The Swiss Federal Laboratories for Materials Testing and Research (EMPA) has developed a cable stayed footbridge with a GFRP girder in order to examine the dynamics of light weighted structures.
The main objectives of the project are to create an experimental research platform that, with the assistance of a variety of specialist partners working on sub-projects, can provide new research on the use of GFRP in bridge and other large structures.
Dr. Daniel Gsell of EMPA, who is co-ordinating the project, refers to the fact that most of the bridges tending to favour a design of large spans with more slender and lightweight structures, which are based on an increased use of high performance building materials. As a consequence of these trends Dr Gsell claims that modern bridges are becoming increasingly prone to vibrations:
“The same external dynamic excitation energy causes larger vibration amplitudes since the inertia mass of the structures is reduced. Due to the slender design the stiffness reduction of the structural components shifts the eigenfrequencies of the structure towards excitation frequencies containing higher mechanical energy. Therefore the structural components are subjected to an increased fatigue loading and the lifetime decreases”
Gsell adds that “the application of GFRP profiles in bridge construction, especially in bridge decks, accentuates the trend towards lighter engineering structures. The light and nevertheless high strength GFRP profiles in combination with stiff structural elements, e.g. steel cables, carbon fibre reinforced tendons, or steel girders, enable the design of very slender and lightweight structures. Compared to conventional building materials, a slightly differentiated dynamical behaviour is being observed in GFRP constructions. Since the material is light, this results in high vibration amplitudes but the smallest resonance frequencies of the unloaded structure are relatively high. As a result of the high ratio life load to dead load, the eigenfrequencies of these constructions are strongly dependent on the actual loading. In the case of high live loads the resonance frequencies decrease, the above mentioned dynamic deficiency becomes relevant again”.
Gsell claims that this behaviour is a real challenge for conventional vibration mitigation systems, such as Tuned Mass Dampers (TMD) or common viscous dampers.
In order to avoid these drawbacks, the development of appropriate vibration mitigation set-up’s and health monitoring systems become essential.
At the EMPA-laboratory a cable stayed footbridge with GFRP-girder has been built in order to examine the effects vibrations have on the structure. Because of the limited space in a laboratory, a footbridge was designed with dimensions of 20.0 x 2.5 x 7.5 m3. In order to get a realistic structure, the bridge was constructed on the basis of the load assumption of the European Union standards.
The bridge girder consists of five plate modules which are composed of pultruded structural GFRP profiles with a cable stayed structure.
Gsell states that despite the use of the soft GFRP profiles, suspending the bridge deck by stiff cables in relatively short spans enables the design of a slender girder.
The free span is suspended by three pairs of seven wire steel strands. The total weight of the longitudinal girder amounts to 1.3 ton. Compared to an ultimate loading of 15 tons an extremely light structure has been designed.
The project has finished the preparatory stages of testing but will now commence work on investigating the dynamics of the bridge in greater detail in order to acquire a detailed knowledge of light weighted structures.
The next step will integrate some of the other sub-project studies, whose developments are validated experimentally within the footbridge. Some of these subprojects are investigated in collaboration with external research groups of industrial partners. As the cable stayed footbridge will remain in the EMPA-laboratory for several years, Gsell is looking to invite new partners into the project.
One of the sub-projects is examining the use of Tuned Mass Dampers (TMD’s), which are commonly used with skyscrapers and bridge girders. TMD’s operate only in an efficient way, if their frequency is well tuned. The resonance frequencies of lightweight structures are strongly dependent on their actual loading. In order to damp such construction by well established Tuned Mass Dampers, the frequency of such a damping system must be adapted permanently to the actual frequency of the structure.
Another sub-project is concerned with fault detection by curvature estimation with fibre optic sensors. The sensors will be integrated into the structural elements in order to assess the structure based on directly measured curvatures. This approach is based on the fact that structural curvatures are much more sensitive to damages than displacements are.
The bridge project is financially supported by the Gebert Ruf Stiftung and the ETH council. The companies Fiberline S/A in Denmark and Maagtechnic AG in Switzerland are supporting the project. The project is also integrated into the 6th European Union Framework Programme for Research and Technological Development (Sustainable Bridge).
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