Analytical And Experimental Methods in Engineering Seismology

The arithmetical steps in the computation of the spectrum are extremely lengthy. A mechanical analyzer was developed by the author at Columbia University, in New York, N. Y., to avoid this numerical work. Some of the writer's earlier work with the earthquake spectrum is reviewed briefly in this paper. It also stresses engineering applications, and presents some new results, in particular regarding the effect of the foundation. Sections 1 and 2 introduce the definition of earthquake spectrum and show the results obtained for various earthquakes with the mechanical analyzer. Section 3 is a treatment of the spectrum curves obtained with the analyzer in relation to some observed facts and to the problem of stress prediction in actual structures. Section 4 considers examples of structures with more than one degree of freedom and shows how the stresses may be computed by means of the effectiveness factor. (The expression “efficiency factor” instead of “effectiveness factor” was used in the previous paper.) The danger of a phenomenon referred to as the “whip effect” is also pointed out. Some attention has been given to another aspect of the problem in section 5; the effect of the foundation on the rocking motion of a rigid structure is taken into account. It is shown that in this case the same methods using a spectrum and effectiveness factors can still be applied by introducing an additional degree of freedom and a natural period corresponding to the rocking motion on the foundation.