Earthquake floor response and fatigue of equipment in multi-storey structures

Abstract

Consideration has been given to electrical and mechanical equipment exposed to seismic shaking in multi-storey structures. Typical buildings of various heights were dynamically analysed, floor responses calculated at the top level, and equipment responses to the floor motions were examined. A method of evaluating effects of cumulative low-cycle fatigue was used to characterize equipment responses and to obtain comparisons of intensities of response for the structures when all were exposed to the same eartquake. Three single frequency shake table tests were discussed, each having time domain characteristics similar to actual structural responses and each capable of producing fatigue effects equivalent to those of a real earthquake. The following points are worthy of mention: 1. (1) Although the structural models used in the analyses had dynamic characteristics representative of a fairly wide class of low, medium and high-rise buildings, further characterization of multi-storey seismic environments through the use of several benchmark structures would be usefll. Existing structures selected as benchmarkswould be subject to both experimental and analytical investigations to determine structural responses and validate analytic models. The information gained would aid in further development of guidelines for seismic test programes for sensitive equipment designated for installation in multi-storey buildings. Detailed seismic analysis for individual cases or for limited classes of structures would likely be warranted only under specialized circumstances (see (3) below). 2. (2) A statistical evaluation of tructure-equipment response using several earthquake records could extend the research described in this paper. However, the use of a single event has demonstrated significant levels of response observed when the natural frequency of the structure closely matched the dominant frequency component of the ground motio. 3. (3) Specialized structures such as nuclear power plants warrant more detailed seismic study because of the stringent safety requirements and the complexity involved with mathematically modelling a reactor structure. In this case, several different models may be examined, and from the ensemble of results, upper bounds on floor response levels can be developed. 4. (4) Test requirements often specify only spectrum test levels without full consideration to time domain aspects of a seismic event. Accompanying the spectrum with a fatigue equivalence parameter could provide a convenient means of incorporating an earthquake's time and intensity characteristics. 5. (5) Single frequency laboratory tests can closely simulate the quasi-harmonic floor responses observed in multi-storey structures. The three tests outlined in this paper are relatively easy to perform, provide a significant degree of operator control over test specimen response, permit an identification of dynamic properties of the test specimen and are suitable for incorporating into test specifications and requirements for both nuclear and non-nuclear applications.