vaulted roof load path

[PE_JRM]

I am renovating a small very old cabin (for myself) and am spinning my wheels on the issue of wind loads. The place is 680 sf (34x20) and has a 3:12 roof slope. I would like to put a vaulted ceiling in the great room to enhance the feeling of it being spacious even though it isn't. I'm planning a structural ridge beam supported by a post in an exterior wall and a post in an interior wall. The ridge beam span is 20 ft (plus or minus).

So, I've been looking for a simple way to do the wind analysis and then design the structure. The AISC procedure seems ridiculously involved for a small building which is basically the size of a shed. Is there a simple analysis approach that gets me there? This is a simple structure and simple hand calcs, well thought out load paths and span table should be sufficient.

Residential construction was never my area. I changed careers from civil structural to aerospace structural in the 1980's so my knowlege and my technical resources are both outdated.

 

[jerseyshore]

Idk the wind speed where you're at, but small residential structures like this are rarely analyzed for wind in my experience. If you have good plywood sheathing nailed per the IRC that's basically the extent of 99% of residential wind design.

The main thing to look at wind-wise when vaulting an existing structure is out-of-plane wind on the now unbraced gable end walls. Without the ceiling to brace the top of the end walls, you have a hinge that is likely to cause issues over time. Run a stud calc for the wind pressure. I usually just specify new full height wall studs for that situation.

 

[Eng16080]

For a small structure like this, assuming the exterior walls are well constructed and there aren't too many door/window openings in them, in terms of the actual structure resisting wind loads, I doubt you'll have any issue. I don't think the modification will make much difference in terms of how the structure resists wind loads, with the exception of the end wall issue that jerseyshore mentions above. That's a really good point and something you should not overlook.

In terms of providing an actual wind analysis for this, based on my particular geographic location, I'll sometimes just use a conservative wind load like 25 psf applied to the windward wall/roof area. If the structure can easily resist that, I'll stop there. If these conservative loads are a problem, then I'll refine my approach further by doing a more in depth analysis.

By the book, you want to use ASCE 7, which is what I think you meant above (not AISC), particularly Chapters 26, 27, and 28 for wind loading (for the 2016 version of the code). Chapter 28 provides a simplified method. I would look at "Part 2" of that chapter, which starts at Section 28.4.

In terms of shear wall design, if the cabin is very old, then the walls are likely not plywood, but rather horizontal boards. Per code, horizontal boards have very minimal shear capacity. Still, your wind demands will likely be low, and the boards may well work fine. In terms of the full load path, you'll want to be sure that the shear walls, especially the ends (building corners) are adequately anchored to the foundation.

Assuming the structure is generally in good condition and there isn't evidence of damage from past wind loads, many engineers will avoid an in depth analysis here and will simply conclude that the structure is adequate to resist the expected wind loads considering that it has apparently done so in the past. This is of course a matter of engineering judgement, and I'm not necessarily endorsing this approach, although I can see the logic. If it was my cabin, I'd probably do a quick analysis and look for any weak spots in the structure which might need to be strengthened. Being that you're renovating, this is the ideal time to address that.

Overall, the design of the new ridge beam and it's support down to the foundation is probably a more critical aspect of the structure than the wind resistance.

 

[PE_JRM]

Thanks Jerseyshore. Years ago I lived in Barnegat (Manahawkin. actually. I worked at Scout camp in South Jersey in the 70's

Good point about the gable end wall - I hadn't thought about that. The span from the foundation to the apex of the gable is only 11 feet but that is a long span for a 2x4.

As you noted, properly nailed plywood sheathing should give me enough shear (wall) capability. I do have a sliding glass door and a large window in the gable end wall - hopefully I've got enough remaining wall there. My judgement is that its OK since 10 feet of it will be fully sheathed - I'll have to check it.

Basic wind speed here (Atlanta. GA) is 105 mph I think. I looked it up before we bought the place a while back. I'll have to verify that.

The problem I'm concerned about and one of the reasons for my post is that I'm vaulting the great room but not the rest of the space. So one end of the structural ridge beam will be supported by a column enclosed in an interior wall. Since the place is so small and almost everything runs through that one interior wall I really didn't want to have to sheath it in plywood if I could help it. Actually, though I don't think there is another way to do it right. Ideas appreciated.

 

[PE_JRM]

Thanks Eng16080.

The existing structure is as minimal as it could be. 2x4 walls with no sheathing on the outer surface and hardboard paneling on the interior (no drywall backing). It is covered in aluminum siding which in my judgement has minimal if any shear resistance. The roof is corrugated metal, no sheathing, no insulation, supported by purlins that run longitudinally and are in turn supported by the gable ends and one interior wall. Like I said, its built like a shed.

What you suggest for a basic analysis is what I had in mind. I had planned a wind pressure (postive and negative) applied to the walls and roof. Chosing an appropriate pressure is the issue. Is there a prescriptive pressure required by some codes? I'll check out the ASCE simplified method you suggested ... thanks. You are right about meaning ASCE instead of AISC. I spent my construction design days designing steel structures. Among other things I was the company platform "expert" so AISC was my main resource.

Honestly the structure is not well designed OR in good condition. It would be easier to tear it down but I am not permitted to do that. It is located in a community organized like a condo association even though its a seperate building. I can renovate it but not tear down and start over. The project is an exercise in engineering creativity. The foundation is an unreinforced slab on grade (below grade actually) and I have to be very careful where I send loads. The columns supporting the structural ridge beam will both get new footings. I'm planning to pour a reinforced concrete perimeter beam on top of the existing slab to raise the floor elevation about 8" (its below grade now). I'm not allowed to change the grading. A new slab will be poured on top. Flowable fill and rigid insulation will fill the void between to old slab and new slab.

 

[jerseyshore]

I frequent Manahawkin often; not sure if you have been back in the last 10 years, but Stafford Twp is a completely new place compared to 15 years ago. Lots of new homes and a busy busy area now.

Like I said I usually just add new full height studs on those gable end walls. Not hard to do and cheaper than any facing connection details. Hinges in walls will always show up over time.

Adding some plywood is probably your best bet on that interior wall. It's okay if the ridge stops short, but you just want to make sure the gable side is well constructed still. Collar ties and rafter ties properly installed and nailed are key, even though I'm sure snow loads are minimal by you.

 

[Eng16080]

PE_JRM, Ok, forget some of what I said earlier. I think you'll want to be a bit more cautious than I implied, considering the lack of any substantial exterior sheathing.

In terms of wind pressure, it's not so much that you choose a wind pressure. Rather it's calculated based on several variables.
If you just try to follow Part 2 of Chapter 28, I think you'll probably end up with something reasonable. Off the top of my head, based on your location and wind speed, I wouldn't be surprised if you end up with somewhere around an average wind pressure of 15 psf. You can use that number as a rough sanity check!

Worst case, if you need shear capacity but aren't allowed to modify the exterior, you can add plywood along the inside face of the walls, perhaps strategically placed in a few spots. You might even be able to justify using drywall to resist shear if the demand is low enough.

 

[PE_JRM]

jerseyshore,
I lived at the shore for the 1981 renovation of the power plant in Forked River. I was the field engineer for the steel saddle upgrades under the torus in the reactor building. I lived in Metz trailer parknear the causeway. It seems that its now a WAWA. I was back a few years ago for a traing class at the FAA tech center in South Jersey and the strip from manahawkin up to barnegat was unrecognizeable to me. Probably rebuildingafter the hurricane.

Thanks for the tips. That particular gable end is currently a porch with a lot of rot. I am rebuilding that portion anyhow so no big deal to make a shear wall out of it.

 

[PE_JRM]

Thanks Eng10680. I am familiar with doing wind calcs but I haven't done it in decades. I remember a lot of coefficients etc. Back then I designed tall watertowers, storage tanks and smoke stacks (some in coastal areas, probably one near you). Anyhow, what I meant by the word choose is using a very conservative figure based on the guidance that simplifies the calcs. Te AWWA code was easy and straight forward. Admittedly a watertower or smokestack is a good bit different geometry than a building.

 

[XR250]

I think actually doing wind load calculations on this project is a waste of time. Just add some plywood and be done with it.
The building code has tables for how many feet of plywood you need. I imagine you could just wing it and be fine. I mean the building has held up this long without calculations.

 

[jerseyshore]

I think I stopped at that Wawa just a few days ago. Lots of changes post Sandy, but also post COVID. Working from home has really changed these shore towns from super seasonal to active year round. Stafford into Barnegat are real busy towns with the schools maxed out these days.

 

[PE_JRM]

Thanks XR250. I agree and don't want to do any extensive wind load calcs. The point of the post is getting a handle on how to wing it. You may have missed the fact that I'm a Civil Engineer who switched from typical CE structures 35 years ago. I've done engineering work on airplanes and helicopters since that time and they are similar to other typical civil type structures but certainly not the same. I don't have the gut feel necessary to wing it as you say unless we're talking aircraft repairs. So, perhaps you could give me the building code reference you mentioned so I can look at the tables you mentioned - that would be helpful and is the sort of thing I'm looking for.

Your point about the building holding up all of these years and being fine would normally be something I might say. You may have also missed from the previous posts above that a) I am significantly changing the load paths -all of the interior walls are being removed and the ceiling will be vaulted, b) I am significantly increasing the loads and c) the existing foundation is barely adequate as it is. As such, I need to know what loads I'm dealing with so I can make the necessary changes accordingly. If I assume a certain load path that load path needs to be strong enough to carry the load. This is a one story building with nearly no snow load. Wind may govern - who knows until you at least estimate what the loads are.