STRAW BALE CONSTRUCTION PROCESS

STRAW BALE CONSTRUCTION PROCESS

There are basically two ways of straw bale construction, the load bearing in which the straw bale wall is used as structural load bearing wall to support the roof and withstand the lateral load; and the post and beam straw bale construction in which bales are used as in-fill panels between and around structural frame of any structural material. Although straw bale construction is still very much a developing technology and a little detail explanation of the construction process will be discussed.

FOOTINGS
A straws bale wall requires footings with a similar load – carrying capacity to that required for a masonry wall, although a straw wall is lighter (one mud brick weights about the same as a straw bale.). The footings usually used are concrete strips or slabs. There have been successful experiments with rubble trench and rubber tyre footings and there are several straw bale buildings built on piers, beam and joist. (Paul Downton, 2003).

The California straw bale code (2001) has it that support for bale walls shall extend to an elevation of at least six inches above adjacent ground at all points, and at least one – inch above floor surfaces.

King (2001) opines that the bottom of the bale wall must be well separated from the foundation and at the very moisture water- proof barrier should be laid over moisture below. Additionally, many builders are placing a layer of pea gravel between wood sill plates along the inside and the outside faces so that bales will never be sitting in water.
BALES STACK UP

Typically, the bales are stacked in running or stack bond on top one another; early experiments with adding cement mortar between courses created thermal bridges while giving no clear benefits. In load bearing building, the bales are laid flat, that is with the longest dimension parallel to the wall and the shortest vertical. In other application, the bales can be stacked on edge that is with the shortest dimension horizontal. This saves interior space with slimmer wall, and, interestingly appears to offer the same net insulation value due to the slightly different orientation of the fibres (Bruce king, 2001).

In the same vein, the California straw code holds the view that in load bearing walls of bale, bale should be laid flat and be stacked in a running bond, where possible with each bale overlapping the two bales beneath it. Overlaps shall be a minimum of 12 inches.

STRAW BALE WALL HEIGHT

Australia straw bale expert’s recommends a maximum wall height of 2.5 meters when using standard sized bales in load bearing construction. The California straw bale code (2001) has it that building with load bearing bale walls shall not exceed one story height without substantiating calculations and design by a California civil engineer, and the bale portion of the load bearing walls shall not exceeds a height to width ratio of 5.6:1 (for example the maximum height for a wall that is 23 inches thick would be 10 feet 8 inches or 3.27 meters).

Whereas, the harvest homes (2003) holds the view that “ given that straw bale walls have been shown to be at least 4 times stronger than a conventional 2×6 frame wall there are no special limitation to the heights of a building.

PLASTER

The California straw code (2001) has it that minimum bale thickness shall be 13inches. Also plaster can be lime-gypsum, lime – cement plaster.

Bruce king (2001) held the view that it is essential that, once plaster is applied directly to either or both of the straw bale surfaces, the completed wall assembly is now a hybrid of straw and plaster, in other words a sandwich panel. Plaster is used generically to includetraditional earthen plasters, lime and gypsum plasters shot Crete or gucite,

Also, Paul Downton, (2003) said “that three layers of render should be progressively ‘weaker’ to reduce the potential for cracking caused by having too brittle an external layer.

PINNING

Straws bales are comparatively soft and do not behave like bricks. Except when surrounded by a sturdy frame of post and beam the bales must be braced or pinned during stacking for stability and alignment. Internal pinned of the walls (with bamboo or dowels) has been prescribed in early

Straws bale codes, but it has falling out of favour, for it is under whether internal pins contribute appreciably to the strength of the finished wall assembly. Bruce king (2001).

Likewise, Downton(2003) said ‘the vertical and horizontal stability of straws bale walls generally needs to be guaranteed by tying bales to structural frames or pinning between bales and structural elements, however there is a growing consensus that the extensive use of reinforce steel bars and excessive pinning that characterized early straw bale construction is not necessary.