Crossing over

Sept. 6, 2016

Engineering a bi-national bridge with building boundaries 

The new 390-ft-long international border crossing between San Diego, Calif., and Tijuana, Mexico, may appear to be a simple through-truss structure, but like many signature structures of its kind, the clean lines and aesthetic appeal surround a much more complex engineering undertaking.

The Cross Border Xpress (CBX) pedestrian skybridge is an unprecedented bi-national project that spans the U.S./Mexico border and directly connects a new terminal building in San Diego to the Tijuana International Airport (TIJ). CBX is a toll crossing for ticketed airline passengers only, offering a direct, convenient connection for more than 2 million TIJ passengers annually, enabling them to avoid unpredictable border wait times and often lengthy delays at congested land ports of entry at San Ysidro or Otay Mesa.

The complex, multi-faceted bridge blends engineering and construction excellence with bi-national collaboration. From a public perspective, the structure required many local and federal permits and approvals of both countries, including a U.S. Presidential Permit and approval from the Mexico Secretary of Communications and Transportation. From an engineering perspective, the bridge meets both U.S. bridge codes and building seismic codes.

Bi-national coordination

The bridge is privately financed and operated by the Otay-Tijuana Venture LLC (OTV), a private investment group with both U.S. and Mexican shareholders. To maintain security across the international border, the bridge alignment was selected in collaboration with the U.S. Customs and Border Protection agency.

OTV placed considerable emphasis on creating a cost-effective structure that emphasizes aesthetics and functionality. All solutions were vetted by the OTV as well as project architects—Stantec, in conjunction with Legoretta + Legoretta—and the structural and geotechnical engineer-of-record, Kleinfelder.

Much of the discussion between the owner, the architect and the engineering team about the conceptual design of the bridge focused on exterior/interior facing and finish, which affects the weight and therefore structural cost. In terms of project delivery, the bridge started as a design-bid-build project in 2010, shifted to a design-build and then shifted again to a modified design-bid-build with design-build coordination. The final decision was based on a combination of project cost and speed of construction.

Interior of the pedestrian bridge corridor, which is approximately 37 ft wide and 20 ft high. Photo courtesy of Harrison Photographic.

Trusses and simply supported spans

The 390-ft pedestrian bridge is really a horizontal building installed within a bridge structure. It consists of a pair of steel through-trusses with three simply supported spans ranging from approximately 115 ft to 150 ft.

The use of steel trusses for the bridge was primarily driven by the interface of the bridge to the Tijuana airport. In this area, the bridge needed to be constructed with minimal impact to the major federal highway that runs perpendicular to the bridge and parallel to the border fence and the airport property. Contractors would not be allowed to close down the airport or the federal highway for bridge construction and there was no space to build on-site. The steel truss framework was really the only option to accommodate limited site accessibility.

The overall bridge section is approximately 37 ft wide and 20 ft high with two fully enclosed, physically separated corridors running northbound and southbound.

Seismically safe

Engineering a building within a bridge structure across international lines required that engineers design to satisfy both building and bridge specifications.

Building features such as moving walkways, lights, mechanical systems, fire life safety and HVAC added complexity to the structure. The bridge was designed per AASHTO LRFD Bridge Design Specifications, AASHTO LRFD Guide Specifications for the Design of Pedestrian Bridges, AASHTO Guide Specifications for LRFD Seismic Bridge Design (for the superstructure) and Caltrans Seismic Design Criteria (for the substructure). It also was checked to meet the California Building Code, with particular focus on seismic conditions.

Visibility from the outside looking into the bridge structure is restricted by an architectural facing on the exterior of each truss. The bridge exterior wall façade consists of a combination of aluminum and fritted glass panels that expose some of the structural steel members and allow for natural lighting.

Visibility from the exterior is restricted by an architectural facing. Photo courtesy of Harrison Photographic.

Translating concept to reality

The bridge construction required bi-national and bilingual coordination between the design team and the contractors on the U.S. and Mexican sides. Early in the design, OTV agreed to have U.S. crews build the U.S. side of the bridge and Mexican crews build the portion of the bridge on their side of the border. The general contractors on the U.S. side included Turner Construction and Hazard Construction and on the Mexican side, Grumesa S.A. de C. V.

OTV also asked Kleinfelder to coordinate a parallel construction process with a bilingual engineering coordinator. Although the project plans were permitted in both countries using imperial units to facilitate construction, the Mexican contractors requested that the team translate critical plan sheets into metric units, which generated a number of RFIs and coordination meetings.

The skybridge structure was erected in six sections over a six-week period, beginning in Mexico. The sections weighed an average of 70 tons each. The first two 150-ft-long skybridge sections, weighing a total of 150 tons, were crane-lifted over the six-lane Via de la Juventud Ote in February 2015. The delicate positioning of the massive structures included a fitted entry into an opening of the newly renovated TIJ terminal with only a 9-ft-diam. clearing around the bridge. The 10-hour process required a team of more than 70 experienced professionals to successfully complete the maneuvers.

Two additional 75-ton skybridge sections were crane-lifted into position on the U.S. side of the border in February and March 2015, attaching to the upper level of the CBX terminal then under construction. After hours of preparation and careful calibration, the steel structures were hoisted into place in approximately 30 minutes. The final skybridge section was lifted into place in April 2015. Completion of the bridge and construction on the terminal building continued through Fall 2015.

The unveiling

After eight years of planning and preparation, CBX opened for public travel in December 2015. In the U.S., CBX features more than 65,000 sq ft of indoor and outdoor patio waiting areas for passengers and guests, along with retail, Duty Free, food and beverage venues, and fully bilingual customer services. The facility is open 24/7, with convenient short- and long-term parking on-site, as well as a variety of ground transportation options including rental car, taxi, Uber and shuttle access.

The $120 million project is expected to boost travel and tourism on both sides of the border and serve as an economic catalyst in the area. Users of the bridge are able to access TIJ flights to Mexico, many of which travel with greater frequency or at lower costs than those offered by southern California airports. TIJ also has direct flights to Shanghai, China, and Oakland, Calif., and has ample capacity to expand for regional and international flights.

About The Author: Frost is senior principal for Kleinfelder and served as project manager for the CBX. Morel is principal bridge engineer for Kleinfelder and was lead design engineer. Baillargeon is assistant project manager for Kleinfelder and was bilingual coordinator.