Traditional methods of grading

In document An investigation into the strength properties of reclaimed timber joists (Page 47-51)


3.1 Traditional methods of grading

The traditional method of timber strength grading, formerly known as stress grading, is by visual inspection by individually licensed operatives. Every grade mark contains a unique identifying number of the grader, so total traceability of each piece of graded timber is ensured, thus ensuring timber quality and customer confidence

3.1.1 Visual grading

Before the advent of BS 5268 (BSI, 2008), or its originator CP 112 (BSI, 1952), the only testing regime for timber was by visual grading. This generally led to an over specification of timber joists, used in many older buildings, for the load to be carried. Timber for reuse in structures must be graded, visually or mechanically, just as new timber is graded. The graded timber is then assumed to have characteristic values of strength, stiffness and density. BS EN 14081 (BSI, 2011) specifies a method of strength grading softwood visually for structural use, specifying two visual strength grades - General Structural (GS) and Special Structural (SS). However, the standard gives only the minimum requirements for visual strength grading, so the following characteristics should be taken into account when carrying out this operation:

 limitations for strength reducing characteristics: knots, slope of grain, density or rate of growth

 limitations for geometric characteristics: wane, distortion (bow, spring, twist)  limitations for biological characteristics: fungal and insect damage

 other characteristics, such as mechanical damage.

In order to determine these characteristics, all four faces of each piece of timber must be examined; however, simple economics do not allow for a through examination (in sawmills a piece of timber is machine graded in two to four seconds). Considering these negative points, TRADA (1995) states that there are advantages in visual strength grading of timber and that these are:

 it is simple, easily understood and does not require great technical skill  it does not require expensive equipment

The first phase of this research was to produce an alternative method of visual grading that would still satisfy BS EN 14081 (BSI, 2011); this would serve to weed out

inferior reclaimed timbers from those which would undergo further machine testing. To facilitate this, the existing visual grading process had to be investigated, and additions made to take into consideration any extra physical damage through ‘service life wear and tear’ that older timbers often demonstrate. Visual examination of new timbers under the standard investigates characteristics such as;

 Knots  Slope of grain  Wane  Fissures  Resin/bark pockets  Distortion  Rate of growth

The alternative test proposed by this research also considers;  Damage - caused by insects, fungus, machinery or fittings  Chemical treatments

 Density

 Condition – wet/dry

 Recovered size – cross sectional area and length

Visually strength graded timber, whether new or reclaimed, must have a minimum cross-sectional area of 2000 mm2 and a minimum thickness of 20 mm. However, If regrading is carried out before processing, provided that the processing reduction from the target size is not greater than 3mm for dimensions less than or equal to 100 mm, or not greater than 5 mm for dimensions greater than 100 mm, the visual grade will not change and it will still conform to BS EN 14081 (BSI, 2011).

3.1.2 Static bending or ‘small clear’ tests

The static bending test utilised in this study is carried out by the central loading method, as detailed in BS 373 (BSI, 1957). The dimensions of the central loading test pieces were 2 cm by 2 cm by 30 cm, detailed in the 2 cm standard test.

The specimens should be air dried to constant mass at 12% moisture content, prior to being tested. In the central loading method the distance between the points of support of the test piece are 28 cm, and the load applied to the central point of the test piece, illustrated in Figure 3.1.

Figure 3.1. Central loading for 2cm standard test piece. After BS 373 (BSI, 1957)

The loading head must move as near as possible at a constant speed of 0.11 mm/s, and contour of the head, which is in contact with the joist, must have a radiused form with a 30 mm radius. The deflection of the test piece, at mid length, is measured with reference to the outer supported points, until the test piece breaks completely, or is fractured and unable to hold 60% of the greatest load placed upon it during the test.

Immediately after each mechanical test has been completed, a determination of the absolute moisture content of the test piece must be made. A section should be removed from the test piece; this must be a transverse section from near the point of fracture. The specimen must be weighed and then dried in an oven at a temperature of 103 ± 2 °C until the weight is constant; this may take several hours, dependent upon the moisture content of the test piece. The loss in weight, expressed as a percentage of the final oven-dry weight, is noted down as the moisture content of the test piece.

3.1.3 Indication of strength within the joist

While three-point loading tests, on a complete recovered timber joist, offer an overall strength grade, small clear test results can offer strength grading of a certain area of the cross section of the joist. Utilising this method to illustrate the weak areas of reclaimed timber joists was considered early during the research period, when the idea

of machining out damaged areas was investigated with regard to the viability of recovering heavily damaged timber joists.

The small clear tests offer a method of directly observing the modulus of rupture; hence, being able to calculate the expected, actual strength of the timber joist. However, a problem with this method is that it because of the different stresses sustained during the working life of the timber dissimilar results in the small clear tests occur when these are taken from different areas of the cross section or along the length of the timber joist. This being expected, taking the lowest result from small clear results for a single timber should yield a least possible result for the modulus of rupture in the joist. This is considered further in the research analysis (Section 5).

In document An investigation into the strength properties of reclaimed timber joists (Page 47-51)