A stress ribbon bridge is a tension structure, similar in many ways to a simple suspension bridge. The stress ribbon design is rare. Few people including bridge engineers are familiar with this form and fewer than 50 have been built worldwide. The suspension cables are embedded in the deck which follows a catenary arc between supports. Unlike the simple span, the ribbon is stressed in compression which adds to the stiffness of the structure. Such bridges are typically made from concrete reinforced by steel tensioning cables. They are used mainly for pedestrian and cycling traffic. Stress ribbon bridges are very economical, aesthetic, and almost maintenance-free structures. They require a minimal quantity of materials. At present studies, on combining stress ribbon bridges with cables or arches, to build the most economical stress ribbon bridges. It makes the study of features of these particular bridges an important one.
The construction of the bridge is relatively straightforward. The abutments and piers are built first. Next, the bearing cables were stretched from abutment to abutment and draped over steel saddles that rested atop the piers. The bearing tendons generally support the structure during construction and only rarely is additional falsework used. Once the bearing cables were tensioned to the specified design force, precast panels were suspended via support rods located at the four corners of each panel. At this point, the bridge sagged into its catenary shape.
The next step was to place post-tensioning ducts in the bridge. The ducts were placed directly above the bearing cables and support rods, which are all located in two longitudinal troughs that run the length of the bridge. After the ducts were in place, the cast-in-place concrete was placed in the longitudinal troughs in small transverse closure joints. Concrete is poured into the joints between the planks and allowed to harden before the final tensioning is carried out. Retarding admixtures may be used in the concrete mix to allow all the concrete to be placed before hardening occurs. Once the final tension has been jacked into the tendons and the deflected shape is verified, the ducts containing the tendons are grouted.
After allowing the cast in place concrete to cure and achieve its full strength, the bridge was post-tensioned. The post-tensioning lifts each span closing the gap between the panels putting the entire bridge into compression and transforming the bridge into a continuous ribbon of prestressed concrete.
• Stress ribbon pedestrian bridges are very economical, aesthetical, and almost maintenance-free structures.
•They require a minimal quantity of materials.
• They are erected independently from existing terrain and therefore they have a minimum impact on the environment during construction.
• They are quick and convenient to construct if given appropriate conditions, without falsework.
• A stress ribbon bridge allows for long spans with a minimum number of piers and the piers can be shorter than those required for cable-stayed or suspension bridges.
Applications of the stress ribbon principle
• Eco duct: A tunnel that was built as part of a large network of motorways outside Brno. The theory is the same as a self-anchored arch but the geometry is much more complex. It is 50m wide and spans 70m a finite element program was used in its design.
• Stuttgart trade fair hall roof: The suspended asymmetric roof comprises a regular repetition of stressed trusses with individual I-beam ribbons of steel between them. The trusses function as strut and tie A-frames based on concrete strip foundations and are tied back to the ground with anchors. The stresses in the ribbons and weight of its ‘green roof’ were used to resist wind uplift.
Stress ribbon bridges are a versatile form of bridge, the adaptable form of structure is applicable to a variety of requirements. The slender decks are visually pleasing and have a visual impact on the surroundings giving a light aesthetic impression. Post-tensioned concrete minimizes cracking and assures durability. Bearings and expansion joints are rarely required minimizing maintenance and inspections. There are also advantages to the construction method since erection using pre-cast segments does not depend on particular site conditions and permits labor-saving erection and a short time to delivery. Using bearing tendons can eliminate the need for site formwork and large plant, contributing to fast construction programs and preservation of the environment. There is a wide range of different topographies and soil conditions found and a number of areas that require aesthetic yet cost-effective pedestrian bridges to be built: Stress ribbon bridges could provide elegant solutions to these challenges.