ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.
Issue link: http://www.aspiremagazinebyengineers.com/i/306864
On May 15, 2009, the world's longest stress ribbon bridge opened in San Diego, California. The David Kreitzer Bicycle Pedestrian Bridge uses high- strength cables embedded in an ultra- slender concrete deck to span nearly 1000 ft across Lake Hodges, and it does so with grace and respect for the site and sensitive lake habitat. Unique Design The stress ribbon bridge type is unique regionally as there are only six examples in North America and only about 50 worldwide. At Lake Hodges, it has proven to be the perfect bridge for the site. Prior to Lake Hodges, a stress ribbon bridge of this length had never before been constructed. At Lake Hodges, the stress ribbon enabled the long spans required for a three-span design. With spans of 330 ft, it gracefully stretches 990 ft between abutments with only two piers in the lake. And it does this with a deck that is only 16 in. thick. The result is a thin ribbon of concrete with an amazing depth-to-span ratio of 1:248. Asset to the Community The bridge provides a crossing for hikers, cyclists, joggers, and bird watchers and allows access to the extensive trail system on the north and south shores of the lake. It provides a significant enhancement to the region by adding both a transportation and recreational resource. By eliminating a 9-mile detour in the system and improving public access to surrounding trails and recreational facilities, the bridge has become a vital link in the 55-mile-long Coast to Crest Trail within the San Dieguito River Valley Open Space Park. The bridge also provides a safe crossing for bicycles. Prior to the bridge, bicycle commuters had to cross Lake Hodges on the shoulder of the busy I-15 freeway, which posed a safety concern. Now these commuters use the new bridge to safely cross the lake on a dedicated path away from vehicular traffic. Design Complexities Although the bridge is simple in appearance, its analysis and design were very complex. Since the bridge behaves as a cable, it is geometrically non-linear. This made linear-elastic analysis—the standard for most bridges—inappropriate. Since the bridge was constructed segmentally, a stage construction analysis was required to track the stresses that were locked in during construction. Further, since the bridge is made of concrete that is post- tensioned and has no expansion joints, the bridge accommodates expansion and contraction by a rise or fall of each span. Thus, the effects of concrete creep and shrinkage and thermal loading were critical. These effects were accounted for by performing a time-dependent creep and shrinkage analysis that included thermal loading. The bridge was modeled stage by stage to represent the construction sequence, and then live load, temperature, and wind loads were applied. Next the analysis stepped through 50 years of creep and shrinkage and the loading process was repeated. In the end, the critical load case was 50 years of creep and shrinkage plus live load on all three spans and a temperature drop of 35 °F. A dynamic analysis was performed to evaluate the bridge under wind, seismic, and vibrational loads. A detailed wind study was performed, which included wind tunnel testing. Critical in the wind study was the torsional response of the bridge, which can be excited through buffeting as strong winds pass transversely over the deck. The results showed the bridge would be stable at the maximum wind speed of 85 mph predicted for the site. The results of the seismic analysis s h o w e d t h e b r i d g e w o u l d s t a y elastic under the maximum credible earthquake. This is surprising for a bridge in California, but somewhat intuitive for the stress ribbon design. With a 16-in.-thick deck, this bridge has relatively low mass, and is restrained laterally by 12 large cables. In essence, the structure is one large seismic cable restrainer. For the vibration analysis, a simplified a p p r o a c h f r o m t h e B r i t i s h a n d Canadian bridge codes was used. The profile david kreitzer Lake hodges bicycLe pedestrian bridge / SAN DIEGo, cAlIforNIA ConCrete sUpplier: Palomar Transit Mix, Escondido, calif. preCAster: U.S. concrete, San Diego, calif. post-tensioning ContrACtor: Dywidag-Systems International, long Beach, calif. bri Dge DesCription: 990-ft-long, three-span stress ribbon bridge, 14 ft wide and 16 in. deep b riDge ConstrUCtion Cost: $8.5 Million ($550/ft 2 ) by Tony Sanchez, T.Y. Lin International The DaviD KreiTzer LaKe hoDges BicycLe PeDesTrian BriDge The Lake Hodges Stress Ribbon Bridge. All Photos: T.Y. Lin International. 36 | ASPIrE , Summer 2010 Lake Hodges.indd 36 6/21/10 12:22 PM