Strength Grading Of Timber

The strength capability of timber is difficult to assess as often there is no control over its quality and growth. The strength of timber is a function of several parameters including the species type, density, size and form of members, moisture content, duration of the applied load and presence of various strength reducing characteristics such as slope of grain, knots, fissures and wane. To overcome this difficulty, the strength grading method of strength classification has been devised. Several design properties are associated with a strength grade; these include modulus of elasticity and bending strength parallel to the grain, strength properties in tension and compression parallel and perpendicular to the grain, shear strength parallel to the grain and density. The design properties of timber are determined non-destructively through visual strength grading criteria or by machine strength grading via measurements such as the following: flatwise bending stiffness, using a three-point loading system; density, using x-rays or gamma rays techniques; and modulus of elasticity, by means of resonant vibrations (dynamic response) using one or a combination of these methods.

The requirements for strength grading of timber are detailed in the following standards:

Most European Union countries have their own long-established visual grading rules and as such guidance for visual strength grading of softwoods and hardwoods is provided in the following British Standards:

1.5.1 Visual grading

Visual grading is a manual process carried out by an approved grader. The grader examines each piece of timber to check the size and frequency of specific physical characteristics or defects, e.g. knots, slope of grains, rate of growth, wane, resin pockets and distortion.

The required specifications are given in BS 4978 and BS 5756 to determine if a piece of timber is accepted into one of the two visual stress grades or rejected. These are general structural (GS) and special structural (SS) grades. Table 2 of BS 5268-2:2002 [10] (reproduced here as Table 1.2) refers to main softwood combinations of species (available in the United Kingdom) visually graded in accordance with BS 4978:1996 [8].

1.5.2 Machine grading

Machine grading of timber sections is carried out on the principle that stiffness is related to strength; where the relationship between the modulus of elasticity, E, and the modulus of rupture of a species of timber from a certain geographical location is determined from a statistical population, based on a substantial number of laboratory controlled tests. There are a number of ways for determining the modulus of elasticity, including resonant vibration (dynamic response), but the most common methods are either load- or deflection-controlled bending tests. The machine exerts pressure and bending is induced at increments, along the timber length. The resulting deflection (or

Table 1.2 Softwood combinations of species and visual grades that satisfy the requirements for various strength classes*

Timber species

Grade and related strength classes

British grown timber

0 0

Post a comment