Plastic Design of Multi-Storey Sway Frames

Abstract

Ultimate load methods have become increasingly popular in recent years, and a now widely used in preference to the traditional allowable stress approach. In particular, the simple plastic theory has been applied to the design of a large number of steel structures. Its use has, however, been restricted to frameworks with comparatively few storeys, since, in tall buildings, the presence of high axial loads in the columns invalidates one of the basic assumptions of the theory. In addition, simple plastic design is prohibited for tall frameworks which are required to resist wind loading without the aid of bracing, since the large sidesway deflections which are produced again violate the basic assumption that changes of geometry are small and may therefore be neglected. This thesis describes the development of a rational form of ultimate load design for multi-storey sway frames. It is shown that the simple plastic equations only require slight modification in order to take account of the instability effects. Furthermore, iterative use of these modified equations produces a design which is economical, whilst satisfying an exacting set of design criteria. A variety of design aids have been developed in order to reduce the design time and the quantity of areEneticel work involved, with the result that frameworks of any size may be designed without the aid of an electronic computer. To assess the validity of the proposed design equations, a range of frameworks designed by the method has been analysed using an accurate computer program, which traces the complete load-deformation behaviour up to collapse. The economy of the design method has been examined by comparison with several frameworks designed by other methods. Due to approximations in the design approach, it is not possible to produce the minimum-weight. solution, but a safe, extremely economical framework is obtained.