COMPOSITE GIRDERS WITH PARTIAL RESTRAINTS - A NEW APPROACH

Most designs for buildings with steel frames are based on girders with simple connections. To eliminate the problems associated with traditi onal construction (such as deep and heavy girders and large deflection s during construction) structural engineers have been searching for a new design system for a long time. The stub girder system is one examp le of such efforts. However, the stub girder system proved to be uneco nomical for most common buildings. A new and different approach to com posite steel/concrete designs was undertaken by the writer resulting i n light building frames and cost savings. This new design system is ca lled partial Restraint Girder System (''RGS'') (Figure 1). (A composit e section is obtained in buildings with metal deck and concrete floors by welding steel studs to the top flange.) With RGS two types of rest raint are possible: the first makes use of moment connections to colum ns; the second includes concrete reinforcement. In buildings utilizing composite girders, deflections were controlled by either shoring, cam ber, or further increase in girder size. RGS has arisen as a viable an d cost effective alternative. In traditional designs, the engineer det ermined the building's moment diagram from a moment distribution or st ress analysis. In the RGS method, the Structural Engineer can control the maximum and minimum values of moment on the moment diagram (the go verning design values) from the outset to fit his design, by establish ing the amount of restraint. Although composite girders with partial r estraints improve the moment resistance of composite girders significa ntly, such design is commonly ignored and the codes of practice give n o guidance as to procedures that might take advantage of the improved properties.