In-plane floor flexibility effects on torsionally unbalanced systems

This paper presents the results of an analytical work addressed to understand the effects of in-plane floor flexibility on torsionally unbalanced (TU) systems subjected to bidirectional firm-soil earthquake records. The study uses a structural system consisting of a linear-elastic diaphragm supported by non-linear frames oriented along two orthogonal directions. The diaphragm is modelled with plane-stress finite elements and frames with stiffness-degrading flexural elements. Results indicate that an increase of in-plane diaphragm flexibility leads to a reduction of frame displacements for systems with initial lateral period of vibration T > 0.4 s. For systems with T ≤ 0.4 s, in-plane Boor flexibility can lead to significant frame displacement increments (50 per cent higher). Results show that these variations on displacements decrease for increasing values of both the seismic-force reduction factor and the system initial lateral period.