11 Assembling the straw bale building
Loadbearing straw bale walls As straw bales will be compressed by an imposed load, the following precautions have to be taken for loadbearing straw bale walls: – The roof load has to be distributed evenly to all walls. Loads must not be concentratedon any spot. – Roof loads have to be transmitted centrically and have to be distributed over at least 50 % of the wall thickness. – The ratio of wall height to wall thickness should not go beyond 6:1. If, however, the wall is reinforced by horizontal braces against buckling, the ratio can be exceeded (11.1). – Straw bales need to be have undamaged stalks, have to be highly compressed and possess a density of at least 110 kg/m³. – Window openings should be rather narrow – but in any event higher than wide (11.2). – As far as possible, lintels above windows and doors should be avoided. Instead, the ring beam should be appropriately dimensioned to accommodate this task. – In case lintels are intended, sufficient tolerance to the ring beam has to be allowed for, as the straw bales tend to creep during the first weeks or months after completion. – Dimensions between wall openings as well as openings to corners have to equal at least one bale length (11.2).
hh
b w 11.1
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Assembling the straw bale building
– For particularly long and slender walls and in the event of very high roof loads, provisions for additional bracing have to be made to avoid buckling. – Roof loads on the bales should not exceed 20 kN/m². – Loadbearing walls must be prestressed. – If walls are not prestressed, a load of 40 kN/m² will cause a settlement of up to 10 % (upright small bales) or up to 14 % (flat small bales). With jumbo bales that have a higher degree of compression, the slump will be less significant. – Prestressed walls can experience slump as well if the prestressing was insufficient or is reduced over time. For buildings with loadbearing walls it is better to use large bales as they provide a larger support surface for load distribution and have a higher degree of compression and therefore experience a lower deformation under load. Since thicker wall constructions also have thicker (and therefore more expensive) foundations, it makes sense to use methods of foundation that do not require to reach the frost-free depth of 80–90 cm. This solution (11.3) uses a foam glass gravel layer that acts against the capillary forces and Is located under a thin strip foundation and reaches at least 50 cm outside of the foundation and is at least 30 cm thick. Figures 11.4–11.6 show options for interesting roof structures ensuring a largely even distribution of forces onto the walls.