Seismic lateral deformations demand in conceptual design of reinforced concrete framed structures

Abstract

In conceptual design of reinforced concrete (RC) structures, the dimensioning under gravity loads is frequently conducted through simplified assessments of loading conditions and static schemes representative of the behaviour of 2D frames. The design of the structures that undergo seismic loads becomes an iterative process, which is often carried out with a subjective approach that is learned over time and with experience, extremely important for realizing sound structures. Indeed, in seismic zone the structures should guarantee adequate resistance and dissipative capacity for high-hazard events and they should also limit the damage to non-structural elements (e.g., infill walls) for earthquakes characterized by greater probability of occurrence. This last goal is generally achieved by limiting the maximum interstory drift of the structure that should exhibit elastic behaviour, in case of not severe earthquake. The limit for maximum interstory drift depends on the ductility of the infill. This limit often governs the dimensioning of framed buildings in seismic zone, together with the limitation of the axial forces in the columns increasing their ductility. The paper shows a simplified procedure for the preliminary assessment of a target value of the fundamental period of vibration of the structure. The target depends on the maximum drift allowed for the infill and the displacement demand expected for the specific site, in the well-recognized design hypothesis of bare frame. The procedure allows simplified and immediate application of experimental fragility curves provided in scientific literature for estimating damage in typical non-structural infill of RC frame buildings.