Stressed Ribbon Bridge

[Rell Jette]

A stressed ribbon bridge (also stress-ribbon bridge or catenary bridge[1]) is a tension structure similar in many ways to a simple suspension bridge. The suspension cables are embedded in the deck, which follows a catenary arc between supports. As with a simple suspension bridge, the weight is taken by the suspension cables, but unlike the simple span, the deck or ribbon is stressed in compression, which adds to the stiffness of the structure (simple suspension spans tend to sway and bounce). The supports in turn sustain upward-thrusting arcs that allow the grade to be changed between spans (where multiple spans are used). Such bridges are typically made from concrete reinforced by steel tensioning cables. Where such bridges carry vehicle traffic, a certain degree of stiffness is required to prevent excessive flexure of the structure, obtained by stressing the concrete in compression.

A stressed ribbon bridge (also stress-ribbon bridge or catenary bridge) is a tension structure (similar in many ways to a simple suspension bridge). The suspension cables are embedded in the deck which follows a catenary arc between supports. Unlike the simple span, the ribbon is stressed in traction, which adds to the stiffness of the structure (simple suspension spans tend to sway and bounce).what is a ribbon bridgestress ribbon pedestrian bridgescancer symbols and colorsbridge materials for salematerials used to build bridgesused bridgematerial used in constructioninteresting civil engineering topicscivil engineering topics for presentationseminar topics pdfbest seminar topics for civil engineeringcivil seminar topics pptcivil engineering seminar topics 2019seminar topics for mechanical engineersmechanical engineering seminar topics 2018Read less

The stress-ribbon design achieves a bridge that is a thin ribbon of concrete with very low visual impact to its natural setting across the lake. At Lake Hodges, this structure type uses a 16 inch thick concrete deck to span 330 feet between supports for an amazing depth to span ratio of 1:248. The bridge can achieve such a remarkable depth to span ratio since it is actually a cable supported bridge with the bearing cables embedded within its concrete deck. Post-tensioning is applied to close the transverse joints between the precast deck panels and to give the bridge its required stiffness for live loads.

The bridge was constructed by first building the abutments and piers, then pulling the bearing cables over the piers and anchoring them to the abutments. Once the bearing cables were stressed to the specified tension, 87 precast deck panels were hung from the bearing cables, which fit into longitudinal troughs formed into the panels. After all deck panels were erected and towed into position, ducts for post-tensioning tendons were secured within the longitudinal troughs. Once the cast-in-place concrete for the closure regions reached the specified strength, the bridge was post-tensioned. Finally, a textured surface was applied to seal the seams between the 87 panels and provide a non-slip, attractive surface.

At the north end of the bridge, the horizontal force from the stress ribbon is resisted by a rock anchor abutment, which is standard for stress ribbon bridges. At the south end, there is a deep layer of alluvium, and bedrock is some 50 feet below the surface. This made rock anchors impractical there, and four 8-foot diameter cast-in-drilled-hole piles were used instead.

In North America, the stress ribbon bridge type has only been used a handful of times, and world-wide an example of this length has never before been constructed. This special stress ribbon design required dynamic analysis to investigate effects of live load induced vibrations, dynamic wind loading, and seismic loading. Considerable effort went into the analysis and design of this bridge. However, the extra effort involved has resulted in a world-class bridge that complements its natural setting across Lake Hodges and within the San Dieguito River Park.

The San Dieguito River Park Joint Powers Authority is the owner and operator of the bridge. T.Y. Lin International is the Engineer of Record for the project. Jiri Strasky, Consulting Engineer, collaborated on the conceptual design and performed the independent design check. Safdie Rabines Architects is responsible for its distinctive look. Contractor was Flatiron West.

Electrical Power to the Bridge is provided by solar power. The solar array enables lighting (trained downward on the bridge surface) in the evenings, along with powering the automatically timed gates on each end of the bridge. Partial funding for the lighting project ($8,000) was provided by the Rancho Bernardo Community Foundation in 2013.

The bridge is located approximately 1000 feet west of the I-15 freeway bridge at Lake Hodges, between the City of San Diego on the south and the City of Escondido on the north. It connects the Coast to Crest Trail on the north with the Piedras Pintadas and Bernardo Bay Trails in the Bernardo Bay Natural Area on the south. The easiest access is to take the West Bernardo Dr./Pomerado Rd. exit from I-15 and go west onto West Bernardo Drive, up the hill. Park in the Bernardo Bay Staging Area on your right at the top of the hill. Walk or bicycle back down West Bernardo Dr. along the bike path that connects the staging area to the bridge.

The bridge is a suspension type structure divided into three 100m (330ft) long spans, the post-tensioning tendons of which are integrated into an extremely thin, only 410mm (16in) strong bridge deck. Thanks to the post-tensioning systems used, the bridge is supported by two piers and blends into the surrounding environment.

Post-tensioning work was performed by DSI USA from a temporary trestle that extended approximately 213m (700ft) from one end of the structure. Initially, approx. 39,000m (128,000ft) Type 19x0.6" DYWIDAG Strand Tendons with MA Anchorages were installed. The primary tendons required for this purpose were prefabricated on site and installed across the open spans using temporary post-tensioning tendons.

Once stressed to a predetermined sag, the precast deck panels were hung into their final position and additional Type 27x0.6" Post-Tensioning Tendons with MA Anchorages were installed in troughs formed in the precast sections.

Following installation, the primary strand tendons were adjusted to the final sag required by the engineer, and final concrete was placed in all troughs and closures. As the concrete cured, the Type 27x0.6" Secondary Tendons were stressed incrementally to control shrinkage. Secondary tendon ducts were fully grouted following completion of stressing.

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Stress-ribbon bridges are one of the oldest forms of bridges that have been extensively used as a cost-effective solution to footbridges, especially in Europe. The classical form of these bridges follows the suspension bridge mechanism, however, the deck is directly loaded over the tensioned catenary cables suspended between the two abutments. Other advantages of these bridges are these bridges are easy to assemble and erect, involve minimal falsework, have a very low self-weight, require minimal long-term maintenance and blend well with the environment. Prestressed concrete segments combined with the use of high strength cable materials provide enough rigidity to stabilise the shape of the bridge against overturning and oscillations.

The main drawback of these structures is that the cables transmit large horizontal forces to the abutments. To overcome this, pile foundations are usually used to support the abutments. Recent developments have also employed innovative methods to solve this problem. Arches are used as a form of support in some cases to provide a smooth transition between the spans. A more straightforward solution is to use towers to take the vertical load from the deck or vary the sag in the cables.

The design and construction of the new crossing builds on known technology for this type of simple catenary bridge, introducing HDPE grouted ducts to improve durability of the tendons that support it.

The footbridge is situated in a rural environment on the edge of the town, hence the commissioning authority sought a design that would have a minimum impact on the surroundings, to suit the low-key nature of its setting.

Design team Ponting, Pipenbaher Consulting Engineers and Jereb & Budja Architects came up with the winning proposal in an open competition in 2017; a 4 m-wide stress ribbon bridge in which the 420 mm-thick deck elements form a slender catenary between the tree-lined banks.

The bridge is formed of 46 precast concrete panels supported on six tendons to create a 120 m-long link between the two banks. Mass concrete blocks and anchored secant-pile walls create the foundations on each side into which the tendons are connected.

Freyssinet Adria was contracted to build the bridge in a joint venture with CGP Novo Mesto, being assigned responsibility for developing the construction method concept and designing and manufacturing the specialist temporary equipment needed to carry it out. Supply and installation of the tendons and erection of 46 precast deck units was also within the scope of works.

Four of the six tendons, each consisting of 47 strands in an HDPE duct, were erected in the first part of the works. This began with 12 m-long sections of duct being clamped to hangers suspended from a steel wire between temporary portals on each bank.

Sections of duct were pulled out over the river one at a time, each being welded to the previous one, until the full length was in place. In order to ensure the strands remain parallel once threaded into the duct, which is critical for this type of structure, Freyssinet Adria used a specialist installation method that is usually adopted on cable-stay bridges.

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