Bracing for the Worst

Reinforcing the West Mission Bay Drive Bridge with precast concrete helps span the budget gap.

By Fernando Pagés Ruiz

As you crest the Mission Bay Drive Bridge heading south into Dana Point, San Diego, you can’t help but feel a rush of excitement. This busily traveled precast concrete girder bridge conveys thousands of tourists daily to one of the world’s largest aquatic playgrounds. Captivated by sailboats cutting blue water and ringed by lavish resorts and endless palm-lined beaches, you might miss the 208 precast concrete skirts that help stiffen the 16 columns supporting your drive over the channel. Good thing, because the last thing you want to notice while crossing a bridge is its substructure - especially in earthquake-prone California.

Following catastrophic bridge collapses during the Loma Prieta and Northridge earthquakes of 1989 and 1994, California engineers began the laborious and costly process of retrofitting seismic reinforcement to all of the state’s overpasses. But along the Mission Bay Drive Bridge, retrofitting became a little less laborious and costly by including precast components.

“Without using precast concrete components, we could not have coped successfully with the tides or the timeline,” explains Bill Kotchi, senior civil engineer for bridges with the City of San Diego Engineering and Capital Projects Department. Forming and pouring cast-in-place concrete during the few hours of low tide and then trying to cure and complete the project under surging salt water made precast concrete the natural choice, but the conditions still presented considerable challenges.

Difficult but doable
Imagine the Mission Bay Bridge as a 1,200-foot-long span arching across an ocean channel between two peninsulas. As originally built, its substructure consisted of eight piers or bents and two end abutments. Each bent had two columns sitting on its own pile and two separate pile caps with ocean surging between them.

Engineers wanted to stiffen this structure by adding additional piles and tying the separate footings into one big cap. “We hammered steel piles about 100 feet deep to reach bedrock on both sides of each bent and then built a large pile-cap over the old and new footings to tie the whole thing together,” explains Kotchi.

The key to tying the separate footings into one structure involved building a boat-shaped concrete skirt around each pair of pile-caps and then filling the resulting hollow with reinforced concrete. Since the boat-shaped skirt had to protect the new, monolithic footing from surging seawater and debris, the contractor - Traylor Pacific, a division of Traylor Brothers in Evansville, Ind. - had to erect the structure at tide level with at least 3 feet of it underwater.

“We considered several alternatives including the option of building the entire structure on floating barges and sinking it into place,” explains Calvin Casey, project manager with Traylor Pacific. “But ultimate considerations of cost, scheduling and risk management made precast more appealing.” What Casey didn’t know at bid time was that precast would actually help complete the project ahead of schedule and under budget.

Pace improves after precast components arrive
Compounding the challenge of working in saltwater surges, Traylor Pacific had to consider the mating habits of a local seabird. Although it’s difficult to believe, Mission Bay used to be a smelly, sandy bog repulsive to humans but a virtual paradise for the California least tern. These pigeon-sized, black-and-white birds have suffered California’s coastal development enough to join the unfortunate club of endangered species. Since the rare tern still roosts along Mission Bay from April to September, Traylor’s contract included an accelerated completion schedule - work had to stop before April 1 - to avoid disturbing the lovebirds’ reproductive cycle.

The project encountered several delays at the outset, and even some cost overruns. Building cast-in-place pile caps suspended over tidal waters proved more difficult and time-consuming than predicted. But while Traylor Pacific struggled with the inherent unpredictability of offshore construction, San Diego Precast Concrete Inc. was busy manufacturing the skirts that would provide form and finish for the bridge’s 6-foot-thick concrete footings.

In a controlled factory environment, San Diego Precast built 208 precision-cast panels to within 1/8-inch tolerances encasing epoxy-coated reinforcement steel bars in a silica fume mix concrete that would provide resistance to the corrosive effects of seawater. Since quality control remained paramount in a structure that would protect the bridge footings from ocean scouring, the City of San Diego had laboratory technicians testing every batch of concrete. “Every cylinder break exceeded specifications,” says Kotchi. Curing was also of prime importance. “But with three days of steam curing at the precast plant, instead of 14 days on site, the city was assured a uniform product with an excellent finish and consistent quality,” says Kotchi.

San Diego Precast delivered the 10-inch-thick, 13-foot-tall, 7-foot-wide panels on schedule. Traylor Pacific closed a traffic lane on the bridge, plucked each panel off the deck with a 100-ton hydraulic crane and lowered it to storage on material barges.

To avoid the difficult coordination of aligning mechanical fasteners, Traylor Pacific cast steel angles into the site-built concrete soffit and had San Diego Precast add steel plates into the precast panels. As workers lowered each panel into place, welders stood ready to fuse the steel connectors.

“We were placing four panels per hour, exceeding any cast-in-place method by magnitudes,” says Casey.

The City of San Diego liked the smooth and uniform finish provided by precast, which exceeded the tolerances allowable with a cast-in-place product. After the joints between each panel were made watertight with a hydraulic caulk, workers constructed the steel reinforcement and poured the 6-foot-thick footings that will help assure the Mission Bay Drive Bridge won’t collapse even in a major earthquake. Precast concrete played an important part in bringing the project in on time and within budget, and the California least terns enjoyed another peaceful nesting season.

To find a manufacturer of this product in your area or for more information, visit NPCA’s Web site at www.precast.org or call toll free (800) 366-7731.

Project Profile
Project: West Mission Bay Drive Bridge Seismic Retrofit
Owner: City of San Diego
Engineer: TY Lin International McDaniel
Contractor: Traylor Pacific, a division of Traylor Brothers, Evansville, Ind.
Precast Manufacturer: San Diego Precast Concrete Inc., Santee, Calif.*
* San Diego Precast Concrete Inc. is a certified plant under NPCA's Quality Assurance/Plant Certification program.