Abstract
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.
Highlights
Cold-formed steel (CFS) sections are produced by rolling or brake-pressing relatively thin metal sheets into cross-sectional shapes at ambient temperature
The large width-to-thickness ratios of CFS members leave them typically outside the limits prescribed by seismic design codes (e.g. AISC 341-16 2016, EN1998-1 2005) for high seismic regions
The optimisation procedure aimed to optimise each CFS cross-section prototype with respect to its energy dissipation E X over the load history up to a drift ratio of 4%. This is the rotation capacity required for Special Moment Frames (SMF) according to AISC Seismic Provisions (2016) and allows a consistent comparison
Summary
Cold-formed steel (CFS) sections are produced by rolling or brake-pressing relatively thin metal sheets into cross-sectional shapes at ambient temperature. Other research on the development of members for CFS moment-resisting frames has shown that the ductility and energy dissipation of the sections can be significantly improved by curving the flanges into a semi-circular shape (Sabbagh et al 2012). In other relevant research Pan et al (2007) developed an optimisation method to obtain hotrolled H-beams with optimal flange shapes which maximize the energy dissipation capacity of the members under monotonic and cyclic loads. To achieve this, they combined a Simulated Annealing optimisation algorithm with detailed nonlinear finite element analyses. It is seen that a mesh refinement to at least 20x20 mm elements is required in order to obtain a good correspondence with the experiment in terms of peak load and overall behaviour
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