Gerald Desmond Bridge Replacement Project: Bringing to life California’s first long-span cable-stayed bridge By Josh Mattheis and Matt Carter After over 50 years in operation, the Gerald Desmond Bridge is at the end of its useful life and will be replaced with a new six-lane cable-stayed main span bridge slated for completion in late 2020. The 2000-foot-long Gerald Desmond Bridge replacement is set to become California’s first long-span cable-stayed bridge. Approximately two miles of new cast-in-place concrete approach via- ducts rise 200 feet off the ground from both the east and the west as they transition to the main span cable-stayed bridge. Composed of two 500- foot back-spans and a 1000-foot main span, the new main span bridge provides increased vertical clearance over the Port of Long Beach back channel for future generations of commercial maritime shipping. Safe, optimized flow of people and goods is underpinned by the bridge’s geometric and structural design, featuring truck climbing lanes and shoulders on both sides of the highway for reduced congestion and a state-of-the-art Type 3 AASHTO global seismic design strategy. An incredible fifteen percent of all North American maritime container traffic crosses the Gerald Desmond Bridge, making it a critical infra- structure link and a vital component of the regional and national econ- omy. The bridge replacement responds to its critical role by providing a resilient, efficient, and aesthetically distinct structure in terms of performance, maintenance and architecture. The aesthetic dimension of the main span bridge is accented by faceted 515-foot-tall mono-pole towers augmented by customizable architectural lighting, making the new bridge a landmark for the Port and the City of Long Beach. Arup is prime designer for the project and Engineer of Record for the main-span bridge Gerald Desmond Bridge Replacement Project and high-level approach viaducts. Arup also provided the cable-stayed bridge erection geometry control and erection engineering support services. Double Texas U-turn The project’s bid package reference design (RID) proposed a grade- separated flyover ramp for west-bound traffic seeking to exit the main roadway and cross to the southern side of the project. Arup’s value engineering identified that the same functionality could be delivered while eliminating the entire flyover structure. Arup proposed a roadway geometry that passed below the main roadway with a dedicated free- flowing two-lane U-turn, facilitated by a new underpass constructed through the existing main roadway embankment. As this is a com- mon geometric configuration in the state of Texas, the arrangement is dubbed the “Texas U-turn.” Through innovative highway engineering, Arup rearranged the Port access roads so that truck traffic accessing the terminal facilities would use the same underpass both to get on and off the bridge, hence the “Double Texas U-turn” moniker.
The proposed solution reduced project costs by close to $70 million while providing numerous functional advantages. Land previously re- served for the RID flyover ramp bridge piers is now free to be used for other, revenue generating purposes. It also reduced the carbon footprint associated with construction volume, as well as reduced environmental risks: A known hydrocarbon contaminant plume in the area meant that deep foundation tailings would have had to been processed as hazard- ous waste. By removing the need for foundations, this cost and risk The Gerald Desmond Bridge Replacement Project is the only cable- stayed bridge of its size on the highly seismic west coast of the United States. Arup designed the bridge towers and end bents to remain es- sentially elastic during seismic events in alignment with an AASHTO Type 3 seismic design strategy. To achieve this, the bridge deck is seismically isolated from the towers and end bents by and array of 34 structurally-fused viscous hydraulic dampers. were eliminated. Seismic design Thanks to integrated structural fuses, the viscous hydraulic dampers only activate during seismic events superior to the one in one-hundred- year return period event. The damper fuses take the form of structural steel tubing encompassing the dampers, designed to release at a force corresponding to the controlling seismic event. After the steel fuse releases, the viscous dampers begin to dissipate cyclic energy in the same way that a car’s shock absorbers do on a bumpy road. The fused damper design reduces maintenance requirements by isolating sensi- tive damper components from ambient cyclical movements, ensuring optimal performance during the design seismic event. Fuses and dampers are designed specifically for ease of maintenance, redundancy and future-proofing. Integrated pressure gages, observa- tions windows, and transducers facilitate routine maintenance. The overall quantity of dampers was determined to make the damper size manageable for installation, maintenance, or replacement. Dampers and fuses are provided by Taylor Devices, Inc. Testing of the full-scale dampers was performed at the University of California, San Diego (UCSD) laboratory. Figure 1: Depiction of tower dampers at maximum seismic distortion. Photo: Port of Long Beach
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october 2020
csengineermag.com
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