Seismic upgrading of vertically irregular existing r.c. frames by BRBs

In several earthquake-prone countries a large part of existing r.c. buildings had been designed before seismic codes entered into force and, therefore, considering gravity loads only. The lateral strength and stiffness of these structures are inadequate to sustain seismic action and their seismic response is aggravated by deficiencies due to vertical irregularity. In particular, the ratio of the demanded storey shear force to the lateral strength presents an irregular distribution along the height of the building. This promotes the development of a storey collapse mechanism. In this chapter, the insertion of Buckling Restrained Braces (BRBs) is proposed both for the seismic upgrading of r.c. frames designed for gravity loads and for the reduction of their vertical irregularity. The effectiveness of a design method previously proposed by the authors is investigated. The method is ruled by two parameters: the behaviour factor and the design storey drift. The design method is applied for the seismic retrofitting of a 6-storey r.c. frame considering the values of the ruling parameters in several combinations. Then, nonlinear dynamic analyses are performed to assess their influence on the seismic response of the upgraded frames.