Table R602.3(5) IRC (size/height/spacing of wood studs) 1

[vitium]

In the 2021 IRC, table R602.3(5) lists the maximum stud height and stud spacing for "bearing walls". I don't see (if it's listed) if there are any restrictions on what species of wood can be used, what type of veneer is on the wall, and possibly most importantly, if there are any restrictions on what the maximum wind speed is for this table.

The way it reads to me is that a any species of wood, with a brittle veneer (or not), in any wind speed region is fine....which seems unconservative given how loose this particular prescriptive design seems to be. For example, this would allow an SPF wall with stucco finish to be built 10' tall with 2x4's spaced @ 24" O.C. in a hurricane prone region. Using design software in our office this doesn't appear to be close to working given the deflection requirements of Table R301.7. Also, it seems that R602.2 says that No.3 grade studs are acceptable for this condition.

The following table R602.3(6) does at least list ultimate maximum wind speeds, but is mum on the other possible variables.

Is there some "catch all" somewhere I'm missing that states "up to 115 mph" or "Southern Pine" UNO. paragraph or something?

Thanks!

 

[phamENG]

No, I don't think you're missing much. Just remember, tables contain information but the code is in the text. So make sure you find the text of the code that references that table to get a more complete picture. And to get the whole picture, you need to understand the code pretty much in its entirety. The IRC is a convoluted web of intermixed requirements with lots of fun holes to fall through. It's really easy to miss stuff, and cherry picking is unwise and mostly wrong.

Most important thing to remember: the IRC (and, really, any code) is the minimum standard. And a structural engineer is not needed to pull from those prescriptive tables. If a single family residence is involved enough to have an engineer designing it, you'd be much better off not relying on those tables. Make sure whatever you design isn't less than what any of the IRC tables say, but if your design says you need more, bigger, or stronger, go with your design. For instance, if a rafter to ceiling joist connection calcs at saying I need 4 nails, but the IRC says 6, I'm going to specify 6. But just because the floor span tables says a 2x8 will work doesn't mean I'm going ignore my vibration calculation that says I need 2x10s.

If the client is insisting, then fire them and tell them to get a home designer to design it prescriptively because they can hold you (or the company you work for) responsible for poor serviceability regardless of code minimums.

 

[XR250]

What Pham said. Unless I am just trying to make something work that is already built, it is rare I use the IRC tables.

 

[jerseyshore]

I too never use the IRC tables unless I need to explain to a client how the existing/ as-built condition doesn't meet the code minimums. Honestly I usually reference the IRC tables to scare people into not raising their ceiling joists more than 1/3 up

 

[Flotsam7018]

I rarely rely on prescriptive design as well, however the more I do residential design I do wonder if some aspects of the IRC's prescriptive methods more realistically capture the performance of these redundant structures within certain bounds than our engineered designs. Oh well, let me get back to specifying these FTAO straps will IRC says I don't need hold downs at all.

 

[vitium]

Thanks for the input. We almost never use any of the prescriptive design tables in the IRC, and I had already told the client that 2x4's don't work in this situation but they brought up the the table in the code and wanted to know why that wasn't acceptable.
I was on my back foot a little since I wasn't expecting to be challenged on my design with a code that was not close to what I was telling her was needed. I told her I'd get back to her with something. Then, after spending some time thumbing around the code was at a loss as to how the code could be giving this minimum but not indicate what the minimum is based on. Perhaps there is some wind zone, and some species of wood where #3 grade 2x4's, 10' tall, at 24" O.C work but it seems like they should at least list what assumptions have been made for this table. What's to prevent someone from using cotton wood in a hurricane zone (as an off the wall extreme example)?

 

[vitium]

No, I don't think you're missing much. Just remember, tables contain information but the code is in the text. So make sure you find the text of the code that references that table to get a more complete picture. And to get the whole picture, you need to understand the code pretty much in its entirety. The IRC is a convoluted web of intermixed requirements with lots of fun holes to fall through. It's really easy to miss stuff, and cherry picking is unwise and mostly wrong.

Most important thing to remember: the IRC (and, really, any code) is the minimum standard. And a structural engineer is not needed to pull from those prescriptive tables. If a single family residence is involved enough to have an engineer designing it, you'd be much better off not relying on those tables. Make sure whatever you design isn't less than what any of the IRC tables say, but if your design says you need more, bigger, or stronger, go with your design. For instance, if a rafter to ceiling joist connection calcs at saying I need 4 nails, but the IRC says 6, I'm going to specify 6. But just because the floor span tables says a 2x8 will work doesn't mean I'm going ignore my vibration calculation that says I need 2x10s.

If the client is insisting, then fire them and tell them to get a home designer to design it prescriptively because they can hold you (or the company you work for) responsible for poor serviceability regardless of code minimums.

Thanks for the reply. I feel like I've spent all morning reading the text that references these tables trying to grock the intent of the code and I can't find anything in the text that I'm missing that would square this. It seems like the code should list what assumptions have been made to allow this table to work, otherwise you could end up in a situation where the code "minimum" is actually inadequate in certain situations and could result in a failure.

 

[phamENG]

It seems like the code should list what assumptions have been made to allow this table to work

For sure. But, then, the people for whom this table is intended wouldn't, on the whole, know what any of it meant. Not trying to be mean, but there's a reason we get paid to do what we do.

What's to prevent someone from using cotton wood in a hurricane zone

Pretty sure cottonwoods don't grow anywhere near hurricane zones? Also, do they grade cottonwood for structural use?
It's a little bit of a leap of faith, but I'd say that you can use pretty much any species so long as it is called out elsewhere in the code for structural use. For instance, Table R60.7(1) restricts its use to douglas fir-larch, hem-fir, southern pine, and SPF. Should they define it more clearly? Absolutely. But you have to remember that the IRC is written more by the National Association of Home Builders than it is by engineers. There was an instance of the IRC more clearly (and strictly) defining anchor bolt spacings for sill plates several years ago. The NAHB (and/or other similar bodies) lobbied all the states to reject the code or edit that part out. It was gone in the next code cycle, back to 6' on center for pinned basement wall connections to a wood floor. Yikes.

For your specific client, I'd inform them that those tables are the minimum standard, and are intended for the cheapest of houses built with fiberboard sheathing and vinyl siding. They are having a custom house built with high quality, brittle finishes that require more robust suppor than that can provide. If they choose to go with that minimum, there's a good chance they'll have issues with their stucco, brick, or whatever, and there's a chance that it could void warranties on some products as many will have their own minimums for installation.

 

[phamENG]

Pretty sure cottonwoods don't grow anywhere near hurricane zones?

I'm completely wrong. Had no idea. They even grow here where I live!

 

[vitium]

For sure. But, then, the people for whom this table is intended wouldn't, on the whole, know what any of it meant. Not trying to be mean, but there's a reason we get paid to do what we do.


Pretty sure cottonwoods don't grow anywhere near hurricane zones? Also, do they grade cottonwood for structural use?
It's a little bit of a leap of faith, but I'd say that you can use pretty much any species so long as it is called out elsewhere in the code for structural use. For instance, Table R60.7(1) restricts its use to douglas fir-larch, hem-fir, southern pine, and SPF. Should they define it more clearly? Absolutely. But you have to remember that the IRC is written more by the National Association of Home Builders than it is by engineers. There was an instance of the IRC more clearly (and strictly) defining anchor bolt spacings for sill plates several years ago. The NAHB (and/or other similar bodies) lobbied all the states to reject the code or edit that part out. It was gone in the next code cycle, back to 6' on center for pinned basement wall connections to a wood floor. Yikes.

For your specific client, I'd inform them that those tables are the minimum standard, and are intended for the cheapest of houses built with fiberboard sheathing and vinyl siding. They are having a custom house built with high quality, brittle finishes that require more robust suppor than that can provide. If they choose to go with that minimum, there's a good chance they'll have issues with their stucco, brick, or whatever, and there's a chance that it could void warranties on some products as many will have their own minimums for installation.

All fair enough. I appreciate your input. I've been doing this for 20yrs and it's just now coming up so... I guess it's not that big a deal overall. Will just tell the client this is what I'm comfortable sealing. If they want something that will crack when the wind blows they can go find some other engineering who's willing to approve some technically code compliant minimum and seal it.
It's just frustrating to go through a design, then to spend all morning looking through the code hoping to find something like "that part of the code only applies at X situations. Since we have Y situation so it wont work", instead my options are "yeah, I don't know why, but the code doesn't apply here. Trust me, or take your business elsewhere" isn't a very satisfying resolution.

I think I'm just going to hang my hat on Table R301.7 and tell her the 2x4's don't meet the deflection criteria based on design and move on with my life.

Regarding the cotton wood, I was just looking through the NDS for the weakest lumber with structural values listed.

 

[jhnblgr]

much of the prescriptive code isn’t compliant with design NDS standards and should not be used for basis of design. The client hired you because of you expertise and if they think they know better than ask them why they hired you. Most places the required insulation in walls determines the wall thickness not structural design. Also when designing beams and headers the bearing area at the beam ends often requires a width greater than a 2x4 or you have a lot of jack studs to make it work or the header extra deep or engineered wood to work also. Point is focusing on one thing is pointless when it might be determined by multiple things.

 

[Mark_J]

All fair enough. I appreciate your input. I've been doing this for 20yrs and it's just now coming up so... I guess it's not that big a deal overall. Will just tell the client this is what I'm comfortable sealing. If they want something that will crack when the wind blows they can go find some other engineering who's willing to approve some technically code compliant minimum and seal it.
It's just frustrating to go through a design, then to spend all morning looking through the code hoping to find something like "that part of the code only applies at X situations. Since we have Y situation so it wont work", instead my options are "yeah, I don't know why, but the code doesn't apply here. Trust me, or take your business elsewhere" isn't a very satisfying resolution.

I think I'm just going to hang my hat on Table R301.7 and tell her the 2x4's don't meet the deflection criteria based on design and move on with my life.

Regarding the cotton wood, I was just looking through the NDS for the weakest lumber with structural values listed.

Just curious. Can you post wind pressure, LF loads, and height? I'd like to put into my calculator and see how much deflection fails.

 

[lexpatrie]

I think it's in the scope of IRC way at the beginning.

Also bear in mind that the IRC in general (and the prescriptive equivalent in the IBC in Chapter 23) are not engineered and are based on "historical successful practice"" or whatever the term is. You're trying to compare cook by feel with a log stove to a twinkie manufacturing plant. Or maybe grandma's sourdough with wonder bread. Then again that kind of disparages the wrong item. It's like comparing rat infested RV "diner" with an A rated McDonalds in New York City where it's inspected frequently. There.

Make sure whatever you design isn't less than what any of the IRC tables say, but if your design says you need more, bigger, or stronger, go with your design. For instance, if a rafter to ceiling joist connection calcs at saying I need 4 nails, but the IRC says 6, I'm going to specify 6.

I think you're in the minority there, and the IRC does not override an engineered design, further when it comes to connections, some spots in the IRC call for unworkably large numbers of nails as it was produced by some guy with a spreadsheet and no practical thoughts. And the provision got voted on by the body of the IRC not the engineers.

One could potentially put 21 nails to connect a pair of 2x4s at 45 degrees (rafter to ceiling joist table) but the wood will split and you'll have no established strength value.

So blindly using the IRC as a minimum after your calculation isn't generally wise.

they can hold you (or the company you work for) responsible for poor serviceability regardless of code minimums.

Not really. You are wildly overselling that. Anybody can sue anybody for anything, it's the ones that survive summary judgement as a complaint against a design professional (i.e. with another engineer swearing the suit has merit) that have teeth. Now, your insurance company may settle to make it go away, that's another story. I haven't seen any floor vibration lawsuits in residential construction "land". Also, establishing some level of vibration performance as actually in a statute, or as a standard of care, that's another challenge for the prosecution.

Incidentally, the older stud tables in the IRC used to have a required modulus of elasticity that only Red Maple could meet as stud grade, as I recall. That MOE requirement disappeared a while ago. Never did track down the code change amendment or when it happened, but it was probably that guy at Reid Middleton who submits hundreds of code changes each cycle. The man's stamina astounds me.

Relevant (somewhat) discussion from earlier this year:

What are the limits of prescriptive design

There was an instance of the IRC more clearly (and strictly) defining anchor bolt spacings for sill plates several years ago. The NAHB (and/or other similar bodies) lobbied all the states to reject the code or edit that part out. It was gone in the next code cycle, back to 6' on center for pinned basement wall connections to a wood floor. Yikes.

There's an FAQ on that over in the wood forum, it's related to unrealistic values from ACI 318-99 appendix D that came out in 2000 after the 2000 IRC was publsihed using the not yet published Appendix D which came out in 2000. Anyway, actual research was done (by SEAOC, not some lobbyists group) and the code eventually got more realistic.

Braced wall anchorage as I recall is how 4 feet which is tighter than the generic wall at six feet as well.

Most places the required insulation in walls determines the wall thickness not structural design.

Only in the North. With the wind load concern I'm speculating our OP is in Florida or the (warm) Atlantic coast.

OP the stud tables probably don't apply, there is a lot of fine print most engineers just don't have the patience to read. Particularly if your design is saying you need more, either for wall finish or for loads.

Last item, Minnesota's code for a fair number of years has had stud tables that go away fair bit beyond the IRC so you may find those interesting as a back check or merely for the oddity of something so structural in the residential code. I practice in Minnesota and even I forget those tables exist, since I think the 2007 Minnesota Residential Code. Exposure C and D as well if I recall.

Sorry if anybody got forty notices about new posts in this thread, I'm on my phone. Should be doing something more productive but I'm not.

 

[lexpatrie]

I just updated the FAQ in the Wood forum, so it's going to disappear for a bit, but here's where it will be:

Anchor bolts in light frame construction (that's not the correct title for the FAQ, I'm going off recall because, again, the FAQs disappear for about four hours if you edit them).

Minnesota Residential Code amendments, R602.3.1, for the curious. Exposure B and C are covered, now I remember, I had an exposure D project on a lake somebody really missed the design on.



As you can see, this requires SPF #1/#2 or better, footnote b. And continuous sheathing, (that's the wall bracing provisions). Also note this is using an L/120 limit, so it's not something you should really use for full on three coat stucco. Vinyl siding only, methinks. 3 feet anchor bolt spacing for the taller walls, and blocking at 10 feet.

And since I'm in the Minnesota code at the moment, here's that rafter to ceiling joist connection table I mentioned with 21 nails....



Personally I think anything beyond 4 is unrealistic, but I'm imagining a pair of 2x6s intersecting at an angle, it's not quite that clearcut.

I should clarify that on sizing, if the code calls for a 2x6 and I can get a 2x4 to work, I'd probably use the 2x6 anyway. It's leaning on the prescriptive nailing that I don't find all that plausible. There's a 39 in that table!

 

[phamENG]

It's leaning on the prescriptive nailing

lex, I appreciate the effort you go through to create long, detailed posts. I do it on occasion, but not much - I have other things to do. So when I say something like "I'll use 6 nails instead of 4", please, for the love of whatever you love, look for the subtext as much as the literal meaning. Saying I'll use 6 nails doesn't mean I'll blindly use 39. I'm not an imbecile. Let me try to say it a different way:

If, for a house, the prescriptive code calls for a connection of a certain capacity, I will not provide a design for a connection or member of a lower capacity in the same or a similar situation unless there's a very compelling reason to do so. If the prescriptive connection doesn't work, I'll design one that at least provides that minimum capacity.

There's a few reasons for this:
1) Why not? We're all familiar with how...sketchy the IRC is in most areas. Why would I provide something more sketchy?
2) Future expectations. We can't, nor should we, design for all future possible changes or conditions. But this is what I call low hanging fruit. To continue the belabored example of the heel connection, how many designers in the future are going to crawl down into that part of the roof and count nails they can't see between the ceiling joist and the rafter? Not many. When I work on a house and I can't see something (which is always), my initial assumption is that it was at least built to the most basic code requirements, and go from there. If it gets opened up and it's not, then we solve that problem. But I want future engineers/designers/contractors working on a house I design to at least have a reliable baseline for assumptions.
3) What if something does fail? I feel like saying "I calculated X should be adequate, but the code said it was X+1, so I designed it to X+1" is a better defense than "the minimum standard is X, but I thought X-1 was okay, so that's what we did. oops?"

 

[lexpatrie]

A lot of what I say is for the audience down the timestream. Once the discussion closes, people can only read the discussion and make their own (erroneous) conclusions as they man read and take from it what they want it to say versus what is meant.

I know everybody wants to be brief and pithy, but I struggle with brevity, as I'm sure everyone has noticed by now. I try, when possible, to provide reasoning and the sources.

With the deterioration of actual mentoring and discussion with more experienced engineers, we are building the foundation for future queries that some computer will eventually misinterpret in doing "design" as well.


With regard to item 1, granted, in concept, that it's wonky, it's not the most reliable backstop for an engineering computation. Going "up" a size between your analysis and the IRC I don't object to, but if the math is correct I wouldn't object to another engineer doing it based on their calculation. Unless there were some other deeper ramification (i.e. weight, or greater stiffness drawing more force).

I don't really follow your item 2. I'm picking this particular aspect of the IRC because I know it's wonky. A lot of those nailed connections don't look like they'd physically fit on the wood rafters or joists involved. Not that ivr calculate it out. Sadly "this provision looks wonky" isnt accepted for code change provisions.

So. Despite being "code", and I haven't traced the origin of that table and have questions. I expect most engineers would.

I deal with older construction more than I'd really like, but there it is, when possible, the IRC could be used for the repair (we'll just pretend fixing "deficient" contruction from 1920 that hasn't collapsed and shows no sign of distress is "repair"). I'm on life safety and that which doesn't calculate out has to be dealt with rationally through well established principles of mechanics, IBC 1604.4, because that's "always acceptable" in the IRC and there's nothing prescriptive the contractor can conform to.

On a related note, code requires joists to be "properly nailed" (or something similar) to balloon framed studs, and I've never found any specific nailing. In that case, I'd suggest adding nailing versus presuming it was "properly nailed". Same issue (as I recall) occurs with the let in ribbon that's under the joist.

But I take your point, philosophically. One can set up the analysis model based on the expected sizes in the IRC and confirm later on site or via the contractor.

On item 3, I feel like you're phrasing that as badly as possible. You performed the work to the standard of care, your calculation showed x minus 1 was what was required for life safety per 1604.4 and you provided x minus 1 on your drawing. You also told them to pre-drill to 75% of the nail diameter to reduce the likelihood of splitting, showed the spacing and stagger between the nails and told the contractor (and the special inspector) to inform you if splits developed. I've been called out to a site once by a very astute (city) inspector who noticed the reinforcing deviated from the supplied drawing. Actually took a lot of effort to confirm the deviation (mostly) worked and put out a revised drawing. (To further explain, somebody added a whole "storage nook" to some wood trusses, built in shelves, sheet rock, plywood floor, etc. They had Christmas ornaments up there in boxes. Commercial building, wood trusses. Same inspector (correctly) mandated a work platform (per IMC) for the new HVAC unit as the roof was sloped. Which added weight...)