Wind design requirements for residential structures 1

[DTS419]

I find R301.2.1 in IRC to be very unclear as to when wind design is and is not required. The first sentence reads "Buildings and portions thereof shall be constructed in accordance with the wind provisions of this code using the ultimate wind speed in Table R301.2 as determined from Figure R301.2(2)." That is basically just a table that is filled out with the wind speed from a map. So at this point it would seem that any residential structure must be designed for wind, similar to how you would under IBC except maybe the wind speed is a bit different as determined by the local building official.

Then we get to R301.2.1.1 which says "The wind provisions of this code shall not apply to the design of buildings where wind design is required in accordance with Figure R301.2.1.1, or where the ultimate design wind speed Vult in Figure R301.2.(2) equals or exceeds 140 mph in a special wind reason." Both those referenced figures basically are high wind regions in coastal areas, and in fact the title of the former is "Regions Where Wind Design is Required."

So, does a residential structure that is not within a high wind coastal area (as shown in Regions Where Wind Design is Required) need to be designed for wind at all? If so, what is the difference between R301.2.1 and R301.2.1.1?

 

[TRAK.Structural]

Residential structures do need to be designed for wind. My interpretation is that if you are NOT in a high wind zone (as defined by the code) AND you follow the prescriptive requirements for all the structural elements, you will end up with a structure that has the required resistance to wind forces. The prescriptive requirements detail things such as braced wall panels, wall stud size/spacing, uplift connections for roof framing, etc; which is where the resistance to wind forces is "baked in". If you don't meet the prescriptive requirements, have a situation that is not covered in the code, or are in a high wind zone than a specific wind design is required similar to what is done in IBC.

 

[DTS419]

TRAK, that is my sentiment as well. However, if following the generic prescriptive provisions yields a structure that can handle moderate wind loads, what is meant by "The wind provisions of this code..." in R301.2.1.1?

 

[lexpatrie]

The lower wind speeds allow the use of the IRC provisions, rather than an outright calculated, engineered, full load-path to ground "wind design".

They are designed for wind by the uh, erm, building designer? (Contractor), because they have provisions for braced wall lengths and braced wall line spacing that account in a non-calculated way, for wind effects. Minimum sheathing thicknesses and nailing requirements, rafter ties to the top plate, etc. Etc. Etc.

The Building code has similar provisions for modest (wood frame) buildings under Chapter 23, though the provisions are not an absolute 100% match between Chapter 23 and the IRC, though there have been efforts to "correlate" them.

 

[Aesur]

I second lexpatrie - IRC may be used for low/normal wind regions and IRC may not be used for high wind regions. There are a lot of instances where the IRC doesn't apply, in fact I would venture to guess that most custom homes for one reason or another don't actually meet the provisions for IRC without at least some IBC (real engineering) design.

 

[DTS419]

Residential structures do need to be designed for wind. My interpretation is that if you are NOT in a high wind zone (as defined by the code) AND you follow the prescriptive requirements for all the structural elements, you will end up with a structure that has the required resistance to wind forces. The prescriptive requirements detail things such as braced wall panels, wall stud size/spacing, uplift connections for roof framing, etc; which is where the resistance to wind forces is "baked in". If you don't meet the prescriptive requirements, have a situation that is not covered in the code, or are in a high wind zone than a specific wind design is required similar to what is done in IBC.

So here's where I think we might have another gap between codes. I have seen many a residential building that was not required to be designed for wind but by quick inspection, it was easy to see would have significant overturning on tall and narrow wall panels. Think garages and barns with large garage doors on the gable end with very little wall space left to act as shear walls. You don't need to be in a high wind zone to generate those forces; the mechanics are the same. Yet there are thousands of those structures being built every day where no one has considered any of that.

 

[XR250]

Many custom houses I work on need explicit shearwalls or moment frames due to the amount of windows/doors and I am in a 90 mph zone ASD

 

[phamENG]

DTS419 - that's not a gap in the codes, that's a failure of enforcement. "Design" as used in R301.2.1.1 is referring to engineered design. You don't need to be an engineer to select braced wall panels or use the uplift tables to figure out what size connector you need. That's anywhere the IRC is adopted without significant editing of that section. Do all plan reviewers look for it? No. They do not. If you're in a high wind zone as shown on the map, then wind resistance must be an engineered solution using principals of mechanics, accepted engineering practice, etc.

 

[gte447f]

DTS419, no, IRC buildings do not usually require "wind design", as in engineered design. Re-read the two sections you originally referenced. Section R301.2.1 says, "Buildings and portions thereof shall be constructed in accordance with the wind provisions of this code...,". Constructed does not mean designed, as in engineered design. Wind design, as in engineered design, is only required as indicated in Section R301.2.1.1, based on wind speed greater than 140mph, etc.

 

[DTS419]

Many custom houses I work on need explicit shearwalls or moment frames due to the amount of windows/doors and I am in a 90 mph zone ASD

XR, who or what triggers those custom homes to be designed by engineer in those cases?

DTS419 - that's not a gap in the codes, that's a failure of enforcement. "Design" as used in R301.2.1.1 is referring to engineered design. You don't need to be an engineer to select braced wall panels or use the uplift tables to figure out what size connector you need. That's anywhere the IRC is adopted without significant editing of that section. Do all plan reviewers look for it? No. They do not. If you're in a high wind zone as shown on the map, then wind resistance must be an engineered solution using principals of mechanics, accepted engineering practice, etc.

I'm talking about buildings that are NOT in a high wind zone that requires design, rather buildings that have features that the basic IRC construction provisions might not be adequate for. I am not aware of any prescriptive requirement that calls for significant uplift connections on tall narrow shear walls that have not been designed as such.

 

[Eng16080]

Not sure if this is helpful here, but as an engineer, I usually just avoid the various prescriptive design crap in both the IRC and IBC. I have more faith in my ability to actually design stuff and follow load paths through the structure based on engineering principles than to understand the prescriptive requirements with all their limitations.

Perhaps it can be useful as a double check for certain things but that’s about it.

 

[phamENG]

I am not aware of any prescriptive requirement that calls for significant uplift connections on tall narrow shear walls that have not been designed as such.

Then it would seem you're not all that familiar with braced wall design. It's a rather convoluted topic, but with perseverance and an open mind it's not hard to understand. If you follow the rules correctly, then you either don't end up with that sort of wall or it has hold downs.

 

[jayrod12]

Not sure if this is helpful here, but as an engineer, I usually just avoid the various prescriptive design crap in both the IRC and IBC. I have more faith in my ability to actually design stuff and follow load paths through the structure based on engineering principles than to understand the prescriptive requirements with all their limitations.

Perhaps it can be useful as a double check for certain things but that’s about it.

We don't have the IRC/IBC debate where I practice, but we do have prescriptive allowable specifications for building that are classified as part 9. If we stray too far from those prescriptive specifications when not warranted, i.e. when the structure ticks all the boxes for the prescriptive designs, then we stop getting the work. In some cases, that's a bonus, but in others it could result in losing a profitable client.

To each their own of course. I'm just pointing out why some people feel obligated to at least try and get the prescriptive requirements to work out.

 

[Eng16080]

jayrod12, my statement above was maybe a little overly harsh. I generally see the prescriptive requirements as being simplifications of engineering, which are applicable in certain limited circumstances. Rather than trying to decipher exactly where that line is drawn, I usually just take the route of engineering most parts of the building.

Where it gets tricky, of course, is where there happen to be big differences between prescriptive and engineered designs, like for sill plate anchor bolts. In cases like that, I’ll admit I still don’t know the right answer. I can certainly appreciate a clients frustration when they see that I want anchor bolts at 12” on center while another engineer might allow 6 feet.

Of course, I believe there are cases where prescriptive design requirements are more conservative, so overall it might come out about the same.

Also, for me, I usually don’t get involved in simple residential projects. Oftentimes for those, there is no engineer and the structure is left to the builder (or designer) to figure out. In that case, the prescriptive requirements are a great resource, as otherwise who knows what the builder might do.

But back to the anchor bolts, if I followed the prescriptive requirements even though the numbers don’t “calc out” and there was later found to be a problem, I’m not confident I wouldn’t be found liable.