The National Transportation Safety Board (NTSB) has issued a second update on their investigation of the I-35 W bridge collapse. A new facet of the investigation is the examination of the de-icing system that was installed on the bridge, the chemical used in this de-icing system, and the properties of this chemical (whether or not it is corrosive). This de-icing system had components installed in the concrete bridge deck. The NTSB Chairman Mark Rosenker stated "The finite element model of the bridge is being revised based on the measured deck thickness from core sections and physical examination of the bridge structure. All structural elements have been incorporated in the model; however, some aspects of stiffness, weight and connections between elements are being modified to match the condition of the bridge the day of the accident." The NTSB has also interviewed 25 drivers and construction workers who were involved with the construction project on the bridge at the time of its collapse. According to the NTSB press release: " The weights of the various construction materials that were delivered to the work site between 11:00 a.m. and 2:30 p.m. on August 1 have been verified at about 383,000 pounds. The combined weight of the loads and construction vehicles is about 575,000 pounds." NTSB has verified the location of these vehicles and materials by means of the interviews and from "a pre-collapse photograph taken by a passenger on an airplane that was departing from Minneapolis on the afternoon of the collapse." View the NTSB press release at: http://www.ntsb.gov/Pressrel/2007/082207.htm
The anti-icing system used on the I-35 W bridge sprays an anti-icing chemical onto the concrete bridge deck when sensors and information from the Road Weather Information System tell it that hazardous conditions are imminent. The anti-icing chemical is potassium acetate. The sensors are ground and deck surface sensors that measure temperature and moisture, and sensors to measure ice formation. The Road Weather Information System are weather stations that measure air temperature and have optical precipitation sensors to detect rain, snow, and sleet. Read the report of the anti-icing system at:
http://www.dot.state.mn.us/metro/maintenance/Anti-icing%20evaluation.pdf
View drawing showing the layout of the anti-icing system on the I-35 bridge at: http://www.dot.state.mn.us/i35wbridge/drawings/BR9340%20Thawspray%20System.pdf
The Minnesota State Senate Transportation Budget & Policy Division and the State House Transportation Finance Division held a joint public hearing on August 15, 2007 on the I-35 W bridge collapse. During that hearing, the members of the Divisions asked Minnesota DOT (MN-DOT) many questions that MN-DOT needed time to respond. MN-DOT has responded to those inquiries and a letter from MN-DOT summarizing the questions and providing the answers is available at:
http://www.dot.state.mn.us/i35wbridge/legislative/Aug15hearing-followup-letter.pdf
MN-DOT has provided some additional information about the Municipal Consent process. This is the law in Minnesota that requires MN-DOT to obtain "municipal consent" from the municipality involved when the highway project alters access, increases or reduces traffic capacity (for example, increasing or reducing the number of traffic lanes), or requires acquisition of permanent right-of-way. View flow chart of the municipal consent process at:
http://www.dot.state.mn.us/i35wbridge/legislative/Attachment-G2-Municipal-Consent-Process.pdf
View timeline of the municipal consent process at:
http://www.dot.state.mn.us/i35wbridge/legislative/Attachment-G1-municipal-consent-process_w-Timeline.doc
MN-DOT has provided information related to their plans for funding the new I-35 W replacement bridge. Abby McKenzie, MN-DOT's Director of the Office of Investment Management, testified about these funding plans at the Minnesota State Senate Transportation Budget & Policy Division and State House Transportation Finance Division joint public hearing on August 15, 2007. View McKenzie's testimony at:
http://www.dot.state.mn.us/i35wbridge/legislative/testimony-McKenzie.doc
MN-DOT has also provided a list of the professional and technical contracts awarded to date associated with the bridge collapse recovery efforts, bridge collapse investigation, environmental concerns, and new I-35 W bridge replacement. View the list at:
http://www.dot.state.mn.us/i35wbridge/legislative/AttachmentI-CONSULTANT-CONTRACTS.xls
The Federal Emergency Management Agency (FEMA) has issued a press release concerning the I-35 W bridge collapse and federal aid programs that are available. In the press release FEMA states that "Initial FEMA-administered aid is limited to $5 million." Read FEMA's press release at: http://www.fema.gov/news/newsrelease.fema?id=38970.
In an unrelated, but important development, MN-DOT closed a bridge today. The Highway 11 bridge over the Red River sharing the border with North Dakota was closed "after a crack was discovered in a support bracket for a girder during an inspection...". Read the press release concerning this bridge at: http://www.dot.state.mn.us/newsrels/07/08/22-hwa11bridge.html.
[Summary compiled by Susan Lane, ASCE]
Wednesday, August 22, 2007
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