knee bracing and earthquakes

Hey all,
My engineer tells me that local codes no longer show knee bracing as an approved method of lateral support due to earthquakes. He says the code doesn't DISALLOW knee bracing, but that the loads are drastically reduced by some kind of ratio that he couldn't explain to me. Something to do with how much the structure will be able to flex with knee bracing alone. I guess it makes sense that you don't want a heavy top load moving laterally because that would be a lot of momentum. Anyway, I don't want to do sheathing or SIPs necessarily. Was hoping to do strawbale or other stackable wall infill (which will not provide much structural support).

Have others ran into this problem? If not, because you are not in an earthquake area? The engineer is trying to figure something out to keep the knee brace method, like running metal cables through the braces or X-bracing metal cables between the columns or something, but he says that is not ideal either. Again, in my project, the building codes seem to be pushing me towards a standardized McHouse using materials that are not appealing or affordable for me. Ok, sorry, I'm whining a bit...

Thanks.

shear capacity can be tough to do with braces alone depending on the size of the structure. and braces tend to work in compression only - unless you have really long tenons (and wide braces and posts or beams to take them). adding hidden or visible steel will work.

what about a moment connection @ the foundation to post? typically a steel shoe that is rigid enough to keep posts from moving about - so braces become a minor part of the overall system?


in addition to seismic you may need to look at wind depending on size and location of the structure. out east (especially on the coast) it will drive many of the decisions.

Hey Paul, That does sound frustrating! So does he subscribe to the idea of having a structure more rigid for earthquakes or a bend but not break idea?

I didn't know the Columbia River Gorge was a terribly seismic area.

Maybe new codes will specify according to the potential of Yellowstone's super volcano erupting.

I wish I new more to your dilemma. Perhaps a look at the styles of the Orient. Some of those have seen their fair share of earthquakes.

Mike, thanks, will check into the moment connection.

Mo, FUNNY. but then of course, how to withstand an meteor impact... The Gorge isn't super seismic but we are only 60 miles from Portland, which has a lot of faults (earthquake faults). I did find that Oregon has strawbale in their building code so I will see what they recommend.

btw, our braces will be attached with metal brackets of some type...which should help with tension.

Hi Paul;

A common type of building in early Colonial settlement was what we now call earthfast, that is the posts were set in holes in the ground, which eliminates the need for bracing. There was a big earthquake in Jamacia around 1691 and I have found a comment to the effect that the Spanish-built earthfast buildings survived and the rigidly built English houses fell to pieces.

What is old is new again?

Jim

Thanks Jim. I had thought about that too, but I didn't like the idea of putting wood down into the ground because of rot. I wonder how the Spanish-built buildings lasted so long with the wood in the ground?!!! What I have read, indicates you can expect the treated wood post to last for only 100 years.

I am wondering if I added bracing from the column to the stem wall would do the trick. Can't remember what that type of brace is called?

Strut?

Tim

There are still a few ancient earthfast buildings on the East Coast, but I am not sure just how old they are. I doubt this type of building would normally last much more than 50 years in the East.

Some trivia for you; earthfast posts must be the modern archeaologists name for piles, an ancient technique for building on soft ground, even underpinning stone foundations! At Plimouth Plantation they are called puncheon posts. In your area, there may still be some box houses, also known as single-wall, California cottages, or board-and-batten houses. With these houses it was standard practice to build them on posts set into the ground. The Dells seems to be an area where these were used in the mid to late 19th century, maybe later.

Take care;
Jim

My engineer and I still haven't come up with anything acceptable to me. He is pushing plywood sheathing (or similar) or steel X bracing pretty hard. I'm not dead set against sheathing. I guess we could use it as backing for our strawbales... However, I am re-interested in timberlinx method again as it looks to provide additional strength. I have an e-mail out to them about response modification coefficient R value when using their product. R value is used to determine load capacity with regard to a structure that is moving (shaking). Straight ahead timber framing gets an R value of 1.5 which reduces my load capacity by 2/3rds! Sheathing R value is 6.5 reducing load capacity by only about 1/7th.
http://www.timberlinx.com/about.php
Not trying to put any timberframers out of biz! Just trying to increase the solution knowledge base for peeps in earthquake country. I'll post later what I found out.

Paul,

I really makes a difference on whether your engineer is familiar with traditional timber frames (not post & beam, metal connectors). It's not something they go over in engineering school. You can still retain his/her services if you'd like, but I'd recommend getting a specialty timber frame engineer. You can look on the TF engineering council website (although they just started collecting dues, so some of their membership might be in limbo??), or in the timber frame resource guide through the Guild. Feel free to PM me as well if you'd like a personal recommendation.

Brad

Just a quick update here. I hired an engineer experienced with post and beam and straw bale. He has even built his own structures so that sealed the deal. He recommends removing all of my knee braces and will engineer in some straps for lateral bracing that will be hidden in the walls. I'm happy with the decision. Those knee braces were kind of hard to design our floor plan around - head bonkers in places etc...plus this will save lumber and labor. tx

Hi Paul,

Alarm bells are ringing in my head already !

Regards

Ken Hume P.Eng.

Hi Ken,

Tell me more!

Apparently, knee braces are no longer in the building code around here as a recommended method for lateral stability. The building code is all about sheathing and "strapping". Every engineer I have talked to has pushed me away from using knee braces.

tx

have fun at the raising......