Optimized retrofit of seismically designed structures to withstand progressive collapse

Seismically designed buildings possess the necessary properties to withstand the design earthquake excitations and limit structural damage to the desired level. This is achieved by appropriately defining individual member capacities, interstorey and global stiffnesses and energy dissipation mechanisms based on the provisions of current design codes. However, such provisions do not suffice, in order to protect a structure from disproportional extent of damage when a local failure occurs. Hence, in order to increase their progressive collapse resistance, existing seismically designed buildings need to be appropriately retrofitted. This paper presents an optimization procedure for the retrofit of seismically designed buildings, in order to increase their capacity to sustain local damage without extensive propagation leading to severe structural failure (partial/full collapse). Two retrofit methods for seismically designed steel-concrete composite buildings are presented and assessed with respect to their cost-effectiveness. The retrofit methods are based on: (a) the strengthening of beams in the structure or (b) the installation of bracings in the horizontal direction. Structural optimization aims to minimize the total retrofit cost, in order to determine the lowest possible need for retrofit material, enabling at the same time the comparative assessment of the two retrofit methods on a 'fair basis'.