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Structural Causes of the Collapse

Page history last edited by Steve Escher 12 years, 10 months ago


Structural Causes of the Collapse


The collapse brought into question the construction and the design of the 35W Bridge.  Several theories have emerged during the collapse investigation to explain what happened.  This page describes general information on the structure of bridges and gives more detailed information about the deck truss design used for the 35W Bridge.  In addition, this page provides information on the different theories behind the cause of the collapse. 


This information provided about the cause of the collapse is preliminary and pending completion of the NTSB (National Transportation Safety Board) investigation of the incident.



General Structural Information




Bridges come in different forms based on the type of terrain being spanned, the length of the bridge, and the proposed use of the bridge.  However, all bridges have four main factors that are used to describe them.  These factors are span, material, form, and surface design. 


Span refers to the structure between two supports of the overall bridge structure.  The basic span types include simple, continuous, and cantilever.  Simple spans are sections supported on both sides.  They require more sections per unit length than other types of spans.  Continuous spans have a single bridge section held up by all the supports.  As bridge length increases, continuous and simple spans become more difficult to construct.  Cantilever spans project from a single support structure and are counterbalanced and supported at one end, but not the other. 


Material is a simple description of what the bridge is mainly constructed of; stone, metal, concrete, etc. 


Form refers to the type of bridge design.  There are numerous design forms, but the three most common are beam, arch, and truss.   The beam bridge design is the simplest of the bridge types. Beam bridges are used to cross short distances as the forces weighted against them build up quickly in this simple design.  The arch design relieves stresses on the beam bridge by adding an arch over the beam to help support it. The beam and arch support each other, allowing greater distances to be spanned.  The truss form is both a bridge design and a type of construction.  Truss design involves a simple skeletal structure focused on the creation of triangles that support weight extremely well.  While beam, arch, and cantilever bridges are constructed with trusses, the truss also serves as an important descriptive design feature.


Surface design refers to the location of the main transport surface in relation to the supporting structures of the bridge.  The three different forms of surface design include deck, pony, and through.  A deck surface design positions the main superstructure of the bridge under the traffic flow across the length of the bridge structure.  The pony surface design puts the main traffic flow through a pair of superstructures that are not connected at the top creating a rough ā€œUā€ shaped channel for the traffic flow.  The through design positions the traffic traveling through the middle of the bridge's superstructure with the structure cross-braced both above and below the traffic.




Deck Truss Design



Deck Truss Design refers to the trusses that are used for support in the superstructure of the bridge along with how travel is conducted across the span of the bridge.  The Deck Truss design, while used to describe the 35W Bridge, is not a specific design and can be used to describe other types of bridges.  The consulting company that originally designed the 35W Bridge was Sverdrup & Parcel.  The general notes that MN/DoT (Minnesota Department of Transportation) has indicate that the design was commissioned in accordance with Division I of the A.A.S.H.O.'s (American Association of State Highway and Transportation Officials) "Standard Specification for Highway Bridges" (1961 Edition)[1]; the name was changed to A.A.S.H.T.O. (American Association of State Highway and Transportation Officials) on November 3rd, 1973[2].  While some of the plans and figures used in the building of the 35W Bridge have been lost, MN/DoT has on file most of the records concerning the design and specifications for the bridge. 





BR9340 Original Plans and Details allows for access to much of the original information that MN/DoT still possesses about the I-35W Bridge.


BR9340 Reconstruction Plan is a PDF from MN/DoT detailing the 1999 Reconstruction plan.


BR9340 Construction Plan is a PDF from MN/DoT of some of the original design specifications of the I-35W Bridge from 1965.


BR9340 Steel Details is a PDF from MN/DoT of specifications on the steel used in the I-35W Bridge as of 1998.



35W Bridge Superstructure as seen from below.

I35W Bridge, before collapse


The 35W Bridge basic statistics include: [3]



Type:  Deck Truss 


Structure Material:  Welded Steel


Deck Structure Type:  Concrete-C-I-P


Year Built:  1967


Average Daily Traffic:  141000


Length of Maximum Span:  139.00 M


Structure Length:  581.30 M


Deck Width:  34.50 M


Location:  1.0 Mile NE of JCT TH 94


Latitude:  44 Deg 58 Min 50.89 Sec


Longitude:  93 Deg 14 Min 40.09 Sec



Theoretical Structural Causes



There have been three main theories put forward for the cause of the collapse, however most sources believe that the overall reason was a design flaw present when the bridge was built in 1967.  Because most sources are focused on the gusset plates due to preliminary reports by the NTSB, most of the following information is focused on the possibilty of a design flaw in the gusset plates.  The cause of the collapse is not a simple matter.  Bridge collapses are the result of environmental conditions coupled with length of use, leading to numerous factors involved as the cause of any collapse [4]. 


More Information






See also:









  1. http://www.ntsb.gov/Recs/letters/2008/H08_1_Design_Adequacy_Report.pdf
  2. http://en.wikipedia.org/wiki/American_Association_of_State_Highway_and_Transportation_Officials
  3. http://www.fhwa.dot.gov/bridge/ta514027sia.cfm
  4. http://minnesota.publicradio.org/display/web/2007/08/06/bridgetroubles/



Comments (2)

brow1769@... said

at 9:54 pm on Apr 20, 2008

Anthony - most of the info was straightforward enough - there were just some wording issues which I edited straight into the page. Let me know if you have questions about this...

brow1769@... said

at 9:56 pm on Apr 20, 2008

PS I was a little confused about the way we were supposed to do edits - now looking over my own page I could have just highlighted the areas needing fixes. Didn't mean to step on your toes! I guess I was unclear on this.

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