alternate brace panels and deviations

[MikeT]

As I look at the windows that I would like to put into my Victoria's Cottage, if I placed each window where I ideally would like it to go, it would mean that I would have only 24" between the corner of the house and a window in the "Great Room".  The plans seem to call for an Alternate Brace Panel (which I believe is 32" per code).  I have SSTB16s embedded in the concrete where I think they should go, but I am very comfortable with epoxying in new bolts in a better location if I need to.

So my question is this:  As I understand the prescriptive path, I need to have 32" between the corner and the window (my panel) and if I do not, I will need to have the solution engineered.   Do you have a sense of what the engineering might be for this situation (a 24" braced panel, rather than a 32")?  

I should note that I have easy access to a structural engineer (who helps me for free), but before I go back to him with a potential field change, I would like to know what yous folks think might be the degree of difficulty in this change and its advisability....

[PEG688]

I wonder if one of these would be OK?

http://www.strongtie.com/products/strongwall/standard.html





I've enver used one and have no idea of $$ but it would seem to be what your after without bugging your engineer .

[glenn kangiser]

Here is the one you are working with now - http://www.strongtie.com/news/industry/wall-bracing/alternate.html

Here is the one you want but it looks like the foundation would have required som special reinforcing and embedded hold downs - it says retrofit brackets not accepted.

http://www.strongtie.com/news/industry/wall-bracing/new-alternate.html

I don't know any more than that - I just remembered seeing them on a job -- actually I think it was some Simpson made - seems like their name was on them-- tying them to the foundation may be the part where the engineer comes in.

[glenn kangiser]

OK PEG -- thats exactly the ones I was talking about -- you found them -- at least I remembered them.

[MikeT]

Thanks to both of you.  I like the idea of knowing what I might face if I need to do something smaller than 32".  I did notice that in the Simpson guidelines for this wall, they assume the SSTB28s for anchor bolts and I have only SSTB16s.  I may end up just having to compromise on the ideal window size and location for something that more easily falls within the prescriptive path.

Stay tuned....

mt

[John Raabe]

Remember that the braced wall panel (alternative or otherwise) doesn't have to start at the corner but anywhere within the first 8' from the corner. Thus you could have two 6' wide windows that join at the corner and then have panels on either side.

Check with your inspector but that's what has worked on some of my custom houses.

[MikeT]

Thanks, John.  I am fine with following directions, but as part of this process I am trying to learn.  Can you either explain what is the physics behind this or direct me to a website or book where I can read more on how the panels work?

mt

[John Raabe]

It's a simple matter of racking resistance - doing things to make the walls rigid so they don't turn into parallelograms when you have a severe wind storm or earthquake. Just having the walls sheathed with plywood or OSB goes a long way toward providing this, but every hole you punch for windows and doors cuts down the rigidity.

Different parts of the world have different design loads for wind and earthquake forces, so it's best to work with the local inspector and find out what they require. You may not need anything more than what is shown on your plans. All my plans have the walls sheathed in structural panels for that reason.

These braced wall panels and the narrow Simpson panels are pre-engineered and prescriptive (cookbook) approaches to providing the work that a structural engineer would do in their analysis of your specific building design. Thus the inspector could say "you can avoid getting engineering if you do X,Y & Z"

On the otherhand, if you go to the engineer and want 10' of glass on each side of the corner they can probably do that! But there will need to be a shear wall somewhere.

[MikeT]

Thanks, John.  I get that part, but I am interested in how one gets 32" as the minimum size for a ABP.  What if it is 30"?   Is the racking strength really reduced?  If so, by how much?  And what about panel/wall height--I would assume the 32" assumes a certain height.  

I think I will sit down with my friend who is the structural engineer over a beer and see if he can take me through some of the calcs.  He and I sat next to one another in high school calculus-- he went on to apply the learnings, and I clearly did not!

[jraabe]

As in all things building, much of this derives from traditional building solutions... 32" is two stud bays when you have 16" o/c framing. 30" is not without strength, of course, but is an "off grid" use of materials.

If you wanted to do 30" wide shear panels the engineer could calculate that solution adding perhaps some thicker plywood, extra nails or a beefy hold-down, and, when stamped, it would satisfy the inspector. But, you would have the design fee for the engineer and no longer be doing something that is on the list of code approve prescriptive (non-engineered) solutions.

Do you see how these traditional code things evolve and yet are still arbitrary, in that they have a clear cutoff point that makes little logical sense?

The width of the rails on a railroad track are arbitrary... there is no real reason why they couldn't be 4" wider. But that size evolved from the width of Roman chariots which had a wheel spacing determined by the width of two horses working side by side. There is a great story about how the width of the space shuttle (which had to travel part way by rail) was determined by the width of a Roman horse's ass.

[JRR]

Could interior walls could be designed for anti-racking when exterior walls are mostly glass?

[glenn-k]

Sounds like an engineer project as most of the small house designs are non-bearing in the interior.  It would just tear out the floor joists in a giant wind or earthquake unless a big foundation was attached to it.

[MikeT]

JRR's question is a good one, but I suspect that an additional force needs to be considered here: the force of the wind and its ability to push and/or suck a wall and in this case, a large field of glass.  I could envision a situation where the a house could withstand the racking force, but a wind could easily suck a window out.

Now that I think about it, this essentially happened this winter up the hill from me.  One of the houses under construction had the windows just sucked out during a storm.  One of the workers told me that while they were trying to get things buttoned up (And he was on a ladder), the window next to him was pulled out and just spun around in the wind for a bit before dropping to the ground.

Then again, I defer to the experts, cuz I'm just guessing here.....

[jraabe]

JRR's suggestion is exactly right.

When you see the slick glass houses on the cover of the glossy home magazines they are likely doing things like this:
• Using steel welded moment frames buried in the walls (Glenn knows about these  )
• Building floor and roof diaphragms that transfer the wall loads to interior shear walls which are in turn anchored to interior foundation footing walls.

All of this is expensive, of course, and unlikely to be of much interest to the practical owner-builder. The small houses we are dealing with on this site rarely have enough area to accumulate the kinds of forces that require anything other than tidy wood frame construction.

One thing about architecture... you can build just about anything you can imagine if you have enough money.  8-)