Chapter 3 Laotian wood-characteristics and its applications 11
3.3 Wood applications and the purpose of strength tests 13
Wood is a most versatile material, it has been used for many purposes, such as lumber, beams, doors, windows, light framing, siding, exterior finish, interior finish, frames, roofing, flooring, sub-flooring, parquet, furniture, fencing, packaging. The principal woods used in this study and their cost are shown in Table 3.1.
Softwoods are commonly used for construction, because they are cheaper and easier to work with. Some hardwoods are used for non-construction products such as furniture, parquet, doors, windows, because they have good appearance, are durable but are more expensive than softwoods.
At the present time, which wood is used as constructional or non-constructional materials is dependent on a customer’s or user’s familiarity with wood and cost in Laos.
The price of wood is usually not considered together with the strength and weight. This present study may help architects and engineers when designing a structure and choosing wood and wood qualities such as strength, stability, and availability in required sizes.
Therefore, the mechanical properties of wood become very significant factors for wood applications, hence the purposes of strength tests [16] [17] [18] [19] which are: tension
compression perpendicular to grain, shear parallel to grain, hardness test, static bending test, specific, and density.
Tension parallel to grain test:
Tensile strength is particularly necessary for woods that are to be bent into curved shapes after steaming. For defect-free wood, tensile strength parallel to the grain is higher than compressive strength in the same direction. Clear, straight-grained used has the highest strength. This test can also be used for determining modulus of elasticity.
Tension perpendicular to grain test:
This test measured the resistance of wood to forces acting across the grain that tend to split a member. This property is useful for wood to be used for purposes such as walls (lumber), boxes, and packing cases. They should be able to take nails without splitting. In this test, the specimens usually fail in splitting, which is similar to the cleavage test.
Compression parallel to grain test:
High strength in this direction is required of wood used as columns, props, chair legs, ie. structures that have to bear loads imposed on them parallel to the grain of the wood. The long length of specimens in relation to their cross-sectional areas may cause buckling at high stresses and failure is from bending rather than true compression.
Compression perpendicular to grain test:
Resistance to compression is an important property for wood used for example, as railway sleepers, rollers, wedges, bearing blocks and bolted wood. These woods are high in density and have high compression strength across the grain.
Shear parallel to grain test:
This test indicates the ability to resist internal slipping of one part upon another along the grain. This is the most important property for wood used in joints. The area surrounding the bolt hole or the mortise should be able to resist high longitudinal shear stresses. Shear strength parallel to grain is less than in any other direction. Failure of specimens in this test is sudden and only the ultimate load is observed.
Hardness test:
Hardness is an important property for various uses, such as floors, decking, mallets, rollers, bearing blocks, furniture, sport items, etc. Some woods are relatively soft, others have a medium hardness and some are hard. Hardness is related to the strength of wood in resisting abrasion and scratching with various objects.
Static bending test:
Bending strength is important in wood used for floor, ceiling joists, roof truss members, table tops and chair bottoms which must resist the high bending loads. Also, static bending is a measure of the strength of a material as a beam. In the resting position, the upper half of a beam is in compression and the lower half in tension. Stiffness (modulus of elasticity) is the ability of a material to resist bending. It is an important property in joists and beams involving high bending stresses, otherwise the ceiling beneath them would crack if the floor above flexed too much under load.
Specific gravity provides the relative weight of wood compared to an equal volume of water. For many engineering applications, the basis for specific gravity is generally the oven dry weight and volume at a 12% moisture content (MC). Specific gravity is used as a standard basis to compare species. A larger number indicates a heavier material.
Density is the weight of wood per cubic cm at a specified MC. Density is important to indicate strength in wood and may predict certain characteristics such as hardness, ease of machining and nailing resistance. A larger number indicates a stronger wood.
The purpose of the tests as mentioned above is related to the strength properties of wood which differ greatly according to the direction of the grain in relation the stress
applied. Wood has different strengths in the longitudinal grain or transverse grain and therefore, the differences in the two directions of the grain wood are dependent on how it is to be used. In this research, the testing of seven types of wood was performed as for the same purposes for wood application as mentioned above.
For hardwoods, eg May Khen Hine is widely used to make tables, chairs, windows and door frames and is usually cut into the different sizes of beams (see Table 3.1). Others such as May Deng, May Tai, and May Dou are most popular for making parquet flooring and furniture because they have fine-colored grain, as well as being hard. Therefore, hardness and compression perpendicular to grain test are more suitable means of testing for them; whereas other tests are less important. Wood for making parquet floors and furniture must have hard surfaces which can resist the applied load without scratching and cracking. Moisture content is an important factor in considering the wood that a member can safely use in flooring without warping and shrinkage. Normally, wood that is to be used for this purpose must have a moisture content of approximately 12%.
Softwoods, eg May Nhang, May Khen Heua ,and May Khe Foy are commonly used to make beams and lumber for use in construction and buildings. These woods are sold in many shops in Vientiane in different sizes (as shown in Table 3.1). For them, the static bending test, shear test, compression parallel to grain test are the most important tests.
Table 3.1 Applications and cost of Laos wood (December, 2002)
Cost
No Species Applications Price
US/m3
Price in length 40 x 80mm, 50x100mm 60 x 120 mm 1 May Deng Interior of the house, window
and door, furniture, parquet 400-600 1.9 – 2.80 US/LPm 2 May Tai Interior of the house, window
and door, furniture, parquet
400-600 1.9 – 2.80 US/LPm 3 May Dou Interior of the house, window
and door, furniture, parquet, framing, tool handles
400-600 1.9 – 2.80 US/LPm
4 May Nhang Beam with different sizes Lumber, table, chair etc
100-200 0.52 – 0.63US/LPm 5 May Khen Heua Beam with different sizes
Lumber, table, chair etc
100-200 0.63 – 0.72 US/LPm
6 May Khen Hine Beam with different sizes Lumber, table, chair, framing
150-300 0.63 – 0.72 US/LPm
7 May Khe foy Beam with different sizes Lumber, table, chair etc
150-300 0.63 – 0.72 US/LPm
Data from five sawmills: Ban Mai km47, Ban Elay km21, Ban Sikai No1, Ban Sikai No2, and Ban Dongsavath. 40x80mm, 50x100mm, and 60x120mm is a size of cross section.
LPm = Length per meter (Length of beam is 2-6 meters).
Figure 3.1 The hardwood tree (from reference [11]).
Figure 3.2 The softwood tree (from reference [11]).
Figure 3.3 Schematic diagram of softwood, illustrating the relative appearance of tracheids (from reference [16]).
Figure 3.4 Schematic diagram of hardwood, illustrating the relative appearance of vessels and tracheids (vascular cells) (from reference [16]).
Figure 3.6 Diagrammatic illustration of the principal structural features (from reference [16]).
Figure 3.5 Cross-section of a tree trunk: A = outer bark (dry dead tissue), B = inner bark (living tissue), C = cambium, D = sapwood, E = heartwood, F = pith, G = wood rays (from reference [12]).
pith