Earthquake ground motion

E a r t h q u a k e ground mot ion is a special kind of load. Other loads ordinari ly act ing on structures are usually in terms of force, bu t an ea r thquake load is in terms of g round mot ion ; other loads are mostly static, no t changing within a short per iod of t ime, bu t an ea r thquake load is dynamic, changing rapidly within a very short period of time, say in one second; other loads act most ly in the vertical direction, bu t an ea r thquake load acts simultan­ eously in bo th the vertical and hor izonta l directions, and also with torsion.