The Physical Evaluation of Seism !

The problem of designing structures to withstand destructive earthquakes is not in a very satisfactory condition. On the one hand engineers do not know what characteristics of the ground motion are responsible for destruction, and on the other hand seismologists have no measurements of seismic motion which are sufficiently adequate to serve for design, even if the destructive characteristics were known. Consequently, engineers have been forced to proceed on an empirical basis. From past experience, chiefly in Japan, it has been found that buildings which are designed to withstand a constant horizontal acceleration of 0. 1 gravity are, on the whole, fairly resistant to seismic damage. It is fortunate that such a simple formula works at all, in view of its inadequacy from the point of view of precise computation. We know that seismic motions do not exhibit constant accelerations; that instead they are made up of exceedingly variable oscillatory movements. A formula based upon constant acceleration may thus lead to large errors, especially when applied to new types of structures which have not been tested in actual earthquakes. In the following paragraphs a new formula for seismic destructiveness is proposed, in the belief that it is more accurate than previous ones. In addition to providing engineers with a more rational basis for design procedure, it determines a new type of seismographic instrument for recording and measuring the destructive characteristics of seismic motion. The actual destructiveness of a given earthquake might be defined as the sum of all the material damage caused by the seismic movements. Such a definition does not lend itself to physical measurement and is therefore unsatisfactory from the physical point of view. The idea contained in this definition may be extended to permit physical measurements. Thus, if strain gauges were erected on all members of all the involved structures, the measure of seismic destructiveness could be taken as the sum of all the indicated strains. These definitions of destructive-