ASCE Los Angeles Section

San Gabriel Mountain, Los Angeles

The East Fork Road Bridge over North Fork San Gabriel River is a 642-feet long, 4-span steel truss bridge built in 1949.  It measures 38-feet in width and carries one 12-foot lane of traffic in each direction plus a 4-feet 6-inch wide sidewalk on each side.  The bridge is located in the San Gabriel Mountains National Monument just north of the City of Azusa and provides access to L.A. County firefighting 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 during a maximum credible earthquake.  The scope of work for the bridge retrofit included replacing existing bearings with seismic isolation bearings; strengthening diaphragms at the piers and abutments; widening the gaps between the abutment backwalls and the bridge and replacing expansion joints; tying trusses across the expansion joint over the piers; replacing some bottom lateral bracings; selective strengthening of connections by replacing existing rivets with high strength bolts, modifying handrails; and installing restrainer cables at the piers.

The project is outstanding and worthy of an award due to its high level of structural engineering complexity, use of innovative technology, safety benefits to the general public, and its sensitivity to the environment.  It also demonstrates how collaborative efforts and open dialogue among owner, engineer, and contractors can resolve project challenges and bring the project to a successful conclusion.
The existing bridge was analyzed using a non-linear time history analysis with site specific ground motions and found to be vulnerable to collapse under a maximum credible earthquake.  The structural deficiencies identified were inadequate bearings, diaphragms, bottom lateral bracings, abutment and pier wall seat widths, and possible footing uplift in the piers under a scoured condition.  Seismic retrofit was deemed necessary to minimize the risk of collapse.
Several alternatives were considered when developing the retrofit strategy.  Ultimately, an innovative approach was selected that minimized impacts on the environment, bridge hydraulics, and the need for foundation work.  Traditional bridge retrofits involve strengthening members so that seismic loads are safely carried to the foundation.  Due to the high seismicity at the site, and the large number of deficiencies identified from the seismic evaluation, traditional methods would require significant and costly modifications to meet the no-collapse seismic performance goal.  So instead of strengthening bridge members and the foundation, a retrofit strategy was developed to reduce the seismic forces by altering the structural response of the bridge during an earthquake. This was achieved by replacing the existing bridge bearings with seismic isolation bearings that increased the structural damping and allowed the bridge to displace and 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.  This 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.

There were many challenges that the project team had to overcome before and during construction of this bridge retrofit project.  Resolution of these challenges and successful completion of the project were made possible through collaborative efforts between L.A. County, the engineer, and the contractors.
There were challenges related to production and prototype testing of the seismic isolation bearings.  Seismic isolation bearings are specialized custom-made products that are designed and manufactured to meet project specific performance criteria.  They have not been widely used for bridge projects and there are only a limited number of suppliers and manufacturers qualified to produce these bearings. In addition, the facilities capable of performing the required dynamic tests for quality control were only available at one location in California.  This resulted in a team that was scattered across a wide geographical area.  L.A. County Public Works and the project site were in Los Angeles, the bearing manufacturer (Seismic Energy Products, Inc.) was in Texas, the Caltrans testing facility was in San Diego, and the Engineer of Record (JMEC Engineering Inc.) was in Northern California.  Despite the distance, the team worked collaboratively over several months to fine tune the bearing prototype design, performed the quality control tests, and manufactured production bearings that met project requirements.
There were also challenges in dealing with traffic management to ensure public safety.  The existing two-way traffic on the bridge was maintained with extreme care and monitored throughout the entire construction duration, including the period during installation of the seismic isolation bearings.  The traffic on the bridge was temporarily halted several times as each abutment and pier was systematically raised to relieve loads on the existing bearings so that they could be removed.
Respecting the environment and ensuring that the project did not negatively impact the surrounding area and wildlife also posed some challenges.  To eliminate impacts to the riverbed below, construction of the project had to be performed without accessing the North Fork San Gabriel River.  This required an extensive falsework and scaffolding system to be constructed from atop the bridge and hung from the bridge members.  Large and heavy steel bracings and the bridge bearings had to be removed and reinstalled from this scaffolding system.  In addition, construction was halted twice for an extended period of time to ensure that birds nesting on the bridge were not impacted.
With great collaborative efforts, open dialogue, and close coordination amongst L.A. County staff, JMEC Engineering Inc., John S. Meek Company Inc., and Seismic Energy Products, the project was safely and 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 manner.

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The project is outstanding due high level of structural engineering complexity, use of innovative technology, safety benefits to the general public, and its sensitivity to the environment.  It also demonstrates how collaborative efforts and open dialogue among owner, engineer, and contractors can resolve project challenges and bring the project to a successful conclusion.

The existing bridge was analyzed and found to be vulnerable to collapse under a maximum credible earthquake.  The structural deficiencies identified were inadequate bearings, diaphragms, bottom lateral bracings, abutment and pier wall seat widths, and possible footing uplift in the piers under a scoured condition.  Seismic retrofit was deemed necessary to minimize the risk of collapse.  In lieu of strengthening bridge members and the foundation, a retrofit strategy was developed to reduce the seismic forces by altering the structural response of the bridge during an earthquake. This 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.
The project is a demonstration of how collaborative efforts and open dialogue among owner, engineer, and contractors can resolve project challenges, overcome before and during construction of this bridge retrofit project.  Seismic isolation bearings are specialized custom-made products that are designed and manufactured to meet project specific performance criteria.  The team worked collaboratively over several months to fine tune the bearing prototype design, performed the quality control tests, and manufactured production bearings that met project requirements. There were also challenges in dealing with traffic management to ensure public safety.  The existing two-way traffic on the bridge was maintained with extreme care and monitored throughout the entire construction duration. Respecting the environment and ensuring that the project did not negatively impact the surrounding area and wildlife also posed some challenges.  To eliminate impacts to the riverbed below, construction of the project had to be performed without accessing the North Fork San Gabriel River.  In addition, construction was halted twice for an extended period of time to ensure that birds nesting on the bridge were not impacted.