Open-Web Steel Joists
In 1953, as U.S. industry ramped up again following world war and depression, the L-Series long-span joists were approved by the SJI jointly with the American Institute for Steel Construction (AISC), allowing spans up to 96 feet with depths of up to 48 inches.
[an error occurred while processing this directive]
In 1959, the S-Series joists replaced the original SJ-Series joists, with joist depths and spans increased to 24 inches and 48 feet respectively, and allowable tensile strength increased from 18 kilopounds per square inch (ksi) to 20 ksi.
During the 1960s and 1970s, the J-Series joists replaced the S-Series joists, the LA-Series joists replaced the L-Series, and H-Series joists were introduced, as allowable steel strengths continued to increase.
The DLH- and DLJ-Series joists were introduced in 1970, including depths up to 72 inches and spans up to 144 feet, and in 1978, these were joined on the menu by joist girders. Joist specifications and weight tables continued to be unified and updated.
In 1986, the K-Series joists replaced the earlier high-tensile H-Series, and in 1994, KCS joists were introduced, providing a constant moment and shear capacity envelope across the entire length of the member.
The publication 80 Years of Open Web Steel Joist Construction, recently published by the SJI, includes a complete chronological listing of the standard specifications and load tables for all of the steel joists, and weight tables for the joist girders, previously made available by the SJI over the time period from 1928 to 2008. This manual can be an invaluable tool for an engineer involved in the analysis of existing buildings constructed with open-web steel joists.
Evaluation and Modification of Existing Joists
The evaluation and strengthening of existing open-web steel joists and joist girders is often required as a result of equipment upgrades or new installations and adaptive reuse or change in use of a facility.
The first step in the process of evaluating an existing joist is to determine the capacity of the member. Ideally, the best method for doing this is through the original construction or shop drawings, which allow the identity of the joist to be established. Similarly, it is also sometimes possible to identify the joist by means of fabrication tags left attached to the joists in the field. However, if a tag can be found, more often than not, the tag identifies only the shop piece mark number, rather than the actual joist designation.
In some instances, it may be possible to establish only the type or series of the joist through the available documentation. In this situation, it is possible to assume conservatively that the capacity of the existing joist is no more than the lightest joist in the corresponding series for the given depth. In addition, if it is not clear whether a J- or H-Series joist is involved, the J-Series joist should always be conservatively assumed because of its lower load-carrying capacity.
However, if a definitive distinction is required, and it is possible to secure a material sample in order to obtain results from a standard ASTM tension coupon test, a determination as to whether the joist is 36 ksi (J-Series) or 50 ksi (H-Series) can be made.
If no drawings are available, it is still possible to establish the approximate capacity of the member by field-measuring the chord and web member sizes, as well as the overall configuration of the joist. This information can then be used to analyze the structure as a simple truss. Critical assumptions that must be made with this approach include the yield strength of the members and whether the existing panel point welds are capable of developing the full capacity of the connected component members.
An alternative method involves filling out the Joist Investigation Form located on the SJI web site. The SJI has indicated considerable success in identifying the series and designation for many older joists with this resource.
The next step in the evaluation process is to determine all of the existing loads on the joist system. The existing and new loading criteria are then used to establish the shear and moment envelope of the individual joist for comparison with the allowable shear and moment envelope based on either the historical data provided by the SJI or an independent analysis of the member as a simple truss.
In the former case, unless the joists were fabricated with a uniform shear and moment capacity over the entire span length (i.e., KCS joists), then it is also necessary to evaluate the location of the maximum imposed moment.
Typically, if the maximum moment is within one foot of the midspan point, and the maximum applied moment is less than the joist moment capacity, the joist is capable of safely supporting the imposed loads. However, if the maximum moment is greater than one foot from the midspan point, the capacity of the joist may not be sufficient even if the applied moment is less than the specified capacity.
This situation can occur for two reasons. First, the moment capacity envelope of the joist may actually be less in regions of the span that are not within one foot of the midspan point. Second, a shift in the moment envelope from that normally associated with a uniformly loaded simple span (and the prerequisite shear envelope) may result in stress reversals in the web members (i.e., from tension to compression) for which the original member was not designed or manufactured.
A similar, although typically more advantageous, condition also can occur with J- or H-Series joists because of variations in the uniform shear capacity of these members.
When the existing joists do not have sufficient capacity to support the new loads, there are three methods that can be used to rectify the condition: load redistribution, adding new joists or beams, and reinforcing the existing joists. Load redistribution involves the installation of a sufficiently stiff member perpendicular to the span of the joist as required to distribute the applied load to enough adjacent joists such that no one joist is overstressed as a result of the new loading.
Adding new joists or beams typically involves the installation of an additional framing member parallel to the joist span such that all or most of the new applied load is supported by the new framing. New self-supporting beams can also be installed perpendicular to the joist span as required to reduce the original span length of the member. Another alternative consists of new independent, self-supporting beam and column frames that avoid the imposition of any new loads on the existing joist framing system.
Reinforcement involves the installation of supplemental material to the original joist as required to increase the load-carrying capacity of the member.
The key to the successful use of load redistribution is the installation of a structural member that can adequately and predictably distribute the applied load to enough adjacent joists to justify the safe support of the load. A method of calculating the relative stiffness of a distribution member is available in the reference material noted at the end of this article.
In general, if the spacing of the joists is less than approximately 78 percent of the calculated relative stiffness of the distribution member and the joists, and the length of the distribution member is less than the inverse of the calculated relative stiffness, then the distribution member may be considered rigid enough to calculate the static load reactions to the affected joists.
Discuss this article in the Architecture Forum...