Economical Design Of Earthquake Resistant Bridges

With the occurrence of every major earthquake, there has been in the past, almost a world-wide tendency to increase the capacity demand of the structure to counteract such events. It is only in the last decade that new strategies have been successfully developed to handle this problem economically. The current international practice has shifted towards a performance-based engineering design, wherein the accent is on serviceability and safety under different levels of magnitude of earthquakes. Also there is an increasing realization that apart from techniques for improving ductility, the structural engineer’s tool-box should include energy-dissipating and energy-sharing devices and those that can control the response of the system. There have also been further advances on appropriate methods and devices of preventing ‘dislodgement’ or ‘unseating’ of the superstructure in the event of severe ground shaking. How these ideas can be used in economical earthquake resistant design of bridges is the subject of this paper.

PLASTIC HINGING AND DURABILITY

There is a marked difference in seismic design aspects of bridges and buildings. The reduced degree of indeterminacy of bridge structures leads to reduced potential of dissipating energy and load redistribution. In bridges, the superstructures (piers and abutments) are the main structural elements which provide resistance to seismic action. For energy dissipation, ductile behaviour is necessary during flexure of these structural elements under lateral seismic loads. This essentially means that the formation of plastic hinges or flexural yielding is allowed to occur in these elements during severe shaking to bring down the lateral design forces to acceptable levels. Since yielding would lead to damage, plastic hinging are localized by design at points accessible for inspection and repair, i.e., parts of the substructure that lie from foundation upwards (see Figure 1). No plastic hinges are, of course, allowed to occur in the foundations or in the bridge deck.