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through 25 when a backing strip or stud is not provided to back up the weld.
The tubular framework is laid out on 4 or 5 ft centers depending on the width
of the steel sheets used, with intervening frame members as required,
typically 2 to 4 ft on center.  For TEMPEST shielded enclosures requiring
electrical isolation from the parent structure grounding, a dielectric
underlayment is used to isolate the sheet steel flooring from the underlying
concrete slab.  This is seen in Figures 7, 12, 20, 21, 22.  The quality of
electrical isolation materials used ranges from sheets of Masonite hardboard,
to 56 mil PVC roof membrane, 1/8-in. (3.2 mm) polypropelene sheet, or  60 mil
chlorinated polyethylene (CPE).  The hardboard is the cheapest, and the
poorest, since it cannot be sealed from sheet to sheet to form a moisture
impervious membrane as the other materials mentioned can.  Further, when it
becomes wet with contaminated water it can lose its electrical isolation.  The
use of hardboard should be limited to its application as a filler material
between bolted or welded seam plates and should not be relied upon for the
necessary electrical isolation.  The dielectric underlayment material should
be overlapped and glued at seams to form a moisture barrier, and should extend
up at flooring edges as seen in Figure 12, or around floor penetrations such
as the column seen in Figure 7, or the anchor seen in Figure 26.
2.11.2.1  Large Welded Structures.  In extremely large welded structures,
thermal expansion can result in significant displacement in the metal
structure, especially during construction, before the shielding is protected
from the external thermal environment.  Expansion joints may be required in
very large welded shielding structures for both vertical and horizontal
expansion in the shielding walls.  The attachment of metal sheets to fixed
metal columns and beams should also allow for some flexibility, as well as
transition thickness of metal between the typically thin shielding sheets and
the heavy steel beam and column members, as seen in Figure 7.  A refractory
material must be provided to protect the dielectric underlayment during the
welding of the seams.  Examples of this are seen in Figures 20 through 22.
2.11.2.2  Warping Due to Welding.  Warping or buckling of floor sheets during
the welding process is a serious problem in most welded room installations.
When concrete anchors or fired pins cannot be used to hold the sheets against
the underlaying concrete slab, typical of electrically isolated TEMPEST
enclosures, then a tubular framework as seen in Figures 20 and 22 can be
utilized under the floor sheets.  Some welded floors have a concrete slab
poured over them, and this can be made thick enough to accept some floor
warpage.  A heavier steel sheet such as 3/8-in. (9.5 mm) plate can be used for
the floor, with a reduced buckling problem compared to floors of thinner
material.  Heating of the floor sheets and quenching with water spray is
another means used to reduce warpage after it has occurred.  Some earthquake
resistant designs require anchoring of the shielded floor to the parent
structure.  With TEMPEST isolation requirements, normal concrete anchors or
pins may not be acceptable, and an isolated anchoring scheme is shown in
Figure 26.
2.12
EMI Conductive Gaskets, Cements and Tapes.  The use of EMI gasket
type materials in shielded enclosure construction is normally limited to
penetrations, access hatch, and filter case closures.  There is some use of
conductive tapes under the seam junctures of bolted panel type construction or
else bronze or stainless steel wool, covering the gap at the ends of adjacent
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