Note: Descriptions are shown in the official language in which they were submitted.
BACKGROUND OF THE INVENTION
(i) Field of the Invention:
This invention relates to open joists, trusses or beams
(hereinafter called "joist", exclusively) made of wood and
comprising top and bottom elongate chords ( or booms ) with
intervening zig-zag struts terminally joined to the chords
tend to each other by means of joints formed of finger
scarfing, the fingers or teeth being of carefully selected
dimensions.
(ii) Description of the prior art:
The basic aim in constructing wooden trusses, truss beams
or joists is to minimize the weight of the beams while
simultaneously maximizing their load capacity. To achieve
such optimization, it is necessary to satisfy
simultaneously certain requirements which are not
necessarily compatible with one another. For example, in
order to transfer high tension forces at a juncture between
the chord and the struts ("also called braces") it is
necessary to increase the surface areas between the tenons
at the ends of the struts and the mortise recesses in the
chords as much as possible. Thus, the surface areas
adhesively bonded between the tenons and the mortise walls
must be maximized. On the other hand, in order to transmit
large lateral forces directed perpendicularly to the
longitudinal extension of the wooden joist, it is necessary
to make the cross-sectional area of each tenon as large as
possible. This is so because the cross forces are taken up
by this cross-sectional dimension of the tenons. Yet,
making the size of the tenons as large as possible is
incompatible with the requirement that the chords should
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not be weakened by large mortise holes. A large tenon
while strengthening the transmission of lateral forces,
weakens the chord because it requires a large mortise hole
which correspondingly diminishes the force transmitting
cross-sectional area of the chord.
One attempt to meet these requirements is found in Canadian
patent No. 819,361 which relates to a joist having an
improved means of joining braces to the chords, which
involves attaching one set of parallel braces directly to
the chords and the other set of parallel braces to the
first set of braces. In this patent, finger tenons or
scarfs are used to make the actual join between timbers.
As noted, tenons in general are very prone to local
stresses and the corresponding mortises also weaken the
timbers containing them.
Canadian patent No. 1,099,473 discloses a mixed finger
scarfing and gusset joint which attempts to overcome this
problem.
Finger scarfing involving dentate tenons and mortises are
also shown in Canadian patent No. 730,348 and can reduce
the above-mentioned weaknesses and local stresses. This
patent discloses a pitch for the sides of each triangular
tooth of 1:6 with an apical angle of 18°. In particular,
this patent discloses a preferred tooth profile shown in
its figure XII of similar dimensions to the triangular
tooth but with a flattened apex and corresponding butt
between teeth.
It has now surprisingly been found that a significant
improvement in the load capacity of an open joist whose
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struts are finger scarfed both to the chords and to
adjacent struts can be achieved by selecting very
particular dimensions for the teeth of the kind mentioned
above.
OBJECTS OF THE INVENTION
It is an object of the present invention to determine the
advantageous dimensions for the teeth forming the finger
scarfing between chords and struts.
It is a further object to provide an open joist of
increased strength, by using such joints.
SUMMARY OF THE INVENTION
In meeting these and other objects, the present invention
provides a wooden joist having upper and lower essentially
parallel longitudinal chords and a plurality of
interconnecting struts in zig-zag arrangement each strut
being terminally joined to a respective chord and to an
adjacent strut near this chord by finger scarfing
comprising complementary and cooperating pairs of at least
two teeth and grooves in inward facing surfaces of the
chords and on ends of each strut and on opposite parallel '
side portions of adjacent struts adjacent said ends, such
teeth and grooves each having a 'truncated isoceles
triangular section defining an apical butt, wherein:
(a) this triangular section has an apical angle
in the range from 8° to 10°;
(b) this triangular section has a perpendicular
height in the range from 7/16 to 9/16 inch; and
(c) the apical butt in the line of the section
is in the range from 0.025 to 0.050 inch.
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Preferably, the parallel side portions are of length in the
range from 5/8 to 7/8 inch at the apices of the teeth.
Preferably also, an outside tooth on each end of each strut
has a flat side defining for this tooth a right-triangular
cross-section, such flat side being flush with the opposite
side of the strut.
Advantageously, the joist has end blocks placed between and
flush with the ends of the chords, these blocks being of
longitudinal thickness greater than 1 1/4 inch. This
thickness is ideally at about 7 1/4 inch to allow the joist
to be safely shortened on the installation premises.
Preferably, the zig-zag arrangement declines 45° between
struts.
BRIEF DESCRIPTION OF THE DRA~rTINGS
In drawings which illustrate embodiments of the invention,
Figure 1 shows a terminal portion of an assembled joist
according to the invention;
Figure 2 shows detail of the embodiment shown in Figure 1;
Figure 3 shows detail of one strut/chord joint in the joist
of Figure 1;
Figure 4 is a perspective view showing two adjacent struts:
Figure 5 is a cross-sectional view of the upper end of a
strut positioned for insertion into an upper chord also in '
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cross section;
Figure 6 shows the assembled joint in the same embodiment
of Figure 5 ;
Figure 7 shows a view similar to that of Figure 5 but of
another embodiment; and
Figure 8 shows precise parameters of the scarfing tooth.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to Figure 1, a wooden joist shown generally by 10
has upper and lower parallel chords 15,20 extending in a
longitudinal direction. The chords 15,20 may, of course,
deviate from their parallel situation when under load.
There are a plurality of interconnecting struts 25 in
preferably 45o zig-zag arrangement, each strut 25 being
terminally joined both to the nearest chord 10,15 and to
its neighbouring strut 25. The interstrut joint partially ,
overlaps the strut/chord joint.
The ends of the chords of the joist 10 are joined by
relatively thicker blocks of wood 30. These blocks 30 are
generally flush with the chord ends. The thickness of the
blocks may be chosen to provide sufficient strength to the
joist 10 while allowing for some wood to be removed by the
workmen in charge of installing the joists, if they need
shorter joists to fit in a reduced space. Thus, it is
preferred that these blocks have a longitudinal thickness
greater than 1 1/4 inches and preferably ranging between
this value and 7 1/4 inches.
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The joint betwen neighbouring struts 251, 25r is shown in
the remaining Figures.
Each joint is formed by finger scarfing made in two
perpendicular edges 5,6 at each end of each strut 25
(Figure 4). Such a scarfing defines fingers hereinafter
called teeth 40 on both edges 5,6, which project in a
vertical plane. The set of teeth in the upper edge 5
projects upwardly while the set of teeth in the lateral
edge 6 projecting towards the set of teeth of a
neighbouring strut. The teeth of the two adjacent edges 6
are complementary and may interlock.
Glue is spread on the surfaces of the teeth before all
joints are made permanent. Preferably R14 Phenol-
resorcinol resin Type 1 glue is used although other
suitable glues may be employed.
There is also finger scarfing in the underside of the upper
chord 15 and on the upper side of the lower chord 20. This
again is complementary to the scarfing on the upper or
lower edges 5 of the struts.
The scarfing in the chords 15,20 may be longitudinally
continuous or may extend for discrete intervals for
individual, joints.
Once a set of two struts 25 are joined as shown in Figure
4, they may together be inserted into the chord~l5 as shown
in Figure 3 to complete the joint. .
In accordance with the invention, the teeth 40 and the
grooves 52 therebetween which together make up the scarfing
are of a particular size as noted above.
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Referring to Figure 8 showing in cross section an enlarged
tooth 40 (or which may as well be a groove to receive a
tooth but for simplicity reference is made hereinafter to a
tooth only) it will be seen that in section the tooth is in
the form of a truncated isoceles triangle. The truncation
of the apex leaves an apical butt 60 in the size range from
0.025 to 0.050 inch. The optimum size is 0.039 inch.
The apical angle ~ is in the range from 9 to 10°
preferably 9°07'50°'. In terms of inclination from the
vertical the angle a' is therefore in the 4 to 5° range
with a preferred value at 4°33'55".
The perpendicular height, h, of the tooth ranges from 7/16
to 9/16 inch with a preferred value at 1/2 inch.
In another preferred embodiment (referring to Figure 7) the
end tooth 41 on one side (always the same side) of the
strut 25 is halved as is the receiving groove 53. Thus the
outside tooth 41 has a flat side meaning that it defines in
cross section a truncated right angled triangle rather than
an isoceles one. At the other end of the strut, this
flattened tooth is on the opposite side of the timber.
This arrangement permits all struts 25 to be
interchangeable since either end may be up. Naturally this
reduces production costs.
