Note: Descriptions are shown in the official language in which they were submitted.
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~ACKGROUND OF THE INVENTION
Field of the Invention
This invention relates to a fastener system of the
type especially adapted for use in the attachment of sheet
insulation material to a corrugated sheet metal base, such as
a steel roof deck. More specifically, the invention is an
improvement in the stress distribution or hold-down plates and
fastener devices utilized in such systems.
Description of the Prior Art
A variety of fasteners have been used to mechanically
secure various materials to sheet metal bases. Generally, these
devices include a rather long nail- or screw-threaded shaft
which pierces the insulation and sheet metal and grips into the
support underlying the sheet metal. Stress distribution or
hold-down plates are either placed between the head of the screw
and the insulation material or incorporated onto the head of
the shaft itself in order to provide a bearing surface area and
prevent the shaft head from tearing through the relatively soft
insulation material when it is under stress.
The primary load which the insulation fastener must
withstand, in the case of a component of a roof, is an uplifting
force induced by winds which tend to unseat the insulation
material upwardly away from the metal base. Resistance to the
w;nd uplift force is, in fact, the primary criterion used by
Factory Mutual System, an industry testing and underwriting
organization, in granting approval to fastening systems for
securing sheet insulation material to a roof.
The present invention presents markedly superior~wind
uplift resistance than the stress distribution devices currently
being utilized. Each of the prior art devices possesses substan-
tial shortcomings in this respect. Those that are round tend
to buckle prematurely under load. Those made of stiffer material
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or those with sharp corne~s tear through the insulation material
before specified ~aximum loads are achieved. For these and
other reasons, the prior art devices have not proved completely
satisfactory in providing the required resistance to the insula-
tion material against the uplift of the wind.
According to the present invention there is provided
a fastener system particularly adapted for attaching sheet
insulation material to a sheet metal base comprising: a hold-
down plate in the form of a regular polygon having at least five
sides; a hole through the center of the polygonal plate; at
least one circular ridge on the plate concentric with the hole;
a plurality of ridges substantial parallel to and adjacent the
perimeter of the plate; radial ridges extending between the
outermost circular ridge and the perimeter of the plate; and
a screw fastener having a shank portion adapted to be received
by the hole and having a head portion adapted to seat on the
surface of the plate surrounding the hole.
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DESCRIPTION OF THE DRAWINGS
FI~. 1 is a view of a face of a stress distribution
plate made according to the inventioni
FIG. 2 is a sectional view of the plate taken through
line 2-2 of FIG. l;
FIG. 3 is a side view of a preferred fastening device
for use with the distribution plate of this invention; and
FIG. 4 is a view of the plate of FIGS. 1 and 2 when
positioned to secure insulation material to a sheet metal base.
Referring to FIG. 4, the environment of the invention
is shown in which a layer of sheet insulation material 2 is
secured to a corrugated sheet metal base 4 suitable for use as
a roof for a building. The sheet insulation 2 is secured to the
metal base 4 by means of hold-down plates 6 and fasteners 8.
The fasteners 8 pass through a central opening in the hold-down
plates 6 and are driven through the metal base 4. In a typical
insulation, after the insulation 2 has been secured in place,
the roof is completed as by covering it with tar or other suit-
able roofing material.
FIG. 1 shows in more detail the preferred configura-
tion of a hold-down plate 6 made in accordance with this inven-
tion. The plate 6 is provided with a hole 10 located at its
center adapted to receive a suitable fastening device 8. The
diameter of the hole 10 is large enough to pass the shank 12 of
the fastener 8, but small enough to retain the head portion 14
of the fastener 8. The plate is also provided with several con-
centric circular impressions 16 located centrally in its face.
The plate 6 is also provided with a peripheral impression 18
located adjacent to and reinforcing the perimeter of the plate
6. Finally, the plate 6 is provided with several radial impres-
sions 20 extending between the outermost circular impression and
the perimeter of the plate. In the preferred embodiment, these
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radial impressions are equally spaced about the plate and connect
the outermost circular impression with each corner of the hexa-
gonal perimeteral impression.
FIG. 2 illustrates the preferred depth and spacing of
the impressions 16 and 18 in relation to the external dimensions
of the plate. The hole 10 must be surrounded by an adequate flat
area 22 in order to properly seat the head 1~ of the fastening
device 12. Outboard of this flat area 22, the circular and hexa-
gonal impressions occupy a significant portion of the plate's 6
remaining surface area. The depth of the impressions depends
upon the size of the plate and the thickness of the plate material.
Generally, the impressions have a depth of roughly ten times the
thickness of the plate material.
The drill screw fastening device 8 is shown in greater
detail in FIG. 3. Basically, the drill screw has a shank portion
12, a formed head portion 14, a pointed end 24, one or more drill
flights 26 and threads 28. The drill screw 8 is designed so that
it may be secured to a workpiece without predrilling a hole since
the drill flights 26 will accomplish this function ahead of the
time that the threads 28 engage the workpiece. In order to
obtain maximum holding power, the drill screw 8 is designed so
that the maximum diameter of the drill flights 26 is no greater
than the minor diameter of the threads 28. It can be understood
that by so designing the drill screw, the threads 28 will be able
to obtain a maximum bite on the workpiece.
Although superficially it might appear that insulation
fasteners are static devices, the dynamic characteristics of
stress distribution plates are essential to their satisfactory
performance. The nature of the typical service conditions in
which these plates operate will clarify this consideration.
First, it must be remembered that the materials used
as roofing insulation are generally quite soft and have a low
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modulus. Even with the protective coating typically affixed to
the upper surface o~ the insulation, the composite material can
withstand relatively little stress.
Second, the amplitude of the w;nd's upward force fluc-
tuates greatly from time to time. Thus, the ability of a fastener
system to secure insulation material under wind uplift loading
is limited not only by its own static and dynamic strength, but
also by the insulation material's dynamic strength and resistance
to tearing. It ls in this aspect of the dynamic performance
lO characteristics of the entire fastener system that the features
of this invent;on interact so as to effect marked improvement in
the prior art.
The important design criteria fundamental to the attain-
ment of these improvements will be illustrated more clearly by
reference to the preferred embodiment. The optimum stress dis-
tribution plate must demonstrate a high resistance to buckling
yet not have a tendency to tear the insulation material. Circular
plates might acceptably prevent tearing of the insulation, but
they will lack sufficient resistance to buckling under load.
Four-sided plates seem to possess greater stability under load,
but tend to tear the insulation easily. The six-sided shape of
the present invention improves both aspects. This shape retains
a superior resistance to buckling, but the shallow corner angles
reduce tearing of the insulation.
In order to improve rigidity, it is not sufficient
merely to stiffen the plate and reduce its corner angles since
the dynamic nature of the wind uplift force further complicates
the design. As gusts of wind cause varying uplift forces, the
stress distribution plate itself, the insulation material, and
the threaded shaft's grip on the metal base are dynamically
impact loaded, which must be accomodated. The present invention
overcomes this problem by providing a plate with stiffness, but
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controlled flexibility. The perimeteral and radial impressions
give the plate adequate strength, stability and resistance to
buckling. The circular impressions, however,enable the device
to flex under dynamic loads and modulate the forces transmitted
to the insulation, the drill screw fastener, and to the stress
distribution plate itself. Thus, in service, the plate absorbs
a portion of the loading energy, prevents its unmodulated trans-
mittal to the insulation material or sheet metal base, and reduces
the likelihood that the insulation will tear or break out, that
the shaft will pull out of the sheet metal base, or that the
plate itself will buckle. In the disclosed stress distribution
plate, the interaction of shape, thickness, flexing impression
and stiffening impression results in marked improvement in the
gastener's resistance to wind uplift forces and permanence of
installation.
