Straw Bale Load Bearing Wall Heights
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.
I've heard of three story load bearing structures in Canada that were engineered and built relatively recently; however, I don't think they have been up long enough and seen enough of nature's forces to give us adequate information as to the long term viability of such a building. I'm eager to watch the time pass on those buildings and see how they fare. I think it really could be a major improvement to the load bearing construction world if these structures prove reliable and safe over the long haul. I'm hopeful they will.
-Andrew Morrison