The first open web steel joist was the Massillon Open web joist, which was first developed in 1923. That joist system was comprised of a single continuous web in a Warren type truss configuration and incorporated two top chord bars and two bottom chord bars. The unobstructed nature of the first open web joist provided a weight-efficient design that could also easily accommodate electrical/mechanical features within the plane of the roof framing.
The Steel Joist Institute was formed five years after the first open web steel joist was manufactured. In 1929, one year after founding the Steel Joist Institute, the first open web steel joist load table was developed. The load table helped unify the design standards and eliminate the product confusion amongst architects, engineers, fabricators, and builders. Although modern open web steel joists are comprised of steel angles and rods or just steel angles, the term ’bar joist’ developed from the early use of continuous round bars and is still used as the common nomenclature.
Since the creation of the Steel Joist Institute, open web steel joists have been a dominant construction feature in the industry offering weight- and material-efficient designs, long spans, simplified erection, and an unobstructed design that allows for electrical and mechanical conduits. Vulcraft was incorporated on June 12, 1946, in Florence, S.C., and has grown to be the United States’ largest producer of steel joists and steel deck products. Vulcraft presently manufactures more than 685,000 tons of steel joists and joist girders annually at seven plants. Currently, there are billions of square feet of both roof and floor structures in the United States that are constructed using open web steel joists
For the practicing structural engineer, open web steel joists can present a challenge as it relates to modifying an existing building to accommodate increased loads, floor performance enhancements, and new floor/roof openings. One common reason for modifying an open web steel joist is the need to install new rooftop equipment that exceeds the original design capacity of the supporting structure. In the event the existing open web steel joists are not capable of safely supporting the new loads, new full-length joists often can not be inserted due to the lack of clearance needed to insert the new joist into the existing cavity.
Historically, some structural engineers have turned to their own engineering creativity to modify an open web steel joist framing system. Some modifications have included welding steel plates, angles, channels, and rounds to the joist chords in an effort to increase the gross section modulus of the joist. While this approach has merit, it often does not adequately address the fact that the weak point may be the web members or the attachment of the web members to the joist chords. Similarly, welded attachments may not be successful due to difficulties with welding access at the top chord and/or quality concerns associated with field welding.
Another approach seen in existing buildings is the installation of new steel wide-flange beams adjacent to an existing open web steel joist. Where this type of modification has been implemented, the end of the steel beam is coped to a depth of 2 1/2 inches and the web is reinforced with steel angles. This installation can present problems similar to the installation of full-length open web steel joists since a wide-flange beam is often too long and cumbersome to fit into an existing opening. Sometimes the beam may be designed as two pieces with a moment connection near midspan. Although this approach also has merit, the size of the steel beam is often too comparatively heavy to match the relative stiffness of the adjacent joists. This match is necessary to ensure that the beam will evenly support the proposed loading without overloading and/or causing a premature failure of the existing adjacent open web steel joists.
Although not widely known, a better approach to strengthening an existing open web steel joist framing system is to install new field-bolted spliced open web steel joists supplied by the joist manufacturer. Field-bolted spliced open web joists are relatively lightweight, they do not require modification of the existing bracing/bridging regime, and may be able to accommodate existing mechanical/plumbing/electrical installations with little modification.
The Grand Prix Motorsports building is a two-story commercial structure that is 34,691 square feet in area. The facility began construction in 2003 and was completed in early 2004. The building has a motorcraft and motorcraft accessories showroom on the first floor and a mezzanine level located on the east side of the building. The west end of the building has a mechanic/repair area on the first floor and a storage warehouse on the second floor. The roof structure was economically designed with open web steel joists to accommodate the locally-prescribed snow load of 30 pounds per square foot.
Due to a design coordination error, the presence and/or height of the rooftop mechanical screen walls was not adequately communicated between the architect and structural engineer of record. The notable height of the screen walls and parapet walls combined with the open nature of the adjacent roof created a condition that could develop significant snow drift loads. In some roof areas the calculated effect of the potential snow drift loads was nearly 100 percent more than the minimum roof snow load. As a result, several of the specified roof joists were appreciably underdesigned for the anticipated loading.
As a precautionary measure, the owner of the facility elected to shovel snow off the roof if the snow accumulation was more than 6 inches. Although 6 inches of snow would not overload the joists, the concern was that blowing snow could create drifts at the parapets and screen walls that could overload the roof joists at those areas.
Several options were investigated to strengthen the roof structure including installing wide flange beams and removing the roof to install new full-length open web joists. Opening the roof from the exterior raised concerns with the impact to the existing electrical/mechanical/plumbing installations as well as concerns over the financial impact to the business associated with the disruption. As a result, the roof strengthening repairs were delayed for nearly four years. After investigating several options to strengthen the roof, the original joist manufacturer, Vulcraft, suggested field-bolted spliced open web steel joists might be an appropriate solution.
Field-bolted spliced open web steel joists can be manufactured to any standard joist size and are shipped from the manufacturer in two segments (see Figures 2 and 3 below). The joists are field-bolted with a moment splice that can be designed to be located at almost any location along the joist (see Figure 6 below). Adjustment to the splice location can help accommodate an existing duct or fire suppression line that would otherwise have to be temporarily disconnected or rerouted to install a standard one-piece joist.
Another benefit of the field-bolted joists is that the joist seats can be manufactured with a 2-1/4-inch height versus the standard 2-1/2-inch height. The reduced seat height provides easier insertion of the joists between the bearing surface and the metal deck. Similarly, the 1/4-inch tolerance accommodates some angular movement as the joists are installed. After installation, 1/4-inch thick steel plates are installed beneath the joists seats to provide a tight connection against the metal deck (see Figures 5 and 10 below).
In new construction, the top/compression chord of open web steel joists is laterally braced to resist lateral buckling via the screw and/or welded connections to the metal roof deck. Of significant concern with the installation of any new flexural member is bracing of the top chord. One option to provide top chord bracing is to remove the roofing and screw the metal roof deck to the new joists. Although removal of the roof membrane is a possibility, the top chord of field-bolted spliced joists can be braced by welding standard steel angle braces to both the new joist and the existing open web joists.
In the example of Grand Prix Motorsports, it was known that existing continuous bridging was installed at the quarter points of the existing open web joists. In order to minimize field welding of additional bridging/braces, the new field-bolted steel joists were designed by Vulcraft to be braced at the quarter points only. The end result was that the roofing system was not penetrated and the appearance from below was relatively unchanged.
Repairs at the Grand Prix Motorsports facility, including the installation of 20 new field-bolted joists, were completed in approximately two weeks. The repair contractor, The Deer Creek Corporation, indicated that all 20 joists could have been installed in two days; however, the project was sequenced such that only one area at a time would be affected by the construction. All repairs were completed during normal business hours without interruption to the business operations.
Design of field-bolted open web steel joists is similar to new construction with some minor modifications. The structural engineer of record for the design should provide the following:
- Existing and new joist layout with joist sizes and layout dimensions.
- Existing/available bearing widths with clear construction span dimensions.
- Preferred splice location.
- Details for the attachment of the new joists to the supporting structure.
- Specifications regarding the permanent bracing of the top chords.
There are several methods to strengthen an existing roof structure. Structural engineers are encouraged not to modify open web steel joists in a manner not accepted by the open web steel joist manufacturer or the Steel Joist Institute. As outlined above, one economical way to increase the load capacity of an existing open web joist roof is to install field-bolted open web steel joists manufactured by the original joist manufacturer. Although field-bolted spliced joists will strengthen the structure in a localized area, additional calculations are required by the structural engineer of record to ensure the metal roof deck and support girders are adequate to support the proposed loading.
|Figure 1: View of the interior of the Grand Prix Motorsports showroom|
|Figure 2: The field-bolted open web joists were delivered to the jobsite in two sections with the red oxide primer paint installed.|
|Figure 3: View showing the unassembled end of a field-bolted open web|
|Figure 4: Due to the 25-foot ceiling height at some areas, the new joists were installed using two man lifts. Each joist end could be lifted and installed by two men.|
|Figure 5: The 2-1/4-inch joist seat depth required a 1/4-inch thick shim be welded into place after the joist was erected.|
|Figure 6: View showing the completed joist installation including the welded angle braces.|
|Figure 7: At some locations the new joists were installed into beam pockets that had to be cut into the walls. The masonry supporting the joists was grouted solid prior to the installation.|
|Figure 8: At some locations the new joists were installed into beam pockets that had to be cut into the walls. The masonry supporting the joists was grouted solid prior to the installation.|
|Figure 9: Erection detail for the field-bolted open web joists.|
|Figure 10: Isometric view showing the steel joist and shim configuration where supported by a steel beam.|
Peter Marxhausen is a forensic structural engineer with Higgins & Associates, Inc. in Morrison, Colo. Marxhausen is also an adjunct faculty member with the University of Colorado at Denver in the Civil Engineering Department. He can be reached at Higgins.Peter@MHO.COM
Dave Henley is the Denver District Manager for Vulcraft Sales Corporation. He has more than 27 years of experience in the design and construction of open web steel joists, joist girders, and metal deck.