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RE: Bent Gusset Plates on MN I-35W Bridge

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I don't think the photograph is all that clear on a "bent" gusset plate. 
Some points to consider:
Some structural systems are intended to have bending in the gusset plates.
A bent gusset plate is not a good indicator of a compression failure. 
We do not know the initial condition of the gusset plate when the bridge was constructed. 
Percipitators of collapse are generally shear or tension failures. 
The critical problems according to the interim report indicate tension or shear as a major problem. 
Tension and shear failures give little warning prior to the local failure and the insuing global collapse.  That is the nature of a truss failure. 

Harold Sprague

Subject: Bent Gusset Plates on MN I-35W Bridge
Date: Tue, 25 Mar 2008 10:25:05 -0400
From: cbanbury(--nospam--at)
To: seaint(--nospam--at)

Does anyone with more experience care to comment on this photo? Is this a smoking gun or are we just looking at the results of initial self-limiting plastic deformation at the connection as the deck beams were loaded during construction? I don’t think I would find this alarming unless analysis and testing showed some evidence of fatigue by stress reversal under moving live loads. I suppose a settlement at an abutment could also cause this type of rotation at a truss joint. It is difficult to tell if the beam is loaded anywhere except at the joints.;_ylt=Au.9y.ThpLGxtpVVarFkZh1H2ocA


Christopher Banbury, PE



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