Application of reliability-based robustness assessment of steel moment resisting frame structures under post-mainshock cascading events

This paper proposes a reliability-based framework for quantifying structural robustness considering the occurrence of a major earthquake (mainshock) and subsequent cascading hazard events, such as aftershocks that are triggered by the mainshock. These events can significantly increase the probability of failure of buildings, especially for structures that are damaged during the mainshock. The application of the proposed framework is exemplified through three numerical case studies. The case studies correspond to three SAC steel moment frame buildings of three, nine, and 20 stories, which were designed to pre-Northridge codes and standards. Two-dimensional nonlinear finite-element models of the buildings are developed with the Open System for Earthquake Engineering Simulation framework (OpenSees), using a finite length plastic hinge beam model and a bilinear constitutive law with deterioration, and are subjected to multiple mainshock-aftershock seismic sequences. For the three buildings analyzed herein, it is shown that the structural reliability under a single seismic event can be significantly different from that under a sequence of seismic events. The reliability based robustness indicator shows that the structural robustness is influenced by the extent to which a structure can distribute damage

Earthquake-induced shear concentration in shear walls above transfer structures

Due to various architectural constraints and multi-functional requirements for modern buildings, combined structural forms, which typically include shear wall systems in higher zones and moment-resisting frames together with core walls in lower zones, are commonly used for these buildings. Transfer structures are often introduced to transfer the loads from higher to lower zones. Previous experimental and numerical studies have demonstrated that the exterior walls above the transfer structure are particularly vulnerable structural members under seismic loading. In this paper, a qualitative model is presented for simulating the shear concentration effect in exterior walls with consideration of the local deformations of transfer structures. A parametric study was carried out to validate the model and to quantify various factors which may influence the shear concentration effect. A shear concentration factor, which can measure the intensity of shear stress concentration in the exterior walls, is defined. Based on the numerical study, design principles are recommended to seismic engineers for minimizing the adverse shear concentration effect on exterior walls under seismic loads. Copyright © 2008 John Wiley & Sons, Ltd.postprin

Seismic response assessment of architectural non-structural LWS drywall components through experimental tests

A research project was conducted at University of Naples “Federico II” over the last few years with the aim to give a contribute to overcome the lack of information on seismic behaviour of architectural non-structural lightweight steel (LWS) drywall components, i.e. indoor partition walls, outdoor façades and suspended continuous ceilings. The tested non-structural components were made of LWS frames sheathed with gypsum-based or cement-based boards. The research activity was organized in three levels: ancilliary tests, component tests and assembly tests. Ancilliary tests were carried out for evaluating the local behaviour of partitions, façades and ceilings. Component tests involved out-of-plane quasi-static monotonic and dynamic identification tests and in-plane quasi-static reversed cyclic tests on partitions. Finally, the dynamic behaviour was investigated through shake table tests on different assemblages of partitions, façades and ceilings. The study demonstrated that the tested architectural non-structural LWS drywall components are able to exhibit a very good seismic behaviour with respect to the damage limit states according to the IDR limits given by Eurocode 8 Part 1. The current paper describes the complete experimental activity within the project

Formulation of risk-targeted seismic action for the force-based seismic design of structures

A risk targeted design spectral acceleration and the corresponding seismic design action for the force based design of structures is introduced by means of two formulations. The first one called direct formulation utilizes the seismic hazard function at the site of the structure. Because the seismic action defined in the codes is often associated with a designated return period, an indirect formulation is also introduced. It incorporates a risk targeted safety factor that can be used to define a risk targeted reduction factor. It is shown that the proposed formulations give analogical results and provide an insight into the concept of the reduction of seismic forces for the force based seismic design of structures if the objective is defined by a target collapse risk. The introduced closed form solution for the risk targeted reduction factor can be used to investigate how the target collapse risk, the seismic hazard parameters, the randomness of the seismic action, and the conventional parameters (ie, the overstrength factor and the deformation and energy dissipation capacity) affect the seismic design forces in the case of force based design. However, collaborative research is needed in order to develop appropriate models of these parameters. In the second part of the paper, the proposed formulations are demonstrated by estimating the risk targeted seismic design action for a six storey reinforced concrete building. By verifying the collapse risk of the designed structure, it is demonstrated that the risk targeted seismic action, in conjunction with a conventional force based design, provided structure with acceptable performance when measured in terms of collapse risk