NTSB URGES BRIDGE OWNERS TO PERFORM LOAD CAPACITY CALCULATIONS BEFORE MODIFICATIONS; I-35W INVESTIGATION CONTINUES

[NTSB Press Releases <NTSBPressReleases@xxxxxxxx>]

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                      NTSB PRESS RELEASE
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National Transportation Safety Board
Washington, DC 20594

FOR IMMEDIATE RELEASE: January 15, 2008
SB-08-02

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NTSB URGES BRIDGE OWNERS TO PERFORM LOAD CAPACITY 
CALCULATIONS BEFORE MODIFICATIONS; I-35W INVESTIGATION 
CONTINUES


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Washington, DC - The National Transportation Safety Board 
today issued a safety recommendation that addresses a design 
issue with the I-35W bridge that collapsed into the 
Mississippi river on August 1, 2007. In this accident, 
approximately 1,000 feet of the deck truss fell into the 
river, and as a result, 13 people died and 145 people were 
injured.  

The recommendation is made to the Federal Highway 
Administration (FHWA) and states: "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."  

"Although the Board's investigation is still on-going and no 
determination of probable cause has been reached, interim 
findings in the investigation have revealed a safety issue 
that warrants attention," said NTSB Chairman Mark V. 
Rosenker.  "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. This review discovered that the 
original design process of the I-35W bridge led to a serious 
error in sizing some of the gusset plates in the main 
truss."     

Undersized gusset plates were found at 8 of the 112 nodes 
(joints) on the main trusses of the bridge.  These 16 gusset 
plates (2 at each node) were roughly half the thickness 
required and too thin to provide the margin of safety 
expected in a properly designed bridge.   

The Safety Board emphasizes that there is no evidence to 
suggest that the deficiencies in the various design review 
procedures associated with this bridge are widespread or 
even go beyond this particular bridge.  In fact, this is the 
only bridge failure of this type of which the Safety Board 
is aware.

"The Safety Board has issued this recommendation, at this 
time, to ensure that the original design calculations for 
other bridges of this type have been made correctly, before 
any planned modification or operational changes are 
accomplished affecting such bridges and before any 
additional stresses are placed on them," Rosenker said.   

The probable cause of this accident will be determined when 
the final report is presented to the Board, currently 
estimated to be sometime before the end of the year.

The recommendation letter can be found on the Board's 
website at http://www.ntsb.gov/Recs/letters/2008/H08_1.pdf.


#

NTSB Media Contact:     Terry N. Williams
                                (202) 314-6100
                                williat@xxxxxxxx







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