2 LITERATURE REVIEW
2.8 Timber strength grading using visual and simple methods
Several pieces of research have analysed the behaviour of wood under stress to determine its strength. In these analyses, the influences of physical parameters, such as moisture content and density, on the modulus of elasticity were considered. The most significant of these is the work of Dinwoodie (2000), which offers a history of timber stress grading, highlighting how this has changed from visual, to a mix of visual and machine grading, to its standing now of purely machine grading. The work also highlights how small clear tests were originally used in conjunction with visual grading of timber joists, and the relationship in results between small clear tests and stress testing of full sized timbers.
Knots in the timber structure are associated with distortion of the grain and since even slight deviations in grain angle reduce the strength of timber appreciably it follows that knots will have a marked influence on strength (Mitsuhashi et al. 2008). The significance of knots, in relation to the overall strength of the timber joist, will depend on their size and distribution both along the length and in cross-section. Knots in clusters are more important than knots of a similar size which are evenly distributed, and knots on the top or bottom edge of a joist are more significant than those in the centre; furthermore, large knots are much more critical than small knots.
Research by Mitsuhashi et al. (2008) also introduced a new method of describing this parameter, the area reduction factor (ARF), which considers the effect of knots on the tension strength of timber. ARF considers both the projected area of knots and the effect of the slope of grains around the knots. ARF was determined as the minimum value obtained when a knot measurement window of 100 mm was slid along the plank.
Dinwoodie’s (2000) work illustrates that density is a function of cell wall thickness and therefore dependent on the relative proportions of cell components and the level of cell wall development. However, variation in density can occur within the same species and even within a single tree. In general, as density increases so the various
strength properties increase, and thus, density remains the best traditional predictor of timber strength; high correlations between strength and density are common
observations in timber strength studies. Dinwoodie (2000) states that: ‘In most of the timbers used commercially the range of the relationship between density and strength can safely be assumed to be linear’.
Accepted literature contains many analyses of moisture content significantly affecting the elastic constants of timber. There are also several studies; Gerhards (1982), and Lenth and Sargent (2004), that show an increase in temperature and humidity results in a decrease in the elastic constants of timber.
Since timber density is influenced by the rate of growth of the tree, it should follow that variations in tree ring width and frequency will affect changes in the density of the timber, and hence, it’s strength. In softwoods, increasing rate of growth results in a lower frequency of growth rings and an increased percentage of low-density early wood; consequently both density and strength appear to decrease as ring width increases.
Exceptionally, it is also found that in certain cases very narrow rings can also have very low density, though this is only characteristic of softwoods from very northern latitudes where latewood development is restricted by the short summer period. Latewood is more dense than wood that is formed early in the season, but the mechanics of tree growth mean that latewood is only formed during the later part of the summer season; hence the greater the proportion of latewood, the greater the density and strength.
The frequency of growth rings can also affect timber strength; this is especially prevalent when considered in conjunction with timber density. This fact is not commonly presented in the literature; however, it can significantly interfere with the values of the modulus of elasticity and rupture. In their study of the parameters that influence redwood crush, Cramer, Hermanson and McMurtry (1996) noted that the number of growth rings per inch may predict the crush behaviour more efficiently than the density alone. This validated earlier findings regarding the changes in elastic modulus and strength within an annual ring as, for example Bodig and Jayne (1982)
discussed, suggesting a variation in the tensile strength of wood within the location of the growth rings.
2.8.1 Small clear samples
Basic stress is that level of loading which can be permanently sustained with safety by an ideal structural component. In the derivation of basic stresses from the tests on the small clear samples consideration was given to both the variability in the strength figures for clear timber and the need to ensure that the imposed load was a safe one for that particular set of conditions.
Timber has been described as a variable material and a measure of this variability was shown in how the frequency distribution of a set of test results approximates closely to a normal distribution curve which can be used to calculate the value below which a certain percentage of the results will not fall.
The apparent strength of timber is influenced by rate of loading, specimen size and shape, and duration of loading. So, rather than apply a series of factors, a single factor derived mainly from experience has been used. Dinwoodie (2000) states that:
‘Generally a value of 2.25 was used for most properties, excepting compression parallel to the grain where it was 1.4. These factors are in effect a safety factor for pieces of minimum strength, and also cover the possibility of slight overloading’. In the static small clear bend test a specimen is supported and a load-deflection diagram plotted. Three strength properties are usually determined from this test. The first and most important is the modulus of rupture (MOR), which is a measure of the ultimate bending strength of timber for that size of sample and that rate of loading. This is actually the stress in the extreme fibres of the specimen at the point of failure. The second strength parameter is work to maximum load, which is a measure of the energy expended in failure and is determined from the area under the load-deflection curve up to the point of maximum load. The third parameter, total work, is the area under the load-deflection curve, and is taken to complete failure.
Before the rise of machine grading the basic stress, therefore, for each property was obtained primarily from the results of the standard tests on small clear specimens by
dividing the statistical minimum by the appropriate safety factor (2.25 or 1.4). Most structural timbers have some defects so it was necessary to apply a factor known as the strength ratio to the basic stress in order to obtain safe operating conditions for these joists.