Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The present invention relates to a truss made of
timber beams of the type having top chord a bottom chord and
a zig-zag web section comprising a plurality of timber beams,
particularly for use in the building industry.
Load supporting structures comprising a plurality of
trusses made of timber beams are well known in the art. Generally,
they consist of top chord, a bottom chord and a zig-zag struc-
ture of web beams fixedly secured to the chords. The trusses
further comprise generally vertical reinforcement beams extending
between the upper and the lower chord. ~en a load supporting
structure is assembled, the trusses are aligned parallel with
each other and mutually connected by bridging members fixedly se-
cured to appropriate parts of the trusses.
The bridging beams are normally nailed to the respec-
tive portions of the trusses to provide the desired stability of
the overall structure in the direction generally horizontal and
normal to the elongation of the trusses.
One of the requirements of load supporting structures
of the above type is that the structures be protected against
"progressive collapse"~ In tfi~ pro~r~ss~Ye collaps~ of a
structure sucE as a roof ~tructure whl'c~ does not ~ave ade-
quate load s~arlng the load of a failed truss will be quite
suddenly transferred to the adjacent trusses through the roof
decking which is relatively light and will sag suddenly until
it reaches a point where the decking must fail allowing the
load to fall between the adjacent trusses. If the decking does
not actually tear or break the load will be transferred to
the adjacent trusses with an impact. Since an impact on a
structure which is already heavily loaded is very severe it is
quite likely that the adiacent trusses will also fail unless
they have an extremely great safety factor. In other words
~f one of the trusses fails at its load, then the load no longer
carried by the failed truss must be completely transferred to the
remaining trusses, otherwise the trusses adjacent to the failed
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one may further collapse due to an increased load caused by the
failure of the former truss.
Most of the known load supporting structures of the
above type do not achieve such load sharing at all. Some of the
known structures achieve the load sharing by having e.g. roof
decking of sufficient strength to transfer the loads. Another
known means of load sharing is through a combination of decking
and bridging. As the bridging beams are normally nailed to the re-
spective portions of the trusses, they are not capable
of reliable transfer of the load from the failed truss towardsthe rest of the structure, unless a great number of nails is used,
which is impractical. The use of relatively strong roof decking,
on the other hand, results in increased costs.
It is an object of t~e pre~ent ~nYent~on ta pxoYide
- - an improved load supporting structure, of t~e above type ~fil-c~
would have an improved resistance to the progressive collapse.
It is a further object of the present invention to
provide a truss made of timber beams which would be capable of a
complete transfer of load from a failed truss to the remaining
portions of the structure by use of regular timber beam bridg-
ing members nailed to appropriate portions of a truss, without
the need for a great and impractical number of nails at the
joinders between each of the bridging beams and the respective
trusses.
The load supporting structure according to the present
invention includes a plurality of elongated timber trusses
arranged in an aligned relationship. Each truss has an upper
chord vertically spaced from a bottom chord and web section
members spanning the chords generally in a zig-zag fashion.
The structure furt~er includes a plurality of timber bridging
beams of an upwardly elongated cross-section, preferably of a
rectangular cross-s~ct~on~ ~h~r~hy~e~ch ~eam ha~ a.top face, a
bottom face and two side faces. The bridging beams pass through
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the respec*ive trusses and extend generally horizontally at an
angle, preferably a r;ght angle, with respect to the elongation
of said trusses. The bridging beams are secured to the respec-
tive trusses such that both said top faces and said bottom faces
of the beams are in a face-to-face engagement with the as-so-
ciated portions of t~e trusses through whic~ the beams pass.
The invention also provides a truss for use in the above
structure. The truss is made of timber beams fixedly secured to
one another and forming an upper chord portion, a lower chord
portion and a web section, said web section comprising a
plurality of wooden beams extending between the cord portions
in a zig-zag fashion. The web beams forming the zig-zag
structure are fi~edly secured with their ends to the respec-
tive cord portions, w~ereby t~e web sectîon includes a plurality
of pairs of the wooden beams w~ose jo~nders are each adjacent
to and generally coincident with t~e lower chord portion.
Each of the pairs thus defines a V-shaped structure. A
generally vertical reinforcement beam extends from the center of
such V-shaped structure and is fixedly secured with its lower end
to the V-shaped structure and with its top end to the upper chord
portion, The vertical reinforcement beam includes generally
horizontal ledge means disposed to one side of the vertical beam.
The ledge means is disposed at the upper portion of the vertical
beam, in proximity to but vertically spaced from the upper chord
portion. In a preferred embodiment the ledge means is unitary
with the vertical reinforcement beam and is, in fact, a bottom
wall of a rectangular recess provided in one side of the vertical
reinforcement beam.
Thé recess is upwardly elongated to receive a bridging beam.
3~ The spacing between the ledge means and the bottom surface of the
associated upper chord is such that with the bridging beam in place,
the bottom surface of the bridging is generally coincident with
the ledge while the top surface of the beam is generally coincident
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with the bottom surface of the associated upper c~ord portion.
T~e invention will now ~e descr~ed ~y ~ay of a pre-
ferred embodiment with reference to tfie accompanying drawings:
Figure 1 ls a schematic front view of a part of a truss
made of timber and incorporating one embodiment of the fea-
tures of the present invention;
Figure 2 is a view similar to Figure l but showing a
truss of a slightly different type; and
Figure 3 is a schematic perspective view of a part of
io an embodiment of load supporting structure according to the
present invention.
The corresponding parts in Figure 1 - 3 are referred
to wit~ the same reference numerals.
Turning now to Figure 1, a truss 10 is s~o~n made of -
timber means fixedly secured to one other by means of gusset
plates ll in a ~ell known manner. The beam com-
... .
prises a top chord 12 which, in Figure l, is horizontal and
generally parallel with a bottom c~ord 13. A plurality of wooden
2~ beams 14, 15 extends between t~e cords 12, 13 in a zig-zag
fashion. It will be appreciated that the beams 14, 15 thus for~
a pl~rality of pairs of beams 14 - 15 each defining, i`n front
view of Figure 1 or Figure 2, a V-shaped structure,
A generally vertical reinforcement beam 16 extends from
the center of each of the V-shaped structures defined by ad-
jacent wooden beams 14, 15, with the bottom end of each of the
beams 16 being fixedly secured to the respective V-shaped
structure 14, 15, the top of each of t~e beams 16 ~eing fixedly
secured to the associated section of the upper chord.
Each of the reinforcement beams 16 is provided with a hori-
zontal ledge 17 which is located in proximity to but vertically
spacea from the respective upper chord 12. In the embodiments
shown in the drawings, the ledge forms a bottom wall of an upward-
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Iy elongated rectangular recess 18 cut out at one side of the
vertical beam 16. The spacing of ledge 17 from the respective
bottom surface 19 of the top chord 12 is such that when a plura-
lity of trusses is aligned to build a load supporting structure,
bridging beams 20 ~not shown in Figure 1) are received in the re-
spective recesses 18 (Figure 3) such that the bottom surface 19
of each of the top chords 12 is generally coincident with the
respective section of the upper surface 21 of the bridging beam
20, while the bottom surface 22 of each of the beams 20 is gen-
erally coincident with the ledge 17.
As best -seen from Figure 2, the depth of the recess 18 is
such that the ledge 17 supports substantial width of the assoc-
iated section of the bottom surface 2~ of beam 20.
The structure as shown in Figure 3 can have the bridging
beams 20 nailed to the respective reinforcement beams 16 in re-
gular manner, without the need for excessive number of nails.
Assuming that any of the trusses as shown in Figure 3 breaks or
otherwise fails, it will be appreciated that the beams 20 read-
ily transfer the load no longer supported by the broken truss to
the adjoining structure. In other words, the nails (not shown
in the drawings) normally used in securing the beams 20 to trusses
lO only operate as means for maintaining longitudinal stability
of the overall structure. The nails are not stressed by vertical
forces, all such vertical stresses being transferred to the re-
spective trusses 10 through the respective ledges 17. In general
terms, Fig. 3 shows an embodiment of the bridging beams being
secured to the respective trusses such that both the top faces
21 and the bottom faces of the trusses are in a face-to-face
engagement with associated portions of said trusses, namely with
the bottom faces l9 of the upper chords 12 and with the ledges 17,
respectively.
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It will thus be appreciated that the present invention
provides an extremely simple yet efficient improvement in the
art of load supporting structures for the building industry
which is relatively light-weight and, at the same time, strong
enough to reduce the danger of the "progressive collapse" if
one of the trusses fails for some reason or another. In a
load sharing structure of the present invention - presuming
there is one weak truss (i.e. a local overload) - the weak or
overloaded truss will be less capable of resisting the load
and will deflect more than the adjacent trusses as the load
is being applied. A typical example of such gradually applied
load is snowfall. As the deflection difference increases
between the weak truss and its adjacent partners the load
sharing bridging will begin to work transferring part of the
load of the weak truss to the adjacent ones before failure of
the weak truss. This may possibly prevent failure as the weak
truss may never reach its yield point. The load sharing
bridging will be transferring a great portion of the load of
the weak truss before the yield point is reached which will
reduce or eliminate the impact type load transfer to the
adjacent trusses. This ensures that the adjacent trusses are
more capable of carrying the extra load - reducing the likeli-
hood is progressive collapse.
Those skilled in the art will readily appreciate that
the trusses shown in the accompanying drawings are not the only
ones in which the present invention can be included. It will
be seen on comparison of Figures 1 and 2 that the upper and
lower cords can be parallel with each other, as shown in Figure
1 or they may diverge as shown in Figure 2. The upper chord may
also be curved, without departing from the scope of the inven-
tion. The ledges 17 supporting the beams 20 are preferably uni-
tary with the respective vertical beams 16 even though an embodi-
ment wherein the ledge would be a separate part fixedly secured
to the beam 16 would still fall within the scope of the present
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invention. Turthermore, lt ~ill ~e appreclated t~at t~e overall
load supporting structure can also differ from the embodiment
of Fig. 3. A bridging member can be a truss with a depth equal
to the distance between the bottom of the upper chord and the
top of the Vee of the web members. Another readily conceivable
embodiment would use the bridging member only in every second
Vee of t~e truss.
However, these and many other modifications of the
disclosed embodiments do not depart from the scope of the
present invention as defined in the accompanying claims.
