Structural Upgrade of Steel Stub Girders

In the early 1970’s J.P. Colaco Ph.D. developed the steel stub girder system. This innovative structural system aimed at providing construction savings through the integration of mechanical, electrical and plumbing service ducts into the part of the building volume that is only occupied by the floor framing system. It was noted that overall depth of floor systems could be large, leading to significant increases in the overall height of buildings, the steel tonnage for the project and the associated cost of façade systems with greater areas. The stub girder system made extensive use of fairly simple shop fabrication techniques and the composite action of the concrete slab and steel load carrying capacity members. This composite floor system had significantly more strength, stiffness and ductility, while the amount of structural steel was reduced. The stub girder system was widely used for a variety of steel-framed buildings in North America (Canada, U.S.A. and Mexico) ranging from 2–70 stories for about two decades. However, due to the relative complexity of the analysis required, labor intensive construction nature and transformations suffered in the international steel market, the use of the system has become nonexistent. Today, structural engineers often get involved in projects where there is a need to upgrade the structural capacity of floors of existing buildings that contain this system, and there is very limited information and knowledge about it. The purpose of this paper is to describe the system, the approach for its analysis, and the techniques to strengthen the system. The paper will also provide some practical recommendations through the analysis of a case study.