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Many different types of purlins are available for use in steel structures. It is important to know what your choices are so that you can make the best selection. Two of the most popular will be discussed in detail here.
Hot-rolled steel beams are a purlin choice used in many steel structures. At the beginning of the 20th century, roof trusses were spanned by hot-rolled channel and I-beam purlins. A considerable amount of time passed before steel buildings came into widespread use, although the use of hot-rolled channels and I-beam purlins can still be seen in use today. This is especially true of production facilities needing substantial interior support. Consumers today tend to select hot-rolled steel beams because of their high load-bearing properties, in contrast to reduced gauge designs. This particular design of hot-rolled purlins can be applied to spans of more than 30 feet and is able to reinforce weighty overhead building loads in the internal structure. Although significantly strong, you may find this design to be pricey.
The wide flanges and channels specific to this design give the hot-rolled pre-engineered roof purlins their unique shape. Hot-rolled purlins can be used with steel decking, enabling excellent bracing and long span intervals. To save on costs, you may wish to use these on top of the primary frame rafters. The deck’s load-bearing ability determines the purlins’ spacing. Use of a roof-deck diaphragm or sag rod bracing may regulate the strain relating to the purlins. As sag rods are employed, they can be positioned up to three inches beneath the highest part of the steel, thus helping to minimize torsional effects. Despite the fact that “C” and “Z” cold-formed framing are not easily customizable; the opposite is true of braced or unbraced hot-rolled steel purlins.
The use of open-web steel joists is another purlin technique you may look at. These are also referred to as bar joists and can reach larger areas than cold-formed or hot-rolled purlins. You may find it helpful to use open-web steel joists in steel structures that are larger than 30 ft. in span, or in those that require large bays.
When pre-engineered steel buildings have open-web joists, a standing-seam pre-engineered roof can be easily attached and the diaphragm operation is aided by horizontal rod or cable bracing. A disadvantage to adding bar joists, however, is that they are unable to withstand considerable twisting or turning pressures because no solid webs are present to assist in relieving this stress.
To sustain standing-seam steel roofing, a couple of proven methods exist in successful set-up techniques that make use of bar joists. One approach is to use a steel deck and affix light gauge hat channels which run atop the steel deck alongside its flutes. A different steel roofing design procedure is to exclude the steel deck in the building but to add tightly spaced cross bridging in its place. The cross bridging along the tight spacing gives firmness so that the entire assembly is able to counteract any energy applications.
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