Development of optimum cold-formed steel sections for maximum energy dissipation in uniaxial bending

Cold-formed steel (CFS) elements are increasingly used as load-bearing members in construction, including in seismic regions. More conventional hot-rolled steel and concrete building structures are typically allowed by the design standards to exceed their elastic limits in severe earthquakes, rendering parameters indicating ductility and energy dissipation of primordial importance. However, insufficient research has yet been conducted on the energy dissipation of CFS structures. In the majority of previous optimization research on CFS sections the ultimate capacity, as typically controlled by local, distortional and/or global buckling modes, is considered to be the sole optimization criterion. This paper aims to improve the seismic performance of CFS elements by optimising their geometric and material highly non-linear post-buckling behaviour to achieve maximum energy dissipation. A novel shape optimisation framework is presented using the Particle Swarm Optimisation (PSO) algorithm, linked to GMNIA ABAQUS finite element analyses. The relative dimensions of the cross-section, the location and number of intermediate stiffeners and the inclination of the lip stiffeners are considered to be the main design variables. All plate slenderness limit values and limits on the relative dimensions of the cross-sectional components as defined by Eurocode 3, as well as a number of practical manufacturing and construction limitations, are taken into account as constraints in the optimisation problem. It is demonstrated that a substantial improvement in energy dissipation capacity and ductility can be achieved through the proposed optimization framework. Optimized cross-sectional shapes are presented which dissipate up to 60% more energy through plastic deformations than a comparable commercially available lipped channel

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