Identification of the hysteretic behaviour of a partial-strength steel–concrete moment-resisting frame structure subject to pseudodynamic tests

In low-rise steel-concrete composite structures, moment-resisting frames can be designed to develop a ductile response in beam-to-column joints and column bases by activating flexural yielding of beams and end plates, shear yielding of column web panel zones and yielding of anchors. To evaluate the performance of these components under differing earthquake intensities, a series of pseudodynamic, quasistatic cyclic and vibration tests were carried out on a two-storey two-bay moment resisting structure. The performance-based seismic design and control of these structures requires that stiffness degradation, strength deterioration and slip are properly modelled. In this context, compact hysteretic models can play a key role and must therefore be striven for. Nonetheless, relevant techniques, like nonlinear system identification, are far from representing standard and reliable tools for the dynamic characterization of full-scale structural systems. With this objective in mind, we present a restoring force surface-based technique applied to pseudodynamic test data, in view of the nonlinear identification of multistorey frames. The technique is developed by means of a parametric approach, where a time-variant stiffness operator is coupled to a modified Bouc–Wen model that allows both for slip and for degradation in stiffness. Strength deterioration is indirectly taken into account too. We also show how model-based parameters can be correlated to the damage process progressively observed both in the structure and in its components. Finally, the predictive capabilities of the identified model are highlighted.