Stub-Girder Composite Structural System
by D. Matthew Stuart
Focusing on structural engineering issues involved in the repair, restoration, or adaptive reuse of older buildings for which drawings no longer exist, this article is the sixth in a series about antiquated structural systems that can be adapted or reanalyzed for safe reuse. — Editor
Most of the antiquated systems discussed so far have been out of popular use for a considerable number of years, with some dating back to the first part of the 1900s. However, the subject of this article deals with a system that was still in use less than 25 years ago.
A stub-girder is a composite system constructed with a continuous structural steel beam and a reinforced concrete slab separated by a series of short, typically wide flange sections, called stubs. The stubs are welded to the top of the continuous beam and attached to the concrete slab by shear connectors.
The spaces between the ends of the stubs are used for the installation of mechanical ducts and other utility systems and for placement of the transverse floor beams that span between the stub-girders. Ideally, the depths of the stubs and the floor beams are identical to allow for the transverse framing to support the concrete slab deck, which spans parallel to the stub-girder, and to facilitate composite action between the floor beam and slab.
Stub-girder construction was first used in 1971 at the 34-story One Allen Center office building in Houston, Texas. The system was developed by Joseph P. Colaco of Ellisor Engineers, Inc., to facilitate the integration of mechanical ducts into the steel floor framing of repetitive, multistory high-rise construction.
This system went on to be used in a large number of high-rise buildings in North America up through the 1980s. The system eventually went out of use because of the increased labor cost associated with both fabrication and the need for shoring until the field-cast concrete slab attained sufficient strength.
Advantages of the stub-girder system that led to its use during the time period in which it was popular included:
- Reduction in steel tonnage by as much as 25% over conventional composite floor framing, due to improved structural efficiency as a result of the greater depth of the stub-girder compared to a conventional system, and to improved structural efficiency due to the ability of the transverse floor framing members to act as continuous beams through the openings between the stubs.
- Reduction of the overall depth of the structural floor framing system by as much as 6 to 10 inches over a conventionally framed composite floor system, which allowed for a reduced floor-to-floor height and overall height of the building and associated cladding.
Prior to the use of the stub-girder system in construction, a load test was performed at Granco Steel Products Company in St. Louis. The test specimen included a W14x48 continuous bottom beam, W16x26 stubs and floor beams, and a 5-foot-wide, 3.25-inch-deep, lightweight concrete slab over a 2-inch metal deck flange, which was attached to the stubs via shear connectors.
Discuss this article in the Architecture Forum...
This article is reprinted from the November 2008 issue of STRUCTURE magazine, with permission of the publisher, the National Council of Structural Engineers Associations (NCSEA).