California’s first long-span cable-stayed bridge

Seismic design 

The Long Beach International Gateway Bridge is the only cable-stayed bridge of its size on the west coast of the United States. To ensure that the bridge is safe during significant seismic events, Arup designed the bridge towers and end bents to remain essentially elastic during seismic events, with the bridge superstructure seismically isolated from the towers and end bents by 32 hydraulic viscous dampers.

The dampers will only activate during a major earthquake. An integrated and replaceable structural fuse comprising a ring of structural steel inside each damper, will shear through at a force corresponding to the considered magnitude. After the steel fuse breaks, the viscous dampers begin to dissipate cyclic energy the same way that a car’s shock absorbers do on a bumpy road. The fused design prolongs the lifespan of the absorbers, as they remain inactive until there is significant seismic activity. 

The reference design provided to Arup and the design-build team at the beginning of the tender included a twin shaft tower with numerous, heavy ductile shear links. Arup’s design will require less maintenance and is far easier to inspect after a seismic event with catwalks provided to allow safe and easy access to every damper. Replacement of the fuses, if activated, is straightforward without the need for hoists or manlifts. On the other hand, replacement of the reference design ductile shear links after an earthquake would have been a lengthy and costly construction operation. Arup’s monopole tower and fused viscous damper solution was less expensive in the initial construction and provides a better long term solution for the owner. 

Wind engineering 

Although earthquake loads dominate in California, a thorough investigation of wind effects was undertaken, as is standard with any long-span bridge. The aerodynamic performance of the bridge has been investigated through a program of wind tunnel testing and numerical wind buffeting analysis to confirm high wind speed strength and stability, as well as low wind speed serviceability against vortex shedding vibrations. 

Innovative roadway engineering 

The project’s bid package reference design also 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 common 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.” 

The proposed solution reduced project costs by close to $70 million while providing numerous functional advantages. Land previously reserved for the reference design 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 had to be processed as hazardous waste. However, by removing the need for foundations, this cost and risk were eliminated.