Less is more: the reduction of beam section for the seismic behaviour improvement of existing steel MRFs

This work focalizes the attention on a problem that often arises when there is the necessity of strengthening a steel moment resisting frame in seismic zone. The idea and the developed example of this work is based on the attainment of improvement of seismic performance by simply trimming the flanges of the beam ends. This strategy is to be applied by considering both the results of the theory of plastic mechanism control and the rules assuring the yielding of reduced beam sections (RBS) when seismic loads are applied to the structure. The results of such strategy is not always effective. In fact, there are several condition that are to be satisfied in order to obtain an actual seismic improvement. Notwithstanding, when these conditions are satisfied, the cost of intervention can be considered as negligible. For this reason this strategy can be very interesting and the rules applied in this work can clarify which is the effect of RBS taking into account all the parameters playing a role in the final design:, i.e. existing column sections, resistance and ductility of existing connections, vertical loads acting in seismic load combination, amount of the reduction of beam section and its distance from the connection.