A Method for the Plastic Design of Unbraced Multistory Frames

In a building without vertical bracing or shear walls, the frames must be able to resist gravity loads and the combination of vertical and horizontal loads, plus second order effects due to vertical load-lateral displacement interaction. Besides, frame stiffness must be sufficient to keep lateral displacements under working loads below maximum allowable values. Two different load factors are used in plastic design— one for vertical loads only and a smaller one for the combination of vertical permanent and horizontal accidental loads. Design of two or three stories at the top of unbraced buildings is generally governed by gravity loads, because the beams and columns necessary to support vertical loads are also able to resist gravity plus horizontal loads under a reduced load factor. The importance of horizontal forces increases in lower stories, and their design is governed by the combination of both types of loads. Design of upper stories is usually made with no consideration of lateral displacements. A revision is carried out later in order to verify that the overall critical load is not smaller than the collapse mechanism load. If necessary, the structure is modified or the critical load is taken as the limit of structural usefulness.