2012 Braced Wall Lines (R602.10.1)

[vitium]

I'll start with the question, for those who dont want to read through a whole story, then elaborate.

Does it say anywhere in the 2012 IRC that ALL exterior walls MUST be a part of a "braced wall line" as defined by Section R602.10 (IRC 2012 page 164)?

In Austin TX US, the city has just adopted the 2012 IRC (we had previously been on the 2006). As a result I'm having to learn all about "braced wall plans", and showing them on our drawings. I have a situation (see illustration -

http://i.imgur.com/t9PXcxa.png)

where creating a BWL (braced wall line) will be practically impossible to do within the limits of the architectural design (especially at the porch). So, my solution is to simply omit these exterior walls as part of braced wall line at all, and rely on larger bracing opportunists at other locations.

There are tons of requirements about spacing of BWLs, amount of bracing panels in BWLs. There are also rules about the maximum offset between bracing walls along a BWL (4' offset from line), but I can't find anything that says "all exterior walls must be part of a BWL" or words to that effect.

Thanks,
Matt

 

[AELLC]

I think you are OK. It is obvious the porch can't be a BWL, and I believe all the fuss and bother about BWL location is mostly about seismic, which I am sure ids not a factor in Austin.

I design a lot of custom homes and I can definitely say most of the homes I see can't have all that BWL criteria met, and here, it doesn't seem to be a problem (Phoenix area, very light seismic).

 

[msquared48]

If you do not have a shear wall line at the outside face of the porch or garage, you have to cantilever the roof and floor diaphragms to take the lateral from each, and the code (IRC/IBC/ASCE7) does limit the size of the cantilevered area. Anything over that area or size limitation must have lateral restraint at the exterior line.

I would have to check the code to see what the current limitation is.

Mike McCann
MMC Engineering

 

[vitium]

@msquared48,

Thanks for the reply. If you don't mind I looking that up it would be really helpful. I have stumbled through much of the code, and do not recall anywhere it saying what the max cantilevered distance of a shear diaphragm is (in this case, it would be the roof sheathing).

I'm really disliking this perspective style of lateral design, and having to show it on the plan. I much prefer the way of just, using engineering judgement to determine if it works, and doing the design where needed. I guess it must be all the houses blowing over in the Austin area these days... :0

Thanks,
Matt

 

[msquared48]

If you use the simplified procedure for seismic see ASCE 7-05 section 12.14.1.1, items #7 and #8 for starters...

As for wind, I could not find a section on a quick search. I will look some more later...

Mike McCann
MMC Engineering

 

[vitium]

Luckily in the Austin area, seismic is not an issue, and really, unless there are unusual circumstances, wind is not typically a major problem either.
My main problem is that the City of Austin Planning and Review Department has decided that engineered drawings MUST show "braced wall plans", of which it often gets quite ridiculous with some of the custom houses we do. In Austin, the entire exterior is always fully sheathed with plywood, and as a result, it is rare that a situation arises where there is actually a structural need for a bracing plan, and the prescriptive methods of the IRC are somewhat restrictive for some of the houses we work on. In the past, when an issue arose, I would do a custom design following the methods laid out in the ASCE-7, however, in the latest IRC (section R602.10.1, regarding BWLs), I don't even think the ASCE-7 is even mentioned in a footnote. Everything is prescriptive. So now I'm forced to discard engineering design, and cram everything into a mold created by the IRC. At the very least, these bracing plans are time consuming and complicated, and often completely unneeded. When they are needed, it's difficult to make them work within the confines of what is laid out in the IRC, so, any ways that I can reduce the amount of rigmarole are helpful.

Regarding the simplified procedure (listed in the IRC, most of the time, the houses we work on do not qualify, I'm not aware of a simplified overall procedure for wind loadings in the ASCE-7, however since the IRC has its own prescriptive method, I'm not sure a simplified ASCE version would fly with the COA building official even if there is one.)

Anyway, sorry to rant, but if you see something in the IRC that clarifies the maximum amount that a diaphragm can cantilever from a BWL, that would be quite helpful.

Thanks for your help.
Matt

 

[AELLC]

Even when the entire exterior is sheathed, in custom homes, there almost always is a need for hold downs at certain locations. You can't just say the sheathing alone is sufficient, and forego any calculation.

All I am saying is since I doubt seismic is an issue, the plans checker may not enforce the BWL criteria at the porch.

 

[vitium]

@Aellc

It's true there are areas that are tricky. But in large part, if the house is fully sheathed in Austin (90mph wind speed, exposure B, no seismic to speak of), there is not much to worry about. I would never just say "Oh its sheathed" and be done with the house as a whole. I look for the areas of concern (typically at garages that protrude, with short return walls, or long/tall window walls etc), and I would always do engineering if required for my own comfort. What I want is to limit the hassle required by the IRC where it pertains to BWLs. I want to show what is required for Building Official where I have to, or where it's needed, and for areas that are of no concern, I want to limit the time and effort. I'm trying to focus my energy where it matters....

Thanks for your help,
Matt

 

[AELLC]

OIC, it's the way you do things there.

Here in Phoenix area it is usually Exp C except one town allows Exp B for one-story, and if the roof pitch is 8:12 those can generate a lot wind load too.

We engineer everything, nothing by "Say OK" except we don't worry about the 2012 BWL criteria as described above.

We do worry about these:

Wall with openings, check for amplified shear
Shear transfer, roof or floor to wall, sometimes need hardware in addition to nails
Interior shear walls, shear transfer detail.
Frequently nailing needs to tighter than standard 6:12" o.c.

Plus the other items you mention, but we do the complete shear wall and hold down design regardless of whether completely sheathed or not.

 

[OldPaperMaker]

Vitium,
Hopefully, the city of Austin doesn't require one to use the IRC to do the structural design of houses. If you have a building that doesn't meet the prescription(s) found in the IRC and you are a licensed PE then you can always use rational design and the IBC. You just have to change the reference codes in your calculations and on your drawings.

 

[Althalus]

There is nothing in the IRC that says anything about distance from the perimeter of any structure to the first braced wall line (e.g. the edge of a covered porch to the exterior wall of the building). However, if you are going with the BWL method and you have a significant distance from the awning to the wall, the plan reviewer may be justified in asking for something more substantial.

The 4' offset doesn't really refer to this situation. But I can see how a building official may use that as a basis to determine a "reasonable" width for a porch prior to asking for something more substantial. But you may counter with the "total" 8' out-to-out offset allowed for such lines.

The simple fact is this: The BWP methodology was developed for very simple designs. Once you start getting into some of these architectural wonders, BWP may rarely be used simply because the geometry does not satisfy the basic requirements. You must use an engineered solution.

An engineered solution is to take place of a braced wall design. If your official is telling you that your engineered solution won't work because it doesn't satisfy BWP requirements, then he obviously doesn't know the code. If that is the case, you've got a problem no one can fix. -- You can't fix stupid.


Althalus

 

[masterthenight]

Agree with OldPaper and Althalus. The IRC is a black box approach for people who aren't qualified to do engineering and also to provide a simplified approach for a basic set of criteria.

Based on my personal experience, well over 50% of truly "custom" home plans that are upwards of 3000sf will likely not meet some portion of the IRC criteria. The IRC is very clear that prescriptive requirements may be used on portions of the structure which meet the criteria and the other portions shall be engineered by rational methods. In my experience, most engineers who are sealing a plan that does not clearly fall within the IRC criteria will just engineer the whole structure anyways.

In regards to the original question, in my experience, there are no explicit requirements for exterior walls to be BWLs as long as you meet the other BWL requirements. I would suspect it is challenging to meet those requirements without using most exterior walls however.

 

[AELLC]

To throw in my 2 cents, frequently a house "designed" by IRC has very high overloads at some of the prescribed hold downs.

It just doesn't make sense to try to save time using the IRC, because a proper lateral analysis takes only a little extra time, however, frequently, some of the BWL requirements are impossible to achieve in a typical custom home - that is where engineering judgment takes over.