Cantilever(?) Wood Sill Plate 1

[JoshH726]

I was provided the attached figure considering for single story, lightly loaded (SDC A, 115mph wind) wood frame office building. My initial concern is uneven bearing stiffness between the concrete and insulation would lead to cantilever or cross grain bending in the sill?

This is similar to Figure 1 in the attached article I think pulled the picture from.

Let's say the minimum "stem" width of 3-1/2" to allow for a screw anchor, and would fit a washer. This would be a 2" "overhang" for the sill plate. Since the bearing stiffness is non-uniform below the sill plate, there should be a nonzero amount of bending in the sill?

Another issue appears to be if shear loads in the wall exceed 400 plf, I'd need the washer to be within 1/2" of the loaded side. This seems to then be the far side of the wall. So custom cut washers?

Anyone else seen a detail such as this? Any thoughts from field install?

Thanks in advance

 

[phamENG]

I understand and appreciate that energy efficiency is important and needs to be a high priority in overall design, but goodness...these people are going maximize the energy efficiency right into a pile of collapsed studs and sheet rock.

There are a lot of things for a structural engineer to loathe about that detail. But I'm not as worried as you are about them provided the things you say are true: essentially zero seismic loads and a hurri-what? For a simple one story house you could probably get away with it. Consider the fact that structurally you don't need 2x6s for that kind of a building. I'm in a decently high wind area and we build with 2x4 walls all the time. So your 3.5" of bearing area is enough. As for the cross grain bending - not a concern for gravity loads and only a concern for uplift if you put your uplift load path on the interior. For the sill to bend like that, whatever is applying the load would also have to bend to KEEP applying the load as it bends. The studs won't do that. They're much stiffer and will control the distribution of load through the connection. That means that you either a) assume the stud is structurally only a 2x4 or b) you design it as a 2x6 with an eccentricity of e = (5.5"-3.5")/2. For lateral loads, you just need to keep it to the exterior (I'm very dubious of the reliability and longevity of gyp shear walls). The Huber ZipR system has insulated panels with OSB skin and published values for shear walls to be used with them. I believe there are other products out there that tout shear wall panel strengths in rigid insulation boards.

Another thing to think about in all this is that the foam can actually have a structural rating. Especially the under slab stuff (which most of us have used for years) comes rated for floor loads. So that helps a bit, too.

Now, the anchor is a bit sketchy. There's about enough room to set it perfectly - and maybe not even that much.

 

[JoshH726]

Good points phamENG, appreciate the response. Yes, I don't like relying on the gyp, but from the photo, showing it inside, that where my thoughts followed. I'm familiar with the Zip boards and have used them on previous projects.

 

[phamENG]

I don't like relying on the gyp for two reasons: 1) it's too easy to remove without realizing and your average handyman will probably laugh away the hold down because they don't understand what it means and 2) it can disintegrate when exposed to water, leaks happen in storms, and storms are when you need shear walls the most (EQ not withstanding, but I don't think gyp is allowed in high seismic anyway).

Unless you can require (and verify) the use of exterior rated, water resistant gyp panels (not just "green board") for interior shear walls, I'd avoid it like the plague.

Anyway - best of luck with that detail...

 

[dik]

They might be better served by having insulation on the outside face of the concrete wall with parging... and 1/2" extruded polystyrene between the wall and the slab...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[phamENG]

dik - good point. There's a nice thermal bridge from concrete to anchor to wood to inside. Run the insulation right down the exterior face and 90% of the problems go away.

 

[Enable]

dik / phamENG if you look at the document the OP links to you will notice an alternative detail such as the one dik proposes. BTW the author of the attached document is Joseph Lstiburek who is more or less the world's foremost expert on building science.

Note that he provides pictures of each detail in action so these are not just theoretical constructions but have been put into practice. I would also favour the monolithic detail, as would Joseph Lstiburek (if you read his literature on continuity), but sometimes that's just not practical for whatever reason. Again note that he takes great care to highlight the need for HL XPS (rated for load and creep) plus well compacted soil with stable properties as prerequisites for the proposed detail.

Would I try this detail? Probably not. But not because it's not sound from either a building science or structural perspective but because I wouldn't trust the dude compacting the soil or placing the insulation.

 

[dik]

Coming from a colder climate... we do that often... rather than have the top 6" or 8" exposed to -40C...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

I didn't look at the attachment... I've not heard of him (maybe a sheltered life) but authority or not... some of the insulation patterns shown in his paper are impractical or maybe even silly... Best to have the insulation as far from the interior as possible (and not in the neighbour's yard)... You want to insulate the building envelope. Also, the only time I insulate under a slab is if the building is unheated or if a cold storage building where the waste 'heat' is recycled beneath to prevent freezing over time.

You can bear on extruded foam... DOW makes products for that... some of them having a fairly high compressive strength... except for pavements, I normally don't do it...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[JoshPlumSE]

dik / phamENG if you look at the document the OP links to you will notice an alternative detail such as the one dik proposes. BTW the author of the attached document is Joseph Lstiburek who is more or less the world's foremost expert on building science.

Never heard of him. However, I did read most of that article. Now, I strongly dislike him. What a horribly written piece of trash that article is. It's clear that he doesn't understand structural engineering at all and also has little respect for it.

That being said, I kind of agree with PhamEng. If you don't need 2x6's then the detail doesn't really bother me.

 

[Enable]

dik the author of the article is Canadian and was the director of HUDAC (Housing and Urban Development Association of Canada) prior to being blacklisted for warning the public about how an energy incentive program was leading to greater risks of CO deaths. Again, many of the details in the linked document will work for the Canadian climate.

As an aside: Joseph Lstiburek started out building in Northern Ontario because there were no building codes at the time and he could try out his modern - for the time - details...not all were good ideas!

 

[dik]

Josh... beat you to it by 2 minutes... same impression about the paper...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Enable]

JoshPlumSE: Agree that Joe has little respect for structural engineers and our profession. And to be fair, I would say probably not without warrant (of course only those in the guild can make jokes about their own so your reaction is not uncommon). Disagree about the contents of the article as it pertains to building science. It is generally correct in those details and his writing style is enjoyable.

 

[dik]

If his other work is of the same calibre... I can see why he may be scorned by 'real' engineers.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

mildly confused...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Enable]

The article attached to the OP is a parody of a real article published in ASHRAE as most of his online articles are. You may not enjoy that style in which case you would be better served by reviewing the ASHRAE equivalent.

BTW he wasn't scorned for his engineering work but rather being a whistleblower. People were literally dying due to ill-conceived energy retrofit programs and he wanted to let the public know about it.

I can appreciate how you may not like the guy when his article starts with "Structural engineers love it when someone talks to them…because things are typically very boring for them." Meh. I suppose I have a greater sense of humor about our profession.

 

[dik]

I guess the new expression is, "Those that can, do, and those that can't, blow whistles." BTW, a lot of the recent deaths in Texas because of the freeze were attributed to CO...

Josh... in cold climates, where you need the insulation, you need 2x6 and 2x8 framing.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Enable]

....if a cursory review of a single article that you took but minutes to evaluate gives you confidence in forming an opinion about the life work of an individual....I mean, good luck with that outlook. It's surely not one I ascribe to. But each to their own.

If anyone wants to actually discuss the building science details feel free. Otherwise I will bow out of this thread.

 

[dik]

Outside front door needs repair... will do it shortly... effects shown on inside door at -37C

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

I don't have a problem with that... he may be a good whistleblower... something fundamentally flawed makes him a 'bit suspect'. What's worse, he may give other people the wrong information.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik