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Str engthening of existing steel joist framingsystemsVulcrafts field-bolted spliced open webjoist solution

June 2009 (/toc/182) FeatureArticle

Although not widely known, there are options for strengthening anexisting open web steel joist framing system if new demands on thestructure require upgrades. When repairs to accommodateincreased roof loading was needed for the Grand Prix Motorsportsbuilding, the original joist manufacturer, Vulcraft, suggested field-bolted spliced open web steel joists might be an appropriatesolution.

By Peter Marxhausen, M.S., P.E. and Dave Henley, P.E.

Tuned for efficiencyMedical research building structuraldesign meets strict vibrationperformance requirements. Read More (http://cenews.com/article/9865/tuned-for-efficiency)

Strengthening of existing steel joist framing systemsVulcrafts field-bolt... http://cenews.com/article/6588/strengthening-of-existing-steel-joist-frami...

1 of 11 09-Sep-14 8:01 PM

History

The first open web steel joist was theMassillon Open web joist, which was firstdeveloped in 1923. That joist system wascomprised 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 aweight-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 wasmanufactured. In 1929, one year after

founding the Steel Joist Institute, the first

open web steel joist load table wasdeveloped. 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 steelangles and rods or just steel angles, theterm bar joist developed from the early useof 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 adominant 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. Vulcraftpresently manufactures more than 685,000

tons of steel joists and joist girders annuallyat 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

Background

For the practicing structural engineer,open web steel joists can present achallenge as it relates to modifying anexisting building to accommodateincreased loads, floor performanceenhancements, and new floor/roofopenings. One common reason formodifying an open web steel joist is theneed to install new rooftop equipmentthat exceeds the original design capacityof the supporting structure. In the eventthe existing open web steel joists are notcapable of safely supporting the newloads, new full-length joists often can notbe inserted due to the lack of clearanceneeded to insert the new joist into theexisting cavity.

Historically, some structural engineershave turned to their own engineeringcreativity to modify an open web steeljoist framing system. Some modificationshave included welding steel plates,angles, channels, and rounds to the joistchords in an effort to increase the grosssection modulus of the joist. While thisapproach has merit, it often does notadequately address the fact that theweak point may be the web members orthe attachment of the web members tothe joist chords. Similarly, weldedattachments may not be successful dueto difficulties with welding access at thetop chord and/or quality concernsassociated with field welding.

Another approach seen in existingbuildings is the installation of new steelwide-flange beams adjacent to anexisting open web steel joist. Where thistype of modification has beenimplemented, the end of the steel beamis coped to a depth of 2 1/2 inches andthe web is reinforced with steel angles.This installation can present problemssimilar to the installation of full-lengthopen web steel joists since a wide-flangebeam is often too long and cumbersome

to fit into an existing opening. Sometimes the beam may be designed as


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two pieces with a moment connection near midspan. Although thisapproach also has merit, the size of the steel beam is often toocomparatively heavy to match the relative stiffness of the adjacent joists.This match is necessary to ensure that the beam will evenly support theproposed loading without overloading and/or causing a premature failureof the existing adjacent open web steel joists.

Although not widely known, a better approach to strengthening an existingopen web steel joist framing system is to install new field-bolted splicedopen web steel joists supplied by the joist manufacturer. Field-boltedspliced open web joists are relatively lightweight, they do not requiremodification of the existing bracing/bridging regime, and may be able toaccommodate existing mechanical/plumbing/electrical installations withlittle modification.

Case study

The Grand Prix Motorsports building is a two-story commercial structurethat is 34,691 square feet in area. The facility began construction in 2003and was completed in early 2004. The building has a motorcraft andmotorcraft accessories showroom on the first floor and a mezzanine levellocated on the east side of the building. The west end of the building hasa mechanic/repair area on the first floor and a storage warehouse on thesecond floor. The roof structure was economically designed with openweb steel joists to accommodate the locally-prescribed snow load of 30pounds per square foot.

Due to a design coordination error, the presence and/or height of therooftop mechanical screen walls was not adequately communicatedbetween the architect and structural engineer of record. The notableheight of the screen walls and parapet walls combined with the opennature of the adjacent roof created a condition that could developsignificant snow drift loads. In some roof areas the calculated effect ofthe potential snow drift loads was nearly 100 percent more than theminimum roof snow load. As a result, several of the specified roof joistswere appreciably underdesigned for the anticipated loading.

As a precautionary measure, the owner of the facility elected to shovelsnow off the roof if the snow accumulation was more than 6 inches.Although 6 inches of snow would not overload the joists, the concern wasthat blowing snow could create drifts at the parapets and screen wallsthat could overload the roof joists at those areas.

Several options were investigated to strengthen the roof structureincluding installing wide flange beams and removing the roof to install newfull-length open web joists. Opening the roof from the exterior raisedconcerns with the impact to the existing electrical/mechanical/plumbinginstallations as well as concerns over the financial impact to the businessassociated with the disruption. As a result, the roof strengthening repairswere delayed for nearly four years. After investigating several options tostrengthen the roof, the original joist manufacturer, Vulcraft, suggested


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field-bolted spliced open web steel joists might be an appropriate solution.

Field-bolted spliced open web steel joists can be manufactured to anystandard joist size and are shipped from the manufacturer in twosegments (see Figures 2 and 3 below). The joists are field-bolted with amoment splice that can be designed to be located at almost any locationalong the joist (see Figure 6 below). Adjustment to the splice location canhelp accommodate an existing duct or fire suppression line that wouldotherwise have to be temporarily disconnected or rerouted to install astandard one-piece joist.

Another benefit of the field-bolted joists is that the joist seats can bemanufactured with a 2-1/4-inch height versus the standard 2-1/2-inchheight. The reduced seat height provides easier insertion of the joistsbetween the bearing surface and the metal deck. Similarly, the 1/4-inchtolerance accommodates some angular movement as the joists areinstalled. After installation, 1/4-inch thick steel plates are installedbeneath the joists seats to provide a tight connection against the metaldeck (see Figures 5 and 10 below).

In new construction, the top/compression chord of open web steel joistsis laterally braced to resist lateral buckling via the screw and/or weldedconnections to the metal roof deck. Of significant concern with theinstallation of any new flexural member is bracing of the top chord. Oneoption to provide top chord bracing is to remove the roofing and screwthe metal roof deck to the new joists. Although removal of the roofmembrane is a possibility, the top chord of field-bolted spliced joists canbe braced by welding standard steel angle braces to both the new joistand the existing open web joists.

In the example of Grand Prix Motorsports, it was known that existingcontinuous bridging was installed at the quarter points of the existingopen web joists. In order to minimize field welding of additionalbridging/braces, the new field-bolted steel joists were designed byVulcraft to be braced at the quarter points only. The end result was thatthe roofing system was not penetrated and the appearance from belowwas relatively unchanged.

Repairs at the Grand Prix Motorsports facility, including the installation of20 new field-bolted joists, were completed in approximately two weeks.The repair contractor, The Deer Creek Corporation, indicated that all 20joists could have been installed in two days; however, the project wassequenced such that only one area at a time would be affected by theconstruction. All repairs were completed during normal business hourswithout interruption to the business operations.

Design requirements

Design of field-bolted open web steel joists is similar to new constructionwith some minor modifications. The structural engineer of record for thedesign should provide the following:


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Existing and new joist layout with joist sizes and layout dimensions.1.Existing/available bearing widths with clear construction spandimensions.

2.

Preferred splice location.3.Details for the attachment of the new joists to the supportingstructure.

4.

Specifications regarding the permanent bracing of the top chords.5.

Conclusion

There are several methods to strengthen an existing roof structure.Structural engineers are encouraged not to modify open web steel joistsin a manner not accepted by the open web steel joist manufacturer or theSteel Joist Institute. As outlined above, one economical way to increasethe load capacity of an existing open web joist roof is to install field-boltedopen web steel joists manufactured by the original joist manufacturer.Although field-bolted spliced joists will strengthen the structure in alocalized area, additional calculations are required by the structuralengineer of record to ensure the metal roof deck and support girders areadequate 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 weredelivered to the jobsite in two sections with thered oxide primer paint installed.

Figure 3: View showing the unassembled end of

a field-bolted open web


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Figure 4: Due to the 25-foot ceiling height at

some areas, the new joists were installed usingtwo man lifts. Each joist end could be lifted andinstalled by two men.

Figure 5: The 2-1/4-inch joist seat depth requireda 1/4-inch thick shim be welded into place after

the joist was erected.

Figure 6: View showing the completed joistinstallation including the welded angle braces.

Figure 7: At some locations the new joists wereinstalled into beam pockets that had to be cut into

the walls. The masonry supporting the joists wasgrouted solid prior to the installation.

Figure 8: At some locations the new joists wereinstalled into beam pockets that had to be cut into

the walls. The masonry supporting the joists wasgrouted solid prior to the installation.


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Figure 9: Erection detail for the field-bolted open web joists.

Figure 10: Isometric view showing the steel joistand 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 facultymember with the University of Colorado at Denver in the Civil Engineering

Department. He can be reached at [email protected](mailto:[email protected]?subject=SE0609

%3A%20Strengthening%20of%20existing%20steel%20joist%20framing%20systems)

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.


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