Roof Purlin Support 1

[LPPE]

I have an existing prefab metal building. An insurance carrier has questioned the lack of roll clips at the roof purlins.

The roof purlins are 9" deep 16 ga. C sections. They are lapped 48" over the supports. Between the lapped C purlins is a 16ga. plate. The plate is sandwiched between the C purlins, and is attached to the top flange of the support girder. The C purlins are not welded or screwed to the support girder, just bolted through to the 16ga plate.

The roof is only sloped at 1/4" per foot.

I took the reaction from the purlin, and looked at the bending it would induce in the 16ga plate. From my analysis, the 16ga plate is not even close to sufficient to resist the "rolling" of the purlins due to gravity loads.

What am I missing in my analysis? This building is about 30 years old, and has survived many heavy snow loadings with no problems.

This picture is one side of the C purlin sitting on the girder. The 16ga plate is right behind it, then the other lapped C purlin.

 

[Bobber1]

Sometimes only the ridge purlins are supported from rolling. The roof panels then tie the other purlins to the ridge purlins and prevent them from rolling.

Check to see if the ridge purlins are supported.

Or could the roof act as a diaphragm and transfer the "rolling" forces to the rake framing?

 

[LPPE]

No ridge, flat roof from the front of the building to the back, at a 1/4"/ft slope.

 

[LPPE]

After some creative assumptions, I was able to "verify" the design as is. Right on the ragged edge, though. Then again, I guess all metal buildings are.

Thanks for the help!

 

[jheidt2543]

OK, LPPE, don't keep us in suspense! What creative assumptions did you make to work it out?

 

[BAretired]

The flanges of the C purlins bearing down on the girder prevent the pair from rolling.

BA

 

[LPPE]

I used BAretired's assumption and analyzed the C purlin as an additional bending member (in addition to the "knife" plate), and also moved the roof reaction location from the flange of one of the C purlins (first back-of-envelope approach) to the centroid of the web of the C purlin, which reduced the moment arm, and hence the required bending capacity of the purlin assembly due to the slight slope of the roof.

Overall, I'd say that relatively flat roofs on prefab'd metal buildings would likely not require roll clips if the purlins are C shapes. Z shapes might be an entirely different story! But as the roof slope increases, you may also need the roll clips for C shapes.