Spacers shall be made
from high density polyethylene (not less than .96 specific gravity). Spacers
shall be double wall construction and shall be of Formex Manufacturing, Inc.
make, or approved equal.
REASON: High density
polyethylene (.96 specific gravity) has been chosen by Formex for the manufacture
of all interlocking module spacers because of its flexibility, strength, and
resiliency. These characteristics afford at least two important advantages,
i.e. (a) under compressive stress (ex: workmen walking on duct bank or when
concrete is poured) there is sufficient give in the material to prevent the
spacer from causing a kink in the duct wall, and (b) the spacer modules can
be freely handled on the job site without breakage. Since many utilities install
underground systems year round, and most large projects carry through more
than one year, it is important to use a formulation of plastic for spacers
that will not be adversely affected by the extremes of summer or winter temperatures.
Finally, double wall construction makes it possible to design a spacer which
will hold safely the vertical loads normally encountered when men work atop
large duct banks, especially when concrete is poured.
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Spacers shall consist
of interlocking modules, i.e. bases, intermediates, and caps. Base pads shall
be used to assure specified dimensions between trench floor and bottom of
first tier of ducts. The interlocking modules shall include an internal vertical
channel on both side edges of the spacers. When the duct bank is assembled,
a No. 3 reinforcing rod shall be passed through the internal vertical channel
on one side of the spacer bank and driven into the trench floor. At the next
spacer location, the No. 3 rod shall be inserted on the opposite side, etc.
The reinforcing rods shall be bent inwardly at the top of the spacer bank
sufficiently to squeeze spacer cap so duct assembly will not float or move
in any direction during the concrete pour. When soil conditions negate the
use of reinforcing rods for this purpose, then an alternate method shall be
to place a trench jack directly above the spacer location and use adjusting
wedges between the spacer cap and the trench jack to hold the duct assembly
in position during the concrete pour.
REASON: The features
specified in this sub-paragraph have been designed into the Formex spacers
mainly to do the best job at the lowest possible cost. A duct bank erected
to specification and firmly anchored against floating will (a) save concrete,
and (b) assure a straighter duct line thus making cable pulling easier.
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The interlocking module
spacers shall provide independent support for each duct.
REASON: If each duct
is not independently supported, it will rest upon the ducts directly underneath.
In larger duct banks this often causes the ducts in the bottom tier to become
oval and many times this condition prevents the use of the bottom tier of
ducts.
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When spacer modules are
locked together the openings shall provide approximately 1/16" clearance
over the O.D. of the duct. The interior surface of the duct spacer opening
shall be convex to prevent kinking of the duct.
REASON: The suggested
clearance is desirable because duct is not always perfectly round. The additional
clearance in the spacer opening permits the spacers to snap into locking position
with a minimum effort. The surface in the spacer opening upon which the duct
rests should be slightly convex because the spacers are bound to tilt to some
degree under pressure of the concrete pour. The convex profile eliminates
sharp edges that could kink the ducts.
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The openings of the spacer
modules shall be positioned to assure the horizontal and vertical separations
for each duct as specified on the project drawings.
REASON: For power distribution,
the specified separations for ducts is important to adequate heat dissipation.
For all duct lines specified separations are important for a variety of reasons,
i.e., cables and/or conductors, when pulled through duct, may cut into duct
walls at bends, sweeps, and changes in elevation. Concrete encasement is recommended
even if the duct is direct buried; with certain soil conditions it is necessary
to use reinforcing rods and sufficient concrete.
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Spacers shall be installed
at ____ foot intervals.
