Dear Tim,
…Well, the honeymoon is over: in many jurisdictions local building officials refuse to approve a plan without an engineer’s stamp, even though it’s been designed using the prescriptive tables in the IRC. From snow loads to soils to foundations, either because of their lack of understanding of the code or unwillingness to accept liability, this issue has created a disconnect with builders and code officials. Unnecessary engineering is adding to the cost of the house (both in the fees for services and materials for over-engineering), delaying production time and quite frankly is often looked upon as a ‘nuisance’ by engineers, who place these small residential projects at the bottom of their priority list…
… Any insights, help or direction you could provide would be greatly appreciated.
Jan Rohila, Education Program Director
Building Industry Association of Washington
In a nutshell, this issue boils down to: When is engineering required and when not? As far as being a nuisance, shame on any engineer who projects that sentiment. That would be like a builder considering nailing boards together a nuisance. Yes, some boards are easier to nail than others, but that’s what builders are for, for crying out loud.
I think the code folks have gone overboard in their efforts to eliminate engineering of the simpler things. In fact, I believe those very efforts are the ones causing so much confusion. There are simply too many ways to build a house to say this way doesn’t need engineering but that way does. Trying to make that distinction—that line in the sand between shall be engineered and doesn’t have to be is what this hullabaloo is all about. What is the solution? I have an idea, but you’ll have to slog through the rest of this column first to get to it.
I, no stranger to building codes, spent more than a day researching the International Residential Code (IRC) (read: “groping desperately to unravel a hopelessly tangled pile of kite string”) and still am not sure I’ve got this issue completely nailed down. The following, I think, begins to answer the question. I’ve included code references in case some Very Brave Person wants to investigate further.
IRC vs. IBC. Right out of the chute, there can be confusion as to which code to use. The below Scope paragraph describes when the IRC can be used. However, the International Building Code (IBC) has many similar sections and lots of its own prescriptive requirements. One would hope that the prescriptive requirements in both codes are the same, which I believe is the case, though I didn’t specifically check them all.
To be clear, this article addresses only the 2003 IRC prescriptive requirements.
Scope. The IRC applies only to detached one-family, two-family (duplexes), and multiple single-family (townhouse) structures, three stories and less in height. Anything outside that definition must use the International Building Code [R101.2].
Conventional Construction. To evade engineering, the building, from stem to stern, must be “conventional construction”; i.e. in compliance with the structural chapters of the IRC [chapters 4,5,6 and 8 primarily]. This includes, among many other things, certain minimum amounts of shear walls—both exterior and interior in many cases—and proper connection of them to foundations, floors, and roofs.
If your overall building qualifies to use the IRC but certain structural elements aren’t “conventional”, then you only have to engineer the unconventional part(s), not the entire structure [R301.1.3].
Wind. If your structure is located where basic wind speeds are 110 mph or greater per the IRC’s map [Figure R301.2(4)], you can’t use the IRC. You’re instead directed to one of four other publications (good luck finding them). Concrete construction is an exception here—you can still use the IRC for formed concrete and Insulated Concrete Form (ICF) construction in wind areas up to 150 mph [R612.1, R611.2 and R301.2.1.1].
Wind Exposure. Surprising, to me at least, is that the requirement for engineering doesn’t depend on wind exposure. So it doesn’t matter whether you’re in exposure A (very well protected from the wind) or exposure D (on the naked shore of an ocean or other large water body), the only thing about wind triggering engineering is basic wind speed. [R301.2.1.4]
110 MPH Wind. Areas of 110 mph or greater winds generally include Alaska’s seaboard and the southern – eastern seaboards (from Texas to Rhode Island). Most of the continental U.S. is in the 85-90 mph region [Figure R301.2(4)].
Seismic Design Category. Engineering is required if you are in seismic design category E, usually [R301.2.2]. The other design categories—the ones that don’t trigger engineering—are, from mildest to strongest: A, B, C, D1, and D2. If you are in category E and you can get an expert to say you’re not, then you may still get out of engineering by being demoted to D2. Or, if you’re in category E and you design a plain plywood box with no “irregularities” (see below), you may again avoid engineering by demoting to D2. [R301.2.2.1.2].
How To Determine Seismic Design Category. The handy map in the IRC [Figure R301.2(2)] shows seismic design categories for the U.S., including Hawaii and Alaska. However, it assumes a site class of “D”. But how do you know if you really are in site class D? Or, for that matter, what is site class D? Unfortunately, the IRC doesn’t give you either answer—it refers you to the IBC. Great.
Site class has to do with soil type. If your soils are stiff, dense, or rock, you’re okay with site class D. But if your soils are soft or spongy, you’re site class E or F, and now you can’t use the IRC maps to determine seismic design category—you need to hire an engineer. Cripes!
Seismic E. Seismic E areas generally include the west coast, the Sierra Nevada mountains, Hawaii (south part of the big island only), isolated pockets in the Rockies (eastern Idaho, western Wyoming, and northern Utah), southern Alaska, and a pocket along the Mississippi River in the western Tennessee, southeastern Missouri area [Figure R301.2(2)]. But, again, this is predicated on soil type.
Seismic Limitations – Dead Weight. If you use really heavy building materials for roofs, floors, or walls, engineering for seismic design will be required. Standard residential construction, including masonry and ICF, generally do not fall into this category [R301.2.2.2.1].
Seismic Limitations – Irregular Buildings in Seismic Design Categories C, D1, and D2. If your building has any of the following irregularities and you’re in C, D1, or D2, engineering is required [R301.2.2.2.2]:
- Shear walls don’t line up vertically from foundation to roof. An example might be a 2nd floor exterior wall cantilevered beyond the exterior wall below it. Certain exceptions apply for mildly cantilevered or set-in wood framed walls.
- A portion of roof or floor is not supported by a shear wall. An example could be a portion of house supported on posts. There is an exception for projections of 6’ or less.
- The end of an upper floor shear wall occurs over an opening in the shear wall below. An example cold be a 2nd floor shear wall that ends over the middle of a garage door below. Exceptions abound.
- There is an opening in a floor or roof that exceeds 12 feet in any dimension, or is greater than half the roof or floor’s smallest dimension. An example could be a very large skylight or a large opening in a 2nd floor to accommodate stairs.
- Split level floors, a.k.a. vertically offset floors. Basically, if each level of floor has shear walls all the way around below, or if the floors are overlapped and tied together well, you could be exempted from this.
- Shear walls are not at right angles. This could apply to a house with a 45 degree V-shaped footprint , or any other non-90 degree exterior wall arrangement.
- Above grade stick-frame shear walls are mixed with masonry or concrete construction. However, masonry or concrete fireplaces and brick veneer don’t count, i.e. they’re exceptions. An example could be a mostly stick-framed house that has a window wall of ICF. This is the only irregularity that causes the entire affected story(ies) to be engineered.
Snow Load. Buildings located where ground snow load exceeds 70 pounds per square foot (psf) are required to be engineered [R301.2.3].
Floodway. Buildings located in floodways, as designated by the local building department, may not be designed per the IRC—they must follow IBC regulations. Note, this is different than being in a floodplain—which is allowed under the IRC without engineering [R301.2.4].
Story Height. First of all, understand that there is a difference between story height and maximum stud length. The IRC’s definitions [Chapter 2] tell us story height is a floor-to-floor distance. But in Chapter 3, story height is defined as stud height plus up to 16” of floor framing. Make sure you and your code official interpret this correctly.
The IRC is more restrictive about story height than stud height of certain individual walls. For example, is it possible to have a story height of 10 feet but also have a 20 foot balloon framed entry wall, without engineering.
Weaving one’s way through the IRC’s spaghettian layout to get to the bottom of this story height issue is no simple matter [R301.3, Table R602.3(5), and Table R602.3.1]. Here’s the upshot.
- Wood Framed Walls. Maximum story height without engineering equates to 10 foot studs plus up to 16 inches of floor framing. However, there is an exception allowing 12 foot studs if you pump up shear wall requirements 20%.
Now, certain wall’s stud heights can go all the way to 24 feet without engineering, if wind speed is less than 100 mph, and you’re in seismic category A,B,C, or D1, and you follow the spacing and footnotes in Table R602.3.1. Regardless, the story height must not exceed that described in the above paragraph.
- Steel Studs. Maximum story height without engineering equates to 10 foot steel studs plus up to 16 inches of floor framing. I found no exceptions.
- Masonry Walls. Maximum story height without engineering equates to a floor to ceiling height of 12 feet plus up to 16 inches of floor system. Gable ends can extend another 8 feet without engineering.
- ICF (Insulated Concrete Forms). Maximum story height without engineering equates to a floor to ceiling height of 10 feet plus up to 16 inches of floor system. Maximum number of stories above grade without engineering is two [R611.2].
Construction on Fill. Engineering is required for fill soils under footings or foundations [R401.2].
There’s more prescriptive stuff in this code, but the above are the structural major-ticket-items. As I alluded to earlier, finding answers here is kind of like capturing the snitch in Harry Potter’s quiddich: The snitch, very hard to see in the first place, flits and darts about while at the same time bludger balls barrel recklessly around aiming to bodily remove you from your broomstick.
Now for the solution. I think the code folks ran awry when they invented prescriptive design. Most people aren’t even aware that prescriptive design violates most state’s engineering law, which paraphrased, states that engineering shall be performed only by engineers (also architects in some states). Deciding what beams or shear walls to use is engineering. Period. I don’t care what IRC tables tell you, still, it is engineering. Span tables are a classic example. Say you’ve got a 16 foot garage door header that you size using an IRC span table but fail to notice the footnote that says only applicable to uniform distributed loads. Most non-engineers don’t even know what this means and won’t care that a girder truss is bringing a huge point load to the mid-span of their header, making it grossly undersized.
I prefer the good old days when the code simply described minimum standards and it was up to designers and code officials to decide when engineering was needed and when not. This required some basic understanding of the code, and common sense. I think today’s codes strive extravagantly to eliminate any possibility of someone actually engaging their brain and using common sense. I’m a big fan of common sense. You see, as soon as you try to define things so precisely, so exactly, that common sense is no longer needed, two things happen: 1) Your descriptions become so dense and perplexing, no one can understand them; and 2) People disengage their brains, throwing any attempt at common sense out the window.
Probably very few code writers will agree with the above, so here is another idea. How about a computer program that lets you input the type of structure you’re planning and where it will be located, then it spits out appropriate limitations and restrictions. Although I’m in the software business, this is a bit more ambitious than I would wish to tackle. If someone else wants to beat me to the punch, please do.
Tim Garrison of ConstructionCalc.com, is a professional engineer, author, and software producer for the building industry. Check out his latest book, “Cracks, Sags, and Dimwits-Lessons to Build On” at www.constructioncalc.com
I’m an engineer, and asked my local building official how his department decided if engineering was required for a residential building permit. His answer: “When all the building corners are square (90 degrees), no engineering required. If corners are not square, engineering is required.” (!)
Go figure (literally).
Bill