Among various harbour structures, a trestle type pier with vertical steel piles is unique in respect that it is very flexible. Hence, it seems inadequate to design it by the seismic coefficient method which could be applied to other types of harbour structures.
　In the present paper, first the characteristics of a pier both in its elastic and plastic ranges are investigated experimentally and the results are summarized into following two items: 1. dynamic properties of a pier can be described by that of a one degree-of-freedom system, 2. restoring force can be approximated by the family of smooth hysteretic curves proposed by P.C. Jennings. Then the earthquake responses of the above mentioned model are computed for three earthquakes: EI
Centro (1940), Shimizu(1965) and Kushiro(1965). Final results are arranged into one chart by which the earthquake response of a pier can be easily evaluated. Finally, the ductility factor of a pier is suggested as a safety factor in the proposed method. The new method is actually applied to nine working piers, and the comparison between the seismic coefficient method and the new method is also included.