Use CA Bldg Code not CA Res Code for residence

[beautifulhills]

I am a designer working in conjunction with my engineer (out of State, specializing in modular steel homes) I' ve designed a 3 story less than 60' long, single family. The home will be in structural steel, with lateral bracing. We ll install the same model home several times. For most all purposes the home fits neatly into the California Residential Code 2016. However, many of the sites are Seismic Design Category E with D soils.
R301.2.2.4 of CRC says Buildings in SDC E shall be designed to resist seismic loads in accordance with CBC
Am I correct, we have no choice - we must use California Building Code, and not any of the California Residential Code? (I wonder what the additional burden will be of not having the benefit of the CRC? what extra things we ll need to do) The code states we cannot use both codes.

 

[wikidcool]

Structurally, anything that isn't covered in the CRC has to be done per the CRC. It doesn't sound like your structural system is prescriptive anyway, is it?

R301.2.2.4 says just the seismic system has to be per CBC. It explicitly states components not required to be designed to resist seismic loads can still be per CRC (gravity system, non-structural, etc.). All E category requirements regarding seismic design in the CBC will have to be complied with.

 

[beautifulhills]

Hello wikidcool, Thanks for your informative reply. Just be clear, your first sentence is to read" Structurally, anything that isn't covered in the CRC has to be done per the CBC" ? Sorry to be so pedantic/fussy.
You're correct the system is not prescriptive - it will be cold formed steel, with lateral bracing, I m assuming per ASCE 7-10. Yes, a useful clarification that the gravity system does not need to be per CBC.

 

[wikidcool]

Not sure I fully understand your question, but another way to say it is CRC R301.1.3 -

R301.1.3 Engineered design. Where a building of otherwise conventional construction contains structural elements exceeding the limits of Section R301 or otherwise not conforming to this code, these elements shall be designed in accordance with accepted engineering practice. The extent of such design need only demonstrate compliance of nonconventional elements with other applicable provisions and shall be compatible with the performance of the conventional framed system. Engineered design in accordance with the California Building Code is permitted for buildings and structures, and parts thereof, included in the scope of this code.

In other words, only the portions of the building design that don't comply with CRC need be done per CBC. Other elements, even structural elements, can be CRC. I've had wood-framed homes that only needed the garage door header sized, and one non-complying shearwall designed and the rest was prescriptive. Per your SDC E, the entire seismic system is kicked out of CRC - you're right, you don't have a choice on that. But everything that doesn't have CBC seismic design requirements for SDC E can still potentially be under the CRC if they aren't kicked out for other reasons.

Keep in mind that there's a myriad of things not part of the lateral force system that have seismic design requirements in SDC E (non-structural component anchorage, etc). You probably don't have many of them in a small pre-fab house, but I suggest reviewing the requirements. ASCE7-10 also has requirements for such things (chapter 13, etc). Some of those things are not listed specifically in the CBC, but since it's incorporated by reference, they do apply.

 

[beautifulhills]

wikidcool:-

After making this post, I had a call from an official at San Mateo County, who in less detail than you, confirmed what you are saying. He was quite emphatic about using the CRC for everything, except where CRC says it cannot be used, and then reverting back to CRC for anything that was not part of that 'component' as he called it.

Yes, good point on things other than lateral for SDC E that are required - albeit as you say may be small.

It seems particularly unfortunate that CBC 1 1 7 3 1 says you can only use one code or the other, with some very unclear exceptions to that stated in that paragraph - at least unclear to me - the sentence almost tails off at the end without finishing.

One thing that was causing concern was compliance with fire ratings given CRC did not cover the seismic design - I understand that it's only seismic design that we go out of CRC, but for fire rating we go back to CRC.

I have not been familiar with CBC, but am quite familiar with CRC, so that this has been a learning curve for me.

Great input wikidcool