ASSESSMENT OF THE RESPONSE ENVELOPE OF REINFORCED CONCRETE MEMBERS WITH DIFFERENT FAILURE MODES

The expected lateral response of reinforced concrete (RC) elements strongly depends on their failure mode. Code-conforming RC members are supposed to exhibit a flexural (F) ductile failure, with complete exploitation of their post-peak displacement/deformation capacity. However, substandard RC elements, which are typical of existing structures, may undergo a shear failure after yielding (FS) or even before yielding (S) of longitudinal reinforcement. In this case, the element is classified as “shear - sensitive” and may be characterized by an abrupt collapse after the attainment of the peak load-capacity, with a fast loss of lateral load-bearing capacity up to the onset of the progressive reduction of the axial load-bearing capacity. This condition may trigger structural collapse, as also shown from surveys of damages due to strong earth-quakes in past and recent times. So, the determination of the failure mode of RC members is paramount for a reliable assessment of the seismic safety of structures. In general, static criteria are applied to pre-classify the expected failure mode of RC members and their response backbone is defined dependently on the determined failure mode. However, a comprehensive approach for modelling, with a unique set of predictive equations, the response envelope of RC members without pre-determining with a deterministic approach their failure mode is missing. In this paper membership functions are defined to determine for RC columns the possibility that they exhibit an F, an FS, or an S failure. A set of basic predictive equations are defined, based on experimental data, to define a multilinear response envelope. The proposed equations may be combined with membership functions to use a unique set of equation able to reproduce the behaviour of RC columns with different expected failure modes.