In a press release issued yesterday, the National Transportation Safety Board (NTSB) made reference to its investigation of the anti-icing system on the I-35W bridge, prompting Minnesota Department of Transportation (MnDOT) officials to respond with their own release.
MnDOT released a fact sheet describing its automatic anti-icing system, installed on the bridge in 1999.
From MnDOT’s website:
• The anti-icing system prevents the formation of frost and bonding between snow and ice and the bridge deck’s surface
• Weather stations mounted on the bridge, along with sensors and thermometers in the bridge deck, monitor weather conditions such as dew point, wind speed, air and ground temperatures, wind direction, dampness and freeze point
• When monitors determine frost will form or there is a chance of freezing, they activate spray discs imbedded in the bridge deck to spray potassium acetate onto the bridge surface, usually before or early in a snowfall
• After the chemical has been applied to the bridge surface, it is then further distributed by traffic
• Potassium acetate is used because of its ability to melt snow and ice at low temperatures and because it is environmentally friendly, less corrosive to steel and concrete and has a low freeze point
• The spray units are integrated into the road surface, are traffic-proof and distribute anti-icing agent at the same volume and pressure over the entire treated surface
• Advance warning signs warn motorists as they approach the bridge that anti-icing is in progress when signs are flashing
• All activity of these systems are monitored via software located at the Mn/DOT’s Regional Transportation Management Center in Roseville, Minnesota
• The system can also be activated manually from the RTMC or by cell phone if needed.
The NSTB also stated in its press release that it has thus far received 184 calls on its witness line.
“Two callers identified themselves as people who were on the bridge and escaped immediately prior to the collapse. In total, 314 witnesses have been contacted and or interviewed,” according to the statement.
The statement also noted that NTSB has interviewed 25 construction workers and drivers who delivered materials to the bridge site that day. NTSB has determined that “the combined weight of the loads and construction vehicles is about 575,000 pounds.”