Estimation of Relative Displacements on Equipment Or Secondary Structures Under Severe Earthquake Excitation

Abstract eng:
The design of equipment or substructures mounted on industrial structures is commonly addressed from the perspective of the adequacy of the anchoring system. The results for the design of the supports and anchoring devices in terms of forces, accelerations, and displacements and also for the study of the behavior of the equipment are obtained from the same analysis carried out for the primary structure, which is usually a response spectrum analysis (RSA) that considers a response modification factor to take into account in a very approximate manner the energy dissipation behavior of the primary structure. The codes allow estimating the “expected displacement” of the structures through the amplification of the displacements obtained from the RSA (for reduced earthquake action) by a factor similar or equal to the response modification factor, based mainly on the “equal displacement rule”. However, the nonlinear behavior inherent to the equipment or substructure, if it exists, is completely independent from the nonlinear behavior of the primary structure. Furthermore, in many cases the configurations of the equipment or substructure are not comparable to the configurations of the primary structures and therefore the applicability of the provisions of the codes is at best uncertain. A parametric evaluation of the displacements of a range of secondary structures has been carried out using timehistory analysis considering different levels of nonlinear behavior in the primary structure while the secondary structure has been made to remain elastic. Average results over a set of 10 ground motions have been compared with the estimations obtained from the RSA results considering the average spectrum of the ground motions, for a range of period of the primary structure and a range of ratios between the secondary and the primary structures. The results from linear and nonlinear analyses are compared first in terms of the story shears and interstory drifts of the primary structure and also in terms of the drifts in the secondary structure. The primary structure results confirm the trends in the behavior already observed by many researchers, while the set of results for the secondary structure drifts show that using the same approach for the estimation of "inelastic" values is always a "safe side assumption" but in some cases can grossly overestimate the results.

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