12TH ISET ANNUAL LECTURE – 1991 APPROACH TO EARTHQUAKE ENGINEERING- OF A PRACTICAL THEORY N.C. NIGAM Professor of Civil Engineering and Director, Indian Institute of Technology, Delhi EMERGENCE in the Earthquakes seismically active are a regions source of critical loading condition for structures located loading is a large measure of the earth. A significant feature of the earthquake To establish of uncertainty associated with the earthquake phenomena. anticipate the the number, design size seismic and loading condition for a structure it is necessary to the site, during the service location of future earthquakes in the region surrounding life of the structure. Further, this assessment should be coupled with the prediction of structural response, and damage, due to random vibration induced by ground motion of a given intensity. Both these steps involve uncertainty at strong several motion stages. earthquakes The problem is compounded by the fact that potentially damaging for such events is statistically occur after long intervals of time and the available data engineering problems, insufficient. A probabilistic treatment of earthquake analysis, therefore, provides which involves assessment of seismic risk and random vibration 1947; Rosenblueth, 1951; a rational and consistent basis for aseismic design. (Housner, and Cornell, 1977; and Bolotin, 1960; Corneil, 1967; Vanmarcke, 1977; Whitman Kiureghian, 1981).