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 futon 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
W hat is the strongest futon frames are made from? This is the most frequently asked question. According to current market statistics there are three sources North America, Europe and Asia. Chart below shows most commonly used hardwoods that are used for futon furniture today. Hardness of wood is shown top to bottom rotation.
A - Specific gravity1 B - Modules of rupture PSI C -Modulus of elasticity Million PSI D - Work to maximum load IN-lb per in3 E - Strength maximum shearing PSI F - Origin
Pine, Western White
For a complete chart scroll down the page
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?
(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.)
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.
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
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
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.
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
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.
A - Specific gravity1 B - Modules of rupture PSI C -Modulus of elasticity Million PSI D - Work to maximum load IN-lb per in3 E - Strength maximum shearing PSI
Oak, Northern Red
Oak, Southern Red
Fir, Pacific Silver
Pine, Eastern White
Pine, Western White
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, Eu = Europe
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. 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.