East Fork Road Bridge over North Fork San Gabriel River Project
Location
In the San Gabriel Mountains, Los Angeles County
City or County Responsible for Project
Los Angeles County
Category
Bridge: Efficient and Sustainable Bridge Maintenance, Construction and Reconstruction Projects
Author
Leslie Schenk
Los Angeles County Department of Public Works
900 South Fremont Avenue, Alhambra, CA 91803
(626) 458-5946
Project Description
The East Fork Road Bridge over North Fork San Gabriel River Project (Project) is a 642-feet long, steel truss bridge built in 1949. It is located in the San Gabriel Mountains National Monument just north of the City of Azusa and provides access to Los Angeles County Fire facilities and recreational amenities such as the East Fork Trail that leads to the “Bridge to Nowhere”. The Project consisted of seismically retrofitting the bridge to prevent collapse after an earthquake. It demonstrates a high level of complex structural engineering and innovative use of technology. Respecting the environment and ensuring that the Project did not adversely impact the surrounding area and wildlife was a high priority. Several alternatives were considered when developing the retrofit strategy. Ultimately, an innovative approach was selected that minimized impacts on the environment. Due to the high seismicity at the site, traditional retrofit methods required significant and costly modifications to prevent collapse. Instead of strengthening bridge members and the foundation, a retrofit strategy was developed to reduce the seismic forces by altering the bridge’s structural response. This was achieved by replacing the existing bridge bearings with seismic isolation bearings that allowed the bridge to behave more flexibly during an earthquake. As a result, the number of members that required strengthening was greatly reduced and the need for foundation retrofit was eliminated. A construction method was also developed that did not involve accessing the river. This required an extensive scaffolding system to be constructed from atop the bridge and hung from the bridge members. Large and heavy steel bracings and bridge bearings had to be removed and reinstalled from this scaffolding. In addition, construction was halted twice for an extended period of time to ensure that birds nesting on the bridge were not impacted. The innovative use of seismic isolation bearings combined with selective member strengthening proved to be a cost effective and environmentally friendly solution that did not require any construction work in the riverbed. With great collaborative efforts, open dialogue, and close coordination amongst Los Angeles County staff, JMEC Engineering Inc., John S. Meek Company Inc., and Seismic Energy Products, the Project was successfully completed in May 2016. The risk of a bridge collapse from a strong earthquake has been greatly reduced, public safety has been improved, and access for critical firefighting facilities and recreational amenities have been preserved by this bridge seismic retrofit project that was constructed in an environmentally responsible and sustainable manner.
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