There are several styles of purlins that work well for steel structures. It’s important to understand each one in all their intricacies. We’ll discuss a couple of them below.  

Many systems use hot-rolled steel beams. The growth in pre-fabricated, pre-engineered steel buildings occurred a number of years after the introduction of hot rolled steel purlins. In the early 1900s, building roof frameworks were manufactured of trusses spanned with hot-rolled channel and I-beam purlins. Even today, for applications requiring substantial internal reinforcement, this approach is still utilized. The growing popularity of hot-rolled steel beams as compared with lighter gauged designs is due to their large load-bearing ability. Hot-rolled beams are especially favored when traverses exceed 30 feet. They can also support substantial overhead loads from within the interior. Although this option is super strong, it can be very expensive.  

Braced or unbraced hot-rolled steel purlins are easily adjustable for uplift, whereas “Z”� and “C”� cold-formed framing are not readily adaptable to this force.

A wide flanges and channels design creates hot-rolled steel structure roof purlins. A combination of hot-rolled purlins and steel decking results in optimum reinforcement and can span long distances. It is less costly to install these above the primary frame rafters. The particular purlins’ spacing is established by the desired load carrying ability of the steel deck. A roof-deck diaphragm or sag rod bracing can be employed to supplement the purlins to deal with building stresses. Sag rods can be installed up to three inches below the topmost part of the steel, which lowers torsional strain.

Another purlin style uses open-web steel joists. Steel building systems that span more then 30 feet as well as buildings that require wider bays are best-suited for open web joists. Also called bar joists, they can reach larger areas than cold-formed or hot-rolled purlins.

A standing-seam rooftop can be easily attached to a still buildings when open-web joists are used, and cable or horizontal rod bracing increases diaphragm effectiveness. One drawback to bar joists, unfortunately, is their inability to withstand substantial torsion burdens, as there are no solid webs to aid in transfer of this stress.

There are a couple of dependable engineering techniques to support standing-seam pre-engineered steel roofing that use bar joists. One way is to install a steel deck and add thin gauge hat channels over the deck and upright to its flutes. Another is to include closely spaced cross bridging instead of incorporating the steel deck into the configuration. The stability is controlled by the cross bridging along the compact intervals, allowing the complete assembly to withstand any energy forces.
Choosing the correct purlin option requires that you and your qualified steel building engineer take into account the building design and all of the potential stresses and forces that will act upon it.