Note: Descriptions are shown in the official language in which they were submitted.
CA 02530799 2005-12-28
WO 2005/005741 PCT/US2004/021105
TITLE
EQUILATERAL STRAND COMPOSITE
LUMBER AND METHOD OF MAKING SAME
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of U.S. Provisional Application No.
60/484,068 filed July 1, 2003.
Inventors: Russell A. Edgar, Stephen M. Shaler, and Habib J. Dagher.
BACKGROUND OF THE INVENTION
[0001] This invention relates in general to composite lumber products and in
particular to an improved method of manufacturing equilateral strand composite
lumber products.
[0002] In its natural setting a tree is optimally designed to resist the
forces of
nature. Its composition is a compromise to the various vertical and horizontal
forces to which it is subjected. When taken from the forest, sawn and used as
a
structural building material; however, its composition is no longer optimal.
For
example, knots and cross-grain can often be found in the areas of greatest
bending
stress. Density gradients within the wood can lead to zones which are
susceptible
to shear failures.
[0003] Despite these undesirable characteristics, wood has been the
traditional
structural building material of choice in North America. Its advantages are
many,
including high strength-to-weight ratios, workability, renewability, aesthetic
value,
and cost. However, after a decades-long decline in the quality of solid-sawn
CA 02530799 2005-12-28
WO 2005/005741 PCT/US2004/021105
lumber, wood is in danger of losing significant market share to non-wood
products.
(0004] The response from the forest products industry was the development of
structural composite lumber (SCL). SCL is an attempt to re-engineer the tree
to
best resist forces subjected to it as a structural member. The tree is broken
down
into smaller components which are rearranged and reconstituted. The result is
a
product that possesses design values that are often significantly higher than
even
the top grades of solid-sawn lumber. Examples of SCL include Laminated Veneer
Lumber (LVL), Parallel Strand Lumber (PSL), and Laminated Strand Lumber
(LSL).
[0005] The enhanced properties and higher design values are obtained
primarilyYthrough two techniques: defect randomization and densification of
the
wood fiber, both largely functions of the size, shape, and composition
(species) of
the wood element. For reasons to be explained below, certain geometries and
species have traditionally been chosen.
[0006] Typically, SCL manufacturers have used thin (defined herein as having
a thickness less than about 0.25 inches) elements having a generally
rectangular
cross section, and produced from either moderate~density softwoods or low-
density hardwoods. Higher density hardwood species (e.g. maple, birch and
beech) have typically not been used, as they require higher pressures to
adequately
densify and consolidate the wood fiber. These higher required pressures can
result
in an undesirable increase in manufacturing costs, damage to the wood fiber,
problems with dimensional instability, and/or a product that is too dense, and
therefore heavy and, unreceptive to common mechanical fastening techniques.
Additionally, the rectangular strand cross section, as well as method of
layup,
creates a product with differential properties in the two transverse
directions. It
2
CA 02530799 2005-12-28
WO 2005/005741 PCT/US2004/021105
would therefore be advantageous if there could be provided an improved SCL
product that possessed isotropic behavior in the transverse directions, while
also
allowing for use of higher density species.
SUMMARY OF THE INVENTION
[0007] This invention relates to an improved method of forming a composite
beam. The method includes cutting an elongated piece of wood to produce
strands
having cross sections with a substantially symmetrical equilateral polygonal
shape.
Resin is then applied to the strands, and the strands are formed into a
composite
beam. '
[0008] In another embodiment of the invention, a composite beam includes
wood strands having cross sections with a substantially symmetrical
equilateral
polygonal shape, adhesively bonded together.
[0009] In another embodiment of the invention, a composite beam includes
wood strands having cross sections with a substantially triangular shape,
adhesively bonded together.
[0010] Other advantages of this invention will become apparent to those
skilled
in the art from the following detailed description of the invention, when read
in
light of the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011) Fig. 1 is a schematic perspective view of a first portion of composite
lumber manufacturing operation capable of making composite beams of the
invention.
3
CA 02530799 2005-12-28
WO 2005/005741 PCT/US2004/021105
[0012] Fig. lA is an enlarged cross-sectional elevational view of a portion of
the composite lumber manufacturing operation illustrated in Fig. 1.
[0013] Fig. 2 is a schematic elevational view of a second portion of composite
lumber manufacturing operation capable of making composite beams of the
invention.
[0014] Fig. 3 is a is a schematic perspective view of a first embodiment of a
strand manufactured according to the method illustrated in Figs. 1 and 2.
[0015] Fig. 4 is a is a schematic perspective view of a second embodiment of a
strand manufactured according to the method illustrated in Figs. 1 and 2.
[0016] Fig. 5 is a is a schematic perspective view of a third embodiment of a
strand manufactured according to the method illustrated in Figs. 1 and 2.
[0017] Fig. 6 is a is a schematic perspective view of a fourth embodiment of a
strand manufactured according to the method illustrated in Figs. 1 and 2.
[0018] Fig. 7 is a is a schematic perspective view of a fifth embodiment of a
strand manufactured according to the method illustrated in Figs. 1 and 2.
[0019] Fig. 8 is a schematic elevational view of a second embodiment of a laid-
up billet formed in accordance with the method of the invention.
[0020] Fig. 9 is a partial schematic view in elevation showing a strand having
one scarfed surface.
[0021] Fig. 10 is a partial schematic view in elevation showing a strand
having
two scarfed surfaces.
[0022] Fig. 11 is a schematic elevational view of a third embodiment of a laid-
up billet formed in accordance with the method of the invention.
4
CA 02530799 2005-12-28
WO 2005/005741 PCT/US2004/021105
[0023] Fig. 12 is a partial schematic cross-sectional view of a beam formed in
accordance with the method of the invention illustrated in Figs. 1 and 2.
[0024] Fig. 13 is a schematic perspective view of a beam having the cross
section illustrated in Fig. 12.
[0025] Fig. 14 is a schematic perspective view of a prior art strand.
[0026] Fig. 15 is a schematic perspective view of a prior art beam formed with
the strands illustrated in Fig. 14.
[0027] Fig. 16A is a schematic plan view of a first alternate embodiment of
the
mat illustrated in Fig. 1.
[0028] Fig. 16B is a schematic perspective view of a beam formed with the mat
illustrated in Fig. 16A.
[0029] Fig. 17A is a schematic plan view of a second alternate embodiment of
the mat illustrated in Fig. 1.
[0030] Fig. 17B is a schematic plan view of a third alternate embodiment of
the
mat illustrated in Fig. 1.
[0031] Fig. 17C is a schematic perspective view of a beam formed with the
mats illustrated in Figs. 17A and 17B.
DETAILED DESCRIPTION OF THE INVENTION
(0032] As shown in Figs. 1 and 2, an apparatus for making composite beams
according to the process of the invention is indicated generally at 10. A log
12,
preferably a high density hardwood, such as a wood having a specific gravity
greater than about 0.55, is placed on a conveyor 14. Examples of suitable high
CA 02530799 2005-12-28
WO 2005/005741 PCT/US2004/021105
density hardwood include, but are not limited to, maple, such as red maple and
sugar maple, beech, such as American beech, and birch, such as yellow birch.
The
log 12 moves in the machine direction, indicated by an arrow 15.
[0033] The log 12 is then canted by a canting apparatus 16. The use of a
canting apparatus 16 to cant logs 12 is well known in the art. The canted logs
or
cant 18 is then cut, such as by a plurality of saw blades 20, to produce a
flitch or
stack 22 of cut boards 24. Preferably, the boards 24 are within the range of
from
about '/4 inch to about 1 inch thick. More preferably, the boards 24 are
within the
range of from about 3/8 inch to about 7/16 inch thick. It will be understood
however, that the boards 24 can be cut to any desired thickness, such as for
example, less than'/4 inch or greater than 1 inch.
[0034] Subsequent to cutting the cant 18 into a stack 22 of cut boards 24, the
boards 24 are un-stacked and each board 24 is then separated into strands 26
in a
stranding machine 28. Preferably, the stranding machine 28 includes a
plurality of
saw blades, however any other desired means of separating the boards 24 into
strands 26 can be used. Other such means for separating the boards 24 into
stands
include, for example, clipping, such as with a guillotine, or slicing.
[0035] The strands 26 have a symmetrical and substantially equilateral
polygonal cross-sectional shape. As used herein regarding strands 26 separated
by
the stranding machine 28 and otherwise unmodified, substantially equilateral
is
defined as all sides of the cross-sectional shape having equal length.
Substantially
equilateral also includes equilateral cross-sectional shapes and slight
deviations
therefrom which are due to slight variations caused in the separating process,
such
as by the stranding machine 28. Such slight variations can include strands
having
cross-sectional shapes wherein one side length is slightly smaller or slightly
larger
than a desired side length. For example, a substantially equilateral cross-
sectional
6
CA 02530799 2005-12-28
WO 2005/005741 PCT/US2004/021105
shape can include one side length which is within the range of from about 0.7
to
about 1.3 times the desired side length. Preferably, the cross-sectional shape
of
the strands 26 is that of an equilateral triangle, as best shown in Fig. 3.
Strands
having other symmetrical and equilateral polygonal cross-sectional shapes can
also be formed, such as for example, strands having square 26', pentagonal
26",
hexagonal 26"', and diamond 26'" cross-sectional shapes, as shown in Figs. 4
through 7, inclusive.
[0036] Regardless of the symmetrical and equilateral polygonal cross-sectional
shape of the strands 26, a side length S 1 of the cross section of each strand
26 is
preferably within the range of from about'/4 inch to about 1 inch.
[0037] As shown in Fig. 3, the strands 26 have any desired length L 1, and
therefore the strands 26 can be formed from logs 12 of any desired length.
Preferably, the strands 26 have a length within the range of from about 2 feet
to
about 12 feet. More preferably, the strands 26 have a length of about 4 feet.
If
desired, one or more boards 24 can be joined together lengthwise to form
boards
of increased length. The boaxds 24 can be joined together by any desired
method,
such as for example, with a finger joint. Once joined together, strands 26 can
be
formed from the finger jointed boards as described above.
[0038] Preferably, the strands 26 have a slenderness or length-to-depth ratio
within the range of from about 100 to about 300. As used in the context of a
length to depth ratio, the term depth is defined as the side length S 1 of the
cross
section of the strand 26. For example, a strand 26 having triangular cross-
section,
a side length S 1 of '/4 inch, and a length of 48 inches, has a length to
depth ratio of
192. As would be known to one skilled in the art, length-to-depth ratios are
positively correlated with bending strength and stiffness, up to a maximum
length-
to-depth ratio above which little or no increase in bending strength and
stiffness
7
CA 02530799 2005-12-28
WO 2005/005741 PCT/US2004/021105
can be observed. For example, in the strand composite product known as
waferboard, one study indicated that bending strength and stiffness increase
for
length-to-depth ratios within the range of frbm about 120 to about 300. Known
strand composite products commonly have length-to-depth ratios within the
range
of from about 100 to about 400.
[0039] After the strands 26 are formed, they are preferably dried in a dryer
30
to reduce moisture content to a desired level, such as below about 10 percent.
The
use of a dryer 30 for drying strands 26 of wood is well known in the art.
[0040] After drying, a plurality of strands 26 is then laid-up or arranged to
define a mat 32. The strands 26 are arranged substantially parallel to one
another,
such that each strand 26 is in contact, or near contact with an adjacent
strand 26.
The number of strands 26 arranged to define the mat 32 depends on the desired
final product density, preferably within the range of from about 39 to about
47
lbs/ft3.
[0041] As best shown in Fig. 1; the method of forming the mat 32 includes
arranging a plurality of strands 26 of substantially equal length side-by-side
on a
surface 34, such as a forming surface or the conveyor 14. The strands 26 are
placed side-by-side such that first ends 36 of each strand 26 are
substantially
linearly aligned with one another, and such that the end surfaces of the first
ends
36 of each strand 26 are substantially coplanar.
[0042] Once the, plurality of strands 26 are arranged, at least one bead of
adhesive 38, such as a hot-melt type adhesive, can be applied transversely
across
the arranged strands 26, thereby adhering the strands 26 to one another and
defining the mat 32. Such a bead of hot-melt type adhesive 38 allows the mat
32
to be easily handled and moved. It will be understood that the hot-melt type
8
CA 02530799 2005-12-28
WO 2005/005741 PCT/US2004/021105
adhesive 38 will melt early in the curing process, as will be described below,
thereby allowing the strands 26 to move relative to one another and nest.
(0043] If desired, vibration means 40 can be applied to the surface 34 prior
to
applying the bead of adhesive 38. Such vibration means 40 will vibrate the
strands 26 such that adjacent strands 26 do not overlap, and that each strand
26 is
arranged substantially parallel to, and in contact with, an adjacent strand
26. It has
been demonstrated through experimentation that strands 26 having substantially
equilateral polygonal cross-sectional shapes, such as triangular, respond
advantageously to such vibration. The vibration causes the strands 26 to
become
longitudinally aligned such that each strand 26 ~is substantially parallel to,
and in
contact with, an adjacent strand 26.
[0044] Although not illustrated, it will be understood that the strands 26 can
be
formed into a laid-up billet, such as the laid-up billet 48, without first
forming a
mat 32. For example, strands of equal or varying length can be randomly
dropped
(i.e. allowed to drop or fall), to a surface, such as a forming surface or the
conveyor 14.
[0045] After applying at least one bead of adhesive 38 to the arranged strands
26, resin 42 is then applied to the mat 32 to define a resin-coated mat 43.
Preferably liquid resin, such as phenol-formaldehyde resin is sprayed on a
major
surface 32A of the mat 32 by a sprayer 44. More preferably, liquid resin is
sprayed on both major surfaces 32A of the mat 32 by a plurality of sprayers
44.
Although spraying is shown in the exemplary embodiment illustrated in Fig. 1,
it
will be understood that any other desired method of applying the liquid resin
42 to
the mat 32 can also he used. Further, reference is made to using liquid resin
42,
although it will be understood that powdered resin can be applied to one or
both of
the major surfaces 32A of the mat 32. It will be further understood that any
other
9
CA 02530799 2005-12-28
WO 2005/005741 PCT/US2004/021105
desired resin can be used, such as for example, diphenylmethane diisocyanate
(MDI).
[0046] A plurality of resin-coated mats 43 are then stacked to define a laid-
up
billet 48. As used herein, laid-up billet is defined as the billet prior to
being cured,
as described herein below. A first embodiment of a laid-up billet 48 is
illustrated
in Fig. 1. Preferably, the resin-coated mats 43 are stacked in a stepped
arrangement such that a first end 50 of any mat 43 overlaps a second end 52 of
the
adjacent lower mat 43, thereby defining a lap joint 54. Preferably, first end
50
overlaps the second end 52 by about two inches. The resin-coated mats 43 are
further stacked such that the first end 52 of any~mat 43 overlaps the end 50
of the
adjacent lower mat 43, thereby defining an overlap region 56. Although 14 mats
43 are shown in the exemplary embodiment of the laid-up billet 48 illustrated
in
Fig. 1, it will be understood that any other desired number of mats 43 can be
stacked to define the laid-up billet 48. The lay-up method, as described
above, is
well known in the art.
[0047] If desired, a layer of reinforcement material 59 can be disposed
between
layers of mats 43 adjacent the lap joint 54, as best shown in Fig. lA. The
layer of
reinforcement material 59 can also be disposed adjacent the overlap region 56.
Any desired reinforcement material can be used, such as for example a woven
fiberglass fabric.
[0048] A second embodiment of laid-up billet 48' is illustrated in Fig. 8. The
laid-up billet 48' is substantially identical to the laid up billet 48, and
includes a
plurality of the mats 43. In the laid-up billet 48' however, the first and
second
ends 50 and 52, respectively, of the mats 43 do not overlap, but abut one
another.
This practice is well known in the art.
CA 02530799 2005-12-28
WO 2005/005741 PCT/US2004/021105
[0049] If desired at least one surface 33A of at least one end of the mats 32
can
be beveled or scarfed, as best shown in Fig. 9. The practice of scarfing is
well
known in the art. Alternately, two surfaces 33B and 33C, respectively, of at
least
one end of the mats 32 can be scarfed, as best shown in Fig. 10. Such scarfing
minimizes void space which can occur after curing at either or both of the lap
joints 54 and the overlap regions 56. The scarfing further minimizes the
potential
density profile of the cured billet 66 at the lap joints 54 and the overlap
regions
56; i.e. scarfing increases density of the cured billet at the lap joints 54
and the
overlap regions 56. It will be understood that the strands 26 can also be
scarfed
prior to forming mat 32.
[0050] A third embodiment of laid-up billet 48" is illustrated in Fig. 11. The
laid-up billet 48" includes a plurality the mats 43. The mats 43 are
preferably
stacked such that the first ends 50 of the mats 43 are vertically aligned.
[0051] If desired, a laid-up billet can include stands having a plurality of
sizes.
For example, strands having equilateral triangle cross-sections and side
lengths S 1
of about'/4 inch and about 7/16 inch can be used to form a laid-up billet. It
is
believed that once cured, billets having strands with a plurality of sizes
will be
stronger than billets made with strands of only one size. It has been shown,
for
example, that composite beams made from triangular cross-sectional strands
having a side length of about'/4 inch have a higher shear strength than
composite
beams made from strands having a side length of about 7/16 inch. Additionally,
composite beams made from strands having a side length of about 7/16 inch have
a higher bending strength than composite beams made from strands having a side
length of about'/4 inch. Accordingly, for a bending member it would be
advantageous to use '/4 inch strands in the core of the beam (where the shear
stress
11
CA 02530799 2005-12-28
WO 2005/005741 PCT/US2004/021105
is highest) and 7/16 inch strands on the top and bottom of the beam (where
bending stress is highest).
[0052] For example, for a beam intended for use as a bending member, such as
a girder, the mats can be formed from strands having at least two different
side
lengths. As shown in Fig. 16A, a mat 110 can be formed from strands having a
larger side length 112 on the sides or outside portion of the mat 110, and
strands
having a smaller side length 114 centrally disposed in the mat 110 (i.e., the
strands
having a smaller side length 114 define a central portion of the mat 110). A
plurality of mats 110 are then stacked, cured, and cut to a desired width and
length
as described herein, thereby forming a beam 116. When the beam 116 is rotated
90 degrees such that the strands having a larger side length 112 define the
top and
bottom layers of the beam 116, as shown in Fig. 16B, the beam 116 thereby
defines a bending member believed to have improved bending strength. Although
illustrated schematically, it will be understood that the strands 112 and 114
can
have any of the substantially symmetrical polygonal cross-sectional shapes
described herein, such as triangular.
[0053] For a beam intended for use as a plank, such as in scaffolding, at
least
two different mats can be formed. A first mat 118 can be formed from the
strands
having a larger side length 112, as shown in Fig. 17A, and a second mat 120
can
be formed from the strands having a smaller side length 114, as shown in Fig.
17B. A plurality of the first and second mats 118 and 120 are then stacked to
form
a laid-up billet such that the first mats 118 are disposed on the top and
bottom of
the laid-up billet, and the second mats 120 are disposed intermediate the
first mats
118. The laid-up billet comprising the plurality of stacked mats 118 and 120
is
then cured, and cut to a desired width and length as described herein, thereby
forming a beam 122 having improved strength, as shown in Fig. 17C.
12
CA 02530799 2005-12-28
WO 2005/005741 PCT/US2004/021105
[0054] The laid-up billet, such as the first and second embodiments of the
laid-
up billets 48 and 48', respectively, is then cured with a combination of
pressure
and energy. For purposes of illustration, only the laid-up billet 48 is shown
in Fig.
2. The method described herein however, also applies to other laid-up billets,
such
as the laid-up billet 48'. Pressure can be applied to the laid-up billet 48 by
any
desired means, such as a plurality of rollers 60. Preferably, pressure can be
applied to the laid-up billet 48 in a continuous press operation, as best
shown at 62
in Fig. 2. In such a continuous press operation 62, the laid-up billets 48 are
moved
by the conveyor 14, such that the plurality of rollers 60 and/or plates (not
shown)
exerts a force on at least one surface 48A of the laid-up billet 48, thereby
compressing the mats 43.
[0055] Although not illustrated, it will be understood that pressure can be
applied toythe laid-up billet, such as the third embodiment of the laid-up
billet 48',
in a batch process, wherein a predetermined length of the laid-up billet 48'
is
disposed between two platens of a,press. The platens are then moved toward one
another, thereby compressing the mats.
[0056] Preferably, energy is applied to the laid-up billet 48 during the
pressing
step to heat, and therefore cure, the laid-up billet 48. Preferably, the
energy is
applied by an electromagnetic heating operation, such as with radio frequency
energy, as schematically illustrated at 64 in Fig. 2. Use of electromagnetic
heating
in the manufacture of wood composites is well known in the art.
Advantageously,
electromagnetic heating provides relatively fast and uniform heating
throughout
the laid-up billet 48. With the application of such uniform heating,
plasticization
and densification of the wood is substantially constant throughout the laid-up
billet
48. The combination of pressure and energy thereby cures the laid-up billet 48
and forms a cured billet 66.
13
CA 02530799 2005-12-28
WO 2005/005741 PCT/US2004/021105
[0057) It will be understood however, that any other desired method of
applying energy to the laid-up billet 48 can be used. Examples of such other
methods include applying energy by microwave, electrical resistance and/or
steam
injection techniques.
[0058] As best shown in Fig. 12, even after curing, the strands 26 retain a
substantially equilateral cross-sectional shape. It will be understood that as
used
herein regarding compressed strands 26, substantially equilateral includes
equilateral cross-sectional shapes and slight deviations therefrom which are
due to
distortions caused in the pressing process. Such slight deviations can include
strands having cross-sectional wherein one side length is slightly smaller or
slightly larger than its original side length S 1. For example, a
substantially
equilateral cross-sectional shape can include one side length which is within
the
range of from about 0.5 to about 1.5 times the original side length S1.
[0059] Referring again to Fig. 2, once the laid-up billet 48 is cured, a
cutter 68
then cuts the cured billet 66 to any desired length, thereby defining a
composite
beam 70. It will be understood however, that the cured billet 66 can be cut in
any
desired direction, including longitudinally and transversely, so as to define
a beam
70 having any desired length L2, height H, and width W, as best shown in Fig.
13.
[0060] The method of the invention is shown as a continuous process occurring
on a continuously moving conveyor 14. It will be understood however, that each
step of the method can also be satisfactorily performed at one or more
independent
workstations.
[0061] As would be understood by one skilled in the art, known strand
composite products commonly use thin, generally rectangularly shaped wood
pieces, often with a thickness less than 0.25 inches. Although use of such
thin
14
CA 02530799 2005-12-28
WO 2005/005741 PCT/US2004/021105
strands often beneficially provides for considerable distribution of defects
and can
minimize void space, using thin strands reduces the natural strength of the
wood in
the direction of the grain.
[0062] Further, such known strands 80 are often cut tangentially from a log
82,
as shown in prior art Fig. 14. The strands 80 are then stacked to form a beam
84
such that the growth rings 86 of adjacent strands 80 are substantially aligned
in the
same plane P, as best shown in prior art Fig. 15. Such a beam 84 has
anisotropic
properties, wherein the mechanical response to a transverse force in the
direction
of an arrow xl is thereby substantially different to the .mechanical response
to a
transverse force in the direction of an arrow yl~ Such a difference in
mechanical
response or strength provides a beam that is substantially stronger when
subjected
to a force in the direction of one of the xl and the yl arrows than when
subjected
to a force in the direction of the other of the xl and the yl arrows.
[0063] When strands 26 having symmetrical equilateral polygonal shapes, such
as the triangles shown in Figs 3 and 13 are used, the growth rings 90 within
each
strand 26 become oriented in an orthogonally randomized fashion relative to
the
growth rings 90 in adjacent strands 26, as best shown in the beam 70
illustrated in
Fig. 13. Accordingly, one advantage of the invention is that the use of
strands 26
having cross sections with a substantially symmetrical equilateral polygonal
shape
provides a beam, such as the beam 70, having transverse near-isotropic
properties,
i.e., a beam wherein its strength or mechanical response is independent of the
direction of the force applied. The press-induced density profiles which also
cause
anisotropic behavior are minimized with the use of electromagnetic curing as
described above. It is believed that such orthogonal randomization of the
growth
rings 90 within a composite beam will produce a beam, such as the beam 70,
having transverse near-isotropic properties. Near-isotropic, as used herein,
is
CA 02530799 2005-12-28
WO 2005/005741 PCT/US2004/021105
defined as the effect wherein the mechanical response to a force in the
direction of
the arrow x2 is substantially identical to the mechanical response to a force
in the
direction of the arrow y2.
[0064] Another advantage of the invention is that beams having such transverse
isotropic properties, such as the beam 70 shown in Fig. 13, can be
manufactured
without regard to the orientation of the mats 32 which comprise the beam 70.
Further, the beam 70 can be used without regard to the orientation of the mats
32
within the beam 70.
[0065] It is believed that an advantage of the invention is that the stands
are
relatively thicker, i.e. the strands 26 have a side length S 1 within the
range of from
about'/4 inch to about 1 inch. Such a thicker strand 26 allows each strand 26
to
maintain the wood's natural strength in the direction of the grain.
[0066] Another advantage of the invention is that strands 26 having a
triangular
cross-sectional shape become substantially longitudinally aligned during the
arranging step shown in Fig. 1, such that at least one face of each strand 26
is in
contact with a face of an adjacent strand 26. After the billet 48 is cured,
the
strands 26 are nested or arranged such that undesirable void spaces between
adjacent strands 26 are minimized, as best shown in Figs. 1 and 13.
[0067] Known strand composite products have generally been produced using
either moderate density softwoods (having a specific gravity within the range
of
from about 0.4 to about 0.55), or low density hardwoods (having a specific
gravity
less than about 0.4). Higher density hardwood species (having a specific
gravity
greater than about 0.55) have been avoided, as they require higher pressures
to
adequately densify arid consolidate the wood. As used herein, specific gravity
is
defined as oven dry weight/volume at 12 percent moisture content. The higher
16
CA 02530799 2005-12-28
WO 2005/005741 PCT/US2004/021105
pressures required to process higher density hardwood can lead to increased
manufacturing costs, damage to the wood fiber, problems with dimensional
stability and/or a product that is too dense, making it heavy and unreceptive
to
common mechanical fastening techniques.
[0068] It is further desirable to provide composite beams having a low
compaction ratio (as used herein compaction ratio is defined as product
density/parent wood species density). Products with higher compaction ratios
are
often susceptible to undesirable levels of springback, especially upon
exposure to
moisture, and springback can contribute to undesirable nail pops in drywall
and
sub-flooring. As used herein, springback refers to the permanent residual
thickness swelling which occurs on release of the compressive stress with
absorption. Known composite products generally have a compaction ratio greater
than about 1.35. Advantageously, composite beams formed according to the
present invention have been shown to have a relatively lower compaction ratio.
For example, in a beam formed with equilateral triangular stands of red maple,
the
compaction ratio is 1.26 (wherein the product density is 43 lbs/ft3 and the
red
maple density is 34 lbs/ft3).
[0069] Another advantage of the invention is that the strands of the invention
can be formed from small to medium diameter trees, which are currently not
used
in many strand composite products, thereby providing a lower cost source of
wood.
[0070] As best shown in Fig. 12, a further advantage of the invention is that
in
circumstances wherein adjacent strands 26 are not perfectly aligned, the
application of pressure causes an edge, such as the edges 26A, 26B, and 26C of
the strands 26 to become embedded or wedged into an adjacent strand 26. Such
wedging further reduces void spaces between adjacent strands 26, and increases
17
CA 02530799 2005-12-28
WO 2005/005741 PCT/US2004/021105
densification, thereby increasing the strength of the beam relative to a beam
having a greater volume of void space. It is believed that the occurrence of
such
wedging increases when higher density wood is used.
[0071] Another advantage of the invention is that beams manufactured
according to the methods of the invention have demonstrated desirable strength
and performance values. For example, Table 1 illustrates the strength and
performance values for beams formed with strands having equilateral triangle
cross-sections and side lengths S1 of about'/4 inch and about 7/16 inch. The
_ performance values shown in Table 1 include modulus of elasticity (MOE),
modulus of rupture (MOR), allowable bending stress (Fb), and allowable shear
stress (F"), wherein n is the number of samples. It will be known by those
skilled
in the art that an increase in product density is known to be positively
correlated
with most mechanical properties. The values shown in Table 1 are those of the
lowest probable product density. A substantial increase in these properties is
likely to be achieved with an increase in product density.
Table 1
Property Units N Sl = 7/16 inch Sl =1/4 inch
Void Volume % 12 7.5 3.1
Volumetric Shrinkage% 12 12.0 11.5
MOE (apparent) psi 12 1,750,000 1,618,000
a
MOE (true) psi 12 2,046,000 1,870,000
MOR psi 12 8,520 8,999
pb b psi 12 2,730 2,765
Shears psi 12 1,150 1,295
psi 12 260 345
a Values adjusted to the average density (39.3 Ibs/ft') and a moisture content
(9.88%) of all beams.
b Based on the volume adjusted minimum MOR divided by a safety factor of 2.1.
° Based on the minimum shear strength divided by a safety factor of
3.15.
[0072] Although the method of the invention has been described in the context
of producing a structural member, it will be understood that the method of the
18
CA 02530799 2005-12-28
WO 2005/005741 PCT/US2004/021105
invention can also be applied to the production of non-structural wood
composite
products.
[0073] The principle and mode of operation of this invention have been
described in its preferred embodiments. However, it should be noted that this
invention may be practiced otherwise than as specifically illustrated and
described
without departing from its scope.
19
