Civil Engineering Reference
In-Depth Information
Pre-compressive
loadbearing technique
heavy-duty tarpaulins that waterproof the
floor and cover in the sides of the building to
give the straw protection from driving rain.
The scaffolding should be sheeted on the
outside for the same reason.The straw is
now installed and the columns and beams
are removed by raising the floor with acro
props and taking the timber columns out,
leaving the floor resting on the walls, which
are then compressed by the weight of the
floor itself; and by ratchet straps that go
through the floor.The weight of the floor
itself and the walls and roof above are
usually enough to stabilise the building
without the need for permanent tie-downs
(these would be needed at this level only if
this was a single-storey building with a
lightweight roof, or there were severe
winds). Once the ground-floor walls are
complete, the roofplate and roof are lifted up
above the first floor, on to more columns and
beams, and temporarily held in place while
the first-floor walls are constructed. At this
stage, the roof provides waterproof covering
over the building and tarpaulins or sheeted
scaffolding protect the sides from driving
rain. Again, the walls are compressed using
ratchet straps over the roofplate.
Buildings need to be carefully designed so
that the principles outlined below work in
practice. These techniques will generally
be used for a commercial build, but for
smaller or self-build types of building
you may find them unnecessary. It's still
perfectly feasible to allow the floor and
roof weight to compress the walls without
using pre-compression. All loadbearing
designs are compressive, it's just a matter of
whether you let the building structure itself
compress the bales, or whether you help
it along so you can build faster. Whichever
method you choose, it is best that as much
as possible of the timber work of the building
is prefabricated, because this speeds up the
actual time of the build and therefore its
vulnerability to the weather. The baseplate,
first-floor ring beam and joists, roofplate
and roof can all be done in advance.
Using a crane and temporary props
Prefabricated parts can either be made off-site
and brought to the site later, or constructed
on-site next to the building. Either method
has the tremendous advantage of enabling
the roof to be built at ground level, thus
saving on labour, time and scaffolding costs.
The roof is not completed at this stage, so as
not to be too heavy, only finished up to the
vapour-permeable (weatherproof) layer. A
crane is used to lift the first floor, and tempo-
rary timber columns and beams are placed
beneath it for support, leaving enough room
to install the straw underneath.The floor will
need a temporary stem wall to be built
centrally along its length to create a pitch for
Using scaffolding
An alternative to using temporary props
is to build the scaffolding up to just above
ground-floor height and use scaffold trusses
across from one side to the other. The
floor can then be lowered on to these and
waterproofed while the straw is installed.
When all the straw is in place, the floor can
be lifted up, the trusses removed, the floor
lowered on to the straw, and the trusses
moved up the scaffolding to their next posi-
tion, just above the height of the first floor
