Drift protocols for seismic testing of RC building columns under biaxial bending

Experimental tests have highlighted how sensitive the seismic performance of reinforced concrete columns is to the load path, especially under biaxial bending loading conditions. In particular, it has been shown that the response of a column is significantly affected not only by the number and amplitude of the imposed drift cycles, but also by the level of coupling of the demand in the two horizontal directions, which means by the in-plan shape of the deformation path. Despite several proposals for the latter have been advanced, a well-established protocol for testing columns in biaxial bending is still lacking. In this work numerical simulations are used to generate drift paths representative of the actual seismic demand experienced by columns in buildings. The objective is to understand whether typical orbits of deformation can be identified, and to determine the factors influencing the shape of the orbits among the properties of the building and the seismicity. This is pursued with time history analyses of building models characterized by different periods of vibration, representing both new and existing structures, excited by sets of ground motion records. The obtained orbits are then used to evaluate protocols selected from the literature, by imposing the different drifts histories to a fiber column model and comparing the produced response in terms of dissipated energy.