Seismic Strength Demand Of Vertically Irregular Structures Subjected To Pulse-like Ground Motions

Structural irregularity in elevation and velocity pulse effect of the earthquakeground motions are two disadvantageous conditions for structural design. This studyreports on the investigations into seismic strength demand of vertically irregularstructures under strong earthquake actions，accounting for the coupling effect of thesetwo disadvantageous factors. Based on shear-type multi-degree-of-freedom (MDOF)systems and non-linear time history analysis, this present paper studies the seismicdemand of such irregular structures subjected to near-fault pulse-like ground motions,provides a modification method for ductility reduction factor of these structures, andfinally recommends the suggested values of the modification factors. The mainresearch contents and conclusions are as following:1. Investigation on the seismic demand of structures with vertical irregularitieswhen subjected to velocity pulse-like ground motions. Specifically, the irregularitiesare in strength, stiffness, and combined strength-and-stiffness in the first storey ofstructures. A nonlinear dynamic time history analysis is performed based on eightnear-fault pulse-like ground motions. Both the structural seismic demand anddistribution mode in terms of displacement ductility, drift and energy, have beeninvestigated to account for these two disadvantageous conditions. The ductilitydemand is found to be higher when considering vertical irregularities and velocitypulse effects. Furthermore, strength irregularities have more significant effects onductility demand than those of combined strength-and-stiffness irregularities, whilethe effects of stiffness irregularities are different. In addition, the displacementdemands and dissipation energy at the first storey increases for these irregularstructures, while those at other stories decreases. These increments of displacementand energy can cause the concentration of damages which will lead to poor seismicbehavior for structures.2. Modification method for the ductility reduction factors of vertically irregularstructures. The global seismic strength demand is related to the ductility level inbuilding structures, and the carrying loads and ductility requirements of verticalirregular structures can be satisfied by improving the global seismic strength andlimiting the ultimate ductility demand within the extent of codes. The modificationshave been introduced by a global strength improving factor to make such irregularshear-type MDOF structures maintain the same ductility level as reference regularones. 3. Suggested modifications for the ductility reduction factors of such verticallyirregular structures. Specifically, the structural vertical irregularities are introducedby changing the combined-strength-and-stiffness at the first storey of a referenceregular system, which is closer to practical design and considered as most commonyet severe cases. The five main parameters of irregularity ratio, ductility ratio,fundamental period, storey number of a system and velocity pulse of a ground motion,have been investigated to gain insight into the modification factors. It is demonstratedthat ductility reduction factors for irregular MDOF systems are clearly smaller thanthose for regular MDOF systems. The modification factor is mainly affected byirregularity ratio, and ductility ratio, decreasing with decreasing irregularity ratio andincreasing ductility ratio. However, the storey number and fundamental period areobserved to have little influence on this modification. Finally, based on the statisticalresults of pulse-like cases comparing with non-pulse-like cases, reference values ofthe modifications are suggested, which can somehow accounts for the effects ofvertical irregularity and velocity pulse on ductility reduction factor in engineeringapplication.