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]

Just up too late. Nothing exciting happening, I'm an engineer, after all.

As a side note, that E limitation appears to carry through to 2012 IRC and with the switch from Table 602.3.1 to 602.3(5) the footnote is gone.

Should anyone be interested in a quick link, here's the 2015 IRC, Chapter 6, third printing.

Are you saying within the IRC "there's maybe other provisions elsewhere for anchor bolt spacing due to the lateral (soil) load"? I spent many hours digging thru the IRC at this point, pretty sure there's nothing in here, but let me know if you see something.

Eh, well, no. I didn't find anything special about lateral loads on the sills, there's some fine tuning you could do on your calculation but it is highly unlikely to alter a 1,800% overstress. I just feel compelled to mention, in case that's coming from a design somewhere, but the refinements appear to increase the allowed load so it's conservative as is. I didn't read Stephanie's article, I just saw that comment about load path when I was trying to find it for discussion...

Floor trusses are specifically permitted by 2024 IRC R502.12 and require engineered design regardless of span. You can't get more "in the middle of two prescriptive designs" than that. If you were EOR would you not allow your clients to use floor trusses?

Touche. Agree they are pretty clearly sandwiched between a prescriptive stud and a prescriptive foundation. But isn't that engineering by somebody else? The truss engineer? I'm not too sold one becomes a "full" EOR on a project just by doing a tall stud wall, you know? I'd expect at least 4 out of five dentists would allow floor trusses without batting an eye, similar for roof trusses.

Source: 2018 International Residential Code. (actually the image is from the 2020 Minnesota Residential Code, but I don't think there are any changes.)

You could probably read that to permit a sandwich of an engineered design between two prescriptive systems, but it's got some (mild?) peril to it, but the peril we're talking about specifically is the wall connection at the top of a soil-retaining foundation wall and that's not happening due to the wood truss...

One would like to think a too long span for the truss will have bearing/crushing issues with a 2x4 that would prompt a question, but then again, I've seen that exact defect on an engineered three story building with an occupied roof and a penthouse, all in wood. So, probably not.

Going to the top of your post -

1 - Some sort of "gateway" check would seem in order, since there aren't engineers on these items, and it's difficult for them to a) make the call when it comes to snow load, seismic load, and b) they could then know (the building officials as well) where to expect engineering. Or how to design so as to avoid it.

2 - That or add "engineering required" for the floor attachment beyond where the 6' sill spacing stops working, with some inclusion of possible dead load friction or say a 25% increase to the stresses..? I feel like some states have revisions in that area to provide additional bolts.

Source: 2020 Minnesota Residential Code

That would be the kind of modification to propagate "upwards"?

3 - What about bringing back the footnote on F[sub]b[/sub] and E? Took them what, 6 years to add the superscript on the 1.6 x 10[sup]6[/sup] psi and change the "by" to a 'x' indicating multiplication? Like as a starting point (although maybe it came back after 2015, I haven't looked that far, so much (unnecessary) reorganization and renumbering .... so many adds) In the northern areas, we tend to see 2x6 studs for insulation so the 2x4 parts of the table are of less relevance to us up here so they get less scrutiny. The table is kind of funky anyway, to me it's a bit backwards, but nonetheless. Somewhere an "engineering required" would be nice to see in that table. If they'd just stop rearanging things and moving sections all the time, that would surely help.

4 - Another perfect opportunity for "engineering required" in a table, which would send folks retreating to eliminate the need for engineering and go back to more standard spans. We'll ignore the 25% decrease for clinched nails for now, but wouldn't a double shear connection tend to provide more than a 25% strength increase? I've not tried to calculate it out, when I get stuck with this situation I use hardware that's been load-tested (cough Simpson cough).

How about (A) - Delete all that garbage for light gauge stud and joists and punt it to the light gauge people to publish their own bloody standard a la the WFCM? Do the light gauge studs calculate out better as somebody put them into the code? Why did they sandwich all that light gage garbage between two wood framed sets of provisions (Floors, Decks), that's just a bad decision from the go.

Yes, there are a few refinements that could be made to my basement wall calc, but it doesn't change the conversation

Granted. I just have to point out the little stuff.

1. R507 is for decks. I'd eliminate those at your peril. Lateral Loads Generated by Occupants on Exterior Decks, Parsons, Bender, Dolan, Woeste, Structure Magazine, Jan 2014. As an engineered design, code calls for it to be "designed for lateral loads" (not really a quote, sorry), without any guidance.
2. Single top plate is possible without engineering in both codes, I think.

Top plate nailing requirements of table R602.3(1) lines 13 and 14 are extreme overkill and can be reduced or deleted.

3. Which part is excessive? I suppose cheaper with fewer nails, sure, nothing dramatic, though, and would the framer actually look at that or just do it per "code minimum" like they've done forever. The splice (1422 lbs?), last I checked, is the same in the two codes, unless the options in the IRC are different. If we're comparing R602.3.1 and Chapter 23, they tend to be pretty similar. The IRC doesn't control a top plate splice location so it's presumably in the worst case location between two braced wall panels at the ends, it's a collector/drag strut, and the nailing is probably unchanged since, say, 1926.

Item 4 isn't an IBC/IRC difference, it's an Engineered versus not difference.

Are you saying you would be okay with specifying 1/2" bolts at 6' to brace the top of your basement wall just so long as you noted on the detail that the connection was selected from the prescriptive requirements of IRC only. Would you include a note requiring the owner or contractor to hire an engineer to verify the connection?

Never been in that particular boat, so to speak. The last situation I had that was reasonably close was a precast plank and a cast in place retaining wall with a house above it. So the sill bolt doesn't really do much as to the restraint of the top of the foundation wall, there. The next closest I have is a concrete floor with a masonry wall under it from let's say 1940. The sill anchor isn't doing much there, either.

That situation is where you should probably flag the issue and if it's not your design, write so explicitly. That's more of a before you issue drawings item, however.

This is a wide ranging discussion, even in the U.S., we've gone from liability, standard of care, prescriptive design, etc. As Greenalleycat mention, who gets sued isn't constrained by "I followed the code" as an absolute defense, though it does usually help the defense. To briefly divert, something like vibration, in the U.S. isn't even in the code, it's "governed" by some research and publications published by various suppliers and standard setting organizations but they aren't generally code requirements. You'll find calculation guidance for steel framing, wood framing, and steel joist framing, but to my knowledge these are guidelines for appropriate practice (i.e. standard of care) but they are not code in the statutory sense. In that situation for a suit against a design professional, there usually needs to be another similarly licensed design professional's report indicating negligence, deviance from a design standard, etc.

The very first words in this thread are "The IRC...." This is the Residential code that most of the U.S. functions under. That's what we're talking about.

As to the horizontally spanning masonry foundation wall, is there a requirement for a bond beam anywhere in the IRC that might provide that, or are we expecting the mortar bedding in tension to do something or ladder reinforcing (if required?).

 

[RBODGE1234]

All really good comments and true depending on ones location.

I practice on the Western side of Washington State and we have both wind and seismic loading that are both very real concerns. What I have experienced over the years is that the IRC is a minimalistic approach to building and I have never actually seen a house plan that can be built locally that is capable of being built fully code compliant just using the IRC. There is inevitably some item or items that do not fall into place under solely the IRC code. If those items are inadvertently missed or ignored by the municipality, the plan slides through permitting. If whatever the item is gets flagged, the municipality will then have the applicant try to get the non-compliant item handled or bought off by an engineer.

I suggest that you do not let that engineer be you. Often I will be asked if I can just provide the lateral analysis ("Lateral Only") or if I could engineer their set of plans to the IRC code for them. Sometimes they want just a garage header sized.
I have learned that on residential houses especially, it is best practice for all parties, the engineer, contractor and homeowner, to just say no to partial participation.

Either you are the engineer of record or you are not the engineer. When you let yourself get hooked into just doing one little item, you have set yourself up for the liability on it all. No matter how clearly you try to limit your scope and liability. Anything goes sideways on that project, you are going to get hoovered up by the attorneys no matter what the issue is regarding. It will cost you way more than what your fees were to get out of the litigation, even if you have absolutely nothing to do with whatever was the actual issue.

Besides, say you agree to just do the lateral. Do you just provide them with the shear nailing for some three foot panels? Or tell them they need to use the portal frame configuration at certain locations? Maybe they have second floor shear walls that land over the garage header? Each of those items need to have the loads traced down to the foundation. Now you are obligated, or at least should be, to size up the footing under there and any hold downs or drag struts and strapping. So as you can see, one thing leads to the next and pretty soon you say to yourself "I should have just engineered it all from the get go.", It would have been easier. If you try to limit yourself by putting on your blinders and ignoring all of the rest well, its only a matter of time before it will bite you.

So to answer your initial question, you are ultimately responsible to decide the limits of the IRC code. I personally have only found it to be a hassle bouncing between the two codes. I find contradictions to various code items like concrete footings and retaining walls that we "engineer" to ACI in the IBC, but when using the IRC code we can go off from a table that is less stringent. Is it better for the client or home owner to scrimp on the design? One might say yes, but only until the home is subjected to the design force loading that is anticipated at its location. Then, maybe they would prefer the greater factor of safety? It is exactly those kinds of decisions we make or are allowed to decide that makes us ultimately responsible to decide. (You may wish to ask and take into consideration the preference of your client) I usually choose to go with how I would do it if it were my own personal project. When in doubt, I use a simple litmus test. I imagine myself in a courtroom on the witness stand being asked, why did you choose to do it this way? And my answer will always be "because my education, training, experience and calculations have shown it would meet or exceed the minimum code requirements for anticipated design force loading". Watch out for that "minimum" requirement. If you know that's not enough, then require better. Never be coerced or tricked into accepting a lesser standard. Ultimately you are the engineer of record and when something goes sideways...it is your name and stamp that is on the line.

 

[DTS419]

I deal with this foundation bolt issue all the time in my practice. I have clients who need a foundation engineered because it is required in some jurisdictions, or because there is something unusual (large custom home, deep basement, etc.) The problem is when you run numbers on the prescriptive requirements that the job was priced on (the engineering often isn’t sought until the last minute, but that’s a whole other topic), you often cannot get the numbers to work, as Mike Mike covered several posts ago in this thread.

This puts me in a predicament. Do I specify what my engineering expertise tells me to do, or do I just sign off on the code?

Of course the contractors throw a fit when I double or quadruple the bolts, because they've seen it done 1,000 times at the code minimum and never had a failure. Problem is these guys aren't engineers and don't understand why just because something "works" doesn't make it right.

We can debate the legalities of who is responsible all day long, but I'd bet anyone a fine sandwich that should there ever be a failure and an engineer touched it, it's not going to matter one bit what the prescriptive code says because the argument will be that the engineer should have known better.

Anyway, at the end of the day, the whole situation really undermines credibility of the engineering profession. The perception becomes that engineers are to be avoided because we're too conservative and we drive costs through the roof because everyone with residential construction experience knows there is a huge gap between what works and what the engineer says it needs to be.

And of course that is to say nothing of the fact that there are likely homes being built each and every day with prescriptive connections that are undersized, at least according to "standard practice".

 

[SWComposites]

Ok, I’m in a different industry, but we are heavily regulated also. And I don’t ever want to in a court explaining why we screwed up a design and killed people. And we get bitched at by manufacturing all the time.

Re: Do I specify what my engineering expertise tells me to do, or do I just sign off on the code? if you are signing or sealing or stamping the design, then you go with your expertise and calcs. Period. If contractors complain, too bad. The jurisdiction is asking for an engineered design, presumably because they don’t accept the proscriptive design in these cases. I doubt they are asking for someone to just sign off the proscriptive design blindly.

If there are real discrepancies between the codes, then these need to be brought up with the IBC authors, whoever they are.

 

[DTS419]

If there are real discrepancies between the codes, then these need to be brought up with the IBC authors, whoever they are.

Anyone here remember the debacle several years ago when the IRC did try to address the foundation bolting requirements? I don’t remember the year as it was before I got into residential (2012 or 2015 maybe) but from what I gather there was an uproar in the builder industry and IRC backpedalled after many states refused to adopt the new approach.

 

[Eng16080]

The anchor bolt connection detail is something that comes up a few times a year (about every 5th moon cycle I believe). We all think about it, and then throw our hands up.

While most of the IRC and IBC prescriptive provisions seem to have some engineering basis, this particular detail apparently doesn't. I suppose it's along the lines of it being constructed that way for a long time and apparently not failing. I can understand the pressure on the code writers from the building community to not decrease the AB spacing (or whatever the code change would be), but I think something should be done about this. From an engineering perspective, I don't think something having been done a particular way in the past should be taken as an absolute justification for how it's to be done in the future. Most houses could be constructed without any connection to the foundation, and most would be fine, but that doesn't make it right.

It's borderline embarrassing trying to provide an engineered connection and ending up with ABs spaced at 6" o.c. or whatever. Meanwhile the builder's looking at a table showing 6 feet o.c. and thinking how he never wants to work with you again. Then later that night, he's going to tell his 6 builder buddies how wacked this engineer is that he worked with today, and then they're all going to tell their buddies. (I mean, who doesn't love a stupid engineer story?!)

If the IRC/IBC people aren't willing to revise the code, perhaps they could at least create a short design guide showing an engineering basis for the AB spacings. I've gotta think shear friction between the sill plate and top of concrete wall plays a big role. Realistically, 2-way action of the concrete probably does too, although that's perhaps a lot harder to quantify from an engineering perspective considering there are usually only 2 or 3 horizontal bars in the wall (at least per prescriptive requirements). Then there's also some fixity at the base of the wall. Besides all that, I imagine the biggest discrepancy is that the lateral soil forces that we use in design are never realized, especially if well drained material is against the foundation wall with a foundation drain at the bottom. If my speculation is true concerning that, maybe the code could provide alternative lateral pressures. Of course, some engineers would say that the lesser pressure shouldn't be used because the foundation drain could get clogged, etc, etc.

 

[phamENG]

I was thinking about the anchor bolts this morning. And yes, I think friction has a lot to do with it. In the absence of uplift, friction is sufficient for most houses that are not significantly exposed and are not subject to wind gusts above 130mph. So the anchors at 6' is just a nominal 'positive attachment'.

The problem is that uplift is treated in a very hands off fashion. You get a roof tie down force in Chapter 8, and you can reduce it by 60plf for every floor level below for light frame construction to account for the weight of the wall. Unless you have a 3 story house, there's almost always going to be uplift at the base of that wall going by that. If you can account for floor dead load it's often better, but then you're starting to engineer the load path. Chapter 3 says you have to provide a load path to the foundation. But it doesn't tell you what to do. In a prescriptive code, that's a problem. They need to spell that out clearly. They need a standard strap detail or something. Some sort of uplift anchorage guidance. Because a 1/2" anchor with a cut washer on the inside edge of tolerance for a 2x6 isn't going to do much for uplift.

 

[lexpatrie]

DTS491 is this what you mean?

Anchor Bolts in Light-Frame Construction at Small Edge Distances, Seismology Committee, Structural Engineers Association of California, Structure Magazine, August 2010

If so it's not concerned with the lateral load from soil, it's lateral load along the length of the wall, and it's also not into a masonry foundation, and it sprung from the "cracked concrete" provisions in Appendix D back in the day. As of around 2000, the Appendix D stuff wasn't done, so it didn't make it into IBC 2000, then got published separately, and for light frame, produced a lot of problems with "established practice" particularly in California, hence the California response as it drifted into the IBC circa, say, 2003 - 2006.

 

[DTS419]

lexpatrie, no I was referring to lateral soil pressure. But what you posted is another good example of "standard design practice" being too conservative.

 

[lexpatrie]

Well, I suppose, it was a bit of a dust-up between the California folks who came very late to the IBC party (they held out on 1997 UBC for quite a while, and as a result the revision in Southern Pine values forced the NDS to issue a retroactive change to the allowable stresses back to the 1997 NDS), they'd have had a much easier time if they'd been on the boat back in 2000.

As I recall further, the Appendix D wasn't ready in time for inclusion in the ACI code, so the whole thing got published like a year later by ICC, in 2000, and incorporated into the IBC somewhere in there. I'm a little fuzzy how it trickled into what I thought was primarily a disagreement in the residential code, but it was 20+ years ago and my "facts" might be a little fuzzy, or outright incorrect.

I've seen a lot of revisions in the masonry and concrete wall tables in the IRC over multiple code cycles, but the top of wall reaction isn't part of that picture. The Minnesota reaction force and blocking has been in the residential code for a fair bit, and they tend to have full basements, versus, say, Florida or Texas or maybe even California. The UBC is kind of the main source for provisions in the IBC, so if California didn't have a lot on basements in the UBC, OH never mind, we're talking about the residential code.

 

[kissymoose]

The anchor bolt connection detail is something that comes up a few times a year... I suppose it's along the lines of it being constructed that way for a long time and apparently not failing

It does though, in my area. I wouldn't be surprised if it isn't at least a quarter of houses in my area that have foundation walls that have popped their top connections and started to learn inward. One could provide college tuition for a family of 6 by just running around town spec'ing out wall restraints and similar fixes for every realtor or homeowner buying or selling a house.

 

[Eng16080]

I wouldn't be surprised if it isn't at least a quarter of houses in my area that have foundation walls that have popped their top connections and started to learn inward.

Ok, wow, so this apparently is a problem somewhere. I don't feel quite as crazy now thinking about this load path and detailing.

Are the failures primarily between the top of wall and sill plate or between the sill plate and floor framing?

 

[lexpatrie]

The code "soft spot" is the reaction at the top of wall path into the "diaphragm".

If that connection is too flexible (holes in the plate for the anchor rods, for example), deficient dead load, high water table, etc, the wall moves inward, if I'm correct in my recollection, the soil load goes from at-rest to active and the soil load goes downward, tending to arrest the progression of failure, along with the wall potentially going into horizontal bending with the ladder reinforcement helping out, the sill plate going into bending, if it's not spliced much, wall height and length being a factor, as well as the wall itself turning into a cantilever versus a propped cantilever/ pin-pinned element. 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. Not that you'd do that in a sealed design, but it's possible that's slowing/preventing failure progression.

 

[kissymoose]

Are the failures primarily between the top of wall and sill plate or between the sill plate and floor framing?

I don't know. It's hard to get up in there to see and the fix is the same regardless. In the few I have seen directly, it's been a failure of the joist/rim to sill connection, which is just a few toenails.

 

[DTS419]

if I'm correct in my recollection, the soil load goes from at-rest to active and the soil load goes downward,

My problem with this is that in order for the connection at the top to move enough for the soil to go from at-rest to active, that means the connection had failed.

But maybe that’s too simplistic of a view. Perhaps it is possible for the bolts to cause enough deformation in the wood sill plate to allow movement without complete loss of the connection. Same with the toe nails.

 

[Mike Mike]

lex

I agree Minnesota code should propagate upwards. Perhaps the reason it can't propagate is someone doesn't want to pay for beefier connections? Perhaps someone would prefer to come back and fix your basement wall after it fails than prevent it from failing in the first place? maybe 4 out of 5 dentists say an ounce of cure is worth a pound of prevention? I'm just sayin.

What do 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."?

In terms of prescriptive deck holddowns, I disagree. Your reference Lateral Loads Generated by Occupants on Exterior Decks specifically says the holddowns don't work as expected on page 27: "Tension hold-downs behaved in a counterintuitive way for the deck investigated. The flexibility of the deck allowed significant rotation of the deck joists within the joist hangers. This resulted in a geometric prying effect that caused zero tension in the tension hold-down and significant tension in the compression hold-down". full article here:

https://www.jlconline.com/_view-object?id=00000155-0949-df79-abf7-bd494e380000

for most decks alternate lateral load paths exist and prescriptive holddowns are excessive. this is an example of where an engineer fluent in both IBC and IRC has a competitive advantage over an engineer fluent only in IRC.

All 4 of my examples illustrated where the IRC prescriptive design is more conservative, and engineered design is less expensive. As you mentioned, yes, these are "engineered versus not" differences.

Since you still don't seem convinced that sometimes prescriptive is more expensive than engineered, here's another example: IRC prescribes #2 grade headers and jacks per table R602.7(2). for smaller openings, an engineered design can justify stud grade.

Having said that, I'm not an IRC expert, so please poke some holes in my musings. I do see the IRC prescription for single top plate now, thanks for pointing that out.

rodge

I agree it sux being the engineer sucked into the middle of an otherwise prescriptive design. hard to estimate and weigh the potential future cost of litigation when we're bidding these jobs. If asked by the prosecution my response would be similar to you: "because my education, training, experience and calculations have shown it would protect public life and safety".

DTS

you bring up a great point on undermining the credibility of the engineering profession. perhaps all we can do is communicate with clients and contractors the bind we're in. if your client needs an engineered foundation design, but no one has specifically asked for top of wall connection design, and your walls are 10' tall or less, would you be comfortable indicating on your drawings only the items you're engineering? so instead of specifying "1/2in bolts at 6ft" at top of basement wall, you simply state "anchor bolts per IRC 2018 R403.1.6" or "anchor bolts per applicable code" or something like that? would that keep your clients happy and would this enable permitting?

I don't see anything about the debacle on google. Any chance you could dig it up and send us links?

SWC

Thanks for the outside perspective. As I mentioned we all need to put our words into action and participate in the international code council because these do not appear to be small issues. either ICC needs to rewrite IBC to decrease loads and increase capacities, or ICC needs to rewrite IRC to beef up requirements. I'm not an expert on the subject, but ICC committees are not immune to corporate influence and their recommendations reflect their committee members, just like any large body, for example

https://pubmed.ncbi.nlm.nih.gov/353...d that,had connections with multiple members.

Participating in committees is a great way to counter this trend.

engineer #16080

Shear friction won't help you because wind uplift completely nulls dead for one-story structures with 140mph wind in exposure category D (yes 140mph exp D is within IRC prescriptive design).

even neglecting wind load and assuming 3 stories with 20' trib each story, your dead load is only around 45psf x 20' + 60plf x 3 floors = 1100plf, which means .4 x 1100plf = 400plf of available friction. not enough to resist your 1000plf soil load.

2 way basement wall action won't help because IRC places no limits on distance between wall jogs. does IRC place any limit on building length and width at all? I don't believe it does but I'm not sure, might be hidden somewhere in the back.

I agree basement footing fixity helps and soil pressures have a large factor of safety. But you're not going to establish an engineering basis when you're off by a factor of 18.

I suppose you could argue not many houses have basement wall jogs more than 20ft apart and experience base uplift. probably less than 10% of homes right? 90% aint bad. but I guess I would still prefer we edit the IRC to keep the failures well below 1%.

all

does anyone know of any good cliff's notes to the IRC that assemble all the key take-aways of this thread into a short document to make it easier for contractors to navigate?

 

[lexpatrie]

As a side note I am aware of a "fifth dentist" in the literal sense, they'd rather you not brush or floss or drink fluoridated water so they can get paid to fill cavities. You see any sixteen year old with say, fifteen fillings, and you're either in Appalachia where Mountain Dew is a day one drink, or you've got a fifth dentist somewhere in their medical records, not that difficult to find them, IMHO.

Sorry, couldn't resist.

My problem with this is that in order for the connection at the top to move enough for the soil to go from at-rest to active, that means the connection had failed.

I mean that in a "why they don't all eventually collapse" way, not a "you can seal and design it that way". The bolts are supposed to have some oversize in the wood, so there's the potential for some slip "into bearing" a la the old banging bolt problem in steel, but if the bolt location is random in the oversized hole, some of them will go into immediate bearing, others will have twice the hole radius difference before they go into bearing, and there's also the nailing of the band joist to the sill and the joist to the sill/sole plate, so there are "backup" paths.

It would be of more use to me, personally, for masonry foundations to be looked at in this way, because a concrete stem wall has a more convincing horizontal bending option, especially if the minimum horizontal reinforcing is installed.

I believe the necessary top of wall movement on a retaining wall is something like 1/2". So it's not a lot of movement. That said, I've seen cracked basement masonry walls with no visible inward movement, too. This one is secured to a concrete slab at the top (connection is unclear, one presumes dowels).

Mike Mike -

Somebody has to actually propose the change, that's a start. If I were going to go that route, I'd try to rope the WABO people into carrying the spear. They seem to have passion, zeal, and experience with code changes in that area of the code, as well, so if they agreed, that could carry some weight into the balloting.

At one point there was a code correlation committee for prescriptive provisions between the IBC and the IRC. I am not sure they are still working or if they've declared "mission accomplished".

What do 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."?

Not from a Jedi, if you know what I mean. I mean in a load path to prevent collapse. The board will eventually crush, and it's fairly unlikely the installation is all that tight anyway.

In terms of prescriptive deck holddowns, I disagree. ...... significant tension in the compression hold-down.

I rest my case. There's a tension load developed and the holddown attracted and resisted it. The load path is altered by the deck flexibility (not really much of a lateral diaphragm), in the life-safety sense, it's a code requirement without an engineered design and has documented effectiveness.

for most decks alternate lateral load paths exist and prescriptive holddowns are excessive. this is an example of where an engineer fluent in both IBC and IRC has a competitive advantage over an engineer fluent only in IRC.

Yeah, I dunno, waiving something so obviously intended as a life-safety measure that's based on testing to be shown to be effective "because I feel like it" as a professional engineer isn't to my taste. I'm not the guy to be expert witness to defend that practice as "standard of care." I get people dislike stuff and think it's overkill, but then again, it went through a balloting process, had a code cost increase associated with it (I PRESUME), so it was honestly put through. (cough unlike residential sprinklers cough).

Since you still don't seem convinced that sometimes prescriptive is more expensive than engineered, here's another example: IRC prescribes #2 grade headers and jacks per table R602.7(2). for smaller openings, an engineered design can justify stud grade.

Eh, sure. But I mean whoop de do. Save fourteen cents on a two foot header grade substitution. What's your fee for doing that design? $1,500? $60? So after DR Horton builds ten thousand of that house they've paid it off? That seems like a great location for a code change proposal if they "work" at a lesser grade. Although "stud" grade is called stud grade for a reason. It's not intended for horizontal use.

I'm really unconvinced that an engineered design is "cheaper" than a prescriptive design.

I deal with fixing a lot of residential stuff, and it's a real fuzzy area between making the piece work, for the calculated loads, when the rafter itself is let's say 300% overstressed. (Not a real example, by the way), but I think you can see thematically what I mean, the prescriptive code has a lot of elements that do not work out to an engineered equivalence in any obvious way (elements that are beyond the L/d span limits for engineering, for example, if they exist). It isn't like the repetitive stress of 1.5 in some areas versus 1.15 in an engineered design is "held to" throughout. This is just an area where research (excluding Woeste, Bender, Parsons, and others, who are kind of my ideological heroes) is done. To me this is ripe for discussion in the Journal of the Performance of Constructed Facilities or some other "academic" venue (I don't mean to shut down discussion here, I mean somebody could publish about ten dozen articles on this subject, code section at a time, rafters, ceiling joists, load-bearing studs, headers.... floor to basement wall connection for soil loads, etc.). Ledger "reform" based on testing has been very nice to see....

RBODGE1234 - this is probably good stuff, worthy of discussion, but I'm gonna abstain as my mental energy isn't there today. A "lateral only" review has to at least be able to see the remainder of the structure and in a high seismic region there's GOT to be a lot of float between the practitioners as to what "standard of care" even is. The only feather in your cap you get for these loads is the satisfaction of a job done correctly, and the displeasure of the other party having to meet the code. I tell people from time to time that "engineering is not in the business of client satisfaction". In the normal sense, sure, return calls, talk to people, be respectful and don't threaten people (side note, that looks a lot like hearsay in a disciplinary action, but we'll move on), but that's nothing to do with engineering practice, that's what you're supposed to do as a member of society.

The engineering stuff, well, when the math doesn't work, the design (my design), (not somebody's redline, I guess - fodder for a future discussion I suppose) doesn't get a stamp. Find another engineer or let me fix it.

This probably goes partly to "get a down payment," if you've gotten at least partial payment, the internal conflict between "I won't get paid for this" is at least partly reduced.

you bring up a great point on undermining the credibility of the engineering profession. perhaps all we can do is communicate with clients and contractors the bind we're in. if your client needs an engineered foundation design, but no one has specifically asked for top of wall connection design, and your walls are 10' tall or less, would you be comfortable indicating on your drawings only the items you're engineering? so instead of specifying "1/2in bolts at 6ft" at top of basement wall, you simply state "anchor bolts per IRC 2018 R403.1.6" or "anchor bolts per applicable code" or something like that? would that keep your clients happy and would this enable permitting?

That seems really dodgy. This is almost depraved indifference. You know it's wrong, you let it go ahead. This is the kind of thing you should address in your agreement (or not), in that your review is to cover the appropriate subjects requested ... as well as any needed changes to the load path to support those loads..... you could specify a force and call for another engineer to design it? Hypothetically you could use the table from Minnesota, provided you as an engineer reviewed it and believe it's correct. I suppose alternately you could devise a system such that the wall is designed as a free-standing retaining wall then somehow "slide" the structure above it, or devise a way that the wall sees no soil load via earth work on the exterior.

does anyone know of any good Cliff's Notes to the IRC that assemble all the key take-aways of this thread into a short document to make it easier for contractors to navigate?

No, but the wood frame construction manual might be a bit more deliberate and thought through on the various subjects.

What are the key "takeaways" anyway? I think I've missed them.

 

[Mike Mike]

thanks for the links from Minnesota board lex. definitely tangential to this discussion but very interesting reads none the less. please send more writings from your ideological heroes too!

By WABO do you mean Washington Association of Building Officials? The ICC code correlation committee is still active and met on Feb 14, 2024, but none of their agenda was structural. The IRC was created around 2000 or so based on the precursor, OTFDC, by a drafting committee of 3 architects and 3 representatives of the national home builder's association (NAHB). 1995 OTFDC seemed to do a better job of defining the line beyond which engineering is required. see below for limits on wall lengths that somehow got eliminated. NAHB? (cough cough, of course)

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?

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?

Deck holdown load: the tension was on the compression side of the deck, meaning the holdown further destabilized the deck and resulted in additional tension on the lag screws. FBD sketch below. the holdowns aren't waived "because I feel like it", the holdowns are waived because they satisfy IBC, the body of engineering knowledge, and engineering calculations. not sure if we're speaking the same language here. did you read the article?

why can't a 3ft double 2x4 stud grade header be oriented horizontally? since you're still unconvinced engineered can be cheaper than prescriptive, how about an 8' tall interior 2x6 bearing wall supporting 3 stories with say like a kip per foot of vertical concentric factored load. prescriptive design requires 2x6 at 16. engineered design requires 2x4 at 24. my time spent / design fee is about zero for both these designs because I already know they comply with IBC.

I think the biggest take-away from this conversation, for me, is how wildly designs vary based on if we're using IBC or IRC. I had always figured IRC was off by maybe a factor of 2, and that the limits of IRC applicability must be clearly stated somewhere if I ever took the time to read it. how naive of me. there are a bunch of other great take-aways but that's the main thing for me.

 

[Eng16080]

In terms of prescriptive deck holddowns, I disagree. Your reference Lateral Loads Generated by Occupants on Exterior Decks specifically says the holddowns don't work as expected on page 27: "Tension hold-downs behaved in a counterintuitive way for the deck investigated. The flexibility of the deck allowed significant rotation of the deck joists within the joist hangers. This resulted in a geometric prying effect that caused zero tension in the tension hold-down and significant tension in the compression hold-down". full article here:

https://www.jlconline.com/_view-object?id=00000155...

for most decks alternate lateral load paths exist and prescriptive holddowns are excessive. this is an example of where an engineer fluent in both IBC and IRC has a competitive advantage over an engineer fluent only in IRC.

I consider deck hold-downs a critical (if not the most critical) element of a deck construction. Most deck failures seem to occur due to a poor waterproofing detail at the interface between the deck and the house. After years of deterioration of the fasteners connecting the deck joists/joist hangers to the ledger, there's no longer any withdrawal capacity. Now add a bunch of people for a graduation party/cookout/whatever and you get a failure where the deck pulls away from the house, collapsing inward and downward. Anchoring the deck framing directly into the floor diaphragm of the main building helps to prevent this type of failure. I like to design decks with the mindset that the deck won't fall off the house unless it's tearing out the floor of the house with it.

The article linked above is very interesting, but it's concerned strictly with lateral deck loading, not the type of failure I mention above. Per the article, the hold-downs don't resist lateral loads as we would intuitively expect. Still, the article doesn't suggest not using them (as I read your response to imply), rather:

While code-conforming hold-down devices did not appear to significantly improve lateral-load deck performance in the two decks tested, these devices do provide a level of structural redundancy that improves inservice deck safety.

Also, the test conditions/loads described in the article aren't necessarily realistic. Loading a 12 ft x 12 ft deck with a lateral load up to 7,000 lbs isn't really in line with something that an engineer would design. In fact, in designing a lateral load path from the deck into the house, you would be significantly limited by the diaphragm strength of the deck boards. Per SDPWS, Table 4.2D, this capacity is only 140 plf nominal and 70 plf allowable. And technically speaking, this is for attachment with nails not screws. Further, I doubt this diaphragm strength is accounting for the typical 1/4" to 1/2" gap between deck boards. So, although the deck was tested for up to 7,000 lbs, the diaphragm used in the test is only really suitable for 70 plf x 12 ft = 840 plf, if that. Considering how deficient the diaphragm is within the overall system, I'm not surprised the resulting load path at failure isn't necessarily intuitive.

 

[Eng16080]

Shear friction won't help you because wind uplift completely nulls dead for one-story structures with 140mph wind in exposure category D (yes 140mph exp D is within IRC prescriptive design).

I wanted to disagree with you on this, but then I ran some quick numbers, and it seems you're correct. Still, my point was less in defending prescriptive code requirements and more in speculating why we don't see more real world failures. Under normal conditions, there are of course not wind loads close to this, so there would normally be some friction. Anyway, great points above.