A simple approximate formulation for the first two frequencies of asymmetric wall-frame multi-storey building structures

A simple approximate method is presented to estimate the first two coupled natural frequencies of uniform asymmetric wall-frame multi-storey buildings . Firstly, the fundamental uncoupled torsional-to-lateral frequency ratio i s determined by using the maximum structural static deflection resulting fr om the gravity load acting horizontally. Secondly, the concept of equivalen t eccentricity is proposed and determined by means of the frequency ratio o f the second to the fundamental mode in the same direction of a building st ructure. Finally, a combination of the fundamental uncoupled torsional-to-l ateral frequency ratio and the equivalent eccentricity is used to extend th e simple formulae, which are limited to coupled natural frequencies of prop ortionate structural systems, to estimate the first two coupled natural fre quencies of asymmetric wall-frame buildings. Numerical examples show that a good agreement between results from the proposed method and FEM has been a chieved. (C) 2000 Academic Press.