Under tension, the pieces all had similar strength values. This took many tests, but in every other test, the material exhibited buckling as well as compression. The three tests which ran the best were used for Table 2. Since the test of the design will be under compression, this data is very relevant for the final design.
Even oven-dried wood retains a small percentage of moisture, but for all except chemical purposes, may be considered absolutely dry. The general effect of the water content upon the wood substance is to render it softer and more pliable.
A similar effect occurs in the softening action of water on rawhide, paper, or cloth. Within certain limits, the greater the water content, the greater its softening effect.
Drying produces a decided increase in the strength of wood, particularly in small specimens. The greatest strength increase due to drying is in the ultimate crushing strength, and strength at elastic limit in endwise compression; these are followed by the modulus of rupture, and stress at elastic limit in cross-bending, while the modulus of elasticity is least affected.
There are no vessels "pores" in coniferous wood such as one sees so prominently in oak and ash, for example. The structure of hardwoods is more complex. In discussing such woods it is customary to divide them into two large classes, ring-porous and diffuse-porous.
The rest of the ring, produced in summer, is made up of smaller vessels and a much greater proportion of wood fibers.
These fibers are the elements which give strength and toughness to wood, while the vessels are a source of weakness. Examples of this kind of wood are alder basswood birch buckeye, maple, willowand the Populus species such as aspen, cottonwood and poplar. The latewood will be denser than that formed early in the season.
When examined under a microscope, the cells of dense latewood are seen to be very thick-walled and with very small cell cavities, while those formed first in the season have thin walls and large cell cavities. The strength is in the walls, not the cavities. Hence the greater the proportion of latewood, the greater the density and strength.
In choosing a piece of pine where strength or stiffness is the important consideration, the principal thing to observe is the comparative amounts of earlywood and latewood. The width of ring is not nearly so important as the proportion and nature of the latewood in the ring. If a heavy piece of pine is compared with a lightweight piece it will be seen at once that the heavier one contains a larger proportion of latewood than the other, and is therefore showing more clearly demarcated growth rings.
In white pines there is not much contrast between the different parts of the ring, and as a result the wood is very uniform in texture and is easy to work. In hard pineson the other hand, the latewood is very dense and is deep-colored, presenting a very decided contrast to the soft, straw-colored earlywood.
It is not only the proportion of latewood, but also its quality, that counts. In specimens that show a very large proportion of latewood it may be noticeably more porous and weigh considerably less than the latewood in pieces that contain less latewood. One can judge comparative density, and therefore to some extent strength, by visual inspection.
No satisfactory explanation can as yet be given for the exact mechanisms determining the formation of earlywood and latewood. Several factors may be involved. In conifers, at least, rate of growth alone does not determine the proportion of the two portions of the ring, for in some cases the wood of slow growth is very hard and heavy, while in others the opposite is true.
The quality of the site where the tree grows undoubtedly affects the character of the wood formed, though it is not possible to formulate a rule governing it. In general, however, it may be said that where strength or ease of working is essential, woods of moderate to slow growth should be chosen.
In the case of the ring-porous hardwoods, there seems to exist a pretty definite relation between the rate of growth of timber and its properties.
This may be briefly summed up in the general statement that the more rapid the growth or the wider the rings of growth, the heavier, harder, stronger, and stiffer the wood. This, it must be remembered, applies only to ring-porous woods such as oak, ash, hickory, and others of the same group, and is, of course, subject to some exceptions and limitations.
In ring-porous woods of good growth, it is usually the latewood in which the thick-walled, strength-giving fibers are most abundant. As the breadth of ring diminishes, this latewood is reduced so that very slow growth produces comparatively light, porous wood composed of thin-walled vessels and wood parenchyma.
The latewood of good oak is dark colored and firm, and consists mostly of thick-walled fibers which form one-half or more of the wood. In inferior oak, this latewood is much reduced both in quantity and quality. Such variation is very largely the result of rate of growth.
Wide-ringed wood is often called "second-growth", because the growth of the young timber in open stands after the old trees have been removed is more rapid than in trees in a closed forest, and in the manufacture of articles where strength is an important consideration such "second-growth" hardwood material is preferred.
This is particularly the case in the choice of hickory for handles and spokes.
Introduction - Introduction to the Structure Challenge; Take a piece of 1/8" x 1/8" x 36" long balsa wood Any errors in the construction of your jig will magnify during the construction of your structure because no one is a . 1. Introduction: This report is the first stage of the design, construction and testing of a balsa wood structure. In April, the design will be tested against classmates' designs, where the design with the highest load/weight ratio wins. The Balsa Bridge Competition offers participating teams the opportunity to demonstrate knowledge of structural design principles in the construction of a three-dimensional balsa bridge structure. Students will design and build a bridge structure using specified balsa wood. The structure will be weighed and judged and then loaded until failure.
Here not only strength, but toughness and resilience are important. Forest Service show that:Structure Building Jigs. After a while I created a jig using pieces of cut up wood shelving screwed to pieces of MDF board (Medium Density Fiberboard). Any errors in the construction of your jig will magnify during the construction of your structure because no one is a perfect builder.
1. Introduction: This report is the first stage of the design, construction and testing of a balsa wood structure. In April, the design will be tested against. 1. Introduction: This report is the first stage of the design, construction and testing of a balsa wood structure.
In April, the design will be tested against. Structure. Magnified cross-section of black walnut, showing the vessels, rays and softwoods are not necessarily soft. The well-known balsa (a hardwood) is actually softer than any commercial softwood.
Conversely, some softwoods Wood to be used for construction work is commonly known as lumber in North America. Aircraft Structures Chapter 1 structure development by building a glider with stacked wings incorporating the use of wires as wing supports.
Mosquito, used a balsa wood sandwich material in the construction of the fuselage. [Figure ] The fiberglass. Balsawood Structure Design. 1. Introduction: This report is the first stage of the design, construction and testing of a balsa wood structure.
In April, the design will be tested against classmates' designs, where the design with the highest load/weight ratio wins.