Assessing purlin "slip" in wood post frame roof diaphragm with flush purlin system

[fastline12]

Still working the design of this post frame building. It uses wood roof trusses on 12' centers, and purlins on 3' centers, on edge and in joist hangers. This system is well proven for good roof loading and good strength but apparently takes a hit in stiffness due to the way the purlins are attached, causing the purlins to try and roll. Another common build method is 2x4s or 2x6s laid flat over roof trusses. The vertical deflection of this system is very poor but performs well as a diaphragm.


So far we have considered doubling up the purlins in a double hanger at the ends of the diaphragms or other methods to mitigate the rolling effect. Even considering the lack of stiffness, we can make the system work but not as efficiently.

Would anyone have any data or info regarding these issues?

 

[jayrod12]

If you provided blocking between the purlins at each end along the truss, i.e. install the purling with hanger then install blocking between the purlins that is nailed at a reasonable spacing to the truss, would likely stop the rolling you're concerned about.

 

[BUGGAR]

I've used vertical blocking at the trusses, face nailed to both truss and purlins, to keep the purlins from rolling at the truss supports. This works well at the doubled trusses on the intermediate posts. Between supports we rely on the roof panels to act as sag rods - theoretically suspect but it works with a little help from panel diaphragm action. I've also seen angled straps used over the tops of purlins down to the truss to keep them from rolling.