Determining Unbraced Length of Compression Flange
Determining Unbraced Length of Compression Flange
(OP)
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
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






RE: Determining Unbraced Length of Compression Flange
RE: Determining Unbraced Length of Compression Flange
Thanks,
Juston Fluckey, E.I.
Engineering Consultant
RE: Determining Unbraced Length of Compression Flange
RE: Determining Unbraced Length of Compression Flange
A brace is deemed adequate if it can provide 2% of the compression in the flange to be braced without excessive deflection.
You could ensure that the clip angle has the required resistance and you could estimate the deflection of the clip angle and the beam web by making a few simplifying assumptions.
BA
RE: Determining Unbraced Length of Compression Flange
I know the flange cannot move laterally without taking a portion of the web with it, but where to draw that line? Sorry, I was that kid in class that always asked "why?".
Juston Fluckey, E.I.
Engineering Consultant
RE: Determining Unbraced Length of Compression Flange
RE: Determining Unbraced Length of Compression Flange
RE: Determining Unbraced Length of Compression Flange
This link has a similar thread. Again, it's not in the code, but it has a link to AISC. Check it out.
ht
RE: Determining Unbraced Length of Compression Flange
In the Simplified Analysis, an initial misalignment, Δ0 is assumed. The brace is required to resist a lateral force of 2% of the factored compression in the element being braced. Moreover, the displacement, Δb resulting from that force must not exceed Δ0. For simplicity, members may be considered hinged at brace points.
In the present case, the deflection of the clip angle must be considered as part of the displacement. If it is light gage material, it is likely to be the most flexible part of the connection.
BA
RE: Determining Unbraced Length of Compression Flange
RE: Determining Unbraced Length of Compression Flange
I say so with a light heart.
In general, it appears that you have a very light roof system with a light rafter and light gage pulins. I'd agree with others that your comp. flange is braced.
I noticed that you are an E.I. but call yourself an "Engineering consultant". You sound on the up & up, but be careful with titles, they can get you in trouble.
RE: Determining Unbraced Length of Compression Flange
Perhaps you should have been that kid in class who asked "why not?"
BA
RE: Determining Unbraced Length of Compression Flange
-Toad, my company put "Engineering Consultant" as my email signature and on my business cards.
Juston Fluckey, E.I.
Engineering Consultant
RE: Determining Unbraced Length of Compression Flange
RE: Determining Unbraced Length of Compression Flange
BA