Straw Bale Construction in Earthquakes
I want to start by sending Gabriella's and my love, prayers, thoughts, and hope to all those effected by the recent earthquakes in Japan and New Zealand. Clearly the tsunami in Japan was behind the majority of the death and damage, but the impact that earthquakes are having around the world on human populations cannot be missed. As is often the case with deaths associated with earthquakes, the housing in which people live can either be the shelter they so desperately need or a deathtrap.
Over the years, many people have looked long and hard at how straw bale structures perform in earthquakes. From the early pioneers such as Bruce King (who identified the seismic resistance qualities of straw bale construction) to the recent work done by Darcey Donovan (who spearheaded the shake table study at the University of Nevada, Reno shown above) these engineers and many others like them have long touted straw bale construction as a smart choice in earthquake prone areas. Let's take a look at why.
Shear Design. When building post and beam structures, the most commonly utilized straw bale design, the frame itself is braced to resist lateral shear. Lateral shear is the force that is applied to a building in the same plane as the wall it is being applied to. In other words, if you stand at the corner of a building and push, you are applying lateral shear force to the wall you are pushing. The frame itself will be engineered to resits this shear force as the primary defense.
Redundant Design. The bales, attached to and notched around the framing, act as secondary shear resistance. This "backup system" adds significant strength to the overall wall assembly.
Natural Structural Insulated Panels (SIPs). The bales and the plaster are joined as one during the construction process. The combination of the two materials makes a natural SIPs panel which is amazingly strong. It is the combination of the rigid plaster skins with the soft bale interior that makes this structural assembly extremely strong.
Wide Footprint. The bales are wide, much wider than a conventionally framed wall, and as such, they have a wider footprint onto which they can spread any load applied during an earthquake.
Soft and Supple. The bales respond to seismic forces differently than conventional construction materials. In most of the conventional construction systems used here in the United States, the materials are designed to resist external forces (wind, earthquakes, etc...). Bales, on the other hand, are very good at simply absorbing those forces. Their ability to move, even in the most subtle ways, allows them to deflect much of the energy that would otherwise cause failure in rigid building materials.
Stronger Sills. I always use 4x4 sill plates/toe ups. These thicker, stronger boards are able to resist more force than a single or even doubled up 2x board. This is often the difference between a structure holding firm and breaking away from its foundation. (Another potential cause of such a failure is too small of a washer on the anchor blot itself, but that's another story).
It's important to remember in construction that a building acts as a unit. In other words, you cannot think only about the shear strength of the walls or the ability of the roof to resist winds. When wind pushes up on the roof, the roof wants to lift off of the walls and the walls want to either go with the roof or collapse into the home. The connections at every point through the house have to be considered and built to withstand the forces that can and will act upon them.
I've heard from many people who live in straw bale homes that their properties have done well in earthquakes. There are a lot of engineering numbers out there to support this idea too. Like I said earlier, a lot has been done to understand just how safe a straw bale building can be. I, for one, would like to hear more as we see the level and intensity of earthquakes increase. If you know of any other studies of value, please let me know. I plan to undertake a comparison study in the near future testing the strengths and differences of post and beam bale walls, load bearing bale walls, and conventionally framed walls.
If you are interested in helping out with this, please let me know. I already have an engineer on board, Nabil Taha of Precision Structural Engineering, Inc. He is very excited and passionate about this project and straw bale construction in general. We are looking for help finding funding, most likely by means of writing a grant request. In addition, we would love to have some people who are excited about this idea on board to help with all the many details that will arise from the project. Exactly what that means, I don't yet know! Please leave a comment here if you are interested in helping in any way or if you know of other folks currently working on a similar project/study.
Thanks,
Andrew Morrison