Factors affecting strength

1.7.3.1 Density and specific gravity

The density, or weight per volume, of wood varies considerably between and within wood species. Density of wood is always reported in combination with its moisture content due to the significant influence moisture content has on overall density. To standardize comparisons between species or products, it is common to report the specific gravity, or density relative to that of water, on the basis of oven dry weight of wood and volume at a specified moisture content. In general, the greater the density or specific gravity of wood, the greater its strength, expansion and shrinkage and thermal conductivity.

Density is the best single indicator of the properties of a timber and is a major factor determining its strength. Basic specific gravity of commercial timber ranges from 0.29 to 0.81, most falling between 0.35 and 0.60.

Approximate relationships between various mechanical properties and specific gravity for clear straight-grained wood of hardwood and softwood are given in Table 1.5 as power functions.

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Table 1.5. Mechanical properties as a function of specific gravity for wood (G) at 12%

moisture content (Woodhandbook, 1999).

Wood at 12% moisture content

Property Softwood Hardwood

Static bending

MOR (kPa) 170.7 G^1.01 171.3 G^1.13

MOE (kPa) 20.5 G^0.84 16.5 G^0.7

Compression parallel (kPa) 93.7 G^0.97 76.0 G^0.89 Compression perpendicular (kPa) 16.5 G^1.57 21.6 G^2.09

Shear parallel (kPa) 16.6 G^0.85 21.9 G^1.13

Tension perpendicular (kPa) 6.0 G^1.11 10.1 G^1.30

(*) Compression parallel to grain is maximum crushing strength; compression perpendicular to grain is fibre stress at proportional limit. MOR is modulus of rupture (bending strength); MOE is modulus of elasticity.

The average data vary around the relationships, so that the relationships do not accurately predict individual average species values or an individual specimen value. In fact, mechanical properties within a species tend to be linearly, rather than curvilinearly, related to specific gravity; where data are available for individual species, linear analysis is suggested.

1.7.3.2 Natural defects

Sawn timber in structural dimensions normally contains defects of various kinds, such as knots, slope of grain, shakes and checks, compression wood, decay, bark pockets, wane and resin pockets (Fig. 1.12). The seriousness of these defects is not only a matter of how much each individual defect reduces the strength, but also how often they occur. In particular, knots are regarded as very serious defects as they can greatly reduce strength and are almost always present in large numbers in a piece of timber.

a) b) c) d) Figure 1.12. Structural timber − Natural defects: a) Knot; b) Ring shakes; c) Crack;

d) Fungi decay.

Knots

Knots are formed by the change of wood structure that occurs where limbs grow from the main stem of the tree. The limb, extending approximately radially in the main trunk, has its own annual rings and rays, and this local arrangement of cells interrupts the normal pattern for the main portion of the tree.

Most mechanical properties are lower in sections containing knots than in clear straight-grained wood because: the clear wood is displaced by the knot;

the fibres around the knot are distorted, resulting in cross grain, the discontinuity of wood fibre leads to stress concentrations and checking often occurs around the knots during drying. Hardness and strength in compression perpendicular to the grain are exceptions, where knots may be objectionable stress distributions at contact surfaces.

Knots have a much greater effect on strength in axial tension than in axial short-column compression, and the effects on bending are somewhat less those in axial tension. For this reason, in a simply supported beam, a knot on the lower side (subjected to tensile stresses) has a greater effect on the load the beam will support than does a knot on the upper side (subjected to compressive stresses).

The knot has a weakening effect on stresses, as it is illustrated in Figure 1.13. The grain direction changes severely as the wood fibres pass around the knot (cross grain). In this region, the applied load causes tensile stress components normal to the grain of the knot. Since tensile strength normal to the grain is very low, the knot weakens the member significantly. Tensile strength is affected most by the knot, but compressive strength is reduced also.

Bending strength is reduced too, the amount of reduction depending on where the knot is located on the beam cross section. In drying, wood shrinks more radially than longitudinally, so knots frequently become loose and may even fall out. Then, obviously, they are as damaging to bending, compressive, or shear strength as they are to tensile strength.

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f_t⊥

f_t⊥

P P

P/A Away from knot, average tensile stress

(principal stress) Away from knot, average tensile stress

(principal stress)

Figure 1.13. Structural timber − Natural defects: effect of knot on stresses.

Slope of grain

Grain is the longitudinal direction of the main elements of timber, these main elements being fibres or tracheids, and vessels in the case of hardwoods.

In many instances the angle of the grain in a cut section of timber is not parallel to the longitudinal axis. It is possible that this variation is due to poor cutting of the timber, but more often than not the deviation is due to poor cutting of the timber, but more often than not the deviation in grain angle is due to irregular growth of the tree. This effect is of lesser consequence when timber is axially loaded, but leads to a significant drop in bending resistance.

The angle of the microfibrils within the timber also affects the strength of the timber, as with the effects of the grain, if the angle of deviation increases the strength decreases.

The relationship between grain angle and properties has been empirically described by Hankinson equation:

where N is strength at angle θ from fibre direction, Q strength perpendicular to grain, P strength parallel to grain, and n an empirically determined constant. This formula has been used for modulus of elasticity as well as strength properties. Values of n and associated ratios of Q/P are given in Table 1.6.

Table 1.6. Constants for calculating effect of grain angle on strength of wood.

Property n Q/P

Tensile strength 1.5-2 0.04-0.07 Compression strength 2-2.5 0.03-0.40 Bending strength 1.5-2 0.04-0.10 Modulus of elasticity 2 0.04-0.12

Shakes and checks

Sometimes the generic term splits is used to cover both shakes and checks.

Shakes and checks are longitudinal planes of separation (cracks) in the wood (Fig. 1.14). They may be in the longitudinal/radial plane, in which case they are called heart shakes or rift cracks. When they are in the longitudinal/tangential plane they are called ring shakes, the plane of the break being parallel to the annual rings. Ring shakes are thought to be caused by shear due to wind.

Checks occur as the wood season after the tree is felled. They are longitudinal cracks, usually in the longitudinal/radial plane but occasionally in the longitudinal/tangential plane. They occur because of stresses resulting from differential shrinkage in the tangential and radial directions during drying, or from uneven drying in different portions of the lumber. Thus, checking can be reduced by careful attention to drying procedures during lumber production. The major structural effect of checks is to reduce longitudinal shear strength, often the deciding factor in designing timber beams.

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surface shake

through shake

ring shake

pith shake surface

shake

through shake

ring shake

pith shake

Figure 1.14. Structural timber − Natural defects: type of shake.

1.7.3.3 Moisture content

Many mechanical properties are affected by changes in moisture content below the fibre saturation point (FSP). In fact, most properties increase with decrease in moisture content (Fig. 1.15). In fact, if the adsorbed water softens the cellulose/lignin material of the cell wall, weakening the wood properties, the free water within the cell cavity has no effect whatever, except to increase the weight of the wood members.

Wood is hygroscopic, which means that its moisture content adjusts to reach equilibrium with the temperature and relative humidity of the atmosphere in which it is used. The moisture content that remains constant under a given atmospheric condition is called the “equilibrium moisture content”, sometimes abbreviated EMC. In the present European standards, the

“normal ambient condition” corresponds to 20±2 °C of temperature and 65±5

% of humidity. For softwood elements, this condition implies a moisture content of 12 %.

Figure 1.15. Effect of moisture content on wood strength properties: a) Tension parallel to grain; b) Bending; c) Compression parallel to grain; d) Compression perpendicular to the

grain; e) Tension perpendicular to grain (Woodhandbook, 1999).

Concerning the strength and stiffness properties variations, due to the values of the moisture content, they mainly depend by wood species and by mechanical parameter which is considered. For softwoods, the ratios between the values of these parameters measured in “normal conditions” and beyond the fibre saturation point, can be assumed equal to as follow (Woodhandbook, 1999):

- Modulus of elasticity parallel to the grain: 1.1-1.3;

- Bending strength: 1.4-2.0;

- Compression strength parallel to the grain: 1.7-2.3;

- Shear strength: 1.1-1.7.

This increase in mechanical properties with drying assumes small, clear specimens in a drying process in which no deterioration of the product (degrade) occurs. For 51 mm thick lumber containing knots, the increase in property with decreasing moisture content is dependent upon timber quality.

Straight-grained timber may show increases in properties with decreasing moisture content that approximate those of small, clear specimens. However, as the frequency and size of knots increase, the reduction in strength resulting from the knots begins to negate the increase in property in the clear wood portion of the lumber. Very low quality timber, which has many large knots,

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may be insensitive to changes in moisture content. Figures 1.16a, b show the effect of moisture content on the properties of lumber as a function of initial lumber strength.

Figure 1.16. Effect of moisture content on timber strength properties: a) Tension parallel to grain; b) Compression parallel to grain (Woodhandbook, 1999).

1.7.3.4 Time-dependent behaviour of timber

In this section the time-under-load effects on the mechanical property of wood and structural timber are discussed, focusing on the following aspects:

rate of loading, creep and duration of load.

Rate of loading

Mechanical properties values are usually referred to as static strength values. Static strength tests are typically conducted at a rate of loading or rate of deformation to attain maximum load in about 5 min. Higher values of strength are obtained for wood loaded at a more rapid rate and lower values are obtained at slower rates. For example, the load required to produce failure in a wood member in 1 s is approximately 10% higher than that obtained in a standard static strength test. Over several orders of magnitude of rate of loading, strength is approximately an exponential function of rate.

Figure 1.17 illustrates how strength decrease with time to maximum load.

The variability in the trend shown is based on results from several studies pertaining to bending, compression and shear.

Figure 1.17. Relationship of ultimate stress at short-time loading to that at 5-min loading, based on composite of results from rate-of-load studies on bending, compression and shear

parallel to grain. Variability in reported trends is indicated by width of band (Woodhandbook, 1999).

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Creep

When initially loaded, a timber member deforms elastically. If the load is maintained, additional time-dependent deformation occurs. This is called creep. It occurs at even very low stresses and continues over a period of years.

At typical design level and use environments, after several years the additional deformation caused the initial, instantaneous elastic deformation.

For illustration, a creep curve based on a creep as function of initial deflection at several stress levels is plotted in Figure 1.18. As it is shown, the creep is greater under higher stresses than under lower ones.

Figure 1.18. Influence of four levels of stress on creep (kingston, 1962).

Duration of load

The duration of load or the time during which a load acts on a wood member either continuously or intermittently, is an important factor in determining the load that the member can safely carry. The duration of load may be affected by changes in temperature and relative humidity.

The constant stress that a wood member can sustain is approximately an exponential function of time of failure, as illustrated in Figure 1.19. This relationship is a composite of results on small, clear wood specimens, conducted at constant temperature and relative humidity.

For a member that continuously carries a load for a long period, the load required to produce failure is much less than that determined from the strength properties. Based on Figure 1.19, a wood member under the continuous action of bending stress for 10 years may carry only 60% of the load required to produce failure in the same specimen loaded in a standard bending strength test of only a few minutes duration. Conversely, if the duration of load is very short, the load-carrying capacity may be higher than that determined from strength properties.

Figure 1.19. Relationship between stress due to constant load and time to failure for small clear wood specimens, based on 28 s at 100% stress. Variability in reported trends is indicated

by width of band (Woodhandbook, 1999).

Furthermore, duration of load tests are commonly performed under constant load acting until failure occurs, or alternatively with some type of ramp loading applied at different rates. For a timber structure in practice, a significant part of the load will vary in a random manner over the life-time of the structure. It is still an open question as to how results from duration of load tests should be interpreted and used for arbitrary time variations of loads.

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