BuildingsToday.Org

            There are some design and fabrication approaches in steel buildings that can be controversial in their application. These revolve around issues of torsion, tolerances, and that of single-sided welding.
            Anytime structural components in steel buildings are attached to one another the force of torsion will come into play. This is also accentuated by the components’ unique shape. Design shortfalls and the misapplication of structural members can also introduce torsion. Torsion is present in many locations of a steel structure but, most notably, when door jambs or exterior masonry walls are attached to the eave strut’s flanged bottom or the columns in the endwall are  framed into the sides of the primary frame. Notably, the cold-formed steel sections that do not make up a welded pipe are very substandard in their ability to withstand larger torsion forcing.  “Kickers”, which are flange bracing with a diagonal presentation, are employed to remedy the problem. These are utilized in endwall framing that uses a “Z” purlin and flush girts and requires that the expandable endwalls use both sides of the rafter in order that they may be braced at expansion. Another scheme uses endwall framing and a rigid frame along with the use of bypass girts and open-web joists. If flange bracing is not seen as practical, the use of closed tubular sections to replace cold-formed components should be considered.
            The MBMA Manual gives the tolerances for fabrication and erection for most steel; building cold-form components and any built-up structural portions. The tolerances are important to note as there are specific calculations applied to any pre-engineered steel frame. The efficiencies of a steel building framing system can be designed to a level well over ninety percent. Overstressing of the structure can result once loading is introduced if certain tolerances are not taken into account during the planning stages. Critical observation and correct calculations for web sweep and the actions of camber on built-up members are necessary, for example, to engineer correct erection tolerances into the building at erection.
            The next approach to be considered is that of single-sided welding. Steel building systems rely heavily on welded plates and bars for the integrity of the primary frame. Welding equipment at the manufacturing facility provides the welds between the web and flanges on just one side. Some designers and engineers think that single-sided welds are not adequate for correct structural support. Certain studies have shown that single-sided welds do not negatively impact primary frames except for some seismic engineering conditions which can result in a weld failure in the frame rafters near the end plates. This type of welding approach is generally seen as allowable, but not for frames that will undergo fatigue, larger loading forces, and lateral force activity. In these three instances a double-sided weld should be strongly considered. Conversely, rigid frames, as a classification, must be inherently tolerant of all gravity and lateral loads at work.