An investigation of the effects of earthquakes on buildings

Abstract

The forces induced in buildings by earthquakes are here investigated. The effects of some of the physical properties of structures are determined and from an analysis of earthquake records the general character of earthquakes is deduced. An investigation is also made of the dissipation of energy of vibration to the propagation of elastic waves in the ground. This thesis was written to contribute to the program of earthquake research that Professor Martel has been carrying on for the past ten years. As such, it is a continuation of the work done by M. A. Biot and M. P. White. Acknowledgment is made of the financial support of the County of Los Angeles which made possible the analysis of the earthquake records. Sincere appreciation is expressed to Professor R. R. Martel for the advice he has given and the interest he has shown. SUMMARY All the earthquakes analysed are of the same general character. There are no special features distinguishing any one earthquake from another. There is no evidence that the physical properties of the ground have any effect on the character of the earthquake. In the range of periods from 2/10 seconds to 2 seconds, all ground waves are of approximately equal importance. There are no predominating periods of ground waves. The analysis of records provides a convenient method of measuring the intensities of earthquakes. A scale of intensities constructed in this manner is shown in the following table: Scale of Earthquake Intensities El Centro, May, 1940 -- 100 Long Beach, March, 1933, (at Vernon) -- 65 El Centro, December, 1934 -- 60 Helena, Montana, October 31, 1935 -- 55 Ferndale, September, 1938 -- 40 Los Angeles, October, 1933 ? 17 The analysis of the records shows that there is no so-called dominant ground period. For undamped structures, with periods of vibration longer than about 2/10 of a second, the maximum shearing force at the base is, for practical purposes, independent of the height of the structure and independent of the period of vibration. For such structures the maximum shearing forces are independent of the total mass of the structure but for ordinary construction vary in direct ratio as the mass per floor level. When the height of a structure is increased, the shearing forces in the upper portions of the structure are increased. Only the first few modes of vibration are of importance in producing shearing forces. For undamped structures flexibility and lightness of construction will reduce the magnitudes of the shearing forces. However, the flexibility of the first story alone has little effect upon the maximum shearing force at the base of the structure, although it does reduce the shearing forces in the upper portions of the structure. Considerable elastic yielding of the ground may take place at the base of a structure without having sufficient effect on the accelerogram to be distinguished by analysis of the record. The energy dissipated into the ground may be an important factor in reducing the shearing forces. The analysis shows that the reduction in shearing forces due to energy lost in wave propagation is greater for high frequencies than for low frequencies, and is greater for ground of low rigidity than for ground with high rigidity. Tables are constructed which indicate that structures with high frequencies may have their shearing forces appreciably reduced by this energy dissipation. It appears, however, that structures with periods of about 1 second will be little affected.