PROJECT PROFILES : LAKE HODGES PEDESTRIAN STRESS RIBBON BRIDGE
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Precast concrete played a prominent role in 2009 with the building of the World's Longest Stress-Ribbon Bridge. . The bridge is located in San Diego, California. Built in an environmentally sensitive area, the structure comprises three 330 ft spans for an overall length of 990 ft. The ultrathin precast concrete deck was chosen because it minimized disruption to the local ecology and blended well into the sensitive surroundings. The strength and versatility of precast concrete was thought to be the best option.
Altogether, 87 precast concrete panels (29 in each span) were used in the deck. The deck panels were post-tensioned to ensure continuity in the spans, close the transverse joints, and give the bridge its required stiffness for live loads.
Officially named the David Kreitzer Lake Hodges Bicycle Pedestrian Bridge, the structure is situated in the City of Escondido in San Diego County. The bridge provides a vital link to the Coast to Crest Trail, eliminates a 9 mi (15 km) detour, and furnishes a safe route for bicycle commuters who otherwise would ride on the shoulder of the busy and dangerous interstate freeway.
The major design challenge was to build a bicycle pedestrian bridge over an environmentally protected lake some 1000 ft. The structure could not be massive; it had to have a light and airy look instead. It also had to be free of vibration.
The stress bridge with a precast, prestressed concrete deck resembles a suspension bridge with cables embedded in an ultra-thin concrete deck. The cables follow a catenary path, and as they interact with the concrete deck a complex nonlinear behavior develops, which affects long-term stresses and deformations. The design required fairly complex analytical methods to evaluate the nonlinear behavior of the cable system and the time-dependent effects from concrete creep and shrinkage as well as to stage construction analysis to evaluate the locked-in stresses as the bridge was constructed.
Only six stress ribbon bridges exist in North America, with just 50 worldwide. The longest prior to the Lake Hodges Bridge is a 500â€TM stress ribbon bridge in Bulgaria.
No falsework was needed during construction of the bridge that has an overall width of 14 ft and a total deck area of 12,180 Sq. Ft. All of the 87 panels are 14 ft wide, 10 ft long, and 16 in. thick resulting in a span-to-depth ratio of an astonishing 1 to 248.
Once the precast concrete panels were in place and the closure pours were filled with concrete, the entire length of the bridge was post-tensioned by a second set of prestressing tendons. The completed bridge features widened decks (or belvederes), over the two intermediate pier supports on the lake and includes benches for prime viewing spots with informative displays of the lake.
Extensive coordination was required between the U.S. Concrete precast Group-San Diego, the general contractor, Caltrans, the City of San Diego, and the River Park (owners), all of which had jurisdiction over various portions of the project. Construction of the bridge was only allowed during the winter months due to restrictions imposed by nearby breeding habitats for endangered species. Therefore, construction was split into three phases. Precast concrete erection took place during the final phase of construction, starting in November 2008 and ending a year later.
This pedestrian bridge was also a co-winner of the Non-highway Bridge category in the 2009 PCI Design Awards Program.
The total cost of the bridge project was $10.5 million.
The precast portion was approx. $850,000.
