Torsional Behaviour of Irregular Buildings with Single Eccentricity

Abstract

Torsional response in building structural systems arise from two sources: a) eccentricity between the centre of mass and centre of stiffness of vertical lateral force resisting elements and (b) accidental torsion due to the rotational component of ground motion about a vertical axis, the eccentricity between assumed and actual stiffness and mass, uncertainty in live load distribution, and uncertainty in dead loads due to variations in labour and materials. This paper addresses the effect of the ratio Ω between structure uncoupled torsional frequency (ωƟ) to uncoupled transition frequency (ωx) or (ωy), on the torsional behaviour of the structure. A parametric study is carried out using the equivalent static force (ELF) method applied to one story building with single eccentricity by examining different values of Ω. The study shows that structures with ratio Ω > 1.0 are torsionally stiff and displacements values are not sensitive to the increment in eccentricity ratio, in other words, the structure shows planer frame behaviour and the deformations are mainly transitional rather than torsional. Conversely structures with ratio Ω < 1.0 are torsionally flexible and displacements values are sensitive to increases in eccentricity ratio, therefore, the deformations are mainly torsionally twisting rather than transitional motion. It is also observed that when Ω = 1.0, no significant increase in torsional response occurs. Torsional irregularities coefficient as defined by ASCE 7-10 building code, i.e., the ratio between maximum story drift at one end of the structure to the average story drifts at the two ends of the structure, have been calculated for different values of the ratio Ω. Results shows that irregularity coefficient increases as the ratio Ω decreases. In addition, structures with Ω < 1.0 are more sensitive to the value of eccentricity between centre of mass and centre of rigidity. Subsequently irregularity coefficient increases dramatically, and excessive torsion occurs as the value of eccentricity increases for structures with ratio Ω < 1.0, while minor increment is observed on the value of irregularity coefficient when eccentricity increases. This paper discusses the significance of utilizing the frequency ratio Ω to control the torsional behaviour of building structures.