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 viaducts 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 infrastructure link and a vital component of the regional and national economy. 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 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” moniker.
The proposed solution reduced project costs by close to $70 million while providing numerous functional advantages. Land previously reserved 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 hazardous waste. By removing the need for foundations, this cost and risk were eliminated.
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 essentially 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.
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 sensitive 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, observations 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.
Reducing the need for maintenance
Arup’s monopole tower and fused viscous damper solution provides a non-invasive post-seismic remediation plan where the bridge deck is repositioned with jacks, and broken fuses are replaced without the need to alter the bridge substructure. Towers and end bents are simplified to be less congested with fewer items to inspect and maintain, while means of access are provided to conveniently access each viscous damper for inspection or fuse replacement without the need for hoists or manlifts.
Tower geometry, aesthetics and practicality
The two 157-meter high main span bridge mono towers are a dominant aspect of the project’s visual impact. Tower geometric form is critical.
Once the project team decided on a single-shaft tower design, the tower’s geometric form began to take shape. A conical form was originally developed, because the circular form is visually pleasing and responds well to the pattern of the stay cable anchorages and governing seismic demands, which can be of the same order of magnitude in any direction.
The team reviewed a similar conical geometry adopted for the 308-meter tall Stonecutters Bridge towers. In that case, the conical form was constructed with a self-climbing formwork system designed to adapt to the reducing radius of the section. However, given the shorter height of the Gerald Desmond Bridge Replacement Project towers, such a specialized formwork system would not be economical. The conical form was not developed further.
Two further sections were contemplated:
- A modified cone with constant radius corners and a tapering flat section
- An eight sided geometry transforming from an octagon base to a square form at the top.
Of which the octagonal base was retained.
The octagonal transformation is carried out by tapering four of the eight sides while maintaining the other four at a constant dimension. This approach lends itself to an efficient climbing formwork arrangement because out of the eight jumping vertical formwork components, only four change dimensions at each jump.
A design decision was made to taper the faces which are orthogonal to the bridges primary axes. Tapering faces at 45 degrees to the primary axes would have resulted in a “square geometry” that is incompatible with stay cable geometry: the fan of cables will intersect with the corner of the tower meaning that some of the anchorages will pass through the section’s corner. Keeping the diagonal faces constant resulted in a “diamond geometry”, simultaneously resolving the geometric conflict between cable stays and section corners and creating a unique and instantly recognizable tower form.
The octagonal tower geometry uses light and shadow to define the form while bringing practicality to the construction. The team considers the “diamond geometry” solution to be aesthetically superior to the modified cone and more efficient in construction.
The tower cross-section is a great example of form meets function and exemplifies the total architecture perspective of design and construction.
Many technological and performance innovations were achieved during the design and construction of the Gerald Desmond Bridge Replacement Project, of which only a very few have been touched upon here. The end result is a structure that successfully rationalizes performance, maintenance, aesthetic and environmentally efficient objectives into a best-fit product, to the benefit of all.
Matt Carter is a Principal in Arup’s New York office, and serves as the Bridge and Civil Structures Skills Leader for Arup’s Americas region. He has 23 years’ experience in the conceptual and detailed design of long span and complex bridge structures in North America, East Asia, Europe, Africa and Australia. Notable projects include major cable stayed bridges such as the Samuel de Champlain Bridge in Montreal, Gerald Desmond Bridge Replacement in California, the Queensferry Crossing in Scotland and Stonecutters Bridge in Hong Kong. Through his work on major bridge projects, Matt has gained significant experience in the seismic and aerodynamic design of bridges and ship collision risk. In recent years, he has worked primarily on design-build projects and public private partnerships with both owners and contractors as clients, gaining unique insights by working from different perspectives.
Josh Mattheis is an Associate Principal in Arup’s Los Angeles office. He has 17 years of experience in design and design management of large rail and roadway turn-key design and build contracts. After working for international design firms and contractors, his experience is balanced between both. Josh enjoys leveraging this experience to create optimized and integrated designs that provide value to all parties. Recent projects include design and design management work on the Gerald Desmond Bridge project at the Port of Long Beach, followed by the Port Miami Tunnel project and the Gautrain Rapid Rail project. Typical design activities on these (and previous projects) include alignment optimization, selection of structure types, development of adapted construction methods and detailed design production in collaboration with state, municipal and county agencies.