Rafter Ties in Utility Buildings Necessary? 2

[IlliniPE]

I have a client that builds portable shed buildings, and I'm wondering the necessity of rafter ties in high snow load areas (50-70 psf). I understand the building code is pretty straightforward on the need for rafter ties and/or a ridge beam, but I was wondering if there are alternative solutions, or perhaps the sheds are exempt from this requirement given the low risk to human life. The max width on the buildings is 16'. Ridge beams are not an option as the buildings are up to 60' long. The deflection at midspan would be far too much to handle. As always, they'd like to get as much headspace as possible, and it appears other shed companies in the area are not using rafter ties, only collar ties. That's not to say what they're doing is correct, but an engineer is signing off on the plans which raised questions for me.

So my questions are: Are storage sheds and other U buildings exempt from rafter tie requirements? If not, would the roof sheathing be enough to transfer in-plane stress to the support walls and negate thrust? I feel as though the small building width is compact enough such that deflections necessary to cause thrust aren't likely if possible at all. Thoughts?

 

[XR250]

Well the laws of physics still apply to storage buildings.

 

[jayrod12]

How low down could you have the ties before they become an issue? Perhaps on only 16 feet wide, having the ties above the top plate might not be the end of the world.

 

[XR250]

You would be surprised how many engineers around here still think "collar ties" in the upper third of the rafter space is all you need to mitigate thrust despite the code (and math) telling them otherwise. In some cases, it does work for small buildings, steep enough pitch and large enough rafters.
I imagine with the 2x4 rafters I see most often on storage buildings, the rafter tie would have to be pretty low down.

 

[Eng16080]

I'm currently working on a project with a low pitch which also happens to span 16 ft. The ceiling joists (or rafter ties, or whatever we're calling them here) are located not quite half the way up the rafters. Similar to OP, a structural ridge beam won't work. I ran the numbers, and to resist most of the outward thrust due to this condition I ended up needing to use LVL framing and connect the pieces with 24 wood screws at each end. In addition, my snow load is a little less than OPs. Also, I'm relying on other means to resist some of the thrust, as this connection alone doesn't quite work. At each rafter spaced at 16" o.c. the thrust force is about 3,000 lbs.

I'm not sure how you could justify not resisting the outward thrust in some manner for this utility building, especially considering its length of 60 ft. If it was shorter, maybe you could justify looking at the roof diaphragm as a deep beam of sorts.

Here's the detail I used:

 

[lexpatrie]

Collar tie is in the top third, is not really meant to do much besides satisfy the residential code, and are meant to keep the rafters together during roof uplift, in compression they'd buckle and they're in an area where compression doesn't develop anyway, if you analyze it. You don't have to have a collar tie as the IRC gives alternatives (a plywood gusset, [with no design guidance], or a continuous strap across the two rafters is also allowed in the IRC (not that a shed is IRC, but it's agricultural so it's in the neighborhood).

Rafter ties replace ceiling joists and both resist thrust on hand-framed roofs. The larger spans and the shallower roofs develop more thrust and that gets challenging to get a viable connection there (generally hardware is the way I go), as there's usually not enough depth for the forest of nails needed for say, 1,200 lbs of lateral load.

Your competitor not having them is disheartening, because I think they aren't "getting it." That or they don't allow for any snow or live load and exclude it from the warranty and hey, if it never snows it's all fine, right?

I can't find the thread I'm looking for, but this one is a close second, despite the incorrect term in the title (Roof Rafter Collar Tie Calculations/Design, 2019 (1,500 lb thrust there)

For a storage building there's a reduction on snow loads, that's where I'd focus so you can get the tie force low, and if they'll tolerate some deviation with the roof slope, those two variables are the most influential if you ask me.

I haven't put together a FAQ entry on them, this topic comes up often enough.

The Mathematics of Rafter and Collar Ties, Math Encounters Blog, Nov 29, 2010.

This thread has some nice analysis results (graphics) on the rafter tie location, ceiling tie location ("default"), and the collar tie showing it isn't effective at assisting the rafter. Collar Tied Roof Fail, 2019.

Yeah, once autocomplete in Firefox catches on after a few characters it's a sign....

Rafter Ties and Shallow Pitch Roofs, Jordan Truesdell, Journal of Light Construction, October 2008. 2,100 lb thrust there from the calculation.

Note there's quite a few articles on this subject and the notation has to be paid attention to. There are a lot of funky variable definitions that aren't obvious so comparing two approaches can yield different results if you don't read the manual. The one's I've looked at all "match" as I recall, but pay attention.

There's another thread that's pretty detailed but I'll have to hunt for it.

Source thread:
Wood Roof - Thrust, (June 8, 2020) (You can also get a taste for the modelling issues with this construction, it's a bit tricky)..

Mine (closed and with minimal adds):
Raised Rafter (rafter tie) analysis, Sept 26, 2023.

Part of the problem with this topic is the terminology is inconsistent and the folks here tend to call rafter ties collar ties and vice versa, which makes it harder to search for.

Also - I see a lot of "eh just use the decking as some kind of deep beam" while this is appealing in a thematic sense (and may actually account for some of the non-failures), this isn't anything that's been researched and I've never seen a rigorous analysis put forward (I guess the rafter at the end wall has to somehow "absorb" the load via the discontinuous planks.....

 

[Eng16080]

That's a really good summary above by lexpatrie.

In one of the threads referenced, a bunch of us show how the thrust force is being calculated. Based on my own calculation, I arrived at:
Horizontal Thrust Force: Fx = wL*L / [8*(SL/2 - h)]
Vertical Reaction and Each End: Fy = wL / 2
L - roof span
S - roof pitch (rise / run)
w - uniform load on rafter
h - height of collar tie (or whatever we're calling it) measured from bottom of rafter

(I'm not sure you're asking how to actually calculate the force, but in any case, there it is.)

 

[phamENG]

this isn't anything that's been researched and I've never seen a rigorous analysis put forward

Prepare to have your mind blown.

(Note: don't try this at home. The analysis is complex, the detailing onerous, and the chances of satisfactory execution with the typical residential contractor is somewhere between being struck by lightning and waking up tomorrow on a colony on Pluto.)

 

[IlliniPE]

Well the laws of physics still apply to storage buildings.

This is less of a question about physics, and more of a question about life safety. That's why importance factors, risk factors, and building classifications exist. There are many exemptions for utility buildings, so I was wondering if I had missed it.

I'm not sure how you could justify not resisting the outward thrust in some manner for this utility building, especially considering its length of 60 ft.

I agree 100%. The numbers just don't support it. However, if the code allowed it, I would see how they're getting away with it, but I don't see anything of the sort. The only thing that I could possibly think of is some sort of group action taking place with the plywood roofing to resist deflections enough that thrust isn't actually going to happen. I saw another thread that discussed plywood folded plates, but I've never explored design in this way. Also, I appreciate you providing the thrust calculation.

I see a lot of "eh just use the decking as some kind of deep beam" while this is appealing in a thematic sense (and may actually account for some of the non-failures)

Firstly, I greatly appreciate all the resources you provided! I think they may be playing the laws of probability on these and praying the buildings never actually see the snow loads they're "designing" to. That or they're on the life safety side of it and assuming minimal risk in case of failure. Nonetheless, I would be curious about the justification that the plywood is keeping the roof from failure. Common sense tells you that it cannot resist thrust any better than rafters can, but perhaps the answer is in a limitation of deflection.

Prepare to have your mind blown.

Too late, mind blown. THIS is what I was trying to refer to and what lexpatrie alluded to. However, the single bay example still notes using a rafter tie, so I don't see the purpose of the paper as its introduction states that there is no need when using folded plates. Perhaps I'm missing something or single bays just isn't a proper application.

 

[lexpatrie]

I suspect folded plate is the "sheathing fixes all" approach. I'll have to look.

Interesting document, I can't find it on the APA web site, but it's crazy detailed.

 

[phamENG]

IlliniPE - the thrust load still exists and still has to be resisted - somewhere. This method dumps it all out at he ends of the roof. So instead of a 1200lb thrust load at each rafter, you'd end up with about 27kips at each end of your 60ft shed. It's really not very practical at all in low slope roofs, but can be done if you use some steel tie members. (To use wood you'd be looking at a pair of Select Structural SP 2x10s with some nasty connections...)

lex - Sort of. It actually identifies the load path that many buildings actually rely on, and allows you to quantify it, detail, and hopefully have it constructed in a reliable way. Somebody on the forum has used it and shared some sat photos of the house. Said it's been standing for 20 or 30 years with no complaints. Anecdotal, sure, but still pretty nifty.