Top chords are usually braced by roof sheathing. Roof sheathing provides effective lateral bracing when nailed directly to the top chord. In cases where roof sheathing cannot be directly applied to the top chord, purlins are used to prevent lateral buckling of truss top chords. The Truss Designer must take this into consideration when choosing the size and properties of the members during the design process. [For all figures, See PDF or View in Full Issue.]
One such case is the flat area of the top chord on a piggyback base truss. Piggyback base trusses have a flat section where a smaller piggyback or cap truss will set on it. You can either have a flat section of top chord of piggyback base truss flush with the piggyback bottom chord as shown in Figure 1 or raise the bottom chord of the piggyback 1-1/2” as shown in Figure 2.
For the scenario in Figure 1, the cap truss sets directly on the base truss so that the connection between the cap and base can be made directly. To prevent lateral buckling of the top chord of the base truss, the purlins must be installed below the top chord. For Figure 2, the purlins are installed on the top of the top chord of the base truss and the cap sits on the purlins. In each scenario, if continuous sheathing is run on the top chord of the base truss there is no way to connect the trusses together due to the sheathing being in the way. Also, by sheathing the interior chord of the base truss, ventilation would be limited between the base and cap section of the truss.
Another case is trusses with a dropped flat top chord. Flat top chord is usually dropped specifically for purlins and must be run that way in the engineering program. Figure 3 shows how the top chord lumber could fail if designed as sheathed and then checked with purlins instead of sheathing.
To switch from sheathing to purlins in MiTek 20/20 Engineering, see Figure 4.
To switch from sheathing to purlins in Structure with Truss Design, see Figure 5.
Note that when switching to purlins, always start with a maximum of 2’ o.c. spacing. When running the cap trusses as continuous bearing, it is required that the purlins below are spaced at no greater than 2’ o.c. for it to be considered continuous bearing. With purlins spaced greater than 24 inches on center the cap trusses must be analyzed with bearings where the purlins are. Along with the cap trusses being analyzed without continuous bearing the base trusses would need to be analyzed with point loads from the cap bearing locations instead of the uniform load from the caps. Therefore, we recommend limiting the purlin spacing to 2’ o.c.
For additional information, or if you have questions, please contact the MiTek Engineering department.