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    Open-Web Steel Joists

    continued

    For load redistribution solutions, it is my preference to use trussed distribution members, rather than individual beams, to ensure adequate transfer of the applied load. Trussed means continuous members located perpendicular to both the bottom and top chords of the existing joists in conjunction with diagonal web members connected to the continuous members at the intersection of the joist chords. The resulting configuration looks like a truss and provides greater stiffness than an individual beam connected to either the bottom or top joist chords alone.

    I also recommend that no more than five joists be engaged by any one redistribution member. In addition, the use of pipes for the continuous redistribution truss chord members can be advantageous, as this type of section fits neatly through the V-shaped panel point openings created at the intersection of the existing chords and web members. However, load redistribution solutions may be difficult to install, depending on accessibility and the presence of existing MEP systems, ceilings, or other appurtenances.


    Other Steel Joists

    In addition to the steel joists presented in the SJI's 80-year manual, there were also a number of joists produced by manufacturers that were either never members of the SJI or joined it later.

    Some of these manufacturers include: Ashland Steel Joists (manufactured by Ashland Steel Products Co., Inc, Ashland City, Tennessee); Vescom Structural Systems, Inc., Westbury, New York; Ridgeway Joists (manufactured by Continental Steel Ltd., Coquitlam, British Columbia); Northwest Joist Limited (a Division of Brittain Steel Limited, New Westminster, British Columbia); Cadmus Long Span and Joist Corporation (affiliated with Alexandria Iron Works, Inc., Alexandria, Virginia); T-Chord Longspan Joists (manufactured by the Haven Busch Company, Grandville and Grand Rapids, Michigan); and the Macomber Steel Company, Canton, Ohio. Table 1 (PDF) provides a summary description of the joists produced by these manufacturers.

    In addition, some manufacturers, prior to becoming SJI members, produced products other than the historical standard SJI joist series. Some of these manufacturers include: Truscon Steel Company, Youngstown, Ohio; Macmar and Kalmantruss joists (manufactured by Kalman Steel Corporation, a Subsidiary of Bethlehem Steel Company, Bethlehem, Pennsylvania); and Gabriel Steel Company, Detroit, Michigan. Table 2 (PDF) provides a summary description of the joists produced by these manufacturers.

    In addition, it should be noted that Bethlehem Steel Company also produced cold-formed joists with hat-channel sections for the chord members, and Gabriel Steel Company also produced unique V-shaped top-chord and single round-bar bottom-chord members.

    Additional manufacturers not included in Tables 1 and 2 are: Berger Steel Company (double V-shaped chord members); Armco Steel (cold-formed hat-channel chord members); Raychord Corporation (cold-formed hat-channel and U-shaped chord members); Republic Steel (cold-formed hat-channel chord members); and USS AmBridge (cold-formed U-shaped chord members).


    As indicated above, adding new joists or beams to an existing system can also be used to accommodate new loads on an existing joist structure. When new members are added parallel to the existing joists, the new framing can be used either to reduce the tributary area of the existing joists or to provide direct support of the new loads such that there is no impact on the existing joists. Methods used to install new parallel framing often involve manufacturing, shipping, and erecting the new members using field splices.

    However, it is possible to install new full-length manufactured joists by means of loose end bearing assemblies. In this scenario, the joists are first erected on a diagonal to allow the top chord to be lifted above the bearing elevation. The joist is then rotated into an orthogonal position, with the lower portion of the bearing assembly then dropped and welded into place. Typically, in this situation, a shallower bearing seat is also provided for ease of installation and then shimmed once the new joist is in its proper position.

    When new beams or other similar members are added perpendicular to the joist span, the new framing serves to reduce the span of the existing members, thereby increasing the load-carrying capacity of the joists. However, it is still necessary to analyze the existing joists to ensure that no load reversals have occurred in tension-only web members and that the actual applied moment falls within the remaining existing moment capacity envelope of the joist. As with load redistribution solutions, both of the above new framing approaches may be difficult to install.

    New framing that involves the installation of independent, standalone beam and column frames is intended to provide direct support of the new loads such that there is no impact on the existing joist framing. This type of new framing can involve beams located either beneath or above the impacted existing framing and supported by new columns and foundations, or beams that frame between existing columns. This type of solution can also involve new beam frames supported from posts located directly above existing beams or columns. The above solutions are typically more adaptable to the presence of existing MEP systems, ceilings, or other appurtenances.

    Procedures for reinforcing joists, expertly described in the SJI's Technical Digest No. 12, involve two basic approaches: 1) ignore the strength of the existing member and simply design the new reinforcement to carry all of the applied load, or 2) make use of the strength of the existing member when designing the reinforcement. Both of the recommended approaches typically involve significantly more labor costs than material costs because of the expense associated with field-welding.

    I prefer to avoid the use of field reinforcement for the following reasons. A manufactured open-web steel joist is basically a pre-engineered product; however, when an engineer involved with the modification of an existing joist specifies new field-installed reinforcement, that same engineer assumes the responsibility for the overall adequacy of the joist. This liability extends to not only the reinforcing modifications but also, inherently, to any pre-existing, unknown conditions or deficiencies in the joist.

    In addition, field-welding associated with the installation of reinforcement also poses concerns for the design engineer. Problems associated with field-welding include temporary localized loss of the material strength of the existing steel due to heat generated by the weld, induced eccentricities, inadequate load path mechanisms, and lack of access, particularly at the top chord.

    The only exceptions that I make include the installation of supplemental web members as needed to transfer concentrated loads greater than 150 pounds on chords that are located greater than six inches from a panel point to the closest adjacent panel point, and reinforcement designed by the original manufacturer's engineer. It should also be noted that a new, proprietary "weldless" joist-reinforcement system is now manufactured by Lindapter.

    The analysis of existing open-web steel joists can be a challenging undertaking and often involves a considerable amount of detective work. Unfortunately, there is typically little or no documentation available concerning the capacity of a specific existing joist under investigation. However, it is hoped that the reference information provided in this article will assist in increasing the likelihood that the capacity of a joist can be determined using the historical data that is available from the SJI.

    Typically, the investigation of an existing joist results in the need to modify the structural system to provide for the support of new imposed loads. At this juncture, the engineer must then determine if he or she is more comfortable with assuming the responsibility and liability for modifying a pre-engineered product or employing a possibly less risky option, such as load redistribution or adding new joist or beam framing. To assist structural engineers with the evaluation and modification process, I have developed a flowchart based on experience with numerous projects that involved existing joists.

    D. Matthew Stuart, P.E., S.E., F.ASCE, SECB, is a licensed structural engineer in 20 states. He currently works as a structural department head at Pennoni in Philadelphia, 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 adapted from the June 2009 and November 2009 issues of STRUCTURE magazine, with permission of the publisher, the National Council of Structural Engineers Associations (NCSEA).

    References

    Fisher, James, Michael West, and Julius Van de Pas. Designing with Steel Joists, Joist Girders and Steel Deck. Charlotte, North Carolina: Nucor Corporation, 1991. 2nd ed., 2002.

    Steel Joist Institute. 80 Years of Open Web Steel Joist Construction. Myrtle Beach, South Carolina: Steel Joist Institute, 2009.

    Steel Joist Institute. Joist Investigation Form (online).

    Steel Joist Institute. Standard Specifications, Load Tables and Weight Tables for Steel Joists and Joist Girders, 42nd ed. Myrtle Beach, South Carolina: Steel Joist Institute, 2007. Includes a brief history of open-web joists.

    Steel Joist Institute. Technical Digest No. 12: Evaluation and Modification of Open Web Steel Joists and Joist Girders 2007. Myrtle Beach, South Carolina: Steel Joist Institute, 2007. An excellent resource for the evaluation and modification of existing joists and joist girders. Expertly describes procedures for reinforcing joists, and describes problems associated with field-welding.

    Steel Joist Institute. Miscellaneous Steel Joist and Joist Girder Specifications and Load Tables. Myrtle Beach, South Carolina: Steel Joist Institute. SJI Archives.

    Another Resource

    Higgins, Robert. SlideRuleEra web site. Provides civil and structural engineering information, including out-of-print materials, public-domain documents with limited availability, and older, usually conservative methods for solving technical problems.

     

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    Flowchart diagramming an engineer's decision-making process for evaluating the reuse of existing open-web steel joists.
    Image: Courtesy D. Matthew Stuart Extra Large Image

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    Open-web steel joists have been manufactured using a variety of standard steel members.
    Photo: Courtesy D. Matthew Stuart Extra Large Image

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    Detail drawings of the Allspan Steel Joist manufactured by Macomber Inc.
    Image: Courtesy D. Matthew Stuart Extra Large Image

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    Diagram drawings showing the methods for attaching lathe to Truscon O-T Steel Joists.
    Image: Courtesy D. Matthew Stuart Extra Large Image

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    Axonometric diagram showing the use of a steel joist in a composite system with steel decking and a concrete slab.
    Image: Courtesy D. Matthew Stuart

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    Detail drawings of a Macomber joist system intended for use in light wood-frame residential construction.
    Image: Courtesy D. Matthew Stuart Extra Large Image

     

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