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Bridges equipped with dampers against earthquakes

08.09.2005


Researchers at the Universitat Jaume I of Castelló and at the University of California have devised a system to reduce the damage caused by earthquakes in bridges. Installed between the piers of a bridge and their respective foundations, it is a flexible device that helps to mitigate the effects produced by the movement of the Earth’s surface, working in a similar way to the shock absorbers in cars. The study has been published in the journal Earthquake Engineering and Structural Dynamics.

Because of the horizontal accelerations brought about by earthquakes, the bridge deck is subject to displacements and forces that could damage its structure. To avoid this, a new type of damper whose stiffness and damping features adjust to the deck position at each moment is introduced, absorbing the energy introduced into the structure by the earthquake and therefore reducing the structural damage. To date, similar damping systems have been designed, but the novelty introduced by the one developed by the Universitat Jaume I and the University of California is that, as the earthquake takes places, the bridge piers do not move freely. Instead, the devices incorporated cause a swaying type of movement of the bridge deck, which is very stable under seismic conditions. This is possible because the dampers very easily show an increase in length but offer high resistance to compression. Then, as the bridge sways sideways, one leg stretches while the other bears the weight of the bridge, and when the movement changes direction, the opposite happens.

“We are thinking about an A-shaped frame with two legs. When the ground motion starts to take place horizontally from left to right, what happens is that one of the piers forming the A elongates and all the weight of the bridge is transferred to the other one. In the commercial systems that are usually installed, the spring of this second pier yields and is compressed, that is, the spring is compressed as easily as it is stretched. What we have proposed here is that piers can stretch but offer great resistance to compression, thus significantly reducing motion at the bridge deck, as we have proved with virtual simulations”, explains M. Dolores Martínez Rodrigo, a lecturer at the Department of Technology at the Universitat Jaume I of Castelló.



One of the most striking conclusions of the study, according to scientists, is that with this damping system it does not matter how high the ground acceleration is during the earthquake, as the bridge motion velocity is always kept within safe limits.

“This system yields two immediate results: on the one hand, lateral acceleration is limited as if the bridge were an inverted pendulum and, on the other hand, the point of support of the superstructure rises and thus kinetic energy is systematically transformed into potential energy. At the end of the earthquake, the system is slowly self-centring”, explain the authors of the study, which has been published in Earthquake Engineering and Structural Dynamics.

The researchers have also published the results from the application of this kind of system to conventional buildings, although further research remains to be conducted in this sense.

Hugo Cerdà | alfa
Further information:
http://www.uji.es/ocit

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