When is an Engineer Required for Residential Construction 1

[Loui1]

New to IRC and am struggling to understand why a developer is requiring a PE stamp for the design of multifamily homes. We're talking about typical two two story units, back to back separated by a party wall, totaling 4000sq ft....a whole neighborhood full of them. This is located in the midwest, so no coastal wind zone or major seismic. No crazy roof overhangs or layout. The developer seems to anticipate having to construct shear walls with hold downs. I've never seen a house with holddowns. Any insight would be appreciated.

Are there certain provisions in the IRC that typially cannot be met, which therefore require engineering??

FYI: I dont want to shoot myself in the foot and argue my way out of a project, but would like to understand why I'm designing these things.

 

[JStephen]

The PE laws in the different states generally have exemptions for engineering and architecture of structures below a certain size. But just because these are exempted from a PE requirement, doesn't mean they can't be designed by a PE if someone wants it done. You can design an outhouse if someone wants to pay you to do it.

I'm not familiar with the construction in question- but is it possible the builder has recently moved from another part of the country where things were done differently?

 

[youngstructural]

Loui;

I was very supprised to read that you have never seen a house with hold-downs... Hold downs are required in homes by all of the codes I am familiar with (this does NOT include the IRC, or any other American code like the UBC). Typically the code will prescribe hold-downs for walls that are shorter than a certain length (ie: have a high probability of rocking under lateral loading) or in the case of all walls above a certain height (quite rational, as the rocking is actually due to a profile height/length of the wall, but this can easily be missed on a plan view).

I know the NBCC, OBC and the CAN/CSA O86 quite well, and have now been working with New Zealand codes for about six months. All of them require hold-downs in residential construction based on some variation of the above. I will do a little looking (because you've got me curious!) and I will let you know if I can put my hand on any similar IRC clauses. If not, and your client still wants you to consider hold-downs (which I think is wise), I can scan you the pertinent portioned of a few codes for your consideration.

One further question for you: We are talking timber construction, right? Stick-framed housing? Either way it will be a requirement, but will differ depending on which materials you are dealing with. Remember to detail your foundation appropriately to allow for the vertical forces imposed by the hold-down anchor under longitudinal shear.

Regards,
Will post again,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[youngstructural]

Well that was quick.... First try, first thing to show up....

http://www.awc.org/wood-design/2006/07/where-can-i-find-information-on-wall.html

Quote: "Basic Requirements
Section R602.10.4 of the International Residential Code (IRC) provides basic wall bracing requirements and specifies the minimum length of braced wall panels for different types of exterior sheathing - in general this is 4 feet. Therefore, where an opening exists near a corner, the minimum length of braced wall panel required (at the corner and elsewhere) is 4 feet for an 8 foot high wall."

Further Discusion and guidance regarding the applicable clauses from IRC and the aspect rations involved is offered. Looks to me like your IRC uses length/height aspect ratios. Have a good read through that, I only read through quickly, but I think it's exactly what you need...

Hope that helps!

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[seattlemike]

If the house does not conform to all the minimums and restrictions of the IRC, then an engineer is required to do an IBC design. For example, on a two-story house, the bottom story would be required to be 55% (I think) plywood shear walls on each of the 4 sides. There is some mention of how the wall panels need to be close to the corners, and parallel walls (your parallel exterior walls) can be no more than 25' apart. Otherwise, an engineer is required.

I recommend getting on engineer. Especially if you are doing multifamily, and reusing your plans. Being Multifamily alone may bump you out of the IRC, and into the IBC requirements for structural design.

And there are some areas of the midwest that have high seismisity in the IBC, and there may be local requirements for Tornados. Also, since you are unfamilar with these codes, it's not a bad idea to hire someone who is familiar with them.

Mike

 

[Loui1]

To clairify, the construction is stick framing. Yes I understand if you dont meet perscriptions of IRC, you get kicked into IBC, BUT as my main question is, typically what prescriptions cant be met??

youngstructural , thank you for the link. It really does cover what I'm talking about. One would think the developer would request wall openings laid out so that non-engineered systems could be used. This would save them big bucks on the labor of installing tiedowns in 250 houses.

 

[youngstructural]

No worries Loui, we all need pointing the right direction from time to time...

They Seattlemike, just out of curiosity (and NOT out of any kind of malicious sarcasm!) I'm not familiar with the IRC whatsoever, so this may be a dumb question, however: Is it really a set of minimums in the IRC that dictate applicability? I would be most interested to see them, since all of the residential codes I know set maximums (ie: Not more than two above ground floors, not more than 600m^2 floor area, footprint not more than 320m^2, etc.

Again, NOT being sarcastic... I accept that a code could be worded as a litany of minimums (ie: No wall shall be less than x length, foundations not less than 220 thick, etc). I would expect that you're going to say that it's like most: Upper and lower bounds are defined, but since I could be wrong, I thought I would ask.

Thanks in advance,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[kenvlach]

Re "This is located in the midwest, so no coastal wind zone or major seismic...The developer seems to anticipate having to construct shear walls with hold downs. I've never seen a house with holddowns...would like to understand why I'm designing these things."

Tornadoes

"Category F2: Considerable Damage (113-157 mph); Roofs torn off frame houses; mobile homes demolished; boxcars overturned; large trees snapped or uprooted; light-object missiles generated; cars lifted off ground.

Category F3: Severe Damage (158- 206 mph); Roofs and some walls torn off well-constructed houses, trains overturned; most trees in forest uprooted; heavy cars lifted off ground and thrown.

Category F4: Devastating Damage (207- 260 mph); Well-constructed houses leveled; structure with weak foundations blown off some distance; cars thrown and large missiles generated.

Category F5: Incredible Damage (261- 318 mph); Strong frame houses lifted off foundations and swept away; automobile sized missiles fly through the air in excess of 100 meters (109 yards); trees debarked; incredible phenomena will occur."

http://www.noaa.gov/tornadoes.html

Missouri Tornadoes
"Compared with other States, Missouri ranks number 8 for frequency of Tornadoes, 12 for number of deaths, 15 for injuries and 9 for cost of damages."

http://www.disastercenter.com/missouri/tornado.html

 

[archeng59]

in my opinion, those types of residential buildings do not fall into the category of single family and two-family dwellings, which can be designed/constructed using the prescriptive provisions of the IRC. Multi-family dwellings are governed by the International Building Code (or any of the other model codes) as "R" type construction. The framing must be designed by licensed professionals per code.

 

[jhoulette]

You should be very careful not to blindly use the prescriptive methods briefly described above. Many residential structures do not meet the requirements to use the prescriptive method, (plate heights, wall spacing, wall lengths) so a true lateral design is required thanks to todays architects and trends homeowners want. I have deisgned well over 100 single and multi-family homes where about 90% of them have several areas that did not meet the criteria. Also, all of the houses had at least 4 holddowns (wind speed of 110C in my area).

 

[JAE]

kenvlach - the IRC doesn't require tornado design. But if the developer wants to, he/she can certainly require additional design measures....just needs to realize that you pay for those additional measures.

 

[seattlemike]

RE: tornados..

yes, for tornados, you just build concrete walls around your bathroom (because the plumbing anchors a bit), and pray.

but if you don't get hit with a tornado, you can still get hit with high winds, so game on.

RE: IRC
ulp, yes, okay, maximums AND minimum, ... I was thinking that your floor and roof weights have to be minimal (10 psf) to go with IRC. And there are a MINIMUM lengths of sheathed exterior walls (55% bot floor, all sides, 25% top floor). No topographic wind effects in area (shouldn't be a problem in the midwest)... MAXIMUM 25 ft between wall lines.... or that could be higher...30 ft or 35 ft... it's all in there. There's several more MINs and MAXs as well.

 

[JAE]

There may be a desire to have a separate legal entity to share liability. It's pretty common knowledge in the structural engineering realm that multi-family residential is one of the highest risk areas for engineers.

You may be asked to provide a fee to design ONE or TWO types of townhome combinations, and then they may use those plans for ...how many?... times. This adds to your liability as the probability of something going wrong in 100 units is higher than the probability in one unit. Your fee should almost linearly reflect this exposure to risk.

 

[Loui1]

Thanks for the input guys. I'm going ahead and just designing per the IBC. I think the final point is made that there's always going to be something not comforming to the IRC prescriptions unless it's a really simple house. Along with liability issues, it's better to just kick it into IBC.

As for tornado design, I'd recommend reading the several FEMA publications outlining what is required to resist tornadic loads. You really cant design an entire structure for anything above a Category 2 Tornado anyway, and in my mind, codes that implement anything more than just a "strong room" are wasting a lot of money.....but thats a whole other thread.