In another thread about REMOTE wall systems, a Larsen-Truss system was mentioned. I like the idea of cellulose insulation, and, equally important for me is the ability to construct it on-site with local rough-sawn lumber. While looking for examples of Larsen-Truss walls on timber frames, I came across this link to a home - named Hjälmaren by the owner - in mid-coast Maine. The frame was built by Connolly & Co. (is John a participant in this forum?), with builder Bruce Cummings doing the truss work. As the owner describes, he had to educate the GC and the builder as neither had heard of a Larsen Truss prior to this project. To integrate the Larsen-Truss walls with the timber frame, a 2x deck was built atop the foundation, extending a foot outside the foundation walls. The timbers were inserted between the joists and rest atop the foundation walls, leaving a one-foot perimeter support on which to build the walls.

The example is one option for integration of a Larsen-Truss wall system with a timber frame, but I'd like to find more. I'd be curious if any of you on this forum have feedback (on this house or the Larsen-Truss walls in general), or any other examples to introduce into the discussion.

Best,
~ Kevin

I think Will Beemer used a Larsen system on his home (or was it a Heartwood project?)


Does the Larsen truss offer any shear transfer from the frame? Often in the work I do we rely on the SIPs or the wall system to take wind load / shear.

A friend is entertaining a double offset 2x framing. 2x10 plate and sill and 2x4 studs will be offset except @ windows / doors, which will be framed with 2x10. It will be filled with wet cellulose and possibly have Hi-R 1 1/2" foam on the exterior, over sheathing. Rain screen / strapping over the foam for siding. This doesn't solve the shear issue unless we get creative - but for his needs it will work.

As far as I can tell, the Larsen-Truss is simply a form of double-wall framing that uses plywood gussets to space the walls - 2x4 for the load bearing walls and 2x3 for the outer frame. The wall system used in the Maine house example (initial post in the thread) does not carry any loads and is all 2x3 - merely vertical members to hold the insulation, sheathing, windows, doors, and siding. Would the plywood sheathing (and, to some extent, the drywall) offer any shear resistance?

The issue with the plywood, assuming it is on the outermost layer of framing, is that it is now 12" away from the load bearing structure (the frame), so you have a lever arm without a solid connection between the two frames.

It may not matter in some cases - but whenever I've been involved with alternative systems like this (just a few times) the engineers always want to either see the frame designed for lateral / shear loading - or the wall system set up to take those loads.

Yes, the drywall is technically doing something, but I've never seen it included in any sort of calculation.

I doubt that shear loading will be an issue for the house I'm planning - a small, 20 x 20 x 2-story home. I'm inclined to think that a wall system wrapped in sheathing will be adequate resistance to shear.

What is the most common way to support SIP's on a timber frame? I've seen examples where the posts are supported inside the foundation walls (going all the way to footings in the basement in some cases) with the SIP's resting on sills atop the foundation walls. At first glance that seems a bit unnecessary. Just curious at this point as to what the "best practices" are.

I've often had the deck run over the foundation, then set the SIPs on the deck. With a 6.5" SIP wall and a 10" foundation you have half your post over masonry, and you solid block in the deck as needed. If you have a need you can pilaster up in the basement or build a bump out in the foundation. Depends on loading, etc.... as my usual response.

I'm curious as to what the common/best practice is for thick walls. A Larsen-truss type wall is 12" thick, for a total thickness of 20" once an 8" post is included. I've seen one where a 2' wide perimeter of plywood was laid atop 2x sills. The deck was built to the width of the foundation walls with 1' of the plywood extending out from beneath the deck to support the walls. Granted, the trusses are not carrying any load, but it just seemed a bit flimsy.

What is the method used for a straw bale / timber frame house?

I am going to use a somewhat extreme example to illustrate something here.

I have seen plenty of examples of very old timber frames where the walls have absolutely no direct support whatsoever from the foundation. These are cases where the first floor of the building is built of stonework -so it serves as the foundation- and the second floor is timber framed, but jettied on all four sides well past the stone wall.

Now here is my point. I believe it is possible to have an outboard enclosure system that has no direct support from the foundation at all. The frame would sit on the foundation and the the shell would be 'jettied' out beyond. This could save tremendously on the cost of building an oversized foundation, and also there are 2 huge advantages that this makes possible.
The exterior shell can go down along the foundation past the sills, this can in turn provide excellent insulation in a very hard to insulate spot. It also totally isolates the sill from water ingress. The only 'sill' involved in such an outboard system would be a piece of wood enclosing the bottom. This would obviously have to be used with a wall system especially adapted to this.

I have seen SIPs hanging off the frame and running past the sill. This does insulate the floor framing.

This does not work if you need the shear planes of the panels or walls to be able to push against the foundation for wind loading...

That certainly makes the most sense, as with SIP's. I'm trying to sort out the best way to do it with a wall system that's composed of 12" thick trusses sandwiched between T&G interior wall and exterior sheathing/cedar shake siding.

Wouldn't the frame itself provide the necessary wind shear plane?

Just blasphemous!

Don Wagstaff

If you designed for it.

Hello,

I am just blown away by the implications of that statement.

Don Wagstaff

Don, do I detect a bit of sarcasm there?

This same thought kept coming to me, why can't the frame just be made to handle all that? But I felt as if I must surely be missing something since no one else had mentioned what seemed to me to be the simplest solution. Maybe I am still missing something?

But brave Mr. Pfield-Steader seems to have created a user name solely to share this groundbreaking bit of wisdom with us (I am not at all sarcastic there, just to be clear). I guess he is braver than me.

Or have we fallen into the trap of letting SIPs or whatever enclosure method we may use serve a structural function, and not designing our frames to be able to handle themselves any more? Just a hop, skip, and a jump away from balloon framing I'm afraid! (OK, so that's maybe a bit of a stretch!)

But in all seriousness, why can't the frame be made to handle the wind shear associated with this system? There is no reason immediately obvious to me

DL,

Timber frames are not inherently stiff structures. They flex. Yes, even frames with a lot of knee braces flex when loaded by wind. They are strong but not stiff.

Modern building regulations require stiff buildings that do not flex. It is incredibly difficult (you can substitute "expensive" for difficult) to build an all wood-joined timber frame that meets code requirements for lateral stability. So to build code compliant timber frames buildings, most often we rely on the envelope to provide lateral stability.

This is typically not a problem when the frame is enclosed in any of the typical manners -- sips or stick framed walls. Those systems can be made to meet the requirements quite easily. It becomes an issue with alternate enclosure systems and especially with open air structures.

But there are forms of bracing other than knee bracing that do provide lateral stiffness. This would be lateral bracing.

Lateral bracing is a technique employed in much of Europe, it employs long slanted braces interfacing with horizontal and vertical timbers. For this to work, you must have either a studded timber frame, such as in the English or French styles, or a relatively close posted timber frame with horizontal ties, such as in the German style. I don't know much about the English (I assume it would achieve the same properties) but the Germans say that by having long braces pass over the horizontal members, they achieve a tremendous amount of lateral stiffness. Having the verticality of a cavity interrupted by one or more horizontal ties does a tremendous job of stiffening the structure against wind loading (that in combination with the Windrispen, very long roof braces that cross many rafters and tie them together) The Germans do all this, because they teach that you should rely on the enclosure -be it it boarding or infill- for absolutely nothing other than keeping out the cold. They have also striven to make frames which are very stiff and rigid.

I imagine that having long braces that pass from one bent to the next, with a few studs in between, would do the same thing.

Whenever you have vertical members that can brace against other vertical members, it improves the stiffness of the whole assembly. Vertical members braced against horizontal members is a flexible assembly. The same is true of Horizontal members braced against other horizontal members.

Alternately, you could build the frame as normal, and as part of a modified Larsen-Truss or similar enclosure method include very long 2x4 braces nailed diagonally across the frame, crossing over many of the studs thus tying them together.

The American timber frame is a flexible design, likely designed to be flexible. Keep in mind though that other cultures designed their frames to be as stiff as they could get them.


By enclosure the frame in typical manners you are essential doing the same thing I am suggesting here, tying many vertical members to each other to achieve stiffness. Sheathing can do a decent job of achieving lateral stiffness, I won't say much for relying on it for keeping things square...

As I said, if you design for it, you can do it. It may mean extra long tenons and braces, some tension steel, rigid connections to the foundation, or all of the above. It also depends on location, use, and live, dead, and wind loading.

I've worked on projects where the frame did all the work. On the coast in a hurricane zone it added plenty of steel and rigid connections to the foundation. In the mountains it required stiff posts with strong braces and equally strong tenons or tension joinery. In California it required lots and lots of embedded steel.

And don't forget about seismic. That adds a whole nother level of requirements.

And remember that every action has a reaction - and this is where simply adding braces or increasing their size doesn't work in all situations. That long brace can become an equally strong crowbar trying to pull apart your joinery.

The crowbar effect, that is why you will notice the Germans don't have their braces pushing against the posts, or if they are against the posts they are directly opposed with another brace on that same post. These posts typically extend all the way from sill to plate, or they may start at the sill just inside the corner post and brace against the next post, with another brace directly opposite in the next cavity there to ensure the post doesn't go anywhere.

Use wall ties too, 1 or 2 horizontal timbers that span the cavities between posts. This will have the effect of causing the posts to press against each other. If one post wants to flex, it would have to flex all of the others with it. These don't have to be very big at all, they aren't bearing any roof loads or other structural loads. They just need to be stiff enough to be able to push. They would work with the same dimensions as brace stock.

Hello,

Spent the day yesterday in Arnhem at, what I have to call here, the round wood auction and came home with some I am very pleased with.

My second entry up there DL was not a sarcastic one, it was genuine and comes from my assumption that a timber framed house is built with consideration to structural integrity in and of itself. But that is maybe, or clearly a limited point of view. But it's a question of labels it seems. You have timber frame and you have timber frame like, structures. I wonder how often that distinction is made clear. By the way, maybe it was a blunt reaction. No offense intended.

Greetings,

Don Wagstaff

Don,

Historically most builders did not have to deal with modern codes, and when the frame is buried in the walls you gain the shear by all that other stuff going on around the frame. I've moved 2 German style barns in Eastern PA that were very stout, and later a huge barn in MD that was surprisingly underbuilt. In both cases code really wasn't around when they went up, and even when they were moved the one that became a house (despite how stout it was) needed the shell for the engineers to sign off on it. The large MD barn went back as a barn... with little review required.

Just the nature of the craft. Some places require some pretty strict standards, others do not. It can be tricky depending on locale, code, size, and loading to get what many view as a traditional wood to wood joined frame to pass mystery without taking into account the balance of the structure.

Hi,

Do you think it is too late to change things there where you are? There where the timber frame revival got started while timber frames over here were being covered up with plastic and asbestos?

Greetings,

Don

I'm not sure what to change.

Building codes are here to stay - for better and worse, in many instances for the better, in some not so much. Wishing away the fact that wood to wood joinery needs special consideration to handle lateral and tension loading and that many contemporary frames are just not designed to handle this loading doesn't change things.

Designing a euro style frame as in DL's other thread may solve some issues - but it would increase costs significantly - and I can't see many typical US customers going for having studs, horizontal timbers, and large diagonal bracing running along their walls. (and I haven't studied these options enough to know if they work under load - while the crow bar effect may be minimized from plate or beam to post - there is still a reaction to be had when something pushes on the building from the side - and those forces need to be resolved somewhere in the structure)


I'm all for sane and common sense practices. Before we moved out of our town house we needed to install hardwired smoke and CO detectors as we were attached to 5 other units. This is tricky to do well after the fact (the units were built in the late 80s). Of course - the updated code said that we needed to have a detector (interlinked to all the others) on each floor. Fair enough, one in the basement / garage level, one on the main living space, and one on the bedroom level. The fine print also called out for 1 in each bedroom. So atop our stairs we had 1 in the landing (to cover the upper 'floor') - and 6' away in each direction were 2 more - 1 in each bedroom. The whole place was 1100 sq. ft. - with approximately 450 sq. ft. on each main living level. So on the upper level we had 3 smoke and CO detectors covering 450 sq. ft. of living area, all within a 6' diameter of one another.

Silly, for sure. But someone in Burlington a few years ago hacked up an old Victorian, rented it out to college kids, and jammed way too many 'apartments' into the thing. When the furnace failed and someone died - city council and the fire chief pushed through some new codes. Since then all 'multi' family units were required to undergo the changes at time of a later sale. Even though our 'unit' was single family - but attached to 5 others (with firewalls and no common hallways or entrances...)

I set off the hard wired alarm system in the house I was remodeling in the other day, with a chain saw. I had on ear muffs, I could still hear the beeping, finished the cut and vented the area, it was in the garage, btw. The beeping started within seconds of the saw starting, all through the house.

I like Mike's point on the difference on framing approaches from the American side and the German side. America was at that time booming, they didn't take the time to go into the extra detail as laid out by DL. It worked, they weren't concerned with dry wall cracks, but we are today.

If the alarm systems will save lives, what is the extra effort cost compared to that? Kind of like the road pickle they use today, the roads are bare tar within the day or next, it is a terror for the car itself, but saves lives I am sure. It all comes with a cost. The gray area which Mike points out are the tougher side to accept.

Hi,

Was that the co2 alarm - never heard of that one - the fire alarm, or the burglar alarm?

Yeah, drywall and timber frame - bad combination. But instead of rethinking that one you change the frame to suit the drywall. Or not?

And I don't buy the safety thing. Not that I'm against safety, but that it has anything do with the character of timber framing, quite the opposite.

Greetings,

Don Wagstaff

Agreed on the issue that if the alarm saves a life, great. But someone should have the presence of mind to say that 3 alarms within 6ft diameter might be a bit.... excessive.

Don, not sure I follow you on the latter part of your comment.

We have a habit of taking an extreme case and acting like it is the norm, and passing laws accordingly. Sometimes things fail, things go wrong, etc. and unfortunately people die without it being an inherent flaw in the whole system. Yet we feel the need to over regulate as a result.

All that does is make us dumber in the end. Sure there is something to be said for being safe, but mandating safety doesn't work. You just make a better idiot.
----------------------

Lets talk a little more about lateral bracing. I don't think that this would have to make a building more expensive. I don't really even think it is a guarantee that German framing would be inherently more expensive than American, some forms of it I think have the potential to be more affordable. But this is another discussion altogether.

The long braces aren't lateral bracing, but they do contribute. The primary lateral bracing is the horizontal members between the posts, that tie the posts together into a rigid system. This adaptation could very easily be added to a timber frame without making it much more expensive.

Let's look at Pol barns, a related construction method. Pol builders today are aware of the need for lateral bracing to make their frames rigid. This is accomplished by nailing horizontal 2x4's to the outside of the frame every 2 feet, which conveniently also serve the function of supporting the metal siding. Most builders probably think that siding is the only reason for these, but it's not.

Don has a good point as well, we need to make our frames and our walls compatible with each other. We shouldn't use these that don't work together by virtue of their conflicting natures.
Also we need to be (and I think we usually are) well aware of modern concerns that did not apply to builders of the past.

The solution to this is, either only enclose the frame in ways that are compatible with the way we make our frames, or else adjust the way we make our frames to meet modern concerns. We may have to make the frames in different ways for different situations.

The problem with the typical American frame as I see it is this: Load transfers and bracing occur over only a very small area of any timber, the rest is free to flex and bend all it wants. The flexibility problem does not lie in wooden joinery, but rather in the fact that the posts have no reason not to flex. The major flexing does not occur right at the joint, but gradually along the length of the timbers. By enclosing the frame in a rigid system we make it impossible for the timbers to flex. alternately, if we braced the posts against each other they would be unable to flex, allowing us to enclose the frame in a less rigid shell.

Now here is a thought, what if you flipped your larsen truss-like system to run horizontally, suddenly it would provide bracing to the frame.

Hello,

Just this, to put it in power point terms: Timber frame + regulation = safety

Let me know if that is not clear please.

Wow, the graphics really come across fudged on this forum set-up: That's not helpful.

(Can I just say, as a general observation, that the question of pronoun usage - personal and otherwise - really has a defining influence in this form of media. And I think it sucks, unless, that is, we can all agree that we are pretending to be wizards or something. Ok that was it). (I'm going to bump this down to the end so it isn't out there on the active topics page).

Greetings,

Don Wagstaff

I'm not sure it would work, but I'm open to seeing it proven out...

i thought about that as soon as i posted it. unmodified it wouldn't work, but a hybrid system would work

i think we might be best not to think of thins in terms of systems. 'This is the system and so this is how we are going to do it" I think we get in a rut, and don't advance when we do that. I think we need to be flexible, being willing to adapt and change, and modify our methods. The master is the one who realizes he doesn't and never will know everything there is to know, and is always open to learning and advancing

And Don, what is it you mean about the pronouns? I'm afraid I am a bit confused there

Smoke alarm. It was not my saw, could be a heavy mix to the gas. I just looked through the opaque tank to check the fuel level, checked to see how dull the chain was, gave it a couple pulls and plunged it into the timber.

Hello,

Oh, I came up against the realities of the limitations of UBB threadsâ„¢ and then just ran out of enough time to go any further.

I assumed that these codes are mostly safety standards. In fact we used to call them safety codes.

Does it not seem odd: Larsen-Truss ON a timber frame? Why not, Hand formed brick cross pattern masonry facade on a Larsen-Truss on a timber frame? Just where does the excess end?

Oh yes, Bahler, the pronoun thing. It's just me trying to figure out how to make directed comments, as opposed to general ones, given that I don't like funny made up names and constraints imposed by a binary system, as the case may be. Sorry, not related to anything in particular. You can see that it is also a technical problem given that I exist within this forum as Cecile en Don Wagstaff.

Greetings,

Don Wagstaff

Sorry - hadn't checked the forum for a while.

So, my point was just that I thought the point of a timber frame is to handle all the structural loads associated with then house while the envelope (in this case a larsen truss system) simply supported the insulation and windows.

And thanks for calling me brave...although I'm not sure why my question would make you think that?

Thanks!

So what I'm hearing in this thread is that a larsen truss would not provide enough lateral stability?

As I've been contemplating building my own small timber frame home, I would like to use local lumber, which would preclude the use of plywood.
Would a larsen truss, using 1x lumber on a diagonal for sheathing, be OK?

Thanks!

Diagonal 1x sheathing has a fairly good shear value, and can often be a substitute for plywood. It's really up to your engineer to determine what you can get away with, and how (or whether) the Larsen truss is attached to the frame.

There are plenty of examples of this techique being used on TFs.
If a TF can not support itself and stand the test of time, it's a mere accent.
The two stucture redundancy was discussed a little while ago here:
http://www.tfguild.org/forums/ubbthreads.php?ubb=showflat&Number=18184#Post18184