I am trying to find a way to build a strawbale-enclosed timberframe in Utah, which has a D1 seismic design requirement. It seems that all timberframes in Utah are wrapped in plywood/osb shear walls which I am not particularly interested in. Embedded steel moment frames are also employed frequently, but even if that could be concealed by the strawbales somehow, it relegates the timber frame to ornamental status, and my attraction to timberframing is the elegance of a structure that is useful, beautiful, and efficient.

Perusing some structural engineering forums (I am not a structural engineer, or any kind of engineer for that matter) I found a discussion about timber moment frames (steel connected timber, but timber nonetheless) wherein it was suggested that the posts of the structure could be embedded in concrete and effectively cantilevered against lateral forces, thus creating moment resistance.

Aside from the moisture issues (I'll come back to that), is there any reason why this wouldn't work for a timberframe? Extra long posts could be lowered into prepared holes in the ground, and concrete poured around it. The timbers would have to be large, I'm assuming, but in the end you'd have a true standalone timberframe that also would resist significant seismic loads. And then I could wrap with strawbales without any problems (or OSB).

Of course the main problem with the idea is the wood/concrete connection. What's the point in building a house that will last hundreds of years if the posts rot out in 50? But surely this has been done before? Wouldn't a system that started with lots of drainage, and incorporated multiple waterproof barriers (what about using ICF to form the hole for the beam) prevent the concrete from absorbing moisture and wicking it into the post? The site is at about 6500 feet, so it gets a lot of snow 6 months of the year, which melts and probably makes the ground pretty wet for a bit, but then it's quite dry until the next spring.

Another idea I had was to pour concrete up to 6-12 inches (or whatever) clearance for the post, set the post, then pour some kind of waterproof epoxy around the post that would have the hardness of concrete but not the moisture wicking capabilities. Is there any precedent for this? One of the engineers in the previously mentioned thread said that he had used steel jackets to separate the wood from concrete.

Thanks for your help with this. It's really important to me to do something that is authentic, and not just for show. I'm going to be cutting the frame by hand by myself, so it means more than just a place to live in for a few years.

This is a fascinating idea. I'm really curious to hear what the experts here have to say. I don't have any relevant experience myself, but from an engineering standpoint, this sounds promising. I'm a little skeptical about the ICF idea, as it seems the foam would crush too easily. I like the epoxy idea, and there may be some trick with drainage that can make it work. CB.

I'm not an engineer nor have I worked with seismic concerns but I have a relevent concept that may be worth some consideration.

Since you want to use very thick side walls, much thicker than an ordinary foundation walls, perhaps the posts could extend through the first floor to the basement floor and if the foundation could be thickened with pilasters to meet the tf perimeter, then the posts could be bolted, bracketed or strapped to the foundation to acheive the cantilever. Embedded wood should be avoided and a monitored solution seems prudent. I have no idea if an engineer would approve.

Happy brainstorming.

For us layman builders that are not so adept at looking at computers and finding tension load and such, what is a D1 seismic design requirement? ?

Mo, I'm not an engineer, but I'll tell you what I know. The seismic design categories are a hierarchy of engineering specifications that vary with the building site's probability of experiencing severe seismic ground motion, the site's soil type, and the building's intended use. The determinations are made by the ASCE (American Society of Civil Engineers) and mandated in IBC 2003. The category determines the value of certain variables in the equations that produce load calculations on walls. The higher the category, the higher the load, and the design requirements become more rigorous.

The scale goes from A to F. Although D is right in the middle, it seems that most of California gets a D rating as well, so it's not a trivial classification.

After rereading your question, Mo, it occurred to me that you probably already could guess at the general meaning of the D1 design category, and were asking about specific loads in pounds that the category requires a given wall to resist. I don't know the answer to that question.

Timber frames do not have the ability to resist lateral loads by the requirements of modern building codes. There is a performance factor for each type of frame (ordinary vs special steel moment frames, unreinforced CMU or concrete, and specially detailed versions of CMU and concrete, wood sheathed, etc.). The intent is to capture the ability of the frame to plastically deform and absorb energy without failure (essentially the hysteresis losses).

Timber frames were removed from the tables a few years ago. I am not aware of the specific reason, but I do know that nearly all structural engineers, myself included, will not count the performance of the frame in lateral loading. Wood makes for lousy moment joints over the long term.

The classification is mostly important due to the prescriptive requirements of the code (i.e. - number of stories, special reinforcing of CMU and concrete, etc.) and does not replate to the forces specifically. To find the forces on a building, you have to get the USGS accelerations for the probabalistic earthquake, then multiply by the weight and modification factors in the code (for the simple method). One of those factors is in the table mentioned above - without a factor, no analysis is possible per the code (though a value of 1.0 would likely be appropriate, if wildly conservative).

The cantilevered post method is allowable, but you will find that it is grossly inadequate for all but the lightest structures when your post is wood.

If it is feasible, you might consider tension rod bracing which is hidden in the straw bales. The rods (think of the X brace rods in pre-engineered metal buildings) could be connected to the wood columns, though detailing would be key to their performance.

Thanks Jordan.

So all of this begs the question (at least for me): what is the point of the timberframe in the modern environment? Is it merely ornamental? If the frame is not a factor in resisting lateral loading, then it's not doing the most critical job of the "frame" (holding the static weight of the roof is relatively easy). If you have to bolt steel or nail OSB to it, why not just build it out of steel in the first place?

Don't get me wrong: I love the idea of a timberframe, have been working hard the last year to learn all about to get ready to build a house, and just invested a lot of money in tools for my project. I guess I'm just having a little bit of cognitive dissonance now. I thought I had found something that was beautiful, efficient, and timeless, but now I'm finding out that it can't stand alone, and that it plays about the same role in a house as a stone veneer. Some function, yes, but it's only skin deep. I'm very sad about this.

Joey,

Your original thoughts about adopting a timber frame building system are not misplaced. Do not let the fog of code speak cloud your vision. Engineers are out there to help you achieve your dreams and not to snuff them out.

You do need to recognise the reality that most people do not tend to live in open sided timber framed structures and from a historical perspective there nearly always has been a significant contribution made to the structural performance of every timber framed buildings by the diaphragm stiffening action provided by some form of sheathing, panel infill or even stone walls thereby creating a whole composite structure.

It can be difficult to master the art of employing modelling software to replicate the real life performance of composite structures, especially in respect of load sharing and behaviour assumptions. The verification of modelling assumptions is still a long way from being complete and this is almost certainly the case with straw bale construction.

My suggestion to you is to define exactly what it is that you want to do. Look around for other building precedents and examine them in detail to see how they are performing and only then take counsel from someone who can demonstrate that they really understand the performance of the type of the building system that you are proposing to adopt.

The marriage between timber framed buildings (barns) and straw is one which has a long history, its just that in the past the straw was viewed as a crop rather than a building component. Farmers have long recognised that straw contributes greatly towards the structural peformance of the barn especially when it needs it most (winter). Full barns are much better able to stand up to the rigours of high winds and snow loads than empty ones. Your proposal is therefore akin to building a timber framed building (barn) with no outside walls or sheathing and filled only with a part load of straw positioned around the perimeter. In this scenario the timber frame structure will be expected to contribute a lot more towards achieving suitable structural performance than other similar timber framed buildings located in non seismic regions. Barns usually have some form of sheathing or wall system which in your case you are proposing to substitute with straw bales. For straw bales to work well in diaphragm action it is necesary to contain and restrain them thus some form of additional wall stiffening will most likely need to be provided anyway.

Please don't lose heart. You are now beginning to face up to some of the quite complicated issues that need to be recognised, understood and then overcome in order to achieve a satisfactory outcome.

Regards

Ken Hume P.Eng.

Thanks Ken. My plans are with an engineer now. I'll keep my fingers crossed that he comes up with something that will work and be acceptable.