ABSTRACT
The traditional design of MRFs, based on the use of full-strength beam-to-column joints, requires only the prediction of the monotonic response of connections (CEN, 2005b; CEN, 2005a). In particular, to characterize joints behavior under Serviceability Limit State and Ultimate Limit State, only the prediction of the initial stiffness and of the plastic resistance is needed. Conversely, as the energy dissipation supply of semi-continuous MRFs relies on the ability of connections to sustain a number of excursions in plastic range without losing their capacity to carry vertical loads, it is clear that, in order to successfully apply partial-strength joints, it is necessary to properly characterize and predict the response
1 INTRODUCTION
During the ’90s three strategies have been developed to overcome the problems provided by the brittle behavior of welded connections (SAC, 2000c; SAC, 2000b) which, after Northridge and Kobe earthquakes, have been demonstrated to be unreliable. The first one, namely the strengthening approach, provides the enhancement of the welding techniques, imposing the adoption of properly certified electrodes and specifying, through seismic codes, the welding techniques to be used in dissipative MRFs (SAC, 2000a). The second one, namely the weakening approach, is based on the idea of promoting the plastic engagement at beam ends by cutting a portion of the beam flanges (Reduced Beam Section) in a zone close to the span end, but sufficiently far from joints (Moore et al., 1999). In this way, stress concentrations in welds are avoided and dissipation is provided by cyclic flexural behavior of H-shaped sections. The third approach is constituted by the use of partial strength connections, so that dissipative zones are shifted from the beam ends to the connecting elements, which are designed to be weaker than girders. According to this design philosophy, the dissipation capacity is relied on the ability of end-plates and/or panel zones (Faella et al., 2000; Iannone et al., 2011) to withstand plastic deformations. Therefore, welds are designed to be adequately over-resistant compared to the weakest joint component.
