Earthquakes are unpredictable. One can't tell where they will appear, when they will arrive and with what intensity they will strike. However,
major earthquakes need not have disastrous consequences. If we understand their character we can design our structures to withstand their onslaught.This paper outlines the major considerations relating to earthquake resistant design of bridges. It highlights the need for distinguishing between
buildings and bridges with regard to earthquake design force levels due to the difference in the characteristics of the two types of structures.
Bridges adequately designed for usual loads like dead loads, live loads, wind loads, etc are not necessarily safe for earthquake forces. For usual loads, the structures
remain within the elastic range of the material during service stage. In the case of strong earthquake shaking, it is neither practical nor economically viable to design
structures to remain within elastic limits. Cracking of concrete and yielding of steel - which would be considered unacceptable for usual types of loading - are depended
upon to dissipate the seismic energy without collapse for strong ground shaking that may take place infrequently. How this idea is incorporated in present day codes
(including IS:1893, Ref 1&2), is explained in the paper.
With the experience of many large earthquakes in the past, codes have undergone successive upgradation of design forces. The paper traces some of the changes with time.
How seismic resistance of the bridge structure can be enhanced and what are the vulnerable elements during severe ground shaking have been highlighted in this paper. The potential of
liquefaction has also been discussed. Seismic devices can be used to reduce earthquake effects on the whole bridge structure or on particular elements of the same. An exposure of the
state-of-the-art technology in this regard is presented. Seismic Retrofitting of bridges is an emerging field for preparing the large stock of existing bridges to withstand earthquakes.
The design philosophy in this regard, traditional methods and state-of-the-art techniques are described in the paper. The bridges damaged during the Bhuj Earthquake are discussed in brief
to highlight the vulnerability of bridges during severe ground shaking.
