by Patricia McDaniel Hamilton
Bracing. Cross-bracing at the pilings
is necessary to resist lateral loads
from wind or water (Figure 3). Once
again, we let the engineer do the
head scratching. If pilings top out any
more than 2 or 3 feet above grade, we
usually need 3x8 or 3x10 treated timbers
bolted to the pilings in an X pat-tern,
with two 3 /4-inch bolts at each
Cross-bracing is easiest to install
with one brace on each side of the pilings.
This makes a very thick
brace/piling sandwich. Unfortunately,
if the area below the first floor is to be
enclosed or partitioned, the cross-bracing
may conflict with wall locations.
Sometimes the two pieces can
be installed on the same side of the
pile, and half-notched into each
other. This detail needs to be carefully
considered at the design stage.
Even with bracing in place, a piling
foundation is not an absolutely rigid
structure. When concrete slabs are
poured around the foundation, expansion
joints are needed at each piling.
Figure 3. Bolted two-by X-bracing (top) and interior shear walls (above) help stiffen houses
built on pilings. Both details must be engineered.
Layout and installation of floor
joists is the next step. A "highlift"
(four-wheel drive, extendible forklift)
can be a real timesaver in this and subsequent
steps, especially if the house is
more than one story. We cut joists to length on the ground,
then lift them in groups onto the beams.
Because the girders are rarely directly below the exterior wall lines, and may be
slightly out of parallel, the trick is to securely fasten the joists to the band
joist but only tack them to the girders. Then, when all the joists are in place,
we can square the whole floor and then
fasten the joists to the girders. Each
joist/girder connection needs a hurricane
tie (actual loads engineered) like the
Simpson H6 (Simpson Strong-Tie, 4637
Chabot Dr., Suite 200, Pleasanton, CA
94588; 800/999-5099). It is easiest to
install hurricane ties before the subfloor is
installed, though they can go in later. We use only corrosion-resistant
fasteners and hot-dipped
galvanized nails. If the metal straps were
galvanized before fabrication, all the
edges will begin to corrode immediately.
We field paint all such straps with a cold
galvanizing compound, available in
spray cans. Other options are stainless
steel or Simpson's new Z-Max fasteners with triple zinc coating.
Framing continues in a conventional
manner upwards from the first-floor
deck, but special attention must be
paid to point loads and to bracing the
structure against wind loads. We frame
standard stud walls, except that plywood
is installed after the walls are
raised. We always follow the engineer's
plan for stud size and spacing.
Gable ends are best framed with
continuous studs. This avoids a weak
hinge point at the ceiling line. For
cathedral ceilings, this detail is
required. For extremely high ceilings,
it is sometimes necessary to increase
the stud size to 2x8 or larger. Double
top plates are standard, and we take
special care to stagger the laps and to
secure the plates to the studs.