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    Stub-Girder Composite Structural System

    continued

    The test specimen was loaded beyond the calculated design load, with the initial failure occurring at the exterior end of the outermost stub at one end of the stub-girder. The method of failure included web crippling and delamination of the web from the flange. Application of additional load resulted in crushing of the slab at the inside edge of the same stub. However, separation between the bottom of the slab and the top of the stubs did not occur, which indicated that composite behavior was maintained up to the point of localized crushing of the concrete slab. Web stiffeners added to the failed stub allowed the system to achieve a final failure load that was 2.2 times greater than the calculated design load.

    The methods of design used to determine the capacity of the section included a non-prismatic beam analysis, a Vierendeel girder/ truss analysis, and a finite element analysis. For the Vierendeel analysis, the stubs and transverse floor beams act as verticals and the concrete slab and continuous beam act as the chords.

    All three of these methods of analysis provided a close representation of the actual behavior of the stub-girder; however, the Vierendeel and finite element methods more closely identified the secondary moment effects on each side of the openings. The Vierendeel method of analysis also provided a more accurate representation of the actual steel stress, while the finite element method provided a more accurate representation of the stress in the concrete slab, including the high stresses that resulted in the crushing of the concrete at the inside edge of the first exterior stub as observed in the test specimen.

    Additional tests of stub-girders were performed in the late 1970s in Canada. The primary purpose of these tests was to determine the effects of changes in the spacing and depth of the stubs and to establish the failure modes of a stub-girder. The results confirmed that the behavior of a stub-girder was similar to a Vierendeel girder/ truss. Additional conclusions of the tests included:

    1. The stiffness of the girder increases as the length of the open panel between the stubs decreases.

    2. Shear distortions at the open panels (as a result of the Vierendeel action) were an important parameter in determining the elastic deflection of the stub-girder, but did not influence the rotation of the solid end sections of the overall girder.

    3. Tensile cracking of the concrete slab at the ends of the open panels occurred at relatively low loads, but did not have a significant impact on the elastic stiffness of the girder.

    4. Further extensive cracking of the concrete slab at the ends of the open panels occurred in the inelastic range of the girder. It was further determined that the ultimate strength and ductility of the girder could be improved through the use of internal reinforcement within the slab that was placed to resist the observed cracking.

    5. The precision of the Vierendeel method of analysis was dependent on the accuracy of the distribution of shear forces between the concrete slab and the continuous lower beam across the open panels and the assumptions made relative to the location of the points of contraflexure within the open panels.

    6. Failure of the shear connectors resulted as a combination of shearing and prying effects.

    7. To prevent premature failure due to web crippling at the stubs, stiffeners should be provided.

    8. Five different failure mechanisms were identified: buckling of the stub web, concrete failure in the vicinity of the shear connectors, diagonal tension failure of the concrete slab, shearing off of the headed stud connectors, and combined yielding of the steel beam and crushing of the concrete slab at the ends of the open panels due to the cumulative effects of the primary and secondary (Vierendeel) moments.

    Further research in Canada revealed additional insights into the behavior, design and economical construction of stub-girders. This research indicated that only partial end plate stiffeners, rather than traditional fitted stiffeners, were required to reinforce the stub webs. Furthermore, web stiffeners were not always required at the interior stubs. In addition, a continuous perimeter weld between the base of the stub and the top of the continuous beam was not required. The tests also confirmed that rolled wide flange shapes were more conducive to stub-girder construction than split T (WT) or rectangular hollow tube (HSS) sections.

    Additional conclusions of these later Canadian tests included:

    1. Deflection computations using the Vierendeel method of analysis were typically conservative for service loads and unconservative for ultimate loading conditions.

    2. The amount of internal slab reinforcement, particularly in the direction transverse to the stub-girder span, was established based on Canadian Standard Association (CSA) criteria available at the time of the tests.

    3. The conventional method of calculating the number of shear studs required and the application of standard methods of composite design to the analysis of stub-girders appeared to provide satisfactory results, however, caution was recommended when specifying closely spaced studs, particularly at the end stub.

    Additional recommendations and guidelines emerged throughout the 1980s for the stub-girder system. In fact, the American Institute of Steel Construction (AISC) had plans to develop a design guide for stub-girder construction; however, because deeper wide flange sections became more readily available and guidelines for the design of reinforced and unreinforced web openings became more established (see AISC Design Guide 2: Steel and Composite Beams with Web Openings, 1990), it was never published.

    In order to document some of the final design guidelines that were established for stub-girder construction, the following list of criteria is provided:

    1. Economical spans for stub-girders range from 30 to 50 feet, with the ideal span range being 35 to 45 feet.

    2. Transverse floor beams should be spaced at 8 to 12 feet on center.

    3. The stubs do not necessarily have to be placed symmetrically about the centerline of the stub-girder span.

    4. The use of three to five stubs per span is the most common arrangement.

    5. The stub located nearest the end of the stub-girder (and the surrounding, adjacent truss/ girder elements) is the most critical member, as it directly controls the behavior of the overall stub-girder. In addition, the end stub may be placed at the very end of the continuous bottom beam, directly adjacent to the support point.

    6. The performance of a stub-girder is not particularly sensitive to the length of the stubs as long as the length of the stub is maintained within the following limits:
      • Exterior stubs should be 5 to 7 feet in length.

      • Interior stubs should be 3 to 5 feet in length.

      However, increasing the length of the open panel between the stubs will reduce the stiffness of the stub-girder.

    7. Stub-girders must be constructed as shored composite construction in order to take full advantage of the concrete slab top chord. In addition, because of the additional dead load imposed by shoring from the upper floors in multistory construction, the need for shoring of the non-composite section becomes even more critical.

    8. Stub-girders should be fabricated or shored to provide a camber that is equal to the dead load deflection of the member.

    9. The overall strength of a stub-girder is not controlled by the compressive strength of the concrete slab, therefore the use of high-strength concrete mixes provides no advantage.

    10. It is typical for the ribs of the metal floor deck to run parallel to the span of the stub-girder. This orientation of the ribs therefore increases the area of the top chord slab and also makes it possible to arrange a continuous rib or trough directly above the stubs, which in turn improves the composite interaction of the slab with the stub-girder.

    11. Welds between the bottom of the stubs and the top of the continuous bottom beam should be concentrated at the ends of the stubs where the forces between these two elements are the greatest.

    12. Internal longitudinal slab reinforcement to add strength, ductility, and stiffness to the stub-girder should be provided in two layers, one just below and one just above the heads of the shear studs.

    13. Internal transverse slab reinforcement should be provided to add shear strength and ductility. Placing the transverse reinforcement in a herringbone pattern — i.e., diagonal to the direction of the stub-girder span — will also increase the effective width of the concrete flange/ top chord.

    14. The flexural stiffness of the top chord slab of a stub-girder should be based on the conventional effective width allowed by standard composite beam design criteria, except that the transformed section should include the contribution of both the metal deck and the internal longitudinal reinforcement.

    15. It is not proper to include the top flange of the stubs in the calculation of the moment of inertia of the top chord slab element.

    16. Modeling of the stubs as the verticals of the Vierendeel truss/ girder involves dividing the stubs up into vertical elements equal to one-foot lengths of the section spaced at one foot on center from one end of the stub to the other. The vertical stub elements should be modeled as fixed at the top and bottom, at the top chord (concrete slab) and bottom chord (continuous beam) of the truss/ girder, respectively.

    17. The transverse floor beams should be modeled as a single vertical web member/ element of the truss/ girder. The top and bottom of the member should be modeled as pinned at the top and bottom chords.

    In conclusion, it can be stated that the stub-girder method of construction was and still is an innovative solution to multistory, framed steel floor construction. However, as deeper wide flange sections became more available in the marketplace and design engineers became more accustomed to analyzing web holes in wide flange beams, the use of stub-girder construction waned. In addition, because of the extra labor costs associated with the fabrication of stub-girders and the necessity to construct stub-girders as shored composite construction, the system priced itself out of the industry.

    Discuss this article in the Architecture Forum...

    D. Matthew Stuart, P.E., S.E., F.ASCE, SECB, is a licensed structural engineer in 20 states. He currently works as a senior project manager at the main office of CMX, located in New Jersey, and also serves as an adjunct professor for the master's of structural engineering program at Lehigh University in Bethlehem, Pennsylvania.   More by D. Matthew Stuart

    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).

    References

    Bjorhovde, Reidar. Chapter 18: "Stub Girder Floor Systems." In: Handbook of Structural Engineering, W.F. Chen, editor. CRC Press, 1997.

    Bjorhovde, Reidar, and T.J. Zimmerman. "Some Aspects of Stub-Girder Design." Canadian Structural Engineering Conference, Montreal, Quebec, February 1980. Reprinted in the AISC Engineering Journal, Third Quarter 1980.

    Colaco, Joseph P. "A Stub-Girder System for High-Rise Buildings." AISC National Engineering Conference, May 1972.

    Lam, Y.W., T. Rezansoff and M.U. Hosain. "An Experimental Investigation of Stub-Girders." Behavior of Building Systems and Building Components Conference, Vanderbilt University, March 1979.

     

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    Seen here under construction in 1971, the 34-story One Allen Center was the first of several buildings built in the Allen Center complex of office towers near the western edge of downtown Houston, Texas.
    Photo: © American Institute of Steel Construction Extra Large Image

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    The stub-girder framing system of One Allen Center, shown at an early stage of construction.
    Photo: © American Institute of Steel Construction Extra Large Image

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    At a later stage, the stub-girder framing system with the steel decking installed above the steel I-beams.
    Photo: © American Institute of Steel Construction Extra Large Image

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    Detail section drawings of a typical steel stub-girder system. Detail section drawings of a typical steel stub-girder system.
    Image: © American Institute of Steel Construction Extra Large Image

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    Plan and section drawings comparing conventional and stub-girder steel framing systems.
    Image: © American Institute of Steel Construction Extra Large Image

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    Diagram drawing of a typical wide-flange Vierendeel truss.
    Image: © American Institute of Steel Construction Extra Large Image

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    The 1983 Nova Corporation headquarters building (now the Nexen Building) in Calgary, Alberta, also employs a steel stub-girder structural system.
    Photo: Kevin Cappis Extra Large Image

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    Designed by J.H. Cook Architects and Engineers, the 37-story Nexen Building has an unusual plan, including a sharp, 45-degree corner.
    Photo: Neal Sanche Extra Large Image

     

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