MASS VARIATION WITH DISSIPATIVE STEEL STRUCTURES UNDER SEISMIC LOADS

Abstract eng:
In seismic engineering different levels of structural analysis are used, depending on complexity and importance of the structure and the required accuracy.  Non linear time history analysis is used along with a full model of the structure to give the most accurate results. This method requires most effort and a well predesigned structure.  Quasi static pushover analysis is used to get an insight of the subsequent development of plastic mechanisms under horizontal load patterns. This method requires only the modelling of the retaining system which, in the case of steel structures, could consist only of a pair of columns and the X-type braces in between.  The simplest, but most inaccurate way, is to use the lateral force method (e.g. according to Eurocode 8), which can easily be handled by hand calculations. The dynamic behaviour of the structure is modelled as a single-degree-of-freedom oscillator and adopted to the real behaviour by a behaviour factor q. It is evident, that for predesigning purposes, the number of dissipative elements along with their performance must be adjusted depending on the total oscillating mass, so that the target displacements are met. However, in the respective design codes no rules are given on how to carry out this task. This holds as well for most of the scientific publications, which describe refined analyses for given structures, but do not provide guidance for predesign. The present paper reports on a study on mass variation with a multi-storey steel structure. With a given dissipative structure a variation of the masses in each storey results – apart from a change in the natural frequency – in a different storey drift. This is becoming equivalent to a structure with given masses and variation of the performance of the dissipative elements, e.g. by altering the cross section of the braces. In the study a rule is developed, which describes the minimal required performance of the dissipative elements with a given mass and target storey drift. Thus, the structural engineer can adopt the bracing elements to the given needs.

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