Barrel Framed Roof Joists

[jdgengineer]

When barrel framing roof joists have you typically designed the joists for combined strong + weak axis bending or have you assumed that the sheathing and finish material forms a "deep beam" and therefore you don't need to check weak-axis?

In our case we have a curved roof framed ideally with 2x10 joists @ 24" oc. The max rotation of a joist in the roof is 15 degrees. If we break up the vertical force into strong and weak-axis we have strong-axis cos 15 = 0.97 ~ 1.0 (full load) and weak-axis sin (15) = 0.25.

Designing the 2x10 in weak-axis bending for the 25% of the load doesn't work, and would require 3x10 @ 16" oc. What do you all do?

 

[KootK]

Deep beam, even if it's just 24" deep, and copious blocking. How is the curving of the sheathing being handled?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jdgengineer]

Good point Kootk. I'm not sure, I need to inquire with the contractor. We've done this before, but I can't remember what they did. He would probably prefer to kerf the bottom of the plywood, but obviously that is not ideal structurally. If we block the roof at least the plywood could span in the direction parallel to the kerf cut. I suppose they may be able to use a double layer of 3/8" plywood which may be easier to bend.

Any thoughts?

 

[KootK]

Any thoughts?

Not my area of expertise I'm afraid. It's my wife's though. I'll query her tonight. I just get a bit nervous about relying on diaphragm action in sheathing that will be all bent and kerfed as will surely be the case. Probably just paranoia for small scale stuff though. What scale are we talking here?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jdgengineer]

Not very big. Approximately 28' x 20' roof as part of single-story residence. We are looking at running one curved steel beam in the middle and then barrel framing the 2x10 roof joists 14'-0". As an alternate we could use a few curved GLB beams. I was planning on reviewing with the contractor what their preference was.

 

[EngineeringEric]

Can you provide blocking between your joists and coil strap over and terminate this load somewhere? if so then your weak axis isn't really full span but two-cont. spans this provides much better results.

 

[KootK]

Ohh... refinement to Eric's suggestion. Slightly staggered blocking lines connected with small diameter lags in end grain. Like sag rods in steel, right over the barrel. And yeah, end grain lags are shitty. It's indoors and minor.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

Actually, I think I like the straps better. System will add a bit of strong axis load to all members which is surely fine. Peak load at top.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jdgengineer]

Thanks guys. I'll get the blocking in at 8'-0" oc max spacing. This seems like it should distribute well.

Straps though, are they really necessary? The roof is the bottom half of a circle (more ellipse I guess). Therefore the weak-axis bending should always be putting the blocks in compression and not tension. Any accidental tension caused from wind loading seems like it may be able to be taken by the roof plywood nailing and blocking attachment (I'm not in high wind). If I were to put a strap on the top side, how effective would it really be as if it were in tension it would create uplift on the nails. Straps on the underside would likely get me a lot of flack.

 

[XR250]

Maybe span the 14' with some 3N metal decking and forget about the rafters.....

 

[KootK]

Straps though, are they really necessary? The roof is the bottom half of a circle (more ellipse I guess). Therefore the weak-axis bending should always be putting the blocks in compression and not tension.

Agreed. I can't speak for Eric but I was definitely thinking that your barrel was convex down. Didn't know you were building a giant sushi boat to catch rainwater. Crazy Californians. More GDP than sense.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jdgengineer]

Didn't know you were building a giant sushi boat to catch rainwater. Crazy Californians.

We are in a drought...

Architect is looking at building slope for drainage at the bottom to come to the two sides. No snow loads obviously.

 

[structSU10]

that is an odd choice architecturally. Something you cannot see at ground level, and reducing headroom / forcing a taller structure.

 

[jdgengineer]

Different project, but this is the effect they are going for.

 

[KootK]

It's gorgeous. Your climate permits some sexy detailing that would never fly in colder locales. Nice glass too.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jdgengineer]

To close the loop, the contractor preferred the curved GLB approach, so the barrel framed joists span now 7'-0". These calc out fine now for strong + weak axis bending. He said no problem bending the 15/32" plywood and no need for kerf cuts as the radius of the curve is pretty large.