Optimal insertion of viscous dampers into torsionally coupled structures

Structures characterized by non coincident center of mass and center of stiffness, also referred to as eccentric structures, develop a coupled lateral-torsional response when subjected to seismic excitation. This coupled behavior may increase significantly the local peak dynamic response of such a structure, as compared to that of an equivalent system with coincident center of mass and center of stiffness (equivalent non-eccentric system). In recent years various innovative earthquake-engineering techniques have been implemented in actual building constructions. Among passive protection systems, added viscous dampers have proven so far to be easily applicable and highly effective. In this paper, the effects of inserting a system of added dampers into eccentric structures are investigated with reference to linear elastic one storey stiffness eccentric structures subjected to seismic excitation. The goal of the research study here described is the identification of the “optimal” system of added viscous dampers capable of minimizing the seismic induced torsional effects. In detail, the paper introduces an optimization procedure based upon genetic algorithms and the use of selected response indexes based upon the mean square response to stochastic inputs of the eccentric systems. The stochastic response is here evaluated with reference to a white noise input. Viscous dampers are modeled using a linear constitutive law. The effectiveness of the proposed optimization procedure is here measured with reference to the response of eccentric systems as subjected to historically recorded ground motions.