Design Flaw Identified in Minnesota Bridge Collapse
A serious error in sizing parts of the I-35W bridge may have lead to the deadly collapse last year
Jennifer Roy, Contributing Editor -- Design News, January 15, 2008
National Transportation Safety Board officials today announced serious flaws in the size of several gusset plates used in the main truss of the I-35W bridge.
NTSB Chairman Mark V. Rosenker said the investigation into the collapse is ongoing and would not confirm whether the agency believes the undersized plates caused it, but did say the design flaw was “a critical part” of the bridge’s failure.
A final report on the cause is expected by the end of the year.
Rosenker said undersized plates were found at eight of the 112 joints on the main truss of the bridge. The 16 plates, two at each joint, were about half the required thickness and were too thin to provide the margin of safety expected in a properly designed bridge.
“Although the board’s investigation is still ongoing and no determination of probable cause has been reached, interim findings in the investigation have revealed a safety issue that warrants attention,” Rosenker said during a press conference in Washington, D.C. “During the wreckage recovery, investigators discovered that gusset plates at eight different joint locations in the main center span were fractured. The board, with assistance from the FHWA, conducted a thorough review of the design of the bridge, with an emphasis on the design of the gusset plates.”
The NTSB today also issued a safety recommendation to the Federal Highway Administration (FHWA) stating, “for all non-load-path redundant steel truss bridges within the National Bridge Inventory, require that bridge owners conduct load capacity calculations to verify that the stress levels on all structural elements, including gusset plates, remain within applicable design requirements, whenever planned modifications or operational changes may significantly increase stresses.”
The I-35W bridge collapsed into the Mississippi River Aug. 1, killing 13 people and injuring 145.
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When the bridge was constructed, the center section of the bridge
most probably arrived by barge via the Mississippi River. The center
section of the bridge is lifted into place and attached to the two
sections cantilevered out from the river piers. (See NYTimes photos -I
35W Bridge Collapse www.nytimes.com - photo 2) All the truss
members meeting at the connection nodes may not be in the same
plane. The iron workers erecting the truss must force the connection
plates into place using tapered pins in the rivet or bolt holes. You will
note that the rivet heads appear tight to the plate and it is a simple
matter to determine whether the plate is tight to the truss member.
The purpose of the connection plates is to transfer the load from the
truss member to the rivets and then to the next truss member.
Warpage as shown in the picture dated 2003 would have no effect in
the transfer of the loads.
The weight of the concrete trucks is a legal load. Meaning, the truck
can be placed anywhere, anytime and any number of those trucks. In
fact, the design load of any Interstate system bridge is for bumper-to-
bumper Sherman tanks in all lanes. At the time of the collapse the
actual load on the bridge was considerably less than the design load.
Bill Kallman, PE
www.myspace.com/billkallmanpe
- 2008-30-3 11:47:06 EDT -
If gusset plate is indeed the cause of the bridge collapse, why did the designers made it hard for them to determine the proper size? Gusset plate design is a simple mechanics problem in our college days. Is it a design flaw or simple job stress problem on the part of the designer?
- 2008-9-2 13:00:44 EST
Fatigue Could Loom Large in Bridge Collapses
11/18/2007
