Civil Engineering Reference
In-Depth Information
supported by years of testing. I have concentrated on working with the second alternative
due to personal concerns over the previous method that I shall detail later in this chapter.
For and against relying on the render to carry roof load
When using the carrying capacity of the straw in the bales to carry the roof and ceiling
load it is essential that appropriate compression of the walls be achieved. Once this level
of compression is achieved, the straw bale wall will present mechanically as if it were a
single block. In consultation with our engineers it was established that not only was the
straw of the wall capable of supporting the roof and ceiling load when properly
compressed, but also negated the need for additional metal or timber bracing in infill
construction. Not only does this system hold weight by the recommendation of the
engineer, but it has also been physically proven to work, as my home in Heathcote was
built by this system.
The Heathcote house
The Heathcote house is as an example of not relying on render for carrying strength.
We are in a high wind zone area with the peak of the roof approximately nine metres
high, added to which are three-metre-wide verandahs that also catch the wind. The straw
bale walls are over 4.0 metres to the underside of the rafters and have no additional
bracing. The ring beam that supports the rafters is supported by 100 × 100 cypress pine
posts 2.7 metres apart with nothing but the straw bales to hold them upright. I must
stress that this is totally reliant on adequate compression of the bales to lock them
together.
To achieve this level of compression we utilise flat filament polyester strapping with a
breaking strain of 1000 kg. When using this strapping it is essential that the correct
connector buckles are used, as the system is only as good as its weakest link. When we
first started working with this strapping we were using a lower grade buckle, which was
prone to collapsing under the strain of the strapping. The strapping is tensioned with a
standard strapping tensioning tool available at around $120 (2005).
Compressed straw bale capability
Compressed straw bales are more than capable of carrying the roof load. The structural
capacity of straw bale walls where the top boxing does not extend beyond the bales over
the render is substantiated by the test results of research done at Sydney West University.
The straw bales are more than capable of supporting the load of the roof in load bearing
buildings, so it is unnecessary to extend the top boxing over the bales to transfer weight
through the render. Extending the top boxing over the render does increase the carrying
capacity of the wall, however I believe there are good reasons not to rely on the render for
strength.
Relying on render
I believe there are reasons for concern regarding the reliance on render for strength in a
straw bale wall. Over the years, I have had occasion to see first hand the maintenance, or
lack of maintenance, to Australia's domestic buildings. We live a busy life in a hectic
