30 May 2012
The Aurora Bridge seismic retrofit project has reached a milestone that measures less than an inch.
Crews closed both directions of SR 99 overnight Tuesday to jack up the structure by three-quarters of an inch (19mm)and install new 1t bearings that will cater for movement during an earthquake.
The installation sets the stage for the final phases of work on a US$5.7 million project that will help the 80-year old bridge better withstand seismic loads.
Since April 2011, crews have been under the bridge reinforcing the steel and concrete on the north and south ends. Work includes adding steel to the skeleton, thickening concrete girders and covering existing columns with fibre. Crews also are creating create stronger foundations. Work scheduled in the final phase includes the new bearings and a new expansion joint.
“An earthquake twists and turns a structure. These materials allow the bridge to move but not entirely fail,” said John Chi, WSDOT project engineer. “As we saw in Japan after the tsunami, a bridge that’s bruised is better than a bridge that’s broken when it comes to restoring the transportation infrastructure.”
Work under the north end of the bridge near the Fremont Troll is nearly complete; work under the south end is on schedule to be done by August. Work on the bridge deck itself takes place this week and again in late June and July as crews replace the bearings, lower the bridge into place and then replace the expansion joint that runs across all six lanes of traffic.
“Working on this bridge is a challenge because it’s so beautiful,” said Chi. “We need to make improvements while maintaining the look and feel of this iconic gem.”
Earthquakes pose a substantial risk to transportation infrastructure in Washington state. The purpose of the seismic retrofit programme is to minimize and avoid catastrophic bridge failures by strengthening bridges and structures. An estimated 900 bridges are on the retrofit program list including the Alaskan Way Viaduct, the SR 520 floating bridge. Of those 900, 268 have been repaired, 134 partially repaired, 487 are in need of repair and 12 are under repair.