Here’s an interesting question I received today about straw bale load bearing wall heights. I hope to hear more about this idea of multi-story load bearing structures in the future. Perhaps you have some input to share…
Andrew,
Although you said that you could not have a 2 storey LB building because of the wall thickness/height ratio – could it not be done with a box beam between the floors with a second beam beneath the roof. Surely this would then start the ratio again? From other straw bale builders I gather that the bales would support the weight and therefore with an interim box beam this has to give it sufficient strength?
Here’s what I said in response (with a little expansion):
I would see how this could work. It seems to make sense; however, the biggest issue I hear from engineers is that the overall weight of the walls is still transferred to the foundation in a direct load path and thus the wall height ratio cannot be started over. The middle box beam could ultimately become a hinge point for the wall if under large “out of plane” loads.
In general it’s dangerous to have a lot of weight up high in a wall system or building because the tendency is for that wall to rack and/or tip over if under loads such as wind or earthquakes. That’s why in post and beam structures, and all conventional construction structures for that matter, there is a specific lateral bracing requirement that must be met as well as an “out of plane” resistance requirement. With so much weight in the walls there could be a risk of such a collapse.
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Hi Andrew,
In your load bearing DVD and generally on your website, you describe the use of welded wire mesh on both sides of load bearing walls to brace and strengthen them. What strikes me though is that the mesh is placed horizontal/vertical, which leaves the bracing resistance to the welds on the mesh only! Image the mesh welds would have zero resistance: the wires of the mesh would oppose no resistance at all for the walls “dancing” out of line and shifting like a stack of cards. You could as well remove the mesh. Using mesh aligned with the walls is, in my view, a dramatic waste of material.
If the mesh were placed at an angle (10° would do the job), the bracing would now be done by the welds AND the wires themselves, with a dramatic increase in resistance. The traction resistance of steel is very high and is used in all steel reinforced structures. I am an engineer and this does make immediate sense to me, but maybe I should elaborate to make it clearer.
Practically, the mesh would be placed alternatively at two angles (for instance leaning left on one half of the wall, and leaning right on the other half of the wall, with some overlap in the middle where they meet and on the corners).
Regards and thanks so much for your sharing and work.
Hi Fred. This does make very good sense to me in terms of increasing the shear value of the wall; however, the system as described is also an engineered system and has been tested with solid results. You are no doubt correct that the angled mesh would have a higher value and there may be places where that would be affective. The issue is how to attach the mesh at an angle as it won’t always be able to stretch from bottom plate to top plate if run at an angle due to the size of the sheets and the structure itself. In addition, running the mesh at an angle would take considerably more labor to complete and I imagine would leave a lot of waste pieces that would need to be cut from the sheets/rolls to accommodate the angled installation. I see this as a balance of adequate shear strength to adequate labor and material use.
One last detail, because the mesh is embedded in the plaster, the overall strength of the system is a result of the entire system, not just the mesh. Therefore, the mesh installed as I have described along with the strength of the plaster and the bales themselves (the mesh is sewn through the bales and so a sandwich system is thus created) is much higher overall than if the mesh were tested on its own.
Thanks for your input and ideas.
Hi Andrew,
Leaning the mesh would certainly mean some complications, mainly when bending or cutting the mesh at an angle on the top & bottom of the walls. However, I would consider it seriously because of the massive increase in sheer resistance. In case of an earthquake, it would probably mean the difference between standing up and laying flat.
Overlapping would also not be necessary I think, as the increase in resistance would largely make up for the few narrow triangles that would remain uncovered.
It’s worth considering for sure. I don’t think that the “laying flat or standing up” comment is correct though. This system was engineered in California to stand up during their earthquakes and has been accepted as a viable building technique in high earthquake areas. That doesn’t mean that your idea would not increase the strength as I believe it would; however, I want to be clear that the system, as is, is very much a viable and safe system for building shear. You can read the engineering report at http://www.EcoBuildNetwork.org under the straw bale testing area. It is called In-Plane Cyclic Tests of Plastered Straw Bale Wall Assemblies. The study and system was done by David Mar of Tipping Mar and Associates.
why not use some twisted pair high tensile wire tied in at a diagonal? I would us clean used barbed wire as a cost effective solution.
That is almost impossible to get tight enough to work in this situation. Wire, if not welded, tends to slip. It also creates issues when shaping as you don’t always have adequate anchor points and meshing such a wire into the bales would likely prove top be a weak connection.
Andrew-
At the SB workshop we attended, you emphasized the importance of bracing the corners of the building properly. Would it be worth doing the “angled mesh” idea just there, since the corners are where racking forces are always focused?
This discussion, plus recent events in Haiti and Chile, bring up the general question of the performance of properly constructed SB building in a 6.x or greater earthquake. Have you ever seen (or have pictures of) SB structures after such a quake? I would love to see a post about that. We actually do have quakes in KY (I was awakened by a tremor which rattled things in 2008) and some say we are due for much more… see this link:
http://kyem.ky.gov/programs/earthquake/
Thanks!
It could help increase the shear value for sure. Is it necessary? That’s a question for the engineer who designed the home. Hope you are well Ted.
Hello!
It is funny to hear the different arguments about mesh etc.. in America. In Europe(most countries)it is not seen as necessary to use mesh, in fact it is seen as a bad thing as this may cause condensation due to the cold effect of metal. There are in fact some two story straw bale buildings successfully built in the Uk and Ireland, check http://www.amazonails.org.uk for instance or you could read some Building with Straw Bales by Barbara Jones. She has some very interesting points. This method has been done by using a second base plate on top of the first floor wallplate and building up again from there. This seems to be sufficient enough. It would be interesting to hear why you feel the need to use mesh?
Thanks for the impact. I like mesh for lots of reasons, I use it to attach cabinetry. I use it to shape corners and niche. I use it to attach the bales to the frame and thus eliminate the need for internal or external pinning. I use it for shear strength. I use it for plaster reinforcement. I use it to tighten up the walls after baling is complete. It has so many uses to me that I can’t see why people wouldn’t use it to be honest. I hear the concern about condensation; however, I don’t know that it’s really a viable argument here in the states. Perhaps in the UK where the salty sea air and the overall air moisture is higher, but I don’t see that as a concern here. There are instances of people building multiple story load bearing buildings and other structures without mesh and people say “look, it works.” I want to see those buildings after the ground shakes or after a huge wind storm or snow storm. Anything will work while not under stress, but to me, it’s important to know that it will withstand the pressures of Mother Nature. If it does…GREAT, I’m on board. I simply have seen too many people experiment with building techniques in ways that seem way unsafe to me. I worry about that happening again and the benefits of SB being lost to the news story of “see, building with bales is dangerous.” My two cents and an explanation of why I use mesh.
hi Andrew,
I am certainly NOT an engineer, so I must admit that I make my suggestion like a deer in headlights. 🙂 Can the first floor be post and beam (or whatever) and the second story walls be load bearing?
LOOK OUT! That was the car screaming by you as you stand frozen in the headlights! 🙂
I think that question is best answered by an engineer. I can argue reasons for the answer yes and the answer no. What that means to me is that it’s probably possible and that an engineer would need to design it. Sorry for the “no answer” answer. Perhaps an engineer is listening out there and can chime in.
In Emerald NE, a two-story LB structure was built in the 1960’s (might have been earlier). It still stands, though it is in bad shape these days due to a lack of care. I believe it has been vacant for a number of years. The second story housed a dance floor.
I’ll see if I can’t get out that way with the camera and grab some photos…
Excellent. I’d love to see some pictures of it.
I haven’t written for a while (June 2008), but have on several occasions asked about multistory load-bearing walls. My idea was (as someone stated above) to put a sill around the entire first floor wall. This sill would tie the entire rectangle together, and protect against the sheer forces from pulling the walls apart. The only way the house could fall, would be if all walls moved in the same direction, and they would be braced in part by the thickness of the walls, and (for my design) a wrap- around 2-story (porch & balcony) framework.
I like Rod Zorger answer on the load bearing and Andrew answers on the mesh:
1-For number of stories
Without tests results on two stories strawble walls, the first floor should be post and beam and the second story walls could be load bearing.
Appendix R, StrawBale Construction, of the 2008 Oregon Residential Code allows only single story load bearing strawble construction.
2-For the mesh, It is absolutely important to use mesh for the strawble wall to resist lateral loads such as wind and specially earthquake. Even so there are some building standing without mesh, there is no grantee that they will stand tomorrow. Remember that most of buildings that fallen down looked just fine the minute before they fall.
The bottom line is, mesh will give you a peace of mind so you can sleep like a baby at night. Of course this is assuming that every thing else is fine.
I don’t know if this would be part of this conversation or not. I have been (mentally) planning the building of a straw home for a long while and my thinking has gotten me to believe that (maybe) steel strapping tying thr 1st and then again the 2nd course of bales to the foundation and each succeeding course to the preceding one with re-bar the length of the thickness of two bales (2 pcs re-bar/bale) driven through the bales all the way to the top course would be enough. How far from the truth might I be? Truthful question as I don’t know anything about the subject except my supositions. Jut now found this site.
I think that would work, but also believe it is overkill. I use rebar “staples” to tie the corners together and then use a simple strapping to connect the box beam to the foundation. The strapping, made by Cordstrap, also tensions the box beam and thus compresses the walls. Once the roof frame is on and the bales have been fully compressed over a series of days or even weeks, I add 2″x2″ WELDED wire mesh on both sides of the structure. This mesh is nailed to the toe ups and to the box beam. Because it is welded, it cannot flex or stretch, giving positive connection once again to the foundation. Finally, the mesh is sewn from one side of the bales through to the other, making the entire system (mesh/bales/mesh) one unit. A sandwich of strength. This is the easiest way to secure load bearing walls in my opinion and the best for adding shear strength, plaster strength, support for cabinets, shaping of corners, and more: all with the inclusion of the welded wire mesh.
Question:
On the topic of two-story load bearing SB structures, might not an intervening 2nd floor system, brace the walls if it was tied through the wall to the outside face?
I envision this as possible solution in a situation where the floor joists would be perpendicular to the longest two walls in an highly rectangular building.
This would leave the two end walls without quite so strong a brace (as far as a bowing outward goes), but as most of the roof and 2nd floor loads would bear on the two longer walls, the end walls might be sufficiently braced by the close proximity of the corners of the building? All this with normal mesh construction and strapping.
In the end, I’m not sure what practical difference this design would be from that of a post and beam structure forming the base, and a load bearing SB 2nd floor sitting on top of the base level.
Am I missing something obvious, or does this just need to be tested to see if this would work in practice?
Thanks
Hopefully an engineer can answer this question as it’s a bit beyond my “jurisdiction” if you will. I see what you’re saying and it makes sense; however, I still think there’s an issue with the added weight of the second story on the lower story that needs to be addressed.
I am curious about Load Bearing Building codes in Souther Alberta, more specific the Cardston County. Without having to go to an architect or an engineer for a 25×25 Load bearing stamp of approval. Do you have any specific references I can use to get a building permit. As well I curious what the largest structure I can build without a permit. Any information is welcome, you are all AWESOME!!!
Hi Cody. Thanks for the compliment. It’s hard to talk details of any specific area because there are so many requirements that are applicable to each area. You can look at the approved building codes in Oregon, California, and talk to people ein Canada who have had success to see what they did. I kno what there are many bale structures up North and that the Ontario Straw Bale Builders Association is a very helpful organization. They may be better situated to help you with Canada specific details. Good luck!
How tall can a load bearing straw house be?… And how long can it be?… Was going to be building a 40x 15 with about 6 and 8 foot height for a shed style roof?
Hi Newlin. Much depends on the jurisdiction you’re building in, whether the building department uses Appendix S of the 2018 IRC, and whether engineering is required. The basic answer is that the height of the wall is based on a ratio to its width. In general, a 3-string bale will allow for up to a 10′ tall wall. Unsupported lengths of walls are also limited within the code. I believe that limit is 40′, but it’s been a while since I have reviewed that code section so I suggest you check it out. Of course, if your interior partition walls intersect those exterior bale walls, then you are fine to go longer as the bale walls would then be considered “supported” by the partition walls. Finally, engineering can allow you to build taller and longer runs of walls if the engineer understands how bales work, if you need more lenient limitations (sounds like you won’t though).