Precompression of Loadbearing Walls

Load-bearing straw bale walls are unique in the realm of construction materials for their need to be precompressed. The precompression serves three important functions: getting rid of the "squish"in the walls,leveling the top of the wall, and providing a structural tie from foundation to roof plate.

Getting Rid of "Squish" Straw bales are not rigid blocks, and though they can hold up a roof, they will compress under that load over time. The tighter the bale, the less "squishiness" in the wall, but even very tight bales will settle. Once the plaster is applied (and has cured), this settling stops because the plaster skins are not compressible. But we don't want to apply plaster to a wall that will be getting shorter as the plaster is trying to cure!

The Nebraskan pioneers addressed this issue by building the roof structure on top of unplastered walls and allowing time for the walls to settle under the weight of the roof. Once the settling stopped (this could take four to six weeks), they plastered the compressed bales, creating the structural skin of the stressed skin panel that results in such remarkably strong and rigid buildings.

In most modern construction scenarios, it is not practical to leave a building to settle over such a long period of time. Modern straw builders use a variety of mechanical systems to pre-com-press their load-bearing walls and then prepare the walls for immediate plastering, since roof loads will no longer cause the bales to settle.

There are many techniques for pre-com-pressing, most of which are low-tech and easy to employ. They all share the common concept of tying some form of continuous structural roof plate to the foundation at regular intervals around the building. These ties are then subjected to a mechanical force that draws the roof plate down toward the foundation. This mechanical force is a substitute for the weight of the roof — and for additional weights and forces to which the roof is subject. As a testament to the com-pressive strength of bales, even large mechanical forces usually produce only one to four inches of immediate compression in a wall with seven courses of bales. Properly tensioned, the walls are remarkably strong and resistant to flex in all directions, even before plastering.

Leveling the Wall

Because bales are not perfectly square blocks,and because there will be variations in density between bales, the top course of a bale wall will not necessarily be level. Mechanical precompression systems all use ties that are spaced at regular intervals around the building, and differing amounts of force can be applied to each point to achieve a level top plate.

Attaching Roof to Foundation

Neither plaster skins nor stacked bales provide a whole lot of resistance to lifting forces, to which your roof may be subject.Whatever form of mechanical tie is used to precompress the wall is left in place to provide this resistance to uplift, and this function should be taken into account when materials and methods for precompression are being chosen.

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