Shear walls in a Timber Frame

[AlpineEngineer]

I got a call for a very unique project. It's a timber frame home with a 'cord log' infill. Basically they build the timber frame and fill inbetween the posts with stacked logs. The logs are 16" long and stacked so that you see the butts of the logs, just like you stack firewood. The trouble is this in a very high wind zone, 125mph 3 sec gust. It's my understanding that the timber frame itself can NOT resist lateral forces (then it would be a moment frame). So it seems to me in timber frames the LFRS is obtained with SIP panels or other panel methods.

Do you guys have any ideas on how I can create a LFRS with these stacked logs? I thought of looking into how they do it with straw bale infill homes, I have heard there is a method with straps that tie to the fndn and go up and over the bales. Of course the bales get covered up with a finish. In my case they want to leave the butts of the logs exposed both inside and out, so asthetics is an issue. Maybe I can create some kind of corner bracing with timbers? The structure is only 30'x35'. So I've got that going for me..
Thanks

 

[JAE]

How are the stacked logs connected together? Are they toenailed in as they are stacked? Are they round shaped? Are they angular?

 

[AlpineEngineer]

http://www.daycreek.com/dc/HTML/TMEN_No149.htm

here is a great website that shows construction.. Oh boy, it appears they put sawdust inbetween the mortar layers...

 

[msquared48]

Normal log home construction uses spikes or rods through the logs to transfer the shear.

I would look at installing 18" long lag pins to take the shear, not relying on the mortar - that is really tanamount to chinking - and relying on developing a connection of the logs to the timberframe columns at each end to develop the uplift in the shearwalls.

This is an odd one.

Mike McCann
McCann Engineering

 

[MiketheEngineer]

Any chance you could "X" brace the frames with straps or cables and cover them with the logs?? Might have to do some cuts in the logs to accomodate the bracing but at least you will know exactly how everything works....and it will still look original

Good Luck

 

[AlpineEngineer]

Mike, I thought of the X brace idea also. Maybe they could just set the logs around them. Or I wonder if there would be a way to make the x bracing out of timbers and expose them on the interior? Kind of like pole barn bracing..
Thoughts?? Thanks..

 

[JAE]

For high wind, it may be OK as a type of shearwall in the plane of these walls. Perpendicular it would be a trick to check - although it is pretty thick.

Seismic areas - won't fly in my opinion as you have no ductility at all - a very brittle condition with all that mortar.

If you used diagonal timber braces (exposed to view just like the vertical columns and horizontals) and just stacked the logs around them?

 

[MiketheEngineer]

That is what I was thinking. Use 3/4'' or 1'' tie rods in the plane of the timbers at their centerline. Then stuff the logs around them - grooving them if necessary to get a nice fit and hide the rods.

BTW - do you work for Alpine Engineering - now part of ITW??

 

[msquared48]

I don't know how you would get around the code requirement of no contact between wood and concrete. I do not think this would fly here.

If it wal allowed, then it would have to be treated as an unreinforced masonry wall- just using wood bricks.

regarding the sawdust infill, earth or whatever. Probably a thermal break of sorts. If nothing else, an excellent termite nest location.

Mike McCann
McCann Engineering

 

[AlpineEngineer]

I've got some more info:
The home has a crawl space so the builder has suggested a raised floor system to get around the wood/concrete contact. The walls are mix of round and split logs (as well as bottles )... The mortar is a 'papercrete' product, masons mix with paper mixed in for more thermal mass I guess. There will be a layer of sawdust in the middle (fortunately we are in a very dry climate, sounds like a mold haven to me).

mssquared:
I'm not sure I follow your lag idea. Are you thinking of long bolts like in a log home? Maybe there is a way to have 2 long vertical embedded all thread rods on either end of the shear wall and then at the top of the wall have horizontal steel that connects the cord logs/timberframe/all thread rods together..
The builder is not to thrilled with the x bracing idea or exposed timber bracing, but he'll do whatever he has to.
Thanks for all your input guys.

 

[msquared48]

AlpineEngineer - yes that was what I was initially thinking - without looking at the picture you furnished. DUH...

After that, with the double wall construction and center void, I kinda gave up on that. Plus, with the randomness of the logs thru the wall, it would be very hard to arrange.

Still thinking here...

The thing is probably built, so this is a probably moot point. Nevertheless, I would abandon the center void and use a mortar infill with a rebar grid, vertical and horizontal in the mortar. I would stub from the end wood columns rebar into the center mortar infill to transfer the uplift/downdrag to the columns. I know they like the look, but I would put rigid insulation on the inside of the structure in a furring wall. Perhaps only a portion of each wall could be devoted to a shear wall, leaving the remainder with "the look".

The only remaining issue would be the wood to concrete contact.

Mike McCann
McCann Engineering

 

[JAE]

I don't know how you would get around the code requirement of no contact between wood and concrete.

Mike, what code requirement are you referring to?

Wood can't "hold up" concrete - but basic contact is not denied is it?

 

[msquared48]

JAE:

You would ask for the code reference - I'll have to track that down for you... Some things you just get used to and just do, know what I mean?

Been doing that for years here as we had to use treated wood in contact with concrete for years until they discovered that the preservative in the wood caused the sill plate anchor bolts to deteriorate. Now we have to use building paper between the concrete and the wood plate where we have bolts thru the plates, or stainless steel anchor bolts - yea...like that's gonna happen. I do believe that there are other alternatives used now too.

Seems the moisture in the concrete causes the wood to rot with direct contact in these environs. We even have to design in clear spaces for venting for all wood connections inset into a concrete wall (pocket condition).

Mike McCann
McCann Engineering

 

[JAE]

Understand - I know there's issues with the natural moisture, and moisture transferrence, from the concrete to the wood. Just wasn't sure there was a direct code sentence somewhere I wasn't sure of.

Maybe covering the log building with a very THICK layer of shellac would do the trick...eh?

 

[msquared48]

OK JAE:

I first learned of this from my supervisor in 1979, in the 1979 UBC, section 2517,c,3 under "plates, sills and sleepers." For the air space inferral, look to 2517,c,7 in the paragraph regarding clearances from planter boxes.

In the 2006 IBC, the closest thing I have found so far is in sections 2304.11.2.4 and 2304.11.4.1, where these are referring to wood supported on slabs in direct contact with the earth. By extrapolation, I would automatically extend that to walls in contact with earth, since the earth stops within 6" of the top of most concrete walls here, and the moisture from the concrete can wick.

Could be a stretch, and maybe not applicable to your area, but I chose to err on the safe side, particularly in the wet Northwest.

Cheers...

Mike McCann
McCann Engineering

 

[JAE]

Mike - I agree - wood and concrete and...and the resulting moisture exposure isn't too beneficial to the wood remaining competent.

...what about ceramic logs!

 

[tngolfer]

Is creating a concrete or cmu shear wall and then putting a veneer on each side composed of 1" thick pieces of wood and mortar? The contractor could make an attempt to line up the wood on each face of the wall to make it appear as though they are a single log.