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WOOD:
Nature's Best Building Material
By Joe Tatulli
Over
the years as a retailer, manufacturer, and analyst-at-large
I have concluded that the most critical aspect of the
entire futon furniture industry is the quality and durability
of the convertible futon frame. This quality and durability
aspect is embodied in two separate elements. The first
is the element of mechanics and craftsmanship. The second
is resident in the materials used to make the frame
and the mechanism. When the two elements of craftsmanship
and material are well executed quality furniture should
be the result.
When one or both are poorly executed
problems will follow. It should also be evident to anyone
who makes or sells convertible futon furniture that
when these frames, or any piece of furniture for that
matter, are put under excessive stress they may break.
Breakage, therefore, and the resultant returns, bad
feelings, and negative reputation that follows, is a
major issue.
Having a better understanding
of wood and its properties, as a material used in a
mechanical application, may help every member of the
industry better comprehend how this material performs
under stress, and thereby grasp how to choose a wood
that will naturally lend itself to this most stressful
application. We hope to educate both retailers and suppliers
so they can better discern how to judge the products
they sell every day. - Editor
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| We
all remember the story of Pandora's Box. Well, the box
was probably made of wood, and you will soon see why I've
come to that conclusion. Let me start by saying that my
brief study of the world of wood has been very rewarding,
and I hope you will benefit from it as well. Most of the
information for this article was gleaned from a single
source, a book called Wood Handbook: Wood as an Engineering
Material, published by The US Department of Agriculture's
Forest Products Laboratory. This comprehensive, five hundred
page publication covers a multitude of topics concerning
wood and wood products, including chapters on availability,
physical properties, mechanical properties, fastenings,
structural analysis, bonding wood, finishing wood, wood
preservation and many others. With all of this information
comes some good news and some bad news. The good news
is, after you digest this article you will know more about
wood than you did before you read it (no kidding). The
bad news is the article is not comprehensive, exhaustive
or conclusive in any specific sense. I do intend to present
some very general conclusions regarding the correlation
of weight and strength. But even these simple, generic
conclusions are complicated by the fact that even within
certain commercial species and their families significant
differences exist. Differences which may have a profound
affect on the correlation for a specific piece of wood.
For instance there are some sixty varieties of oak grown
in the US and another one-hundred and forty grown world-wide.
When a product is said to be made from oak, which one
of the two-hundred is it? Even within the category of
White Oak there are a dozen or so varieties, each with
its own properties. Anyway, you can figure it out from
there. Bottom Line: Wood is a natural product and is therefore
subject to the variation common to all natural materials.
"Variability, or variation in properties, is common
to all materials. Because wood is a natural material
and the tree is subject to numerous constantly changing
influences (such as moisture, soil conditions, and growing
space), wood properties vary considerably even in clear
(no knots) material." 1
Presented here as well (in a nearby side bar on page
36) is a short botany lesson regarding trees and the
wood they produce. Most interesting (in regard to this
discussion) is the fact that the terms "hardwood"
and "softwood" are botanical terms and do
not indicate quality of hardness, strength or durability.
You can read the side bar for all the details. Also
worthy of mention here is the orthotropic nature of
wood. By orthotropic we mean that wood has unique and
independent properties in the directions of three mutually
perpendicular axes-longitudinal, radial, and tangential.
The longitudinal axis is parallel to the grain; the
radial axis is perpendicular to the grain in the direction
of the growth rings; and the tangential axis is perpendicular
to the grain running in tangent to the growth rings
(see Figure 1). These axes, when placed under stress,
react differently in relation to each other and also
in relation to the nature of the kind and direction
of the stress. Remember Pandora's box? Wood is just
as tricky.
To begin I am going to define several properties of
wood with this caveat: Many of the ways wood is qualified
do not actually relate to the issues which are important
to the determination of whether or not a specific wood
should or should not be used in convertible futon frames.
Issues like shrinkage (wood volume reduction due to
moisture loss during drying), thermal properties, electrical
properties, weathering and decay, and chemical resistance
have no direct relation to the issue at hand. That issue
being; does a particular species lend itself to be used
as an integral part of the load bearing or mechanical
parts of a convertible futon frame?
Property
1:
Weight-Density-Specific Gravity
(a physical property of wood)
"Two primary sources of variation affect the weight
of wood products. One is the density of the basic wood
structure; the other is the variable moisture content."
2 The density of wood,
without consideration of water content, varies from
a low of 6-10 pounds-mass per cubic foot (pcf) for Balsa
to over 65 pcf for several imported woods. The average
for domestic woods runs from about 20 to 45 pcf. To
help understand and take into account the natural variations,
even within a specific species, a coefficient of variation
of 10 percent is considered suitable for describing
the variability of density within a common domestic
species. 3
Specific gravity, a scientific measuring formula, is
used to more clearly quantify the weight to density
to moisture content ratios at an ovendry moisture content
of 12 percent. (12 percent being a typical moisture
content goal when oven or kiln drying green wood for
use in furniture or other constructions.)
I have concluded, in the most simplistic of terms,
that woods with a higher specific gravity, at 12 percent
moisture content, lend themselves much better to the
stress factors inherent in most convertible futon furniture
frame applications. (See Table 1 for a list of some
common species and their properties as they relate to
specific gravity and several other important [mechanical]
issues on page 40.)
Property
2:
Modulus of Rupture
(a mechanical property of wood related
to strength)
The modulus of rupture (mr) in bending is a measurement
that reflects the maximum load carrying capacity of
a wooden member. This modulus is an accepted criterion
of strength. 4 In layman's
terms, modulus of rupture is the amount of pressure
it takes to break a sample piece of clear wood when
bending it. When you look at Table 1 you will notice
that balsa (an inappropriate wood for use in convertible
futon furniture) has a mr of 3,140 psi while ramin (a
wood widely used in futon furniture) has an mr of 18,500
psi, a number almost six times higher. This means it
takes almost six times as much pressure to break a piece
of ramin as it does to break a similar dimensioned piece
of balsa.
Property
3:
Work to Maximum Load in Bending
(a mechanical property of wood related
to strength)
Work to maximum load in bending represents the ability
to absorb shock with some permanent deformation and
more or less injury to a specimen. It is a measure of
the combined strength and toughness of wood under bending
stresses. The value is shown in Table 1 as pounds per
cubic inch (pci). Balsa's value is 2.1 pci and ramin's
is 17.0 pci, a value almost nine times higher. 5
Property
4:
Shear Strength Parallel to Grain
(a mechanical property of wood related
to strength)
Shear strength is a measure of ability to resist internal
slipping of one part upon another along the grain. Balsa's
value is 300 psi and ramin's is 1,520 psi. 6
Property
5:
Modulus of Elasticity and Poisson's Ratio
(a mechanical property of wood related
to elasticity)
The modulus of elasticity and Poisson's Ratio are measurements
used to derive the elastic properties of wood. Once
again these measurements are derived through testing
materials against some rather complex scientific formulas.
Generally speaking wood will bend and stretch depending
on the structural relationships between the three axes.
These relationships are based upon the botanical structure
of the wood cells, moisture content, and several other
variables. These moduli are the most difficult to calculate
and present the greatest challenge to a meaningful understanding
of their values.
In conclusion, it does appear that in most cases heavier
is better. A heavier weight, as measured by density
in pounds per cubic foot, implies that a specific sample
of a species has more substance and is therefore stronger.
This strength is apparent in correlation to the results
of the various bending, twisting, abrupt pressure of
dropping, abrasion and shearing stresses put upon it
as it is tested.
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|
What
Is Wood?
Botanically
speaking, wood consists of a tissue known as secondary
xylem. Xylem's major function in a living tree is to
conduct water and minerals from the roots of the tree
to branches and leaves. Secondary xylem is also the
primary structural and support tissue of a tree (ie.
the bulk of the tree trunk) The walls of the xylem cells
are composed of two plant polymers, cellulose and lignin.
The strength and rigidity of the cell walls are the
result of the presence of these two compounds. The cell
walls of xylem are responsible for the structural integrity
and strength of wood . Each year, the tree produces
a new layer or ring of xylem, referred to as the annual
growth ring. The width of this ring depends on seasonal
conditions, such as precipitation, temperature, sunlight,
and nutrient availability.
Often woods are classified
as either hardwoods or softwoods. Hardwoods belong to
a group of species known as dicots (plants that have
two seed leaves, true leaves with netted venation, and
form a vascular cambium). Commonly, all broadleaf trees
that lose their leaves during the winter are classified
as hardwoods. Softwoods are confiers (pine and fir trees,
for example). These two kinds of woods have basic structural
differences, but the terms hardwood and softwood are
not accurate expressions of the density or hardness
of the wood. For example, balsa, a tropical hardwood,
is one of the lightest and softest woods, while hemlock,
a conifer, produces wood that is harder than some hardwood
species.8
|
1 Wood Handbook: Wood
as an Engineering Material, Revised 1987, The US Department
of Agriculture, Forest Products Laboratory. Agricultural
Handbook 72. 466 p. LCC No. 85-600532. p 4-2.
2 Ibid. p. 3-16
3 Ibid. p. 3-16
4 Ibid. p. 4-3
5 Ibid. p. 4-3
6 Ibid. p. 4-3
7 Ibid. p. 4-7 to 4-25
8 Raven, P.H., et. al. 1976. Biology of Plants. Worth
Publishers, Inc. New York, New York. pg. 472.
For more information on wood and wood
products contact The Forests Products Laboratory in
Madison, Wisconsin at 608-231-9200. You can also find
out a great deal about wood on the internet. Try the
following web sites: The Forests Products Laboratory
site at www.fpl.fs.fed.us/ or www2.fpl.fs.fed.us/; another
good source is www.woodweb.com.- Ed.
Table 1
Editor's note:
The values presented in this table are taken from the
Wood Handbook as outlined earlier. No claim is made
or implied that any measurement contained herein is
applicable to any wood sample other than the clear samples
used to do the tests themselves. It should also be noted
that for each of the values presented the authors of
the Wood Handbook have furnished an average coefficient
of variation. These values are as follows for the values
presented in Table 1. Specific Gravity - 10%; Modulus
of Rupture - 16%; Work to Maximum Load - 34%; Shear
Parallel to Grain - 14%. Any conclusions about the information
contained herein should be tested by an independent
testing facility or by a bona fide expert in the field
of forestry.
| |
Specific
gravity1 |
Modules
of rupture
PSI
|
Modulus
of elasticity
Million PSI
|
Work
to maximum load
IN-lb
per in3
|
Shear
parallel
to grain - maximum
shearing strength
PSI
|
Domestic Hardwoods
|
| Ash,
White |
.60 |
15,000 |
1.74 |
16.6 |
1,910 |
|
| Aspen, Quaking |
.38 |
8,400 |
1.18 |
7.6 |
850 |
|
| Maple,
Silver |
.47 |
8,900 |
1.14 |
8.3 |
1,480 |
|
| Maple, Sugar |
.63 |
15,800 |
1.83 |
16.5 |
1,780 |
|
| Oak,
Northern Red |
.63 |
14,300 |
1.82 |
14.5 |
1,780 |
|
| Oak, Southern Red |
.59 |
10,900 |
1.49 |
9.4 |
1,390 |
|
| Oak,
White |
.68 |
15,200 |
1.78 |
14.8 |
2,000 |
|
Domestic Softwoods
|
| Fir,
Balsam |
.35 |
9,200 |
1.45 |
5.1 |
944 |
|
| Fir, Pacific Silver |
.43 |
11,000 |
1.76 |
9.3 |
1,220 |
|
| Hemlock,
Eastern |
.40 |
8,900 |
1.20 |
6.8 |
1,060 |
|
| Hemlock, Western |
.45 |
11,300 |
1.63 |
8.3 |
1,290 |
|
| Pine,
Eastern White |
.35 |
8,600 |
1.24 |
6.8 |
900 |
|
| Pine, Virginia |
.48 |
13,000 |
1.52 |
13.7 |
1,350 |
|
| Pine,
Western White |
.38 |
9,700 |
1.46 |
8.8 |
1,040 |
|
| Spruce, Black |
.42 |
10,800 |
1.61 |
10.5 |
1,230 |
|
| Spruce,
White |
.36 |
9,400 |
1.43 |
7.7 |
970 |
|
Imported Woods
|
Sample origin4 |
| Balsa |
.16 |
3,140 |
.49 |
2.1 |
300 |
AM |
| Hondurous Mahogany |
.45 |
11,500 |
1.50 |
7.5 |
1,230 |
AM |
| Ramin |
.52 |
18,500 |
2.17 |
17.0 |
1,520 |
AS |
| White Meranti |
.55 |
12,400 |
1.49 |
11.4 |
1,540 |
AS |
| Yellow
Meranti |
.46 |
11,400 |
1.55 |
10.1 |
1,520 |
AS |
| Pau Marfim |
.73 |
18,9002 |
- |
- |
- |
AM |
| Nyatoh |
.55
|
14,445 |
1.97 |
- |
- |
AS |
| 1. Specific
gravity is measured for domestic dry wood at 12%
moisture content and for green wood in the imported
table. All other measurements, where available,
are based upon wood samples with a moisture content
of 12%
2. The modulus of rupture for
Pau Marfin is measured at a moisture content of
15%
3. Specific gravity for Nyatoh
is measured at oven dry. The value represented
is an average and is taken from Woods of Maylasia,
Burgess. p.374.
4. Key to letter code: AM,Tropical
America; AS,Asia
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© Copyright 2000 Futon Life
401 351-0787
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