P1ENG
Structural
- Aug 25, 2010
- 237
I have a situation where I have a W8x15 rafter beam and purlins attach to the rafter at various locations. The purlins are bolted to a clip through the web, and then the clip is bolted to the web of the rafter.
The purlins have different cold-formed shapes, some where the flanges of the purlins are flush with the flanges of the rafters, but the flanges don't touch. Some of the bolts in the rafter web are ~2" from the top/bottom of the beam.
I have a feeling that I don't have any lateral bracing for the compression flange of the rafter. Perhaps someone can answer whether the bolts used that are ~2" from top/bottom of the flanges can be considered lateral bracing. If so, where do you draw the line? (say that lateral web bracing within the depth/4, or some other ratio, from the compression flange is sufficient for compression flange bracing?)
Also, there is a gap between the purlin and rafter flanges, but since the purlins are located on both sides of the rafter, the lateral displacement of the rafter's compression flange would have to be limited to the gap between the flanges. Once the deflection of the gap is reached, I would then have bracing against lateral buckling. Could I allow this?
Finally, the roof rib decking might provided the bracing that I need for gravity loads, but I am unsure right now of the fastening locations of the roof decking, so please stick to answering the (2) hypotheticals above.
Thanks
Juston Fluckey, E.I.
Engineering Consultant
The purlins have different cold-formed shapes, some where the flanges of the purlins are flush with the flanges of the rafters, but the flanges don't touch. Some of the bolts in the rafter web are ~2" from the top/bottom of the beam.
I have a feeling that I don't have any lateral bracing for the compression flange of the rafter. Perhaps someone can answer whether the bolts used that are ~2" from top/bottom of the flanges can be considered lateral bracing. If so, where do you draw the line? (say that lateral web bracing within the depth/4, or some other ratio, from the compression flange is sufficient for compression flange bracing?)
Also, there is a gap between the purlin and rafter flanges, but since the purlins are located on both sides of the rafter, the lateral displacement of the rafter's compression flange would have to be limited to the gap between the flanges. Once the deflection of the gap is reached, I would then have bracing against lateral buckling. Could I allow this?
Finally, the roof rib decking might provided the bracing that I need for gravity loads, but I am unsure right now of the fastening locations of the roof decking, so please stick to answering the (2) hypotheticals above.
Thanks
Juston Fluckey, E.I.
Engineering Consultant