Wood Framing vs Structural Engineering 3

[JAE]

I’m sure a lot of you have heard about the significant flooding occurring in northwest Iowa, Minnesota and South Dakota recently. I came across this photo of a house that had been significantly undermined by the flooding.

We’ve discussed here in the past about how we structural engineers love to follow load paths. We take the low down discrete, roof, trusses, wall, top plates, studs, and footings, etc. This photos sort of brings home the fact that Wood framing with sheathing act more like a, set of rigid boxes tied together, then individual studs, taking loads like columns.

Just an interesting photo I think.

 

[SinStrucEng]

It’s truly impressive how resilient and forgiving wood framed structures can be.

 

[SWComposites]

Yes, but no snow load ………

 

[phamENG]

"Can" is the operative word. For every house that can do this, I'm sure I've been in 4 that would have come down.

The redundancy involved and 'alternate load paths' are certainly good to help me sleep at night, and helpful to explain real world circumstances, but designs should still follow discreet load paths.

 

[jerseyshore]

I think like pham; seeing structures stand that shouldn't stand on paper helps me feel good about designing a proper structure and maybe getting some not-needed but appreciated supplement strength. Then when I am working on a structure that lacks redundancy, it forces me really dive into all of the details and double up on what I'd normally do.

 

[dold]

@JAE

I get where you're coming from and I agree. I'm sure most of us do. The house hasn't collapsed and killed everyone.

But you can't tell me that you'd want to live in a house with every piece of sheetrock cracked in half and every window is jammed and every tile is broken. Serviceability is half the job.

 

[RontheRedneck]

Like SinStrucEng said, It’s impressive how resilient and forgiving wood framed structures can be. I've seem some really stupid things and wondered how in the world they held up.

Although it's worth noting that this house has only been there a short time. Given some time I'm sure it will start falling apart.

 

[Eng16080]

Without the foundation, the floor and wall sheathing materials (including drywall) are providing an alternate load path.

It is amazing, but I doubt the actual loads on the structure are anywhere near the design loads.

 

[DoubleStud]

Probably has interior walls that act like beams now and the floor is minimally loaded. From the photo it looks like the house sits on a concrete slab?

 

[DaveAtkins]

The power of redundancy! All those things we do not count on but do participate in making a structure stable and safe (but maybe not serviceable...).

DaveAtkins

 

[lexpatrie]

Wish we could have put some instrumentation in it before the event. This one also has the "enough" back span of structure to keep it from outright tipping, as well.

Finally a good use for exercise equipment (and an excuse not to haul it away). "Honey, let's keep it, it works as a counterweight if the foundation washes out." (Plus, I'm tired and it's heavy).

 

[JAE]

But you can't tell me that you'd want to live in a house with every piece of sheetrock cracked in half and every window is jammed and every tile is broken. Serviceability is half the job

No I wouldn’t want to live in a house all cracked up. I also agree that we engineers should stick to discrete load paths in our designs.

But our discrete load paths would tell us that house couldn’t cantilever out that far. Just a reminder that there are other load paths (secondary) that do exist. Sometimes they help and sometimes they hinder.

I once saw a discussion about a concrete slab bridge that had been abandoned (road had been rerouted) and a local university thought it would be appropriate to test it to failure to see how well the calculated design capacity matched the actual. Turned out they loaded it to 8 times the theoretical capacity before it failed. The integral concrete guardrails apparently affected its flexural strength.

 

[phamENG]

JAE - I'll argue semantics here because I think they're important.

No engineer should be saying "It can't cantilever" or it can't do this or it won't do that. Anyone who understands the codes (as I know you do) knows that isn't what the numbers are telling us.

We should be saying "We can't rely on it to cantilever."

 

[lexpatrie]

Well, I'll see your "I'll argue semantics" and I'll raise you one.

But our discrete load paths would tell us that house couldn’t cantilever out that far.

No engineer should be saying "It can't cantilever"

JAE didn't say "It can't cantilever" as you put it in quotes. (I know you meant a paraphrase, but it's not an actual quote, what JAE said with our discrete load paths ..... the house couldn't cantilever out that far.

I'll also put this out there - we don't know age of the structure and further, is it a residential code (or before the code was adopted) building.

In favor of it working, lets also point out - no design snow load, no design roof live load, and perhaps no second layer of shingles, no attic load, and no design live load. And no "contingency" dead load. Given we are looking at (potentially) collapse prevention, one might consider a safety factor of zero. But what we know about the construction is about nil. It's probably wood frame, maybe fiberboard exterior sheathing, lath and plaster interior (or not), but the thickness, nailing pattern, etc., these things we don't know.

 

[phamENG]

JAE didn't say "It can't cantilever" as you put it in quotes.

Nor did I say that he did. That's a hypothetical quotation from 'any' engineer. Note that I did not cite that quotation to JAE. That would have made it incorrect.

Alright. I'm done.

 

[XR250]

Well, along the same line of thought, this is one I ran into today. This is the end of a triple truss girder spanning 23 ft. and supporting the rear of the house (9.5k calculated design load). It has been like this for 15 years. The best part is, it is literally sitting on the subfloor with no foundation below it.
Now that is some redundancy.

 

[phamENG]

uhhh...why are they all at different levels?

 

[XR250]

I have no idea. Maybe not even nailed together?

 

[jayrod12]

Maybe it's just the picture playing tricks on my eyes, but it looks like that double plate is deflecting. I also love the shims. Why couldn't they frame all the studs to the same height. Mind boggling.

 

[XR250]

It def. is deflecting.
I'm having them put a 3 1/2x7 PSL column under it and a 6x6 on a footing