5.6: OTHER TYPES OF PURLINS FOR METAL BUILDING SYSTEMS
5.6 OTHER TYPES OF PURLINS FOR METAL BUILDING SYSTEMS
Hot-Rolled Steel Beams
Hot-rolled steel purlins predate modern metal building systems by decades. A multitude of industrial buildings constructed since the beginning of the twentieth century utilized hot-rolled channel and I-beam purlins spanning the distance between roof trusses, a then-dominant type of primary roof framing. The beams are still popular among many engineers for heavy-duty industrial applications and can be used in pre-engineered metal buildings as well. The main advantage of hot-rolled steel beams lies is their higher load-carrying capacities as compared with light-gage sections. The beams may be useful for spans longer than 30 ft, an upper limit for economical use of cold-formed framing. Also, hot-rolled purlins are quite appropriate for heavy suspended or concentrated loads. Their chief disadvantage is a relatively high cost.
Hot-rolled shapes used as roof purlins include channels and wide flanges. Both can either bear on top of primary-frame rafters or be framed flush. The top-bearing design is usually more economical, since it avoids expensive flange coping. Hot-rolled purlins are frequently used in combination with steel decking, which can span longer distances than through-fastened roofing and makes better bracing. Purlin spacing is governed by the deck s load-carrying capacity.
Hot-rolled purlins at sloped roofs do not escape the parallel-to-roof component of gravity loads (Fig. 5.37a). This component can be resisted either by a properly attached and continuous roof-deck diaphragm (Fig. 5.37b), or by sag rods (Fig. 5.37c), the spacing of which is determined by analysis. A typical...