Optimum Lateral Load Distribution for Seismic Design of Nonlinear Shear-Buildings Considering Soil-Structure Interaction

The lateral load distributions specified by seismic design provisions are primarily based on elastic behaviour of fixed-base structures without considering the effects of soil-structure-interaction (SSI). Consequently, such load patterns may not be suitable for seismic design of non-linear flexible-base structures. In this paper, a practical optimization technique is introduced to obtain optimum seismic design loads for non-linear shear-buildings on soft soils based on the concept of uniform damage distribution. SSI effects are taken into account by using the cone model. Over 30,000 optimum load patterns are obtained for 21 earthquake excitations recorded on soft soils to investigate the effects of fundamental period of the structure, number of stories, ductility demand, earthquake excitation, damping ratio, damping model, structural post yield behaviour, soil flexibility and structural aspect ratio on the optimum load patterns. The results indicate that the proposed optimum load patterns can significantly improve the seismic performance of flexible-base buildings on soft soils

Seismic performance evaluation of deficient steel moment-resisting frames retrofitted by vertical link elements

In many earthquake prone regions in developing countries, substandard steel moment resisting frame (SMRF) systems pose a profound danger to people and economy in the case of a strong seismic event. Eccentric bracing systems with replaceable vertical links can be utilized as an efficient and practical seismic retrofitting technique to reduce future earthquake damages to such structures. This paper aims, for the first time, to demonstrate the efficiency of eccentric bracing systems with vertical links as a seismic retrofitting technique for the SMRF structures with WCSB and to develop fragility curves for such structures. To achieve this aim, first, the effect of the vertical links on the behaviour of 3, 5 and 7-storey frames are studied through conducting the Nonlinear Static Analyses (NSA) as well as Nonlinear Time History Analyses (NTHA) using the artificial accelerograms compatible with the target design spectrum. The analysis results indicate that, as aimed in the design stage, the seismic damage is only concentrated at the replaceable vertical links and remaining structural members work mainly in the elastic range. In addition, the proposed retrofitting technique considerably improves the performance of the deficient SMRF systems by effectively restricting the displacement response and damage distribution in such structures. Following the NTHA, Incremental Dynamic Analyses (IDA) are performed to develop the seismic fragility curves for the retrofitted SMRF systems. The results indicate that the proposed retrofitting technique significantly reduces the fragility of such systems, and therefore, can provide a simple and efficient method to improve the seismic performance of deficient steel moment resisting frames in seismic regions

Seismic reliability analysis and estimation of multilevel response modification factor for steel diagrid structural systems

Diagrid systems are emerging as one of the structurally efficient and architecturally aesthetic solutions for tall buildings. Despite the fact that such systems are increasingly used in modern construction, current literature lacks detailed information regarding their structural behaviour and seismic design parameters to ensure satisfactory performance under different earthquake intensity levels. This study aims to assess the seismic reliability of diagrid structural systems and develop more efficient performance-based design methodologies. Demand and supply response modification factors are calculated for 16, 24 and 32-storey buildings with diagrid structural systems using 65° diagrid angle and designed in compliance with current standards under a set of 12 spectrum compatible earthquakes. The results are then used to develop a novel multi-level response modification factor (R-Factor) for diagrid structural systems as a function of site seismicity and acceptable damage level. Subsequently, comprehensive seismic reliability analyses are conducted to assess the seismic performance of the selected structures under intensity levels corresponding to DBE and MCE hazard levels (earthquake scenarios with return periods of 475 and 2475 years, respectively). In general, results of this study demonstrate acceptable seismic performance and reliability of steel diagrid systems. It is shown that even using an R-Factor equal to 4 in the seismic design process could ensure that diagrid structures remain in a performance level higher than Life Safety (LS) for both DBE and MCE hazard levels. Multi-level response modification factors proposed in this study can be directly used in performance-based design of diagrid structures to satisfy different performance targets under any seismic hazard level

Seismic performance of dual systems coupling moment-resisting and buckling-restrained braced frames

Buckling-restrained braces (BRBs) have proven to be very effective in improving the seismic performance of existing and new structures. They provide strength, stiffness and added energy dissipation to the structure. However, being BRBs characterized by a low post-elastic stiffness, their use may lead to residual deformations hindering the building’s reparability and to excessive cumulative ductility demand possibly compromising the residual capacity of BRBs. To overcome these drawbacks, BRB frames (BRBFs) can be coupled with moment-resisting frames (MRFs) to form dual systems. If properly designed, MRFs acting as back-up frames, allow the control of the residual drifts and the optimization of the performance of the BRBs. The contribution of this study is to provide insights into the performance and residual capacity of BRBFs-MRFs dual systems and to shed light on the influence of the main BRB’s design parameters. To this end, a non-dimensional formulation of the equation of motion is introduced for a single-degree-of-freedom system, and an extensive parametric study is performed for a set of natural ground motion records with different characteristics and scaled to various intensity levels. This allows the investigation of a wide range of configurations, considering different levels of the relative strength and ductility demand of BRBFs and MRFs, and to obtain useful information for their design. Finally, two case study frames, modeled as two-dimensional nonlinear multi-degrees-of-freedom systems, are analyzed and the results compared to those obtained from the non-dimensional formulation to show the capabilities and the limitations of the adopted methodology and of the SDOF approximation