Fonte: Roberto Leon, Dipartimento di Ingegneria Civile e Ambientale, Virginia Tech, Blacksburg, VA
Il legno è un materiale onnipresente che è stato utilizzato nella costruzione fin dai primi tempi. Il legno è un materiale rinnovabile, sostenibile e di grande valore estetico. Oggi, ci sono probabilmente più edifici costruiti con il legno di qualsiasi altro materiale strutturale. Molti di questi edifici sono residenze unifamiliari, ma molti condomini più grandi, così come edifici commerciali e industriali, utilizzano anche cornici in legno.
L’uso diffuso del legno nell’edilizia ha appeal sia da un punto di fatto economico che estetico. La capacità di costruire edifici in legno con una quantità minima di attrezzature ha mantenuto il costo degli edifici in legno competitivo con altri tipi di costruzione. D’altra parte, dove le considerazioni architettoniche sono importanti, la bellezza e il calore del legno a vista è difficile da abbinare ad altri materiali.
Gli obiettivi di questo esperimento sono di condurre prove di trazione e compressione su tre tipi di legno per studiare il loro comportamento stress-deformazione e di condurre un test di flessione a quattro punti su una trave di legno per accertarne le prestazioni di flessione. In una prova di flessione a quattro punti, una trave semplicemente supportata viene caricata con due carichi punti uguali nei suoi terzi punti, risultando in una porzione centrale con momento costante e zero taglio. Questo è un test importante perché gli elementi strutturali in legno sono spesso utilizzati nei sistemi a pavimento e sono quindi caricati principalmente da sollecitazioni di flessione.
The compression, tension, and bending test results are summarized in Table 1. As shown consistently by all results, oak is the strongest wood, followed by spruce and southern pine.
Table 1: Wood testing summary
| Compression Parallel (psi) | Compression Perpendicular (psi) | Tension Parallel (psi) | Tension Perpendicular (psi) | Bending (psi) | |
| Oak | 7382 | 2045 | 4780 | 547 | 8902 |
| Spruce | 6342 | 1534 | 3451 | 412 | 7834 |
| Southern pine | 5437 | 1254 | 2756 | 327 | 7423 |
Table 2: Normalized data
| Compression Parallel (psi) | Compression Perpendicular (psi) | Tension Parallel (psi) | Tension Perpendicular (psi) | Bending (psi) | |
| Oak | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 |
| Spruce | 0.86 | 0.75 | 0.72 | 0.75 | 0.88 |
| Southern pine | 0.74 | 0.61 | 0.58 | 0.60 | 0.83 |
Table 2 presents the same data as in Table 1 but normalized to the strength of the oak material. For the two most important properties, bending strength and compression parallel to the grain, the spruce seems to be roughly about 87% and the southern pine roughly 78% as strong as the oak. Given the very large price differential between woods, it would appear that southern pine, as the cheapest of them, is a very efficient choice.
Wood is a sustainable, natural material that exhibits orthotropic properties. In other labs, materials such as metals, polymers, and concrete have been tested in tension or compression with the assumption that the material acts isotropically, meaning that its resistance to a particular load is the same regardless of the orientation of the material. Steel, for example, has a myriad of randomly oriented grains at the micro scale, giving it homogenous and isotropic properties at the macro scale. However, wood, with its easily identifiable grain direction, does not act isotropically. Thus, a designer must carefully consider the anticipated loadings on a wood member or structure to ensure maximum effectiveness of the material. Additionally, due to its natural origin, wood has mechanical properties tied to the individual species of tree, the moisture content, and the size of the test specimen.
Until recently, wood structures were limited to three or four stories in an apartment or small office building. Developments of cross-laminated timber, wood panels consisting of layers oriented at right angles to one another and then glued, have resulted in the development of structural systems capable of reaching 8 or more stories. Much taller buildings, in the order of 20 stories, are still under development.
Wood is a ubiquitous material that has been used in construction from the earliest times. Renewable and sustainable, wood is a structural material widely used in engineering for construction of single-family residential buildings and also for framing partitions and other nonstructural elements in commercial, and industrial buildings.
Due to its natural origin, wood has mechanical properties tied to the individual species of tree. The moisture content and other variables, for example, the presence of defects. For a specific application, a designer must carefully consider the anticipated loadings on a wood member or structure in order to ensure maximum effectiveness of the material.
This video will illustrate how to test the mechanical properties of different types of wood and determine their stress-strain behavior and flectural performance.
Wood is composed of elongated, round, or rectangular tube-like cells that are much longer than they are wide. Within in the wall there are several layers made out of microfibrils, which are bundles of cellulose polymers.
Microfibril chains are aligned in distinct directions inside the walls’ layers. The middle wall with its chains aligned along the longer dimension of the cell provides most of the strength to the cell, while the inner and outer walls’ diagonal chains provide stability. Lignin binds together the cellulose polymers as well as the microfibril chains and the walls’ cells. The bundling effect of many cells together results in a very strong construction material.
Wood is a biological material, and, thus, is very susceptible to environmental decay and pest attack. Much of the wood used today is pretreated with chemicals to protect it from the environment and insect attack.
Wood is an inhomogeneous material characterized by a large number of imperfections or defects such as, for example, knots and splits. In consequence, large factors of safety or ratios of design strength to actual ultimate strength are used to account for large variation in the engineering properties of different wood pieces.
Due to its cellular makeup, wood is an orthotropic material, having different properties along the longitudinal and, respectively, the transversal axes with respect to the grain direction. In consequence, the material will behave differently to loads parallel or perpendicular to the wood fiber. The orthotropic properties of wood can be ameliorated by different methods.
Laminates such as plywood are made of layers with fibers aligned in perpendicular directions, resulting in an isotropic material. Alternatively, glulam is made of thin strips of fibers aligned in the same direction and glued under pressure, deriving its strength from distributing defects.
The cellular makeup of wood also accounts for the free water inside the cell cavities and water bound to the cell walls. In consequence, moisture content is a key parameter in determining wood strength, and, in general, moisture reduction will result in an increase in strength. Volumetric changes associated with drying may result in nonuniform shrinkage and distortion such as twist, bow, cup, or crook.
As wood is a polymeric material, it is also prone to creep or, under constant load, to continuous viscous-like deformation. When the load is released, most of the deformation is recovered. As a result, wood can generally support much higher stresses if the duration of loading is short. Since all these factors would be too complex for use in everyday design, for structural purposes we use the following: a statistical analysis of the ultimate defect-free strength values for many species, corrections for moisture content, and strength ratios based on wood grade to correct for strength-reducing effects.
The properties commonly given for most woods are published in a tabular form for easy reference. These properties are: allowable bending stress, tension parallel to grain, horizontal shear, compression perpendicular to grain, compression parallel to grain, and the modulus of elasticity. In addition to the basic orientation-specific properties of the species of wood, it should be evident that not all wood behaves the same way under load.
Now that you understand the physical properties of wood and the principles of wood testing, let’s use these to perform a few tests.
Before you begin, choose three varieties of wood to compare. For each variety, prepare two compression cube specimens with nominal edge dimensions of 3.5 inches. Ensure that the cubes are free of defects, and their opposite surfaces are parallel. Mark one specimen from each variety for testing with a load applied parallel to the grain, and the remaining specimens for testing with a load applied perpendicular to the grain.
Measure the height of the direction of loading of each test specimen using a caliper. And repeat the measurement in a few locations to determine the approximate average. When you are finished, use the same procedure to determine the cross-sectional dimensions of each specimen.
Set up the universal testing machine as shown in the JoVE video regarding material constants. Then, carefully center a specimen in the correct orientation on the compression platen. Lower the crosshead until a slight load is applied and then use the fine controls to back the load off to as close to zero as possible.
Now apply the compressive load at a loading rate of 40 psi per second. The compression test may continue for several minutes as the load increases and with significant visible strain in the specimen. Allow the test to continue until an obvious maximum load is reached.
Record the maximum load when the test is finished, and then repeat the procedure for the remaining specimens.
Perform another compression test, and, this time, apply the load perpendicular to the grain of the specimen. Repeat the procedure for the other varieties of wood.
Now prepare some dogbone specimens using the same three wood varieties. Prepare one set of specimens with the grain parallel to the long dimension, and a second set with the grain perpendicular to the long dimension.
Perform tension tests on all six specimens as shown in the JoVE video regarding stress-strain characteristics of steel.
Obtain a two-by-four about 24 inches long of each wood variety. Install the four-point bending test apparatus on the universal testing machine. Once the apparatus is ready, start the testing machine. Adjust the test settings to record the loads and crosshead values, and capture the maximum load. Install the specimen in the apparatus and drop the upper crosshead until the apparatus just begins to make contact with the wood beam.
Apply the load at a rate of 2,000 pounds per minute until the beam fractures. Record the failure load when the test is complete, and then repeat the test for the remaining specimens.
Use a table to summarize the results for the compression, tension, and bending tests. Next, in each column, normalize the data to the maximum value, and make a new table.
Now, take a look at your results. As shown consistently by all results, oak is the strongest wood, followed by spruce and southern pine. For the two most important properties, bending strength and compression parallel to the grain, the spruce seems to be roughly about 87% and the southern pine roughly 78%, as strong as the oak. Given the very large price differential between the woods, southern pine as the cheapest of them is a very efficient choice.
Wood testing is of paramount importance in structural engineering for assessing the capability of final designs to handle stress and strain during routine use in order to ensure product safety and compliance with international standards.
In a four-point bending test, a simply-supported beam is loaded with two equal-point loads at its third point, resulting in a central portion with constant moment and zero shear. This test is critical for floor systems where the wood’s structural elements are primarily loaded by bending stresses.
Until recently, wood structures were limited to three or four stories in an apartment or small office building. Developments of cross-laminated timber have resulted in the development of structural systems capable of reaching eight or more stories. While much taller buildings, in the order of 20 stories, are still under development.
You’ve just watched JoVE’s Introduction to Wood Testing. You should now understand the engineering properties of wood, and how to perform tensile, compressive, and bending tests on wood specimens.
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