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?

 

[lexpatrie]

I still don't understand what you mean by "If you had gypsum board installed flush to the wall it could potentially support top of wall partially from bearing on the board edge as well." you mean gyp on the interior face of basement wall?

I mean the ceiling in the basement, if present. If continuous. And not in a "sealed engineering design" sense, in a why didn't it collapse *more* sense.

OFTDC? One and Two Family Dwelling Code? (so, CABO?) I haven't tracked the DNA on the IRC, or I've forgotten the origin story. That seems like something that should be, like, restored to the code.

what if your custom home drawings leave a blank bubble at top of basement wall, don't indicate any linework, and state "by others"? would you be comfortable stamping it then? are you saying laws generally permit a contractor to build per IRC because of his ignorance? but engineers are not permitted to specify per IRC tables unless they have taken the time to engineer it and verified it also complies with IBC, other codes, standards, textbooks, and calculations?

I would be more comfortable it it calculated out, either way. The difference is a contractor isn't qualified to perform engineering, so they're expected to a) conform to the code, i.e. secure engineering where needed and b) not do engineering unless qualified to do so, c) do the work in a workmanlike fashion (i.e. level and plumb within tolerance), d) safety during construction both for the structure - stability, and the workers, OSHA. e) adequately supervise subcontractors and pass work from sub to sub when they're "done". f) secure permits and comply with hour and wage laws, etc.

As a competent professional engineer, you are expected to hold life-safety paramount. If you specifically know something does not work, i.e. past experience (which might be difficult to prove, say, but the "board" may not be all that constrained by a lack of evidence (as those references at least lend the impression, with what looks like hearsay accepted as fact...), that "by other" is trouble, if you ask me.

"Engineering required for top of wall, XXX force, by other" is a different matter, "Permanent duration load for wood design" also sounds good. I think, the various state Boards may think otherwise, but there's at least an effort, and the notation on the drawing (provided the contractor does not alter the drawing before submitting it) would be a pretty strong prompt for the building official to hold things up until they have that engineering done. Requiring the calculations be submitted to you, the "EOR" (we'll just concede that point for brevity), as a reference submittal, i.e. prove to me you had it done, would be another good step, and when it does NOT arrive in a month or two, then there's a phone call to the building official.

I'm not sure Florida would find that (reference submittal) all that kosher, given that is in the realm of delegated engineering where the EOR is ultimately responsible for a review of the design, but it would at least hold the feet to the fire that you as the "EOR" know somebody did the design and thinks the connection is valid. Think of it in the zone of delegated connection design in steel construction. A bit fuzzy, but there's a framework around the process you're following.

You could design it as a full on self supported retaining wall that the upper structure "slips" on, and then indicate that the top of wall should be designed to accommodate whatever calculated deflection you produce from your design calculations, if that's more palatable. I've had at least one occasion where somebody wanted to put the floor trusses "inside" the wall, and with the presumption that the wall deflects due to soil, this seems problematic.

not sure if we're speaking the same language here. did you read the article?

Not in the last say, seven years. I don't have access to Wood Design Focus, I usually reference the (2014) Structure Magazine and the JLC articles.

What published body of engineering knowledge are you referring to? Regarding eliminating the IRC code-required holdowns/connections at the ends of the deck? If the failure sequence is what you're referring to here, it seems the deck at extreme loads goes into more of a membrane so that's how the "compression" holdown goes into tension. Again, the holdown is INTENDED to resist tension, and the testing showed it goes into tension. Again, that's enough for me. That's what it's there for. It's required by code, that's why I show them on any deck drawing I do. "Engineering judgement" is not "I don't feel like it."

Something tells me that "stud" grade is graded differently than something that's intended to function primarily in constant bending stress, versus say, wind load and incidental seismic load from self-weight. knots, slope of grain, are perhaps, or probably, different. I don't use "stud grade" headers. The stresses may work out, but I think there's more to it than just that.

how about an 8' tall interior 2x6 bearing wall

The table you include allows 10' stud height, what happens to your design there? Is that why the IRC has 2x6 there? Do they include 5 psf "live load" on the surface? Does your design? I don't know. Is that 5 psf interior "wind" partition load only in the IBC? I've forgotten. I am not intimately familiar with these IRC tables because as an engineer I don't use them. I'd run calculations for those situations based on the actual loads. If I did a ton of "sign off" work for say, DR Horton or Dominion or somebody, I don't know, it would either say "studs not checked, designer is responsible for design per IRC" or it would give a design where it makes me responsible for the design, nominal size, grade and species and spacing, so the letter is clear on what I checked as an engineer and what I "accept" liability for, and what's supposed to be "designed" by the building designer, i.e. the homebuilder/contractor, so the building official knows what to check against the IRC.

That takeaway sounds fair to me. The IRC has it's origins, the fundamental is "it worked reasonably well in the past" and as to a factor of safety, it will vary from element to element.

While we are at it, or rather, while I'M at it, has anybody noticed the rather stark difference between the deflection requirements in the IRC and the IBC? I'm referring to the "load case" item.

 

[DTS419]

Mike Mike, I'm not familiar with that system and don't quite understand your FBD. Why are there couples at each end of the deck?

 

[lexpatrie]

That's maybe depicting the results for that load testing article.

As a side note, the holdown also provides a load path that doesn't use the ledger/lag screws in tension which looks like in a lot of cases would require the forbidden cross grain tension in the ledger to provide part of the load path.

 

[Mike Mike]

All, here's a summary of key takeaways for anyone who doesn't have time to read the whole thread at this point. let me know if I mischaracterized anything.

Items I think we generally agree on:
1. IRC is wildly out of line with other codes and standards, creating house performance issues and in extreme cases, life safety issues. It's up to us to participate in committees to bring codes into alignment and counteract entrenched interests.
2. The limits of applicability of IRC are not clear, but appear to be: 10psf max floor dead, 15psf max roof dead, 40psf max live, 70psf max snow, no special wind regions over 140mph, no coastal wind regions, roof rafters up to 48' between supports, floor trusses practical limit of up to perhaps 40' between supports.

Here are some specific IRC provisions that are wildly out of line. We're sure there are more, but these are the one's we've uncovered so far:
1. top of basement wall to sill connections
2. top of basement wall sill to band joist connections
3. wall studs
4. rafter heel joint connections
5. wood floor panel sheathing thicknesses

Items we perhaps don't have consensus on yet:
1. Engineers are held to a "standard of care", so if a homeowner's basement wall tilts in because the engineer of record followed the IRC, the engineer may be held liable. That being said, we don't know of any cases where this has ever happened.
2. I say engineered design can be less expensive than prescriptive design in some cases. lex says engineered design is always more expensive than prescriptive design.
3. I say some prescriptive requirements can be removed if alternate load paths are engineered. lex disagrees. eng16180 disagrees in the case of deck holdowns.

 

[Eng16080]

Good summary Mike Mike. A couple quick comments:

The limits of applicability of IRC are not clear, but appear to be: 10psf max floor dead

Floor joist span tables provide values for 10 psf and 20 psf dead. See Table R502.3.

I say some prescriptive requirements can be removed if alternate load paths are engineered. lex disagrees. eng16180 disagrees in the case of deck holdowns.

I do agree with this, although in most cases I would still use deck hold-downs as part of the load path that I'm comfortable with. There are times I don't use them, and although it's probably a topic for a different day, I generally consider not attaching a deck to the house as being superior. As far as my own purposes are concerned, the prescriptive requirements may as well disappear.

 

[RWW0002]

I do not get pulled into IRC often, but another item for your list of "out of line with IBC" provisions above that has come up alot in the last is "king studs" (i.e. full-height studs @ jambs) @ large openings and tall walls. I may not be fully up to date on IRC provisions but, if memory serves, these have varied over the years from woefully inadequate (from a calculation standpoint) to painfully conservative. Last I checked the pendulum has swung back towards the inadequate side. If I am not mistaken, king studs for "tall walls" are not even really addressed.

 

[phamENG]

no special wind regions over 140mph, no coastal wind regions

See Figure R301.2(5)B Regions Where Wind Design Is Required.

Everywhere with Vult 130mph or greater from Texas to North Carolina and 140mph or greater north of North Carolina (which amounts to Nantucket +/- 100 miles in either direction) is required to have an engineered wind design. So this would cover the 'coastal wind regions'.

Special Wind Regions are scattered around the country, most of them out West but a few in Appalachia and New England.

Special Wind Regions are just localized areas that don't conform to the broader maps. The one in Tennessee, for instance, falls in a valley with an impressive fetch that can generate some pretty good winds during certain times of the year. Special wind regions just get their own wind load based on the locality. If it's above 140, wind design required. If it's less, then you use the appropriate stuff in the IRC.

I agree that they need to tighten that up. Probably set it to 130mph+ regardless of where you are. But it's also important to remember that the IRC is not the code. The code is the law in effect in the jurisdiction. Yes, it is usually based on the IRC, but I have yet to practice in an area with winds above 120mph that just take the IRC without making changes to increase the resiliency of a lot of those connections for wind.

 

[phamENG]

Regarding deck hold downs - I would caution anyone wanting to use that report as a reason to say "see, we don't need them!".

1) This was done in a lab. A threaded rod running inside and fastened to a hold down bracket will age better than a screw in a 1-1/2" piece of wood that gets damp ever night, wet when it rains, and dries every day.

ome level of structural redundancy is recommended, even though in ideal laboratory conditions it was shown that sufficient withdrawal capacity could be provided by joist hanger connections when screws are used. It is important to note that both deck tests were conducted in a laboratory setting where materials were not exposed to environmental factors such as wet/dry cycles, and there was no wood decay or fastener corrosion present.

2) This isn't how most decks are built. The behavior of these decks relies heavily on the fact that everything is screwed together. The prevailing interpretation of the codes is that, with the hold downs, you don't need screws in the joist hangers and nails are fine. So most decks, even those built to code, will have very different performance curves without the hold downs.

3) These curves are taken to failure. Waaayyy past where we expect them to operate. It's important that we understand those failures and consider them in our designs, but who designs a deck to exceed an inch of sway? That's going to be very uncomfortable for anyone on the deck. Besides, many post designs will start to 'fail' if your second order moment eccentricity starts to get that high. d/6 is a good 'rule of thumb' for designing wood columns to account for that (as well as eccentricity in the connection). On a 6x6, that's 0.92". If you look at that spot on the curves in Figure 7 of the article, you'll see that the hold down tensions are all pretty close to the 'expected' tension, and the actual tension doesn't exceed the code minimum capacity of the hold down (3500lbs) until the deck has deflected about 14".

 

[Mike Mike]

RWW

I believe the max wall height for IRC is 12ft based on 2024 IRC R602.3(6). Jaw droppingly, the king stud schedule does not differentiate between 2x4 and 2x6, at least not that I was able to find. And yes, none of the table entries are even close to working for 12' tall 2x4 stud grade kings lol.

phameng

I agree with all points on both posts, and that's a great point about local jurisdictions with high winds not simply adopting the vanilla IRC. Another couple things the deck experimenters could have done to get the expected behavior at both service and ultimate loads: (1) install the holdowns on the first joist in from the edge on the opposite side of the joist, or (2) install double DTT1Z holdowns instead of single DTT2Z holdowns. the DTT1Z is installed on the bottom of the joist and screws into existing double top plates.

that being said, I think there's room for engineering opinion here, rather than blanketing every deck with the same resistance couple. there are plenty of decks that I would feel fine about alternate load paths without holdowns: tension going thru hanger screws or thru lag bolts via cross-grain bending, as lex noted. Example photos below:

Eng 16080

Good catch on the floor joist tables. I was referring to the passage below. Which takes precedence where provisions clash? Am I reading something wrong?

After years of deterioration of the fasteners connecting the deck joists/joist hangers to the ledger, there's no longer any withdrawal capacity.

would you feel better about the connection if hangers are fastened with screws instead of nails? what if hanger fasteners are 3" long? enough to get some bite in the house rim joist. not sure if this is a good solution because I've heard too many little fasteners efs up the flashing and waterproofing, not sure if it's true though.

DTS419

Sorry I should have indicated the location of the house, which is at the bottom of my FBD. The couples resist overturning.

lex

it looks like the IRC deflection limits match up pretty close with the live load limits in IBC, but the IRC fails to clarify as usual. and yes this is another great comment for your drafters someday when you're head of the committee. why is this table even in the IRC anyway?

"studs not checked, designer is responsible for design per IRC"

I recently released drawings where I stated "All structural items based on prescriptive design of IRC. Exceptions: headers and jack studs based on engineering calculations." seems kinda similar to what you suggest. I will probably drift toward your language next project.

The table you include allows 10' stud height, what happens to your design there? Is that why the IRC has 2x6 there?

at 10' stud height, holding other variables the same, I think engineered design would approach prescriptive design. the point is if you are designing a custom home with 8' tall walls there is money to be saved by not prescriptively over-designing the stud walls.

Do they include 5 psf "live load" on the surface? Does your design?

yes, engineered design requires 5psf horizontal per 2024 IBC 1607.16.

all

has anyone found the IRC commentary helpful? I'm done reading the main code, considering purchasing the commentary.

 

[RWW0002]

@ Mike Mike - Check out 602.3.1 Exception 2. Allows studs up to 20' tall for some conditions.

It has been a while since I have chased this one down, but try to get that (2)2x6 king stud to work in a 18' tall 2x6 wall with a 18' opening... Hopefully this has been cleaned up somehow, but last I looked the same header and jamb stud requirements you reference above were the only thing required to my knowledge. There are a couple of threads floating around here dedicated to the topic. In 2015 IBC they tried to address this in a way, but it ended up going back to old wording in 2018 and has not changed to my knowledge..

 

[lexpatrie]

I kind of wonder if any of these tables have been "validated" in the past. There have been revisions to the rafter spans in a few places, that suggests somebody somewhere ran calculations, the code change proposals, if they could be turned up, would probably provide an email for the proponent, if you're interested in pursuing that route, that could simplify some of this proposed work.

I'd say the prescriptive designs are "not calibrated" versus engineering. If were trying to curve fit into a concensus, i think its possible prescriptive is conservative in some location somewhere. As to wall studs, stud stiffness has to figure in there somewhere, too.

Wabo - yes, Washington state building officials. Sorry, didn't answer that before.

As to the commentary, I must have one somewhere. If you weren't looking for the most recent one, I could at least describe what is there circa 2003.

It sounds like this is starting to congeal into an FAQ for wood design? Or do we think it belongs here?

I thought the deflection limits in the IRC were more specific about load cases in the past?

It's been an interesting discussion so far...

 

[DTS419]

lexpatrie, my hunch is that the residential codes have been established more as the result of politics and lobbying than hard science. Exhibit A is the debacle a few years ago when IRC tried to address the foundation anchors and then caved in the face of pushback- I'm surprised no one here remembers this. The IRC is adopted at the state level by bureaucrats who, along with the elected officials who appoint them, are under pressure from the builders and trades industries. This happens with all codes- I saw it firsthand even with ACI. But I believe it's much prevalent in residential construction where there is a greater disparity between those who know and those who think they know.

 

[phamENG]

Anyone have links to documentation of this debacle over anchor bolt changes?

 

[RWW0002]

@PhamENG - I am not aware of true documentation, but I remember the back and forth a bit. There was a new table introduced in IRC 2006 that tightened spacing of plate anchors at the top of basement walls (Table R404.1(1) Top Reactions And Prescriptive Support For Foundations Walls and Table R404.1(2)Maximum Plate Anchor-Bolt Spacing For Supported Foundation Walls). These came alot closer to typical "engineered" basement wall support - at least in terms of actual reactions at top of wall and making sure they can be resisted by the plate anchors. If memory serves, values may have been based on assumptions a bit on the conservative side in an attempt to simplify and unify, but not overly so.

It caught alot of backlash and many municipalities omitted it from adoption. There must have been uniform resistance from builders groups of some sort because many times changes like this would just go unnoticed or ignored, but in this case they were omitted from adoption completely. See links below to GA. and NC adoptions and their respective omissions of these tables. Many other areas followed suit.

https://rules.sos.ga.gov/gac/110-11-1

https://www.ncosfm.gov/2009-north-carolina-residential-code-amendments/open

Later versions of this section of IRC do not contain the table. In IRC 2009 "significant changes" summary sited lack of past performance issues and conservative design values as proponent justification. Take that for what it is worth, but that is the only glimpse I found into the code committee deliberations.

https://www.northamptontownship.com...ternational-residential-code-2009-edition.pdf

Hope this helps.

 

[Mike Mike]

RWW

I ran a quick engineering check on three 20' tall 2x6 stud grade kings at the side of a 14' wide opening for 129mph, exp C, 35psf unfactored C&C wind load, 300% overstressed in bending, 5" deflection. Hopefully the homeowner doesn't mind his walls shaking and drywall cracking. no no it's fine he can just post a sign "careful for falling debris. shut door softly. avoid loud noises. and by all means never turn on any appliances with frequencies matching the house harmonic."

great finds on the debacle. I wonder who spearheaded the "lack of past performance issues and conservative design values" justification. If the committees truly believe IRC provisions are conservative and lack past performance issues, then why would they not update IBC to follow IRC? all I want is a little consistency so I can do my job here. something's fishy.

lex

I was just wondering if you've found the commentary helpful. ACI 318 and ASCE 7 occasionally get into code evolution in the commentary. hoping IRC does the same.

There have been revisions to the rafter spans in a few places, that suggests somebody somewhere ran calculations, the code change proposals, if they could be turned up, would probably provide an email for the proponent, if you're interested in pursuing that route, that could simplify some of this proposed work.

I am interested. I did what I could on Google and failed. I'll keep digging.

 

[lexpatrie]

Is this any use?

https://sbcc.wa.gov/about-sbcc/2021-code-proposals

The old ones are quite difficult to dredge and I tend to archive them, no idea why. Plus when you'd make a proposal they usually sent you a CD of them, but I don't remember when it was, I feel like that CD is around here somewhere.

Code change proposals international residential code would be the search term I'd suggest you try.

 

[DTS419]

Mike Mike, this is what I have to explain to my contractors almost daily when this gap between “past performance” (aka this is how we’ve always done it and never had a problem) and engineering requirements comes up: Many structures never come close to seeing design loads. Case in point, how many times does the wind really blow 129 mph on a building? When that does happen, we have hurricane disasters where many buildings are destroyed. Same with snow storms. But 99% of buildings out there will never see full design level wind loads, live loads, etc. and are never put to the test. So that explains why most things that get built seem to work just fine, even if not per standard engineering practice. And as engineers, we are obligated to design for that worst case no matter how improbable. Believe it or not, many contractors, at least the sensible ones, understand this even if they don’t like it.

Can’t tell you how many times I’ve heard about so and so’s 200 year old farmhouse that has a cobble foundation and no uplift connections and how us engineers are just being ridiculous with our modern requirements. Again my rejoinder: it obviously hasn’t had the misfortune of ASCE 7 loads applied to it. (And by the way, they never admit that old house has settled so far out of plumb that half the doors and windows don’t close properly.)

The real travesty is how ICC basically sold out engineering in caving to the “this is how we’ve always done it” crowd.

 

[XR250]

Jumping in here late. Lots of basements here in central NC and I rarely see issues with the tops of walls kicking in on walls that have anchor bolts. The ones w/o bolts have occasional problems. Even precast walls (Superior etc) with little ability to span horizontally and deep trusses running parallel and some shitty blocking every 8 ft. or so to resist the loads seem to hold up OK. It boggles my mind. I generally call out anchor bolts 24" O.C when the backfill exceeds 6 ft. After 9 ft. I start looking at Simpson FWANZ's
The worst I have seen in a house was 14 ft. of backfill and it basically pushed the entire floor diaphragm with it.
Was one of those cases where the foundation walls were engineered but on one considered global stability.

 

[RWW0002]

I am a bit less cynical about prescriptive design than some. I think there is a time and place for prescriptive design as there is engineered design. I do not know that ICC is "caving in" as much as the views of the codes reflect the makeup of the committee. You only "know" what you know and someone can come from the prescriptive design/construction world and diligently design per the IRC without seeing serious problems over a full career (whether by luck or not is up for debate). I am as guilty as any, but I happily practice in My IBC bubble and only venture into IRC when I have to. I am also not chomping at the bit to participate in the code committees.

Regarding IBC/IRC conflicts, I think this is natural. You have a prescriptive code developed with the building stock of "yesterday" and are trying to apply it to "today's" structures. The same provisions that have worked well for the single story gabled ranch house with 8' ceilings and 7 or 8 total windows around the house are being applied to newer builds full of windows, crazy roof planes, tall walls, etc.. The code does not always extrapolate "up" well.

Likewise you have engineered design which was developed for primarily larger structures where things like interior gyp walls and other less reliable means of load transfer are largely ignored. This may be negligible for a larger structure, however the smaller the building gets, the greater the impact of those ignored items lending to increasingly over-conservative designs. Engineered design may not extrapolate "down" well.

To make all this worse, it takes years for the "problems" to arise to know just where the prescriptive designs may be delinquent, yet the cost implications of an over-conservative design are immediate. I think it is our job to fight the more obvious discrepancies, as noted above. I often wish I had a better file of photos of issues that arise from the common IRC issues (both issues of the code as noted above and common areas of lack of inspection or code adherence - ex. gable wall hinge, ridge beam/board, etc..) If anyone wants to upload a bunch of photos I would be glad to file them away as ammo for later use..

 

[Mike Mike]

lex, thanks for the links. I'd like to focus on the ICC committees rather than the local committees. I assume joining the ICC committees is unpaid volunteer effort. I do see ICC is looking for a new CEO and "Government Relations and Operations Associate" on their website, which would certainly be much more lucrative...

99% of buildings out there will never see full design level...loads

agreed, and I think part of the solution is to reduce IBC safety factor, not just increase IRC safety factor.

pushed the entire floor diaphragm with it

interesting compressive failure. did floor sheathing buckle? I assume the sheathing to joist fastening was insufficient for the ~2klf compressive load?

I rarely see issues with the tops of walls kicking in on walls that have anchor bolts.

if ICC committee investigates the data and determines 1/2" bolts at 6' are fine for 999 out of 1,000 houses, then the solution to code discrepancy is to revise IBC to output similar solutions to the IRC. I would be fine with that. 3.0 or so seems like a reasonable reliability index for this issue since it's unlikely a life safety issue, but I don't even care what reliability index the committee chooses. All I ask is that they do their jobs and be consistent across their codes and stop creating legal gray area.

the codes reflect the makeup of the committee...The same provisions that have worked well for the single story gabled ranch house with 8' ceilings...are being applied to newer builds

I agree ICC does a poor job of clarifying the limits of the IRC. I agree this is the result of the makeup of the committee, by which I assume you mean special interest groups.

interior gyp walls...may be negligible for a larger structure, however the smaller the building gets, the greater the impact of those ignored items

in my experience the ratio of interior "nonstructural" gyp walls to "structural" walls is similar for a large 4-story wood apartment and a 1-story house. I think "nonstructural" interior gyp wall contribution to both lateral and vertical load resistance is similar whether large or small, but maybe I'm not understanding you.