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(AP) - The National Transportation Safety board said it has now interviewed more than 300 witnesses to the Interstate
35W bridge collapse, reviewed a pre-crash photograph taken by an
airplane passenger and calculated the weight of construction
equipment on the bridge when it fell.

      The update on the agency's probe into the bridge collapse that
killed at least 13 people Aug. 1 was released Wednesday. It did not
indicate if investigators were any closer to determining a cause
for the collapse.

      The update did say that investigators are looking at a de-icing
system installed on the bridge in 1997 -- other official reports on
the bridge said it was installed in 1999 -- and whether the
chemicals used had any corrosive properties.

      The Minnesota Department of Transportation wasn't concerned
about the de-icing system used on the 35W bridge -- in fact the
agency is planning to install a similar system on the failed
bridge's replacement, said Khani Sahebjam, the engineer for MnDOT's
Metro District.

      Sahebjam said he wouldn't expect the de-icing system to pose a
structural problem "because those are elements up inside the
concrete deck," not part of the support structure below.

      The automated system was triggered by weather conditions and
kept MnDOT from having to send crews to spread de-icing chemicals,
Sahebjam said.

      "It's a strategy that we use especially on viaducts and long
bridges to help out snow removal and ice control," he said.

      A resurfacing project was underway when the bridge collapsed.
Through interviews with construction workers and delivery truck
drivers, the NTSB has calculated there was 575,000 pounds, or 287.5
tons, of construction materials and equipment on the bridge when it
fell.

      It was not clear from the update what significance the NTSB put
on the weight, and a call to the agency was not immediately
returned.

      State Transportation Department construction engineer Liz
Benjamin said the process followed by Progressive Contractors Inc.
to resurface the bridge was normal.

      She referred questions about the weights to state bridge
engineers, who didn't immediately return a message.

      "As far as storing materials on the bridge, that is normal when
we do a bridge overlay," Benjamin said.

      She added: "The bridge designers come up with a plan for us to
follow, and that's what we follow."

      NTSB Chairman Mark Rosenker said the safety board was continuing
to coordinate the removal of debris from the site while making sure
"all relevant components of the superstructure are not destroyed"
so they can be examined as part of the investigation.

      "Additionally, investigators in Washington are shifting through
materials that have already been gathered," he said in a news
release.

      The NTSB said it expected its investigators to remain on-scene
until November.

Gallery

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NTSB sleuths responsible for finding out why the bridge collapsed

The big question: Why did the bridge fall down?

Northern end of I-35W bridge is now focus of probe

Disasters

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Martiga Lohn

Author

The NTSB says it is looking into the de-icing system on the I-35W bridge, as well as the weight of construction materials in place on the bridge the day of its collapse, as possible factors in the accident.

NTSB to examine de-icing system in Minn. bridge collapse

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