A NATURALLY COOLED FLORIDA HOUSE
(Page 3 of 4)
ENGINEERING CUTS COSTS
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Normally, an earth-sheltered structure will start its
"life" as an assembly of filled concrete block walls
resting upon a reinforced monolithic slab . . .
or—alternatively—8- to 12-inch poured
bulkheads, on footings, surrounding a "floating"
foundation.
However, because the soil found at the building site has
low permeability and is potentially unstable, the designers
decided to utilize post-tensioned components,
which would stand the effect of lateral loads or twisting
resulting from shifting earth much better than would
conventional methods of construction.
Post-tensioning, a comparatively new technique in the
home-building industry, is in many ways ideal for use in
earth-sheltered construction. Most important, it imparts
additional compressive and tensile strength to any member
upon which it's used.
Unlike regular reinforced concrete construction (in which
the steel rebar can begin to resist a load only after the
concrete itself has failed ), a post-tensioned
component incorporates a network of interwoven
cables—encased in lubricated plastic sleeves and cast
in the middle of the slab on three-foot centers—which
are pulled to a tension of about 30,000 pounds after the
pour sets, and locked in place by means of engineered
wedges mounted in stressing anchors.
This fantastic force puts the concrete under constant
compression and increases its strength without the need for
excessive mass (the four-inch floor slab is designed to
support 3,500 pounds per square inch . . . a 40% greater
load than a conventionally built pad of the same thickness
could withstand).
Equally significant, post-tensioning can
be accomplished right at the building site, using little
special equipment.
This do-it-yourself factor, in turn, allows for some
creativity on the part of the builders . . . as
evidenced-in the Tallahassee home—by the exposed
aggregate finish on the solarium's interior walls: They
were poured in forms lined with a one-inch layer of river
rock, then removed with the stone fagade intact.
Finally, the use of the "pulling after pouring" technique
can help cut materials costs. Recent studies of
post-tensioned structures and "typical" earth-sheltered
buildings show that only about half as much steel, and
perhaps 40% as much concrete, will be required when a house
is tensioned rather than conventionally poured . . .
and the resulting shell is less likely to crack.
So, although waterproofing is still necessary with any
post-tensioned wall, the use of exotic (and costly)
materials can be eliminated.
