Influence of Exterior Cladding on Structural Performance

In my specialty of Construction and Structural Forensics, I often run into the influence of exterior cladding on the structural performance of buildings, particularly with wood frame and cold formed steel structures. Deterioration from water intrusion sometimes reduces the structural capacity to the point of collapse.

Do any of you, as structural engineers, provide any input or specification during the design phase to reduce such damage?

Ron… as an SE, other than water stops in concrete, basement wall waterproofing, and Interstitial mastic for tilt-up walls, no. I leave that to the Architect or designer as I view it as their responsibility. Seldom do have sufficient window, door, flashing information to do that anyway.

Also, professionally, I consider envelope design to be outside my area of expertise, although one of my good SE friends in Seattle is also an expert in the field. Just not my cup of tea. However I am game to fix any structural damage.

To quote a non-SE… “A man has to know his limitations.”

In our company, which performs mostly in the heavy industrial space where aesthetics are of little concern, we include architectural considerations and work within the civil and structural group. The SE of the project ultimately reviews and buys in to the architectural designs and choices made on cladding, etc, before the drawings are issued. It’s a process that makes the most sense with the way we perform our work.

In cases where the client retains an architect separately, which admittedly are rare for us, that scope falls squarely to them. We tend to review their work for constructability rather than adequacy in those situations now that you mention it.

Like MSQAURED, I leave it to others. Vancouver was kind of the epicenter of the building envelope crisis. Nowadays, you won’t see a set of structural drawings in that city that doesn’t have a big disclaimer on them about how the structural engineer expects that a building envelope engineer will be engaged to do that stuff. Our insurers also also ask a lot of questions about building envelope and it is my understanding that an SE will be in for a sizeable rate hike if they admit to taking on that scope of work. Many of the larger firms have reacted to this by simply developing/acquiring building envelope expertise and offering it as an appropriately priced additional service.

@KootK
Good to see you back!
I agree that structural engineers should add a note on their drawings putting the responsibility on others; however, they must also be cognizant of the impact it may have on their structures. Many don’t have a clue what affect their structure will have on a cladding and vice versa. To give you an example…

In a recent litigation case, the structural engineer who designed the two-story condominium structures (20 buildings) in wood framing, stated on his drawings that the walls were designed for L/240 deflection, which is the general minimum by code. He knew that the exterior cladding would be stucco, so he should not have put that statement on his drawing (stucco requires L/360 minimum). It got him sued! Fortunately after he and I talked a bit and he ran calculations to show that the walls would also meet the L/360 requirement, he was released from the case, but he had to inform his insurance company and his appointed lawyer got involved and he had to pay out his deductible as well as having another mark against him for making a claim against his insurance. Not good. He’s a good engineer and does a good job with wood framed structure design, but it just goes to show how easily a small issue can become a big issue in litigation.

To play devil’s advocate a bit…

  1. Tracking down the appropriate deflection limits for various systems is quite the pain in the butt. That information seems to be scattered all over, isn’t always consistent, and isn’t uniformly applied by practitioners.

  2. I find it bizarre that a structural engineer could be successfully sued for any design failing that doesn’t fall into one of these two categories:

a) A requirement expressed by the code OR;

b) A requirement expressed by the owner, often via another member of the design team.

In my mind, this means that one of two things should be the case:

c) Codes should rigorously set deformation limits and view deflection related to building envelope performance as a safety / durability issue rather than a serviceability issue.

d) All projects should have someone on board capable of setting the appropriate deflection requirements as they related to cladding and reviewing the structural designs to ensure that they are met.

  1. Obviously I want my projects to be holistically successful and, therefore, I do try to make good decisions regarding deformations that would affect cladding. Most of us do. That said, I certainly sympathize with those who find themselves in hot water over this stuff given the current setup of things.

In the US, two codes are prominent…the IBC and the state variants of the IBC. In Florida, the IBC is the guide but we have the Florida Building Code. In the FBC, as with the IBC, stucco is required to have a deflection limit of L/360…he stated L/240…that’s why he got sued. It is a code requirement to meet L/360 or better for substrates supporting stucco. It is in an administrative portion of both codes (Chapter 1) which states that if a standard is referenced in the code, its provisions are applicable as if contained in the code in their entirety. In the case of stucco, it is required to follow ASTM C926, which states that the deflection limit is L/360. So his being brought into the suit is your provision 2a.

The L/360 criteria has been in play for years in the UBC and IBC for ceilings supporting sheetrock, and, personally, I extrapolate that to window walls having sheetrock (GSB or GWB), and all brittle siding materials, using L/600 minimum for brick. Personally, I prefer L/1000 for brick.

Deflection goes two ways…

That’s kind of my point Ron. If it takes you an entire, meaty paragraph just to describe where one should go to find that information, you can bet that a lot of folks haven’t found that information. Frankly, I myself had always thought that the IBC deflection requirements were intended for horizontal framing. As you well know, conventional EOR work is not much like legal stuff. It’s incredibly fast paced and fee competitive and we typically have to specify five things for every one that we actually “design”. Not many folks will be spending time to go on an ASTM standard Easter egg hunt.

An interesting thing happens with multistory concrete condos, at least on the west coast of Canada. You’ve got at least three constraints in play:

  1. If you ever plan to do another project of this sort, you’d better be making the repeating slab on your current project an 8" thick max plate plate with vertical support no closer than 25’ oc.

  2. If you don’t want to foul up the glazing contract, you’d best design the perimeter for no more than 3/8" differential vertical deflection.

  3. If you don’t want to foul up the glazing contract, you’d best design inter story drifts so that they apply no more than 1/2" racking to the glazing.

Speaking as one who’s been involved in the design of these things, #2 and #3 aren’t realistically achievable in a high seismic environment with #1 as a constraint. Yet you’ll see every set of drawings going out with 3/8" vertical and 1/2" lateral given as direction to the cladding guys.

Without pulling punches, I think that this is actually willful deception. And I think that it’s brought about largely because there really is a disconnect between structural engineers and cladding designers. Either cladding needs to be able to tolerate greater movement or the cladding industry needs to be a bit more forceful in ensuring that their needs are met in a way that is clear, easy to find, and uniformly presented. Structural engineers will do the right thing so long as they are confident that their competitors will also be doing that right thing. That’s how the game works. As with most human activities, a level playing field and clear guidance begets good results.

Regarding brick, Canda’s apparently done some research and backed our recommendation off from H/720 to H/360: https://cssbi.ca/assets/resources/Technical_Bulletins/CSSBI-TB-V7N6-06.pdf

Discussing deflection takes us to a whole discourse between structural adequacy and serviceability. In some respects it is similar to designing concrete for durability as compared to compressive strength. Generally, if you design a concrete mix for durability, the compressive strength will be adequate or even greater than needed. If we designed to limit deflection the strength would almost always be sufficient.

In the codes we give serviceability little attention, but that’s what makes buildings last.

Would it be too much of a cost compromise to pay more attention to serviceability?

@KootK…you make a good point with this; however, the standard of care expectation is the you chase things like this down the rabbit hole.

Sure, that’s the standard of care legally. I would contend that the real question, however, is really whether or not that’s reasonable. For example, consider me:

  1. I probably practiced for about fifteen years not knowing that ASTM C926 existed with a deflection requirement.

  2. About fifteen years ago, I discovered ASTM C926 because I got frustrated trying to chase down all of the various serviceability requirements and finally found a book in which a bunch of them were aggregated.

  3. I subsequently re-forgot that ASTM C926 until you brought it up here because it simply is something that is never, ever discussed by humans in practice. Am I a negligent structural engineer? I don’t feel like I am.

Consider that the requirements of ASTM C926 will usually create an obligation for me without any human being having personal knowledge of my projects ever actually realizing that is the case. Rather, an architect will select a component from some masterspec database and that will trigger the reference to the ASTM spec. And then I’m on the hook, even though the only humans to have given consideration to the issue are the ASTM authors and, indirectly, the folks who created the masterspec database, neither of whom are involved in my project. So, somehow, I have to retrieve all of the relevant project information from robot land and ensure that I, a mere human, have it all covered.

I am of the opinion that there should be no such thing as specifications. If you care about something, find a place for it on your drawings.

Now that I do some precast work, I find myself a consumer of specifications rather than just a provider of them. And it is bloody aweful. I’ll read any precast section of the specs or notes start to finish. And I’ll do a bluebeam search for precast, prestress, PC, P.C., and hollow core to catch other things scattered about the specification. I’ll catch most things but not all. But I certainly don’t have time to read the non-precast sections front to back and keep up with all of the updates to the non-precast sections. That, especially when it’s clear that half of the stuff put into the spec hasn’t really been given any meaningful attention anyhow. Much of it will contradict the drawings.

Not at all. And where problems have arisen, I would say that they are more a function of designer ignorance than they are of cost consciousness.

Over the years, I’ve had the opportunity to review several internal design standards guides put together by top tier firms. But I’ve yet to see a comprehensive section on serviceability requirements other than the rote stuff for horizontal systems. I think that’s telling.

This is the book on serviceability criteria that I mentioned previously. It’s decent, especially in that it has little meaningful competition.

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The standard of care issue relative to waterproofing of the structure reared its ugly head again last week. My partner and I were on a conference call with a very good construction defect attorney. After the litigation issues are resolved I’ll post the discussion and the resolution. It might be interesting in the event you get tagged in a design/construction defect lawsuit.