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On May 15, 2009, the longest stress ribbon bridge in the world opened in San Diego. The footbridge spans some 1,000 feet across Lake Hodges and has a concrete deck that is only 16 inches thick. With spans of 330 feet between supports, the bridge has an amazing depth to span ratio of 1:248. The dramatic crossing fits beautifully into its natural setting and was environmentally friendly to construct. As the sixth stress ribbon bridge in North America and one of only about 50 worldwide, the unique bridge has become a landmark for the region.
Click through additional photos of the completed David Kreitzer Lake Hodges Bicycle Pedestrian Bridge
The elegant and dramatic design of the record-setting stress ribbon bridge fits beautifully within its natural setting.
The David Kreitzer Lake Hodges Bicycle Pedestrian Bridge allows safe and enjoyable crossing for local residents.
Pedestrians and bikers share the bridge paths.
Since the bridge works as a cable, it forms the shape of a catenary between supports. The drape is determined by the span length and how much tensile force can be applied.
As the sixth stress ribbon bridge in North America and one of only about 50 worldwide, the unique bridge has become a landmark for the region.
The bright lights of the city in the background make the David Kreitzer Bridge a favorite nighttime spot.
The bridge is located within the San Dieguito River Park, and is part of California’s 55-mile Coast to Crest Trail, which runs from the beach in Del Mar to the river’s source at Mt. Volcan. The mission of the River Park is to provide recreational opportunities while protecting the natural resources and sensitive habitat along the river.
Before the bridge, the only means of crossing Lake Hodges in this area was by the I-15 freeway. The River Park needed a crossing at this location to provide its users with access to the trails on the north and south shores of the lake. The bridge also provides a safe crossing for bicycle commuters, replacing the dangerous route on the shoulder of the busy freeway.
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Selecting the stress ribbon bridge
The River Park hired T.Y. Lin International as the engineering team for this project. One of the design team’s first tasks was to study bridge types that might work for the crossing. Structure types evaluated included a prefabricated steel truss, precast and cast-in-place concrete girders, a timber glue-laminated bridge, and cable-stayed and suspension bridges. The stress ribbon bridge was also proposed at this time.
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The truss and girder options were limited to spans of approximately 200 feet and would have required at least four piers in the lake. Further, with bridge decks on the order of 10 feet deep, these options would have been aesthetically bulky. The cable-stayed and suspension bridge options were more visually transparent with thin decks and cables. However, these bridges would have towers on the order of 100 feet tall. The visually dominating towers were not desirable for this natural setting either.
The stress ribbon bridge alternative had many structural and architectural advantages. It could accommodate the 330 feet spans required for a three-span design, and only two piers would be required. The bridge would have an ultra-slender profile and its complimentary curves would blend into the rolling terrain around the lake. This bridge’s fewer piers and ultra-slender deck could be built without falsework which would result in less environmental and visual impact to the site.
Aesthetics played a major role in the decision. With fluctuating water levels in this part of the lake, it was important that the bridge look good in both wet and dry conditions. Early hand sketches and computer renderings showed just that. In the wet condition, the stress ribbon bridge appeared to “float” above the water. In the dry condition, it appeared to “nest” above the willow trees. The stress ribbon design was the perfect match for this site.
History and construction of the stress ribbon bridge
Stress ribbon bridges are rather unique. Only approximately 50 of these special bridges have been constructed. The first stress ribbon bridge was constructed in Switzerland in 1965. In the 70s and 80s, more examples were built in Germany, the Czech Republic, other parts of Europe, and Japan.
In North America, the first stress ribbon bridge was not constructed until 1990. This example was built across the Sacramento River in Redding, Calif. Since 1990, five others have been constructed: two bridges in 1999 in Rancho Santa Fe, Calif.; one in 2000 in Grants Pass Oregon; one in 2001 in Blue River Valley, Colo.; and the last in 2009 at Lake Hodges.
The stress ribbon is a type of suspension bridge. It is constructed by running bearing cables between abutments, stressing the cables, and then hanging precast panels from them. The cables are stressed so that the bridge deflects to the proper profile under dead load.
Next, cast-in-place concrete is poured to close the joints between the deck panels. Once the cast-in-place concrete reaches its specified strength, a second set of cables is stressed to post-tension the bridge from end-to-end. The post-tensioning lifts the span, places the deck into compression and creates a continuous ribbon of prestressed concrete. Post-tensioning also gives the bridge its stiffness and keeps live load vibrations to a minimum.
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Since the bridge works as a cable, it forms the shape of a catenary between supports. The drape is determined by the span length and how much tensile force can be applied. The tensile force from the cables must be transferred into the ground at the abutments. Because of this, these bridges require robust abutments and are usually constructed using rock anchor foundations at locations where strong formational material exists near the surface.
Most stress ribbon bridges are of the single-span variety. However, several multi-span bridges have been constructed.
The Lake Hodges bridge
At Lake Hodges, two piers were used to provide a three-span design. With three spans of 330 feet and a total length of 990 feet between abutments, it is the longest stress ribbon bridge ever constructed. It uses 87 precast panels, 29 per span. The panels are 10 feet long by 14 feet wide and only 16 inches thick.
Twenty-foot-long cast-in-place regions were used at the ends of each span. The width of these regions flare to 24 feet wide to provide overlooks at the tops of each pier. The thickness of the cast-in-place regions flares to 36 inches at the face of the piers. The flare was used to resist the large positive bending moments from the upward prestressing force.
The piers were constructed from cast-in-place concrete and are supported on driven HP14x17 piles. A temporary trestle was used for access and coffer dams were used to construct the piers. A steel saddle was placed atop each pier to provide support for the bearing cables.
The bearing cables consist of six tendons of 19 strands each. These tendons were stressed to 4,300 kips to provide a drape of approximately 6 feet. The bridge was post-tensioned with a jacking force of 4,600 kips using six tendons of 27 strands each. Standard 270 kip-per-square-inch (ksi), seven-wire, low-relaxation prestressing strand with a diameter of 0.6 inches was used.
Two different abutment types were used. On the north side of the lake, conditions were ideal. Here, rock was found a few feet below the surface, and rock anchors were used to provide a robust anchorage for the stress ribbon cables.
On the south side of the lake, some 40 feet of alluvium covered the formational material; rock anchors were not practical and a different approach was required. The solution was to construct large diameter drilled shafts to penetrate the week soils. Four 8 foot diameter shafts were drilled to a depth of over 80 feet. The four shafts were constructed in a 2 x 2 pattern and connected by a 10-foot-deep pile cap. The piles resist the pull from the stress ribbon by undergoing double-bending and transferring the lateral load to the stronger soils deep below the surface. The large piles provide the required strength, but as the cables are loaded the abutment is pulled toward the lake. This flexibility had to be accounted for and added considerable complexity to the design.
The bridge was built by Longmont, Colo.-based Flatiron for a cost of $8,800,000. This works out to about $550 per square foot and represents good value for a pedestrian bridge built over water.
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Complex analysis
Although visually simple, the required analysis and design methods are complex. As a cable structure, the bridge behavior is non-linear. Since the bridge has no joints, it accommodates movement from temperature, creep, and shrinkage by rise or fall in sag.
Thus, a non-linear and time-dependent analysis was required. Further, since the bridge was constructed segmentally, a stage-construction analysis was required to account for the stresses that were locked-in as the bridge was built. The flexibility of the south abutment was accounted for by using non-linear p-y springs to discretely model the stiffness of the soil along the length of each pile.
Since the bridge is so slender, it could be susceptible to uncomfortable vibrations under live loads and torsional instability under high winds. So, dynamic analyses were required to verify the bridge would be comfortable for pedestrians. Dynamic analysis combined with wind tunnel testing was used to verify the bridge would be stable under the maximum wind speed of 83 mph expected at the site.
The Larsa4D bridge analysis and design software program was used since it had the capability to account for these complexities. This program had the capacity to perform non-linear, stage construction, time dependent, and dynamic analyses, which were required for this bridge.
The analysis showed the controlling loading for most structural components was “Stage 144.” This stage included live loading of 65 psf applied to all three spans, a temperature drop of 35 degrees Fahrenheit and 50 years of creep and shrinkage.
The design and construction of this bridge was challenging. However, the extra effort has been well worth it. It provides a much needed crossing. Its elegant and dramatic design fits beautifully within its natural setting. Since opening in May 2009, the bridge has been well-used and the public has been extremely appreciative. This special design provides the region with a world-class footbridge, which will serve the people well for decades to come.
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David Kreitzer Lake Hodges Bicycle Pedestrian Bridge Engineer of record Local architect Contractor Construction manager Independent check engineer |
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Q&A with the structural engineer T.Y. Lin International’s Senior Bridge Engineer Tony Sánchez (TS), was the project engineer and the lead designer for the David Kreitzer Lake Hodges Bicycle Pedestrian Bridge. Sánchez shared some thoughts regarding this project with Structural Engineering & Design Editor Jennifer Goupil, P.E. (JG). Q: What types and how many structural systems did you and your team evaluate for this project? A: We evaluated the six different structure types including a prefabricated steel truss, a concrete box girder, a timber glue-lam girder, a cable-stayed, a suspension, and a stress ribbon. Q: How did you select the final structural system? A: The owner of the bridge is a nature park. Their mission is to provide recreational opportunities and to protect the natural resources of the San Dieguito River Valley. The stress ribbon bridge was by far the most environmentally friendly and aesthetically compatible with the site. These were the two essential factors that compelled the River Park to choose the stress ribbon bridge. Q: How was the most challenging aspect of the structural design solved? A: While the bridge is simple in appearance, its behavior is quite complex. As a cable structure, the bridge response is geometrically non-linear. So the linear-elastic analysis that is standard for most bridges was not appropriate here. Since the bridge is constructed segmentally, a stage construction analysis was required to capture the stresses that were “locked in” to each component as the bridge was built. Since the bridge has no expansion joints, it accommodates contraction and expansion by a rise or fall in its drape. Thus, analyzing the bridge for thermal forces and the effects of creep and shrinkage was important. A further complexity came from the soil conditions at the south end of the lake. Here rock anchors were not feasible, so a flexible abutment was used. This flexibility had to be accounted for in the design, which proved to be extremely challenging. Q: What new design innovations were employed by the structural design team? A: We used creep and shrinkage models to show that while the bridge deck has a tendency to shorten and loose its state of precompression, the flexible piles at the south abutment also creep toward the lake and counterbalance this effect. Q: Were any new structural products used or specified for this project? A: No new structural products were used. The bridge is a good example of an innovative way of using conventional construction materials and methods. The bridge is built using standard rebar, concrete and prestressing technology. Q: Were there any surprises? How did you adapt to them? A: When we stressed the first bearing cables the south abutment moved more than our model had predicted. Back in the design office, we softened our soil springs to match the movement we measured in the field and used the calibrated model to reevaluate the design. Our calibrated model showed that less prestressing was required, since the softer foundation was better at counterbalancing the creep effects in the deck. We made the decision in the field to reduce our prestressing force. Q: What owner requirements was part of the project scope? How did these affect the way you managed the project? A: The owner was sensitive to the environmental impact from construction activities. We addressed this concern by restricting construction to outside the breeding season of several threatened and endangered birds that nest along the shore in this area. Construction was only allowed from mid-September through mid-March. We broke the construction schedule into three years to accomplish this. Firm Facts |
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Spotlight: T.Y. Lin InternationalTony Sánchez, Ph.D., P.E., is a senior bridge engineer in the San Diego office of T.Y. Lin International. He is a registered civil engineer in California, Arizona, Nevada, and Utah and has been continuously involved in the bridge industry since earning his degree in civil engineering from UC Berkeley in 1991. Sánchez can be reached at asanchez@tylin.com.
