Vaulted Ceiling Design

[sam37]

Hi all,

Question about a vaulted ceiling design, for a 26' x 52' small church with timber framed 8' tall walls. The roof pitch is very steep, 14/12 and I am tasked with coming up with something that works. The ridge will be at 25', very tall, and the architect wants collar ties 10' down from the ridge at every 8 feet, made to look like logs. The question I have is I want to use LVL Microllams for the rafters, 1.75x14. Upon running some calculations, based on the geometry of the rafter depth and pitch I have enough space for roughly a 32 inch strap at the rafter joint across both rafters. The rafters will be spaced 24 inches apart, 37 plf total combined DL and LL applied. There will be roughly 4000 lbs tension to a flat strap flush at the bottom of rafter joint, and if I use a Simpson MST27 on both sides of the rafter I can get those numbers to all work, with the strap on the other side staggered 1/2". My question is has anyone done that before, and is that an okay method to do it? Will those collar ties every 8 feet just then not be required for structural anymore and just become architectural? Also, the simpson catalog calls out to use (30) 0.162" x 2.5" nails for the strap, I only have a 1.75" member so I checked if that would reduce the capacity and I'm allowed to use 0.148" x 1.5" nails with no reduction in straight straps is not using SPF lumber. Would splitting be a concern for me in my case with that many nails going into both sides of a 1.75" LVL? Also, would it be better to have a ridge board or just to fasten the rafters to each other at the top of the ridge?

If that is not the option I should explore, will I just need to have a ridge beam spanning every 8 feet and use that beefy collar tie 10 feet down from the ridge for structural? How do I connect all the rafters without any collar ties to the ones that do?

Lastly, has anyone ever had to find joist hangers for LVL's with a 14/12 roof pitch? Crazy steep, can find some with ordinary 2x's but not 1.75" LVLs.

 

[DenverStruct]

Well, the big question is, can you get the ridge beam to work to support all of that load from the rafters? That's 52 ft span.

 

[sam37]

Do I need to make the ridge beam span the full length? Is that ridge beam not just going to have to span 8 feet to the next A-frame and be a point load at the ridge at that location? The collar tie is 10 feet down from the ridge, and the collar can be quite beefy, double 2x12 would be my guess. That A frame can take a big load, that ridge beam does not have to be continuous throughout?

 

[Ron247]

You probably need to supply a sketch. Are you using a collar tie or a rafter tie? There is a big difference in the use of these. Collar ties are in the upper third while rafter ties tend to be in the lower third. If the roof is 17' tall, 10' from the top is the middle third.

Ridge boards do not tend to be structural in nature while ridge beams are structural. Ridge boards just make construction easier.

 

[BAretired]

Before getting involved with detailed questions, figure out how you're going to frame the roof.

Will those collar ties every 8 feet just then not be required for structural anymore and just become architectural?

I think the ties at 8 foot centers will be required structurally unless you plan to treat the roof like a folded plate, spanning from end to end, a distance of 52'. Alternatively, you could use a ridge beam spanning 52' but as DenverStruct has noted, that may be impractical.

If the architect wants to use a log like member for a tie, maybe he would be okay with a more substantial rafter member, say a double or triple rafter at tie locations and a single rafter at 2'-0" c/c between. A ridge beam spanning 8'-0" would support the typical rafters adding load to the A Frame at point C, resulting in a substantial moment at points B and D.

The roof could be framed in other ways, but the above suggestion appears best to me.



BA

 

[DenverStruct]

The only reason I mentioned ridge beam is because you mentioned you wanted to make the tie beams as architectural.

 

[dhengr]

Sam37:
I agree with BA, you should work to determine your general framing arrangement before you worry about the number of nails in a strap or connector. The church is 26’ wide and 52’ long, with 8’ high “timber framed” walls, or are they dimensional lumber framed walls? However, it is good to know the approx. sizes of your framing members, and the types of connections you might use. The tie member is more effective when it is lower than 10’ from the ridge, and it induces less bending in the rafters. BA shows it about 4’ above the top of the exterior walls, the rafter bearing elev., and that would be good on this steep a roof. You also have to account for the thrusts from all the rafters at the top of the ext. walls or the walls will really bow out on a 52’ length. Those thrusts will accumulate at each rafter pair with a tie, and add to the tie forces. Their load path is the double top pl. on the wall, acting in horiz. bending. This problem is eased a bit if you use 4’ instead of 8’ spacing for the rafters with ties. This also gives you more places to hang lighting, decorative banners, and ceiling fans, etc. I would add a king post, and make these look like real timber framed trusses, and forget the logs, as ties. I would use a ridge board because it makes installing, spacing and lining up the rafters easier, and it gives a vert. (plumb) surface on which to apply some sort of connector angles as hardware. I don’t think they have to be a full fledged joist hangers of the proper width (1.75”) and seat angle, that seat wouldn’t be effective in any case. It would/could also provide a horiz. bearing seat for the rafter, but I would not consider it a ridge beam, only an erection convenience. That’s a gigantic volume for a small church to heat and cool, all because some Arch. has a big ego and grand esthetic scheme.

I’d look at framing that roof with 2x lumber at 16” o/c. You should be able to get 2x10’s or 2x12’s in 22’ or 24’ lengths out your way. You have some room for insulation, a vapor barrier, and can treat the undersides with any number of ceiling treatments. Then, along with the exposed horiz. tie members, add a couple 3x4 exposed rafters and a king post to make it look like a real timber framed truss.

 

[BAretired]

Some excellent suggestions from dhengr.

Another concept is to eliminate intermediate rafters, using only frames at 8'-0" and a timber deck. Two inch deck (nominally 1.5") will span 8' but is a little limber and 8'-0" is not modular with 52', although 5 spaces @ 8' with 6' end spans would work. My preference would be 8 spaces @ 6'-6" = 52' which is an easy span for 1.5" deck.

Another possibility is a three hinged arch in lieu of A-Frames. Two hinges at ground level and one at the crown is a commonly used concept for churches (points A, C and E below):

BA