Design of Braced Multi-Story Frames by the Plastic Method

Since 1956 an extensive research program has been carried out at Lehigh University to study the inelastic behavior and strength of multi-story frames subjected to various types of applied loads and to develop design methods utilizing the reserved strength in the inelastic range. In this program, because of the difference in load-carrying behavior, multistory frames were divided into two types: braced and unbraced frames, and a separate investigation was made for each type. The results obtained from this research together with the recommended design procedures were presented in a comprehensive set of lecture notes and a design aids booklet, which were prepared for a special Summer Conference on "Plastic Design of Multi-Story Frames" held in August, 1965, at Lehigh University. This paper summarizes the procedure described in the lecture notes for the design of braced frames. A braced frame is defined as a frame in which all resistance to lateral force, sway and frame instability is provided by a specially designed bracing system. The required member sizes for the beams and columns in such a frame are often governed by gravity load. It is therefore possible in the initial stage of design to treat the frame and the bracing as two separate load-carrying systems and to perform independent designs for them. The interaction between the frame and the bracing system is then checked after the approximate member sizes are selected.