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By Martiga Lohn, Published August 23 2007

NTSB to examine 35W de-icing system

MINNEAPOLIS – 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 whether 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 I-94 bridge between Fargo and Moorhead and the I-94 bridge near Barnesville, Minn., have similar de-icing systems.

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 under way when the bridge collapsed. Through interviews with construction workers and delivery truck drivers, the NTSB has calculated that 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.

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