BEAM GLOBAL-MODEL FOR THE SEISMIC ANALYSIS OF RC FRAMES

A new hysteretic beam model based on a moment-curvature uniaxial cycli c law is proposed. The model is characterized by a trilinear envelope curve and a set of cyclic rules. The strength and stiffness degradatio n in the hardening and the softening branches as well as the hystereti c damping are directly dependent on the level and history of loading. Their continuous evolution is described with the aid of a new paramete r based on energy criteria and phenomenological considerations. The mo del is capable of describing the dynamic response of RC frames with a minimum of input parameters. The characteristic points of the envelope curve are defined by three critical strain conditions at cross-sectio n level. A revision of ultimate compressive strain of concrete is used to start the softening behaviour of the envelope curve. The principal phenomena taken into account by the hysteretic rules are: stiffness d egradation and strength reduction due to cyclic loading, pinching of l oops due to shear stress, softening behaviour at failure and the effec t of axial force due to gravity loads. The evolution of cyclic behavio ur is controlled by a new cyclic parameter defined in terms of a damag e index, an accommodation factor and the number of cycles with amplitu de inferior to the maximum strain (curvature) ever experienced. This p arameter allows the description of monotonic and cyclic response for b oth small and large deformations, including the post-peak regime. In o rder to validate the proposed model, the global response of isolated m embers as well as a complete structure subjected to cyclic and dynamic loads are compared with experiments.