# IRC vs IBC Prescriptive Wall Bracing...are they different?



## SDS (Nov 30, 2021)

As the title asks...are there any differences between the 2018 IRC vs IBC in the way prescriptive wall bracing is calculated, specific to IBC R-2 occupancies? Will the results be the same?

This question assumes acceptability by local officials.

Criteria:
110mph wind
Exp B
SDC D1
3-story R2 building

TIA


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## classicT (Nov 30, 2021)

Yes, there is a substantial difference.

Hard to pinpoint exact differences, as you really need to work through the design to see what they are. But one of the bigger differences I have noted is in _IRC Table 602.10.1.3 _vs. _IBC Table 2308.6.1_.


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## classicT (Nov 30, 2021)

But hang on and hold up.... 3-stories and D1?

You are outside the limitations of prescriptive conventional light-frame construction. IBC prescriptive conventional light-frame construction methods are not permitted.

*2308.2 Limitations*
Buildings are permitted to be constructed in accordance with the provisions of conventional light-frame construction, subject to the limitations in Sections 2308.2.1 through 2308.2.6.

*2308.2.1 Stories*​Structures of conventional light-frame construction shall be limited in story height in accordance with Table 2308.2.1.​​*TABLE 2308.2.1*​*ALLOWABLE STORY HEIGHT*​
SEISMIC DESIGN CATEGORYALLOWABLE STORY
ABOVE GRADE PLANEA and BThree storiesCTwo storiesD and EaOne story


For SI: 1 inch = 25.4 mm.

For the purposes of this section, for buildings assigned to Seismic Design Category D or E, cripple walls shall be considered to be a story unless cripple walls are solid blocked and do not exceed 14 inches in height.


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## SDS (Nov 30, 2021)

Thanks ClassicT, that certainly addresses my specific project....so the IBC limits SDC-D prescriptively braced structures, regardless of occpancy, to 1-story.


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## SDS (Nov 30, 2021)

Yes, I just came to the same conclusion reading through the IBC sections you noted in your OP, that was the sort of answer I was looking for, thanks.


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## classicT (Nov 30, 2021)

SDS said:


> Thanks ClassicT, that certainly addresses my specific project....so the IBC limits SDC-D prescriptively braced structures, regardless of occpancy, to 1-story.


Yes... but that is not the only significant limitation. Check out all of the limitations in _Sections 2308.2.1_ through _2308.2.6_.

I have found that the limitation on truss spans (40' max) is also a challenge. See _Section 2308.2.5_.


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## SDS (Nov 30, 2021)

Just for conversation's sake here...

I normally only work with the IRC, exclusively...but this multi-fam project has come up and my 'no' was refused when I turned the project down, so I'm the guy on this one. Not a big deal just not my forte y'know? (I'm just a residential designer, no stamp of any kind)

Originally we presented a townhouse design...3-story 6-plex...I would be fine designing & engineering this prescriptively under the IRC.

However, the local building officials & developer decided the buildings should be apartments instead of townhouses, and the building should be designed as R-2 occupancy under the IBC...one large building instead of structurally independent townhouses. However, the design is still the same, a townhouse plan comprised of six 3-story units with private garages.

So the question arose as to whether it could still be prescriptively braced but clearly not so I'll let my engineer handle the structural design on this one instead.


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## classicT (Nov 30, 2021)

SDS, I see you are a fellow Washingtonian, so I fortunately can help point you in the right direction. Not trying to tell you what to do, but you need to back away gracefully. Even if this is a townhouse under the IRC, a licensed architect is required by State Law.

If you design this, not only will the AHJ most likely kick it back, but they could turn this over to the State Licensing Board. See RCW 18.08.410 and WAC 308-12-340.

RCW 18.08.410​Application of chapter.​This chapter shall not affect or prevent:​(1) The practice of naval architecture, landscape architecture as authorized in chapter 18.96 RCW, engineering as authorized in chapter 18.43 RCW, or the provision of space planning or interior design services not affecting public health or safety;​(2) Drafters, clerks, project managers, superintendents, and other employees of architects from acting under the instructions, control, or supervision of an architect;​(3) The construction, alteration, or supervision of construction of buildings or structures by contractors registered under chapter 18.27 RCW or superintendents employed by contractors or the preparation of shop drawings in connection therewith;​(4) Owners or contractors registered under chapter 18.27 RCW from engaging persons who are not architects to observe and supervise construction of a project;​(5) Any person from doing design work including preparing construction contract documents and administration of the construction contract for the erection, enlargement, repair, or alteration of a structure or any appurtenance to a structure regardless of size, *if the structure is to be used for a residential building of up to and including four dwelling units* or a farm building or is a structure used in connection with or auxiliary to such residential building or farm building such as a garage, barn, shed, or shelter for animals or machinery;​(6) Except as otherwise provided in this section, any person from doing design work including preparing construction contract documents and administering the contract for construction, erection, enlargement, alteration, or repairs of or to a building of any occupancy up to a total building size of four thousand square feet; or​(7) Any person from doing design work, including preparing construction contract documents and administration of the contract, for alteration of or repairs to a building where the project size is not more than four thousand square feet in a building greater than four thousand square feet and when the work contemplated by the design does not affect the life safety or structural systems of the building. The combined square footage of simultaneous projects allowed under this subsection (7) may not exceed four thousand square feet.​
WAC 308-12-340​How do I apply the exemptions contained in RCW 18.08.410?​For the purposes of clarifying the exemptions to licensing contained in RCW 18.08.410, the following brief summaries and definitions apply:​(1) RCW 18.08.410(5) grants exemption for any person to design and prepare construction documents for any residential building, regardless of area, *up to but not greater than four dwelling units;* or a farm building; or a structure as described in RCW 18.08.410(5).​(2) RCW 18.08.410(6) grants exemption for any person to design a building of any occupancy for up to a total building size of four thousand square feet.​(3) RCW 18.08.410(7) grants exemption for any person to design and prepare construction documents for alteration of, or repairs to, a project not greater than four thousand square feet in a building greater than four thousand square feet and when the contemplated work does not affect life safety or structural systems. The combined square footage of simultaneous projects allowances under this section shall not exceed four thousand square feet.​(4) Life safety is affected if the work contemplated includes, but is not limited to:​(a) Alteration of any fire rated construction;​(b) Alteration of any means of egress including barrier free provisions defined by the building codes;​(c) Alteration of a building such that the number of occupants in the affected space(s) would be increased.​(5) Project size is defined as the cumulative square footage of all spaces that contain altered construction in the design under consideration.​(6) Simultaneous projects are projects which have an open permit in the same building, designed or prepared by nonarchitects, the total of which may not exceed four thousand square feet.​[Statutory Authority: RCW 18.08.340 and 43.24.086. WSR 11-11-019, § 308-12-340, filed 5/9/11, effective 7/1/11.]​


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## classicT (Nov 30, 2021)

The other aspect that I will mention, is if it were a townhouse (which it sounds like the AHJ is against), then be weary of _IRC Section R302.2.6_. In essence, each townhouse unit will need to have its own complete brace wall design. You cannot evaluate the townhouse as a building, but have to have the design done individually. The sheathing can carry over the joint between units, but for purposes of the lateral design, forces cannot be shared across dwellings.

*R302.2.6 Structural Independence*
Each individual townhouse shall be structurally independent.
*Exceptions:*​
Foundations supporting exterior walls or common walls.
Structural roof and wall sheathing from each unit fastened to the common wall framing.
Nonstructural wall and roof coverings.
Flashing at termination of roof covering over common wall.
Townhouses separated by a common wall as provided in Section R302.2.2, Item 1 or 2.


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## SDS (Nov 30, 2021)

Thanks ClassicT, it is very sound advice, as I said I told them no, that they needed an architect. I'm meeting with the developer this morning to address several issues, including the designer issue, then on to the city if needed. Loading up on ammunition...


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## classicT (Nov 30, 2021)

What do they have over you to make you do the work? That's not how things work typically... you have the ability to pass on the job, right?


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## SDS (Nov 30, 2021)

classicT said:


> What do they have over you to make you do the work? That's not how things work typically... you have the ability to pass on the job, right?



Well it's never quite as easy as it would seem, but yes, of course I can draw a line and say no, but we've been working together for several years, they like my work...and they don't want to go somewhere else on this one project. So we met up this morning and now we're reconfiguring the project for 4-plexes...


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