Best Projects, Small Project: Bridge Creates New Link To Historic Lighthouse
A mile from the Golden Gate Bridge, the $1.9-million two-hinged suspension bridge for pedestrians replaces a bridge built in 1954 to the Point Bonita Lighthouse. It is the only U.S. lighthouse reached by a suspension bridge.
The new pedestrian bridge, completed in April 2012, stretches over a rocky causeway to the lighthouse and, like its predecessor, is a model of the landmark Golden Gate. The bridge has a 156-ft main span, a 55-ft approach span on the west side and a 33-ft approach span on the east side.
Access to the remote site made the project exceptionally challenging. The land approach is along a steep, half-mile-long trail and a hand-carved 118-ft access tunnel 6 ft tall and 4 ft wide. The lighthouse bridge begins at that point.
The structure was designed for a 50-year lifespan. The support cables, metal fittings and brackets are stainless or galvanized steel. The deck and towers are constructed from ekki, a hardwood imported from Cameroon and used because of its capacity to withstand the effects of saltwater.
Some pieces were 24 ft long and had to be transported to the difficult site by unconventional means and positioned with hand tools and manpower, says Joel D. Dodd, project field coordinator for the general contractor, Flatiron, Firestone, Colo.
In addition, four cables, together weighing approximately 1,780 lbs, also were transported and installed by hand.
Crews drilled holes in the cliff and reinforced them with support rods and shotcrete to help prevent further erosion.
Work on the bridge was precarious, often requiring crew members to hang off the side of the cliff or inch along cables. Crews repurposed the old bridge structure, using it as an anchor for the tie-off fall-protection system.
"Installation aside, the safety of the workers and the U.S. Coast Guard [personnel] who maintain the lighthouse and its fog signal was our utmost priority," Dodd says. "Safety was designed into the temporary works … constructed for the removal and replacement of the bridge."
Point Bonita Lighthouse Pedestrian Bridge
Owner: U.S. Dept. of Transportation, Federal Highway Administration, Lakewood, Colo.
Contractor: Flatiron, Benicia
Lead Design: HDR Inc., Denver, Colo.
Temporary Works Designer: BMA Construction Engineers, Walnut Creek
Rock Anchor Subcontractor: AIS Construction Co., Carpinteria
Submitted by Flatiron