TORSIONAL MECHANISMS IN DUCTILE BUILDING SYSTEMS

It is postulated that in order to estimate torsional effects on the se ismic response of ductile building structures, the associated plastic mechanism to be developed in the three-dimensional system should be id entified. The proposed approach is very different from that embodied i n building codes. Inelastic structures are classified as either torsio nally unrestrained or restrained. It is shown that clearly defined mec hanisms that are to be mobilized, enable the acceptable system ductili ty demand to be estimated. This should ensure that the corresponding d emands imposed on critical translatory elements of the system do not e xceed their established displacement ductility capacity. To this end f amiliar quantities, such as element yield displacement and stiffness, are redefined. Comparisons are made of the intents of existing codifie d design approaches and those emphasising the role of imposed inelasti c displacements. A simple treatment of the consequences of earthquake- induced inelastic skew displacements is also addressed. The primary ai m of the paper is to offer very simple concepts, based on easily ident ifiable plastic mechanisms, to be utilized in structural design rather than advancement in analyses, Detailed design applications of these c oncepts are described elsewhere. The approach is an extension of the d eterministic philosophy of capacity design, now used in some countries . (C) 1998 John Wiley & Sons, Ltd.