous kinked steel girders− 240’, 148’ = 388’− 6 degree kink− Variable Deck overhang (18” min, 5’-6’ max)
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Design of Kinked Steel Girders – Kinked Girder Design Concept
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Design of Kinked Steel Girders – Kinked Girder Design Concept
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Design of Kinked Steel Girders – Requirements
• Stability Effects shall be investigated (4.6.1.2.4b) Not Req’d but check
• X-Frames & Diaphragms are Primary Members (6.7.4.1) @ Kink Only
• 3D Analysis to ensure lateral and torsional effects have been captured.− Individual girders can be designed with line models.− Lateral & Torsion effects concentrated at the Kink.
• State of NYDOT Bridge Design Manual− Section 19.5.3 Item 13 – Guidelines on kinked girders in combination with
field splices.− Used originally for rehabbing existing simple spans into continuous spans
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Design of Kinked Steel Girders – Considerations
• Lateral Bracing is necessary at Kink location to resist lateral & torsional effects.
• Where to put the Kink?− In span away from supports. Analysis could be complicated.− At field Splice. Detailing will be complicated.− At bearing.− If continuous girder section at bearing then erection of this section will be
similar to curved girder. − If field spliced then detailing will be complicated.
• For the I-91 Bridges the Contractor selected placing the kink at pier and field splicing also at the pier. The most complex option.
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Design of Kinked Steel Girders – Details
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Design of Kinked Steel Girders – Details
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Design of Kinked Steel Girders – Details
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Design of Kinked Steel Girders – Details
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Design of Kinked Steel Girders – Details
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Design of Kinked Steel Girders – NB I-91 Erection November, 2011
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Design of Kinked Steel Girders – NB I-91 Deck Pour December, 2011
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Design of Kinked Steel Girders – Status
• Bridge 9N (I-91 NB) Completed January 2012• Bridge 9S (I-91 SB) to be Completed December 2012
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Design of Kinked Steel Girders – NB I-91 Open to Traffic
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Design of Kinked Steel Girders – SB I-91 Steel Erection 10/2012
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Design of Kinked Steel Girders – SB I-91 Steel Erection 10/2012
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Design of Kinked Steel Girders – SB I-91 Steel Erection 10/2012
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Design of Kinked Steel Girders – SB I-91 Steel Erection 10/2012
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Design of Kinked Steel Girders – Summary
• Kinked Girder Guidelines− Maximum Deflection Angle < 6o ±− Deck Overhangs Min. 18” & Max. 72”− Lateral Bracing at Kink
− Primary Structural Member− Takes all lateral and torsional effects
− Kink Location− At field splice− At pier− Other location in span
− Erection Sequence – Start at Kink and work outward• Kinked Girders can provide a superior solution to curved girders in the
right circumstances.
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Design of Kinked Steel Girders – Summary
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