Civil Engineering Reference
In-Depth Information
The one avoidable vulnerability of straw bale walls
Once the walls are in place and compressed they form an extremely strong and stable
structure. The only concern that needs constant reference is its vulnerability to water. It is
for this reason that special consideration be paid to the provision of roofing materials
ideally with wide eaves, if not verandahs. I am also concerned about the treatment of
penetrations through the bale walls of plumbing lines, as condensation on the pipes will
most likely cause an increase in moisture content in the bales in the vicinity of the pipes.
This issue is covered in Chapter 13, plumbing installations.
Straw bale construction: the basics
Straw bale houses are constructed in one of two methods or a combination of both: load
bearing or infill construction.
Load bearing straw bale structures
With load bearing construction the weight of the roof and the ceiling is transferred
through the bales of straw down to the foundations. The load bearing method is, in my
opinion, the cheapest and easiest structure to build, and is an ideal method of building a
simple single storey home.
Infill straw bale structures
The infill construction system has posts and beams that extend up to the top of the walls.
The ceiling and roof are mounted on the upper beams or ring beam. Consequently the
weight of the ceiling and roof is transferred through the posts and beams down to the
foundations. Traditionally, infill buildings have had a timber or steel structure that is
completely self-supporting. More often than not, there are 16 mm steel bracing rods
fitted to the structure to provide the lateral stability to the building.
One of my owner-builders in Ringwood, Victoria, was required by his engineer to not
only provide 16 mm steel bracing rods, but also large steel gussets at the junction of all
his posts and beams. These gussets were triangular sections of steel approximately 6 mm
thick and about 900 mm long down the diagonal.
Structural infill straw bale buildings
Our home in Heathcote is an infill two-storey building but does not have bracing of any
sort to support the timber structure. The lateral stability of the building is provided by
the straw bales only. We refer to this type of construction as 'structural infill' as the bales
form an integral part of the structural integrity of the building as opposed to simply
filling in the spaces between posts to stop the wind blowing through the house.
Two-storey straw bale buildings
Traditionally, two-storey buildings would be constructed by the infill method, with the
upper flooring supported on timber beams that are in turn supported on posts.
There are reports of two-storey load bearing structures, however this method is in its
infancy and, as yet, I have not had the opportunity to view the details of such structures.
