Steel Building Roof Diaphragms
October 23rd, 2006 The roof on a steel building has some important functions besides keeping the occupants and contents inside dry. It plays only a small role in earthquake or wind resistance as the single strength in this area is seen in the one bay which is between the frames. The roof diaphragm, however, plays an irreplaceable role in steel structures with no rigid frame types that include single span beam or a bar joist system because the roof diaphragm covers the distance between the two endwalls. The roof diaphragm also evenly distributes lateral loads to endwalls that are braced.
The most common style of roof diaphragm used in steel buildings is roof braced. However, rigidity of the decking for the roof can also be achieved by using materials like concrete or wood. In metal building construction a corrugated metal roof deck is the most commonly employed design. This corrugated style of roofing has less rigidity, normally, than its steel roofed paneled counterpart because the gauge of the metal is thinner.
A common roof diaphragm construction in steel building systems consists of diagonal steel rods resisting the forces of tension with struts which are compression designed. The roof diaphragm becomes a horizontal truss which contains the primary frame rafters.
Construction of roof diaphragms is relatively simple with the rods being located below the purlins. Vertical distance between rods and purlins should be more than the greatest anticipated vertical deflection that the purlins will have under a full gravity load.
There is a preference by some steel building manufacturers to forgo rods and, in turn, to use steel cables for added building stability. The downside of cable use is that they tend to loosen over times. However, rods are also hard to keep tight for an extended period of time. These “loosening” may allow the building to shift or move, potentially causing damage.
Diaphragm struts may use additional purlins that are laterally braced and designed for compression. Without this lateral bracing the purlin has minimal compression ability. All purlin struts in roof diaphragm construction should have antiroll clips at the supports. Struts should be free of hangars or roof clip attachments because the struts should be free of most load bearing functions.
Pipe struts can be used in place of purlin struts, in certain design demands, because they are normally of a thicker steel gauge. The pipe strut can be bolted to the top flange of the building frame or a web connection can be utilized. The entire layout of compression struts is normally specified on the manufacturer’s roof assembly plan. Any attachment of compression struts should be done in parallel with the purlin bracing placement.
Attention to assembly instructions for any steel building roof diaphragm is essential. This insures that purlin struts are correctly laterally braced to roof purlins and pipe strut attachments to columns or rafters are also according to manufacturer recommendations.