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August 2023 #15289 Page #96
Flat Top Chord Bracing
By MiTek Staff
op 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.
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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.
Figure 1
Figure 2
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