T.Y. Lin International
Construction is under way on world's longest stress ribbon bridge.
Foundation work is almost complete on a 990-ft-long stress ribbon bridge over San Diego County’s Lake Hodges. When finished in March 2009, the $10.3-million footbridge will be the world’s longest of its type, says Wade Durant, resident engineer with San Francisco-based T.Y. Lin International, the designer.
San Dieguito River Park, the project owner, went with a stress ribbon bridge in part because its 12-ft-wide design will have minimal ecological and visual impact. “We could have built a conventional bridge cheaper, but it might never have got approved by environmental agencies because of [so] many foundations across the lake,” Durant says.
T.Y. Lin’s design features only two 43-ft-high foundations in the water to support three 330-ft-long spans with a 15-ft clearance. The north abutment is supported by 15 rock anchors, 75 ft deep in 11-in.-dia holes. The south abutment utilizes four 8-ft.-dia cast-in-drilled-hole columns, 90 ft deep. Each abutment measures 30-ft x 30-ft.
FCI Constructors, Inc., San Marcos, Ca, broke ground in February 2007, working on a 750-ft x 24-ft temporary construction trestle. “At any point in time there is only one thing that can be happening; it is a very linear project,” says Billy Shaddox, FCI project manager.
Because of nesting season, work will stop on May 15 and resume in September, says Susan Carter, deputy director for San Dieguito River Park. Then crews will string up 114 post-tensioned steel cables, each .6-in. in dia and 990 ft long. After the cables are in place, crews will attach eighty-seven 14-ft x 10-ft precast precast concrete panels with steel connector rods. A concrete closure pour will stress the cables. The result will be a bridge with a profile less than 2 ft deep. “A typical concrete box girder bridge of the same span length could be 14 ft deep,” says Durant. The bridge is designed to handle 65 lbs per sq ft and remain stable in winds up to 86 mph.
Durant says the structure will be the sixth stress ribbon bridge in the United States. It will surpass 500-ft-long bridges in Bulgaria and South Africa.