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Theoretical Structural Causes

Page history last edited by Steve Escher 11 years, 7 months ago

Back to Structural Causes of the Collapse

 

Design Flaw:

 

 

Intact gusset plate from the I-35W Bridge collapse.

35W Bridge Collapse-52

The main design flaw continually blamed is the thickness of the gusset plates.  Gusset plates reinforce all the joints that connect the trusses in all truss bridges.  The gusset plates are sheets of square or triangular metal that are bolted into place over joints adding extra support to these critical points.  They are designed to be some of the strongest points on the bridge.  The gusset plates were not checked to see if they could handle the increased weights of recent renovations because they were never considered a possible weak point in the structure of the bridge [3].  The original designs have led investigators to believe that 16 of the gusset plates on the 35W Bridge were half as thick as they should have been, but the original calculations are missing, making determining exactly where the mistake was made extremely difficult.  However, National Transportation Safety Board (NTSB) Chair Mark Rosenker has listed 8 joints that were fractured because they were too thin to provide the margin of safety expected in such a bridge [5].  The gusset plates are supposed to be stronger than the beams that they are helping connect.  Maintenance crews are responsible for scouting fatigue and corrosion and would not have discovered this design flaw.

 

 

The focus of the NTSB investigation of the gusset plates is on the plate that occupied the U10 and U11 junctions of the bridge's superstructure.  In the course of the NTSB’s investigation into the U10 and U11 gusset plates, an interim report has been issued on the adequacy of the U10 and U11 gusset plates design.  This report analyzes the notes and calculations available from bridge design records to determine if the size and thickness of the gusset plates were adequate.  This report by the Federal Highway Administration (specifically the Turner-Fairbank Highway Research Center) concludes that the gusset plate at location U10 was inadequate for the stresses it would have to endure.  While this report states that the gusset plate at U10 was inadequate this is still an interim report and not a final declaration of fault by the NTSB.  The allowable shear and principle stress in the gusset plates was smaller than required, seriously reducing the effectiveness and life of the gusset plates.

 

NTSB report on design flaw in 35W Bridge.

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Fatigue: [1]

 

 

Metal fatigue occurs naturally over the course of a structure's life whenever there are changing forces that overload the structure or elements that promote corrosion.  Fatigue involves micro-fractures in the support metal that continue to build up over the course of the structure's life.  The more fractures in the metal the weaker it becomes.  Repeated cycles of stress on a metal object, even when the levels of stress are significantly under the levels needed to break it, can cause micro-fractures to appear and weaken the object over time.  The overloading of a structure significantly increases the speed at which metal fatigue sets in, but even under normal operating levels fatigue is still building up in the structure, just at a slower rate.  Fatigue is an important consideration when determining the lifespan of any metal structure.

 

 

 

 

 


 

 

 

Corrosion: [2]

 

 

Every structure exposed to the natural elements of the world is susceptible to natural corrosion.  Salt and water are the main causes of corrosion; however bird droppings and de-icing agents can also promote the corrosion of exterior structures.  Corrosion weakens a structure because the oxidized metal usually doesn’t adhere to the structure's surface, causing a disintegration of the metal that may eventually lead to the failure of the structure. 

 

The potassium acetate used in the 35W Bridge anti-ice system may have had a reaction to the zinc and brass on the bridge.  Dan Dorgan, the state bridge engineer and director of the bridge office at the Minnesota Department of Transportation, while talking about a connection between the potassium acetate deicer and zinc based bridge paint stated that “I’m not aware of that connection…I think anything like that would have to come out of the NTSB."

 

 

Bridges can be protected from corrosion with reinforcement coatings, migrating corrosion inhibitors (MCI), and cathodic protection.  Epoxy and galvanized steel can be used to reinforce the coating on bridge rebar. 

 

 


 

 

 

References:

 

  1. http://library.kcc.hawaii.edu/external/chemistry/everyday_corrosion.html
  2. http://www.designnews.com/article/CA6494876.html
  3. http://minnesota.publicradio.org/display/web/2008/01/16/gussets/
  4. http://minnesota.publicradio.org/display/web/2007/08/06/bridgetroubles/
  5. http://minnesota.publicradio.org/display/web/2008/01/15/prebrief

 

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