What are the limits of prescriptive design? 9

[DTS419]

The IRC, as we are all familiar, provides prescriptive design standards meant to cover common construction of “one and two family dwellings and townhouses up to three stories.” Examples of this prescriptive design include connections such as from wall to foundation, headers over openings, etc.

But what are the limits of these prescriptive provisions, and whose responsibility is it to identify them?

Let’s take a closer look at connections to foundations, for example. It’s not uncommon for large custom homes that fall within the IRC’s scope to have finished basements with tall ceilings resulting in deep foundation walls with significant unbalanced soil load. There can also be significant uplift loads that must also be transmitted to ground depending on the proportions of the superstructure. These forces can easily exceed the capacities of the prescriptive provisions, that, if I had to guess, were developed long ago with simpler construction in mind.

It’s also not uncommon for many home builders to skip architects and engineers and simply follow the IRC. I’ve seen too many projects where this happens, and the result is connections that are over capacity, lateral systems without adequate diaphragm and shear wall detailing, etc. This often doesn’t result in total failure, but rather a final product that doesn’t meet current standards of practice, making it hard to call the builder’s attention to flaws with “the way we’ve always done it” that might be code compliant but not necessarily sufficient. And of course, failures can and do happen in the worst cases.

So what mechanisms, if any, are in place to ensure that simply following prescriptive codes are adequate for every situation, and whose job is it to identify when an engineered design is required? And, who is responsible if a code compliant prescriptive design ultimately proves to be inadequate for the situation?

 

[SWComposites]

if you have ever demo'd drywall (gypsum) in a house you would not want to use it structurally. its easy to tear it out with a hammer and small pry bar.

 

[phamENG]

SWComposites - that's not a reasonable judgement of a material's ability to withstand the loads as intended. I can pull plywood off of studs easily enough with the same tools. My issue is more with its resilience. The idea that a tree branch hitting the roof and causing a major leak in a big storm could render my lateral force resisting system ineffective in a matter of minutes when it is needed most is a bit alarming to me. Perhaps the odds of all that happening are a bit too long to worry about, but I don't know...just doesn't sit right with me.

 

[XR250]

It's gotta be better than that cardboard, stapled sheathing in the previous thread.

 

[phamENG]

I will grant you that, XR, but I do try to hold myself to a slightly higher standard than that.

 

[lexpatrie]

I'm lost, are we now talking about gypsum board as wall sheathing being borderline useless? That's not the case outside of high seismic zones, at least, provided the surface is continuous through the interior walls....

I have to reiterate, the gypsum board ceiling isn't allowed to be used as load carrying in addition to another sheathing material, i.e. the plywood/OSB roof diaphragm. The WSP roof is default stronger but comparable in stiffness. Gypsum board may functionally brace a hinge wall, but it's not an engineered path "we" can really use on a design, it just keeps it from failing in some scenarios, as a backup measure. I'm not sure why it's in the code, but there it is, been there since at least 2000. I'd expect it came from the UBC, but I've never bothered to track it down that far.

From my own personal practice, trees usually strike the roof, so the gypsum board isn't even there for the tree to hit. Even so, outside of a hurricane, the wind/rain combo is of some limited duration, and the wind isn't peak forever. A breach in a roof during a hurricane could experience, say, 8" per hour of rain, but we tend to have a vapor barrier, that could limit the water saturation of the gypsum board.

There are scenarios we don't design for.....

 

[phamENG]

Lex, I'm in a hurricane zone, so design winds only happenn during hurricanes.

I realize gypsum shear walls are a thing, but between the ease of modifying them without realizing the impact on the LFRS and their tendency to dissolve when wet, I avoid them whenever possible. We can file it under "engineering judgement."

 

[DTS419]

Perhaps my biggest issue with Table 2508.6: specifying NAILS for the gyp board.

 

[phamENG]

There's an allowance for using screws in lieu of nails. 80% of any table is in the footnotes.

 

[Mike Mike]

lex
great link as usual, basically telling homeowners that yes politicians and special interests screwed you over yet again, sorry maybe I'm too cynical I should reserve judgement. But the way this reads to me is here's a list of shitty things that are going to happen to you and could have been avoided if the limits of prescriptive design were clearly defined.

thanks for sharing the gyp diaphragm section, can't say I've ever perused thru chapter 25. where do you see cold form ties taking the tension load in the detail I shared? I'm not understanding how you're resisting out of plane suction without gyp board tension.

 

[DTS419]

There's an allowance for using screws in lieu of nails. 80% of any table is in the footnotes.

My point was that nails are a terrible way to hang drywall because they pop and become a cosmetic nightmare. That is from a practical rather than engineering perspective.

lex
great link as usual, basically telling homeowners that yes politicians and special interests screwed you over yet again, sorry maybe I'm too cynical I should reserve judgement. But the way this reads to me is here's a list of shitty things that are going to happen to you and could have been avoided if the limits of prescriptive design were clearly defined.

I have appreciated the dialog in this thread as much as everyone else, but here we are 88 posts in and yet without a clear definition of those limits.

 

[phamENG]

I think that's the point

 

[Euler07]

I have appreciated the dialog in this thread as much as everyone else, but here we are 88 posts in and yet without a clear definition of those limits.

The limits are whatever the judge decides on the day

 

[lexpatrie]

There's an allowance for using screws in lieu of nails. 80% of any table is in the footnotes.

Boy did that have me for a minute thinking there was an 80% value if the ceiling was attached with screws. Never mind.

great link as usual, basically telling homeowners that yes politicians and special interests screwed you over yet again, sorry maybe I'm too cynical I should reserve judgement. But the way this reads to me is here's a list of shitty things that are going to happen to you and could have been avoided if the limits of prescriptive design were clearly defined.

Where are those photos from? The document I mentioned by the APA?

Top photo looks like the front has a sort of minor vault in the front half, also note there's a lot of shingles missing from that garage and an apparent sheathing loss on the back side, the sequence of failure isn't all that clear, there's more than one way for that to fail besides it 100% being a defect. Some of this "drive by" inspection stuff gets published as "real."

Second one looks like there's no ceiling, ever.

Third is the classic "window wall" with improvised framing where the building official either caught it, hence the photograph, or somebody fully took it apart after? There are some good guides on tall wall construction, e.g. Weyerhauser, among others.

Even so, to me the takeaway there is this is an awareness on the general contractor and the framer as well as the building inspector. In a hypothetical diagnostic sense, you may see this flaw via thermal imagery without having to take the wall apart, as the solid framing will transmit heat better/faster than the insulation in those areas.

As far as the code goes, the studs need to be continuous, that's the continuous load path we've discussed, so it's a builder's defect/workmanship issue, for one. Bad G.C. supervision of subs, Sub at fault for defective framing, G.C. at fault for passing to the next sub, next sub at fault for "starting work constitutes acceptance" if they know what they are looking at, and a building official at fault for not flagging during the framing inspection (if they even require one) it as "requires engineering if you want to do it that goofy" (hinge wall).

Thanks for sharing the gyp diaphragm section, can't say I've ever perused thru chapter 25. where do you see cold form ties taking the tension load in the detail I shared? I'm not understanding how you're resisting out of plane suction without gyp board tension.

That's where all the stucco information is..... otherwise I'd not have known it was there, either.

Figure R804.3.7.1(1) (OK, I looked at it more closely and that does look like the gypsum board has to be in tension, I think the idea is the ceiling is continuous so there's a lot of it, and it's more of a plate so to me it looks more like a shear diaphragm. We'd still have tension, of course, just rotate the Mohr's circle 45°. But you have that diagonal tension in a shear wall as well.... that's a light gage people Figure they added to the code, not a globally applicable (i.e. it's not "authorized" for wood, although you'd see something pretty similar in the Florida Retrofit guide, or they strap with flatwise 2x from above in the attic.

DTS419 - it's there, you just have to follow the whole thread. We've kind of drifted on and off topic.

Euler - Not Judge - Building official.

 

[DTS419]

DTS419 - it's there, you just have to follow the whole thread. We've kind of drifted on and off topic.

We have indeed covered a lot of specific situations, but ultimately it would be preferrable if there was a concept that could be applied more generally. The codes clearly contain provisions that don't "calc out" and I'm sure we haven't covered them all, but it would be nice to know where we stand as engineers when we find ourselves in those grey areas.

At the risk if sounding naive, I'll admit that when I started this thread, I was hoping someone had already sorted through this and could provide an easy answer. But this has at least been affirming in the sense that I know I am not alone when I scratch my head at certain provisions of the code.

 

[Mike Mike]

lex
the photos are from your link Florida disaster preparation hurricane retrofit materials, maybe I clicked something on the site lol. the article explains that you are correct in that the most common failure is in photo 1: the roof ripped off the top of the gable wall and the issue here has nothing to do with the hinge.

with regards to the cathedral ceiling gable end wall, could contractor argue he provided a complete load path via double 2x6 top plates? in my opinion the IRC was followed perfectly by both the builder and the inspector and I disagree "it's a builder's defect/workmanship issue". "The construction of buildings and structures in accordance with the provisions of this code shall result in a system that provides a complete load path that meets the requirements for the transfer of loads from their point of origin through the load-resisting elements to the foundation. Buildings and structures constructed as prescribed by this code are deemed to comply with the requirements of this section." I agree with you the top plate is insufficient and probably of limited use because of splices, but how does the contractor know that? It's the IRC's job to tell him that and the IRC has failed.

DTS
if you could draw a concept that could be applied more generally out of this conversation I'd love to hear it. are you drumming back to the question of who is responsible when designs that follow code fail? lex says SEOR could be at fault because we are required to follow "standard of care", not the code. However, lex says he doesn't know of any cases of SEOR being held liable for following the code. lex also doesn't know of any failures that followed the code perfectly. kissymoose said he sees a lot of failures that followed the code. nobody else appears to have offered any specific cases one way or the other, but XR seems to lean toward saying failures are rare if IRC is followed in his experience.

I agree I used to be quite naive and the more I dig the more cynical I get. when I started this thread, I hadn't read the IRC and all I knew is other engineers had told me it's a "robust" and respectable code. I think this myth persists among our profession because none of us ever took the time to dig into it... until now.

 

[XR250]

the cathedral ceiling gable end wall, could contractor argue he provided a complete load path via double 2x6 top plates?

As far as I know, the IRC requires balloon framing at cathedral ceilings.

 

[phamENG]

That's R602.3, in case anyone is wondering.

 

[lexpatrie]

I may eventually regret saying this but... not really.

The wording requires a lateral support at the double top plate. Hypothetically if you can justify the ceiling as the support, the double top plate can terminate at the ceiling level and "resume" above with another set of studs. That second set gets lateral support from the ceiling and the roof diaphragms. (I think Gjinolli discusses that, too.)

What you suggest would be the most common non-engineered way to do it. Full height studs to the roof boundary.

I don't want to call that balloon framing, though, because there's no cut in ledger a floor joist lands on, and the floor joist isn't further nailed to the stud.

Edited to add: Wood truss gable end frames, Gjinolli, Vogt, Structure Magazine, August 2007.

Typo in author's name corrected above as well. I also created a FAQ entry for the article.

 

[Mike Mike]

lex, I can't find any Gnijolli on google, can you send a specific reference? I forgot about R602.3 and after re-reading it I agree with XR's interpretation: "studs shall be continuous ... to... foundation... floor, ceiling or roof diaphragm". I think you and XR are saying the same thing. XR was specifically talking about the cathedral framing without ceiling support. I retract my statement "in my opinion the IRC was followed perfectly by both the builder and the inspector". however, I find it odd IRC requires a ceiling diaphragm at the top plates yet provides zero guidance on how to construct a ceiling diaphragm in R802-wood roof framing. let me know if anyone can find any guidance hidden in here somewhere.

 

[lexpatrie]

Mike - I corrected the spelling in the post above, and added a link to it as well.

As a side note, if anybody is looking for "deep background" on the IRC, there's the 1992 CABO code fifth printing here, Table R-402-3a, page 60 of the PDF. (The FAQ for this forum has an entry for the 1992 CABO one and two family dwelling code as well).

It looks like a lot of the prescriptive nailing in the IRC was added, or the CABO got more detailed in the later versions.

Oh interesting, take a gander at R-404.7 and Figure R-404.9.