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TNB Crane Mishap Caused by Retracted Outriggers
Date: March 30, 2006 Contacts: GIG HARBOR – Tacoma Narrows Constructors’ (TNC’s) investigation into Monday’s crane accident on the Tacoma Narrows Bridge determined that the cause of the accident was the turning of the crane’s boom after its outriggers had been retracted, which is not a standard procedure for this operation. The event occurred while a working crew was demobilizing after a night construction operation. In the process of turning the boom, the 30-ton crane toppled over on its side, resulting in a 6 1/2-hour closure of the bridge. Following the incident, WSDOT engineers examined the bridge and found no damage to its key structural components. Damage to the bridge deck itself was minimal and included a few small indents created in the asphalt. Most of the damage was limited to the pedestrian railing, which was struck by the crane’s falling boom. A 32-foot section of railing was damaged beyond repair and was removed from the bridge. Temporary cable pedestrian railing has been installed and will remain in place until WSDOT crews can fabricate and install new permanent railing. That process is expected to take one month. “We know this incident caused major inconveniences to thousands of motorists who rely on the bridge every day,” said Secretary of Transportation Doug MacDonald. “We thank them for their patience and understanding as we worked through the recovery process. We also thank the Washington State Patrol and Washington State Ferries for their efforts to keep people moving. We deeply regret, as does TNC, the disruption the bridge closure had on the traveling public.” Why was the crane on the bridge deck? The crane was being used to remove temporary enclosures surrounding a horizontal strut on the existing bridge. These enclosures allowed crews to make seismic upgrades to the structure while keeping the bridge open to traffic. Was this operation separate from the Tacoma Narrows Bridge project? Seismic upgrades to the existing Narrows Bridge are part of the overall new Tacoma Narrows Bridge project. In addition to building a new parallel bridge, the project upgrades portions of the existing bridge to meet current seismic codes. Why are seismic upgrades needed on the existing bridge? The existing bridge opened to traffic in 1950. It met seismic codes for the time, but seismic designs and construction requirements have advanced since then. The current seismic upgrades include reinforcing areas close to the towers, in the tower struts over the roadway deck, and in the anchorages. This upgrade in seismic reinforcement is not unique to the Tacoma Narrows Bridge - WSDOT is systematically upgrading bridge structures around the state to meet the current seismic design criteria as funding allows. Will the crane that fell on its side be used again to complete the work it started on the bridge? The exact crane that fell will not be used to remove the remaining enclosures on the strut. Other cranes will be used to complete that task. In addition, throughout the life of the project, other seismic and deck work to be done on the existing bridge will require cranes and other heavy equipment. The continuation of the strut enclosure removal work, and associated lane closures and rolling slowdowns, are suspended until further notice. What is being done to ensure the same problem doesn’t happen again? Every work operation of the project is carefully planned before implementing it, including creating a detailed Job Hazard Analysis. The purpose of these plans and analysis is to do the work in correct and safe manner. If despite these efforts an incident occurs, it is thoroughly reviewed to ensure that similar events are avoided. Why did it take so long to reopen the bridge? The precarious position of the crane boom made recovery a delicate and complex operation that required detailed planning and performing work with extreme care. Many preparatory steps needed to take place before the crane could be removed. Those steps included mobilizing numerous pieces of heavy equipment to the site, disassembling and removing the fallen crane’s counterweights and boom, righting the crane body itself, removing damaged pedestrian railing, installing a temporary cable pedestrian railing, and assessing damage to the bridge deck. Who will pay for repairing any damage to the bridge? Costs for the repairs will be borne by TNC and not passed on to tollpayers. For more information on the project, visit www.tacomanarrowsbridge.com ###
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