Residential vs Commercial Code? 3

[SteelPE]

My coworker and I have had a disagreement as of late. It revolves around when you are allowed to use the residential code and when you are allowed to use the commercial code. I have always said that the residential code is to be used for 1 and 2 family dwellings (hence the name). He says that if the project is residential then you can ignore the commercial code and go straight to the residential. The project in question is a 3 family dwelling with an issue regarding calculation of seismic loads. Does anyone have any opinion on the matter? When are you allowed to use the residential code vs the commercial code?

 

[Ron]

SteelPE...you are correct, assuming you are using the International Series of Codes (IBC/IRC). Look in the administrative provisions on applicability and you will see that the residential code applies only to one and two family dwellings; and townhomes under certain conditions.

Apartments and condominiums are considered commercial construction and would fall under the main building code. Further, in some instances, the residential code refers you back to the primary building code.

 

[woodman88]

I would use the IBC even if the building department, by amendment, allowed the IRC.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[msquared48]

I'm with Woodman. The IBC can be used in all cases.

In my opinion, the IRC just allows the contractor and owner to save money by scrimping on the structure.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[spats]

Mike had a bad day... but his comment is not without merit. Does IRC acommodate "standard practice" as being actual design?

 

[Ron]

In most cases, you have no choice. If you are designing a single or two-family residence, the proper code is the IRC. If you are designing a multifamily facility the proper code is the IBC. Most building officials will not allow the IBC to be used for single or two-family residences.

 

[dhengr]

My experience has been that most people would normally use the IRC for one and two family building design, and that you are required to use the IBC for any multi-family building housing more than two families. Most jurisdictions will allow an owner or builder to build one and two family buildings without the involvement of Design Professionals. The same jurisdictions will require a Registered Arch. and/or Engineer on the design of a multi-family building. I’m not aware that the IRC lets the builder or owner skimp on the structure, but if followed scrupulously it might allow them to save on Professional Arch. or Engineering fees on these building. There are limits to what they can do, and still comply with the IRC, in the way of crazy Arch. ideas, spaces, spans, etc. as they might affect structural matters. The IRC conforms with the IBC, in fact it is essentially derived from it, but it is slightly more conservative than the IBC in some respects, and prescriptive, because of this lack of engineering involvement, and the fact that a builder can use this code without the involvement of any engineering scrutiny. I have never had a BO object to my refining my design by using the IBC over the IRC on a residence, when I could use the IBC to advantage, to improve the design; and in some cases do things which the IRC would not otherwise allow. And, I would be surprised if an engineer who usually used the IBC and did some residential design using the IBC had any trouble with the AHJ.

 

[msquared48]

Ron:

In 10 years of doing residences under IBC guidelines, I have never had a design rejected by any building department for not using the IRC. I have always found the IBC more stringent.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[SteelPE]

This disagreement revolves around the requirement of seismic loading. We had a discussion about what lateral load should be applied to the side of a proposed structure. He gave me his number and I asked if that was wind or seismic loading. He said that it is a residential building and the IRC does not require you to analyze residential structures for seismic load. I said that it is a 3+ family structure and you are required to use the IBC not the IRC.... and that is the root of the disagreement.

Thanks for the info.

 

[Ron]

Mike...I agree, but there are building officials who will not allow that. I have also seen design done by the building code, then problems with construction develop and the building official will only hold the contractor to the residential code, not the design......go figure.

 

[RFreund]

SteelPE - What was his defense? "One and Two family really is just a guideline?"

EIT

http://www.HowToEngineer.com

 

[SteelPE]

RFreund

The project is an mess and is a discussion for another time. The reference project is an existing 3 story building that the owner would like to add two additional stories to. We were discussing the amount of additional lateral load that would be added to the existing structure due to the new structure above. His only defense was that the project is residential and not commercial therefore no seismic loading required. He eventually folded, but only after I told him that the first floor is retail space (something he didn't know when I presented the project to him for his opinion). In my eyes, we are now talking about a 5 story building..... what are the odds that only two families would occupy a 5 story building.

No harm done, just a little discussion and I was wondering when IBC takes over from IRC.

 

[RFreund]

ahh, I see.
Well, FWIW we seem to use the IBC for all cases, same as others have noted. Actually, in some 'higher end' villages they wanted to see seismic calcs (maybe I'm recalling this incorrectly) even though they don't usually 'control' around here.

EIT

http://www.HowToEngineer.com

 

[woodman88]

A couple of things about the 2003 IRC (the most current one I have),
1) It does allow the IBC to be used see section R301.1.3 “...Engineered design in accordance with the International Building Code is permitted for all buildings and structures, and parts thereof, included in the scope of this code.”
2) It does require the building to be designed for seismic see “R301.1 Design. Buildings and structures, and all parts thereof, shall be constructed to safely support all loads, including dead loads, live loads, roof loads, flood loads, snow loads, wind loads and seismic loads as prescribed by this code...”
3) The maximum height is three stories see “R101.2 Scope. The provisions of the International Residential Code for One- and Two-Family Dwellings shall apply to the construction, alteration, movement, enlargement, replacement, repair, equipment, use and occupancy, location, removal and demolition of detached one- and two-family dwellings and multiple single-family dwellings (townhouses) not more than three stories in height with a separate means of egress and their accessory structures.”

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[focuseng]

It has always seemed to me that the intent of commercial versus residential is based on risk probability. Commercial structures expect a higher occupancy loading or monetary value thus have more stringent requirements for performance. One and two family dwellings usually are limited in the amount of occupants and the overall probability of being occupied during an event. The more dwellings in a building the higher the risk to loss of life/safety. Thus you should be mandated to comply with the more stringent code as is usually codified by the state. In a multi-family occupancy the building is specifically required to comply with all kinds of things like egress, sprinklering, etc. The structural component is just one aspect. The bottom line is it still falls under the rules of the commercial code.

In WI where I practice, is is very clear that anything not a one or two (other than agricultural) is a commercial structure. The language of the code will tell you if it applies to the building or not. Your state may vary though.

MAP

 

[charliealphabravo]

My take is that the IRC was conceived to allow prescriptive, non-engineered design. I don't have an axe to grind, but I always used the IBC for residential structural design.

Having said that, I think an engineer should have the prerogative to appeal to the IRC even if it is less stringent. But this is not really "design", it is just "selection" which is why it can theoretically be done by a contractor without an engineer.

I'm sure that the IRC is enforced by CBO's for many of the other non-structural sections.

 

[SteelPE]

Here we go again....

We are now looking at a project that is a 4 unit townhouse joined together by common walls. He is saying that the IRC can be followed for townhouses and he does not have to follow the provisions of the IBC. This is the definition he is referencing:

The provisions of the International Residential Code for One- and Two-family Dwellings shall apply to the construction, alteration, movement, enlargement, replacement, repair, equipment, use and occupancy, location, removal and demolition of detached one- and two-family dwellings and townhouses not more than three stories above grade plane in height with a separate means of egress and their accessory structures.

Now I read this as applying to one and two family townhouses. He is saying that this applies to all townhouses no matter how large the project is. What do others think?

 

[msquared48]

And I suppose, since it is a residential useage, as he is implying, that the IRC governs 5 stories of wood framing over three levels of PT parking? Not in my lifetime!

As far as I am concerned, the IRC is aplicable only to one and two story SFR's, not to multiple structures with common walls, or over two stories.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[hokie66]

Not that I know anything about your IRC, but the dwellings you described are not "detached".

 

[wannabeSE]

I know very little about residential codes. However, I just looked at the IRC definition of townhouse:

"A single-family dwelling unit constructed in a group of three or more attached units in which each unit extends from foundation to roof and with a yard or public way on at least two sides."

So, you can't have a two unit townhouse using the IRC definition.