Inelastic earthquake response and design of multistorey torsionally unbalanced structures
Structures exhibit coupled torsional and translational responses to earthquake ground motion input if their centres of floor mass and their centres of resistance do not coincide. However, torsional motions may occur even in nominally symmetric structures due to accidental eccentricity and torsional ground motions. The sources giving rise to accidental eccentricity include the difference between the assumed and actual distributions of mass and stiffness, asymmetric yielding strength, non-linear patterns of force-deformation relationships, and differences in coupling of the structural foundation with the supporting soil. Symmetric and regular buildings that are properly designed have a much higher ability to survive a strong earthquake event than asymmetric buildings and their response to earthquake loading is far more straightforward to predict and design for. On the other hand, even though the response of asymmetric buildings is more unpredictable, designers still have to compromise structural regularity to accommodate functional and aesthetic needs. As a result, serious and widespread damage associated with structural asymmetry has been observed repeatedly in past major earthquakes. In the first studies examining torsional effects in buildings, attention was focused on the elastic structural behaviour of single-storey buildings and the main purpose was to achieve a complete understanding of the effects of mass and stiffness eccentricities and to evaluate them by simple static models. However, as the response of real structures is mainly inelastic, these studies gave poor information on torsional behaviour and interest has moved towards non-linear response studies. In an effort to clarify some of the issues influencing the inelastic torsional response of multistorey asymmetric structures, this thesis presents a series of coherent parametric investigations. These investigations include comparing the response of various reference models to the performance of code-designed torsionally unbalanced structures. An extensive parametric investigation of torsionally responding structures designed as stipulated by a selection of major earthquake building codes is presented and the adequacy of the static torsional provisions is assessed for a wide range of structural configurations and parameters. Detailed investigations of torsionally asymmetric structures incorporating frame elements oriented along both orthogonal axes of the structure are also conducted and the effect of including the second earthquake component to simultaneously excite the structural models is quantified. The relative merits and deficiencies of each code provision are discussed and a new proposed optimised method is tested. All fundamental conclusions from the investigations conducted are presented and various topics for further research are proposed, which are considered to be both necessary and pertinent for increasing and refining the knowledge and understanding the complex behaviour of multistorey torsionally asymmetric buildings.