Hi Stef,
The walls of a straw bale building (just like the walls of
any building!) need to stand up to lateral forces like wind (think hurricanes or tornadoes) or earthquakes so they don’t collapse and cause the roof to come down.
There are at least a dozen ways to build lateral force resistance systems (aka shear elements) into a straw bale building; most of them are described in a book published in 2019 by the California Straw Building Association's
Straw Bale Building Details: An Illustrated Guide for Design and Construction. Chapter 3
Structural Considerations discusses the unique properties of the bale wall assembly, how it differs from walls made with more conventional materials, and then jumps into descriptions and detailed illustrations of the lateral systems in common use with straw bale walls.
This is a really good primer for understanding your options. You might decide you prefer one-over-another, and having worked with almost all of them, I can tell you some are much easier to install than others. If you’re an owner-builder with more time than money, you might consider using the straw bale shear wall—a lateral force resistance system unique to straw bale building. Steel strap tension-only bracing is also comparatively low cost and may be suitable for your area. If you’re working with contractors it might be simpler to use a system they’re more familiar with like a plywood or OSB shear wall.
I have worked with several engineers who have
experience with straw bale construction. You’ll need to check to learn if they are licensed in your state—sometimes they may be licensed in your state but aren’t registered there, and ticking that box can be as simple as completing a form and sending a check, so talk with them in person.
Odisea LLC (
https://odiseanet.com). Jeff Rupert and Lucille Hunter have done the structural design work for lots of straw bale buildings. Jeff was for many years the editor of The Last Straw, a journal devoted to alternative materials and methods like straw bale construction.
Verdant Structural Engineers (
https://verdantstructural.com). Anthony Dente, Kelsey Holmes-Foster, Kevin Donahue and their staff have also done many straw bale buildings. Kevin worked on much of the early structural testing that is now part of the building code pertaining to straw bale construction, and Anthony was a co-author of the book chapter mentioned above, as-well-as being heavily involved in testing for both
cob and hemp-crete walls.
Precision Structural Engineers (
https://structure1.com). This firm also has lots of experience with straw bale construction.
Contact them to learn if they can help you (are licensed and/or can be registered in your state), see if what they have to offer is a good fit for you. They’ll also take a look at the footing design and whether it’s appropriate for your climate and soils, and the roof rafter/truss design. Sometimes local practice can point you in the direction of sizing footings or rafters and trusses, but an engineer will likely look at anything to do with the building’s structure—not just the walls.
Typically, an engineering firm will supply plan pages and calculations that supplement the architectural pages you already have. These pages will address the structural issues only, and unless the original designer or architect (assuming someone else sold you the plans) incorporates the structural changes into the architectural pages, you may see inconsistencies between the architectural and structural plans. This can be really confusing, but you need to follow the structural plans whenever you stumble on a discrepancy.
There’s also a model building code available for jurisdictions in the United States to adopt—the IRC’s
Appendix S Strawbale Construction. Just a handful of states and a few other code jurisdictions have adopted the code since it was first approved in 2015—Tennessee may not be one of them. This code offers a prescriptive path—a way to do the structural design of a straw bale wall assembly by following a specific method described in the code. Called a straw bale shear wall, it relies on the plastered straw bale wall to resist lateral forces in your area. It’s complicated, but in a nutshell, you need to determine the governing force (wind or earthquake) likely in your location, which tells you how much of each wall needs to be detailed as a shear wall, which itself is a function of the type of plaster and reinforcing mesh you use. While you’re building you follow the prescriptive details supplied in the code, e.g., cast a shelf in the
concrete footing for the lower edge of the exterior plaster to bear on,
staple the reinforcing mesh with specific length and width staples using a prescribed pattern, make sure all mesh laps are the required width, apply the lime, lime-cement, or clay plaster required for your shear wall type, etc. I have worked on a dozen straw bale buildings that used this kind of shear wall--it’s not the easiest to install, and by far the most labor intensive, but it’s the only one that’s offered as a prescriptive path in the code. All others require a structural engineer. This code is available as a free down-load from CASBA’s website (www.strawbuilding.org) and it’s also printed in the back of CASBA’s book.
Good Luck!
Jim
Many Hands Builders
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