Building a Staw Bale House
(Page 6 of 10)
December/January 1995
By Athena and Bill Steen
An integral part of a load-bearing building is the roof plate tie-down system. Threaded steel rod, polyester cord strapping, and wire cable are the most commonly used materials for making connections between the roof plate and the foundation. The threaded steel rod and the strapping can be tightened down to help precompress the walls without waiting for them to settle on their own. The amount of compression will vary according to the quality of the bales and the load of the roof. The average range seems to be anywhere from 1.5 to 3.5 inches. If the walls are not completely settled or precompressed before they are finished or stuccoed, there could be cracking and other problems.
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Another point to remember is that load-bearing walls are also comprised of door and window bucks, which are made of noncompressible materials. In other words, sections of bale walls that contain rigid frames will compress less than other sections that contain only bales. Allowance needs to be made for these differences or differential settling can occur. For this reason, we feel that load-bearing designs are best suited to simple square or rectangular shaped single-story structures with a limited amount of window and door openings.
As part of an effort to bring load-bearing construction to a level of code compliance in southern Arizona, panels of unstuccoed three-string bales were tested for compression, transverse lateral loading, and in-plane lateral loading. This testing demonstrated that these unfinished panels could successfully withstand lateral and vertical loads. These tests also helped make possible the development of prescriptive standards for load-bearing straw bale construction for the city of Tucson and Pima County, Arizona, which will be included in their building codes as of January 1996.
Some examples of the requirements for these prescriptive standards are as follows: Load-bearing structures must be limited to one story, with the height of the wall not to exceed 5.6 times the wall thickness (23 inch wide wall = 10 feet 8 inches). Unsupported wall lengths can be 15.7 times the wall thickness (23 inch wide wall = 30 feet) with allowable vertical loads on top the wall not exceed ing 400 pounds per square foot.
Since bales of straw are somewhat elastic in their compressive character, their performance in regions with frequent and heavy live loads of snow is not yet well understood or documented.
Bale In-Fill Systems
In-fill buildings combine a structural framework with bale wall panels. The structure, which can be wood, steel, masonry/concrete or ha bamboo, ca r ries all the vertical loads. Typically the structure is put up first, the bales notched and fit around the posts or columns. In a few buildings, the bales were first stacked like in a load-bearing structure, notched once in place and a post-and-beam structure later inserted. This can speed up the construction process for small, simple structures, but can only be done if the posts are small enough (4x4s) so that the strings of the bales are not affected in the process.
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