Upgrading of Exixsting Steel MR-frames by encasing the columns

The study concerns the behaviour of steel MR-frames subjected to vertical and lateral forces, not properly designed according to capacity design criteria, and upgraded by means of encasing the steel columns with concrete having different grade. The purpose of the paper is to examine if the proposed technique allows one to adequately increase the lateral stiffness and the resistance of the frames. Pushover analyses on multi-storey frames are performed trough a non-linear calculus program, able to take into account of the second order effects. The behaviour of frames with encased columns was compared with those of the bare frames, highlighting that with the proposed technique it is possible to locate the plastic hinges at the ends of the beams, out of the joint. Moreover, it is possible to ensure at the system enough lateral stiffness and strength for the requirements of the limit state damage and to achieve adequate global ductility for a good seismic behaviour.