FIFTH UPDATE: INVESTIGATION INTO COLLAPSE OF I-35 BRIDGE

[NTSB Press Releases <NTSBPressReleases@xxxxxxxx>]

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                       NTSB ADVISORY
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National Transportation Safety Board
Washington, DC 20594

March 17, 2008

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FIFTH UPDATE: INVESTIGATION INTO COLLAPSE OF I-35 BRIDGE


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        The following is an update of the National 
Transportation Safety Board's investigation into the August 
1, 2007 collapse of the I-35W bridge in Minneapolis, 
Minnesota.

        NTSB Chairman Mark V. Rosenker today announced that 
"significant progress continues to be made in the 
investigation," and as a result he believes a final NTSB 
report on the cause of the bridge's collapse and additional 
safety recommendations should be able to be completed and 
presented before the end of the year.  "I applaud the team 
for its expeditious, thorough and tireless investigation 
thus far."

        "We have gotten and continue to receive excellent 
cooperation from the Federal Highway Administration and the 
Minnesota Department of Transportation and have collected a 
large body of evidence to this point."

        As one result of that progress, Rosenker announced 
that the Board's public docket on this investigation, which 
comprises investigative reports of the NTSB group chairmen, 
photos and information submitted by parties, has been made 
available to the public on the Board's website beginning 
today.

        Details of the investigation's progress follow: 

Load Capacity Calculations for Steel Truss Bridges
	
        On January 15, 2008, the NTSB issued a recommendation 
to the Federal Highway Administration (FHWA) calling on that 
agency to require bridge owners (primarily States) to 
conduct load capacity calculations to verify the stress 
levels in all bridge structural elements (for non-load-path 
redundant steel truss bridges), including gusset plates, 
remain within applicable requirements whenever planned 
modifications or operational changes may significantly 
increase stresses.  This was the result of the Board's 
investigation finding that some of the gusset plates on the 
I-35W bridge were under-sized because of an error in the 
original design process.

        Immediately following issuance of this recommendation, 
the FHWA released a technical advisory to State Departments 
of Transportation that supplements the American Association 
of State Highway and Transportation Officials' (AASHTO) 
procedures for load rating steel truss bridges.  The FHWA is 
continuing to develop guidance for load rating evaluations 
of gusset plates in truss bridges, and will work with the 
State DOTs to refine and finalize this guidance.  Also 
following the Safety Board's recommendation and FHWA's 
technical advisory, States have initiated courses of action 
to address the concerns raised.  These actions generally 
include identifying bridges that have undergone changes 
during their lifetime, then analyzing and recalculating the 
load carrying capacity of the gusset plates utilizing the 
evolving FHWA guidance.

        The Federal Highway Administration has provided the 
NTSB with full access to the National Bridge Inventory 
database and worked with Safety Board investigators to 
define the population of highway bridges affected by the 
NTSB recommendation.

Design Issues

        The Bridge Design Group continues to investigate 
design issues, including the review and approval process.  
Sverdrup & Parcel was the original design consultant for the 
I-35 bridge and had been acquired by The Jacobs Engineering 
Group in 1999. Jacobs has provided archival information to 
help investigators better understand what type of system of 
checks and balances would have been in place when the bridge 
was designed back in the 1960s.

        Personnel from the Minnesota Department of 
Transportation (MnDOT) have been interviewed regarding the 
Department's role in quality control for the design review 
process.  These interviews have focused on what types and 
level of review would have been conducted in the 1960s as 
well as how the process works today.  Interviews were also 
conducted of FHWA personnel in the St. Paul District Office 
regarding that agency's role in quality control.  These 
interviews have allowed investigators to better understand 
how the oversight and design review process of the FHWA has 
evolved from the 1960s to the system currently in place.  
Additional private design consultants and State departments 
of transportation are being identified as candidates for 
further NTSB interviews regarding issues of quality control 
and the design review process.

Computer Analysis

        The Structures Modeling Group is continuing a complex 
set of finite element computer analyses of the bridge, with 
technical support from party members.  This effort has also 
involved contracted engineering support from the State 
University of New York in Stony Brook and from Simulia, a 
Rhode Island subsidiary of Dassault Systemes that develops 
the Abaqus product suite for Finite Element Analysis.  
Computer modeling of the bridge in its pre-collapse state is 
well underway to help explore the potential causes for its 
collapse.

        In addition, the Board's Office of Research and 
Engineering contracted with the University of Minnesota's 
Department of Civil Engineering to construct a 1/200th scale 
model of the I-35W bridge to provide investigators with a 3-
dimensional representation of the bridge's supporting 
structure.

Digital Images

        The Video and Photogrammetry Group is working to 
identify and catalog approximately 20,000 digital images 
taken by all parties throughout the on-scene investigation, 
some of which are in the docket.  Additionally, the group is 
completing a study of the data captured by the surveillance 
video camera located at the south lock on the west side of 
the bridge, which recorded a portion of the bridge during 
its collapse sequence.  This report is currently undergoing 
an internal technical review.

Wreckage Examination

        In support of the Structural Investigation Group, the 
NTSB Materials Laboratory is examining specific aspects of 
the bridge, including nodes U10 East and West and the 
corrosion areas on the gusset plates from nodes L11 East and 
West.  The Materials Lab is in the process of completing 
several reports related to these examinations.  These 
sections of the bridge's superstructure have been removed 
from the storage site in Minneapolis and transported to the 
NTSB facilities for preservation, testing and further 
evaluation. 

Public Docket and Future Schedule

        The NTSB maintains public dockets for its 
investigations.  These contain factual information gathered 
by the various Safety Board investigative groups that will 
be used by the Board in analyzing the accident and 
formulating its final report and safety recommendations.

        The following technical investigative group factual 
reports are undergoing peer and managerial reviews, or have 
been completed:

*               Structural Investigation Group
*               Witness Group
*               Highway Construction Factors Group
*               Survival Factors Group
*               Bridge Design Factors Group

These factual reports are being placed in the public 
docket as soon as they are completed.  The docket currently 
contains nearly 300 pages and includes the Structural 
Investigation Group Report noted above.  As of today, the 
docket is available on the Board's website 
http://.ntsb.gov/dockets/Highway/HWY07MH024/default.htm  
Additional reports will be placed on the website as they are 
entered into the docket in the coming months.

        Several additional investigative activities remain to 
be conducted.  These include the Structures Modeling Group's 
efforts, which will continue for several more months as new 
computer models are developed to examine specific failure 
scenarios to help identify the cause of the collapse.  Also, 
the Structural Investigation Group is beginning a sequencing 
study that will incorporate the physical evidence of the 
wreckage, the surveillance video, and the findings of the 
Structures Modeling Group, to determine, to the greatest 
extent possible, the sequence of events that describe the 
initial moments of the collapse.  No determination has been 
made as to the specific cause of the bridge's failure.

        Because of the amount of factual information collected 
during the investigation and the significant progress being 
made, the Board has agreed with the investigative team not 
to hold an interim investigative hearing.  The team believes 
that, barring unexpected developments, it is likely that a 
final report will be ready for the Board to consider at a 
public meeting in Washington, D.C. by the end of the year.  
The final report will include the probable cause of the 
collapse and will likely include additional safety 
recommendations.  That meeting will be open to the public 
and press and available free of charge via webcast.

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





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