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

[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.