W. M.G. Burdzik

Grade verification of SA pine — bending, modulus of rupture, modulus of elasticity, tension and compression

Synopsis The last verification of SA pine timber strengths was done in the early 1980s. Uses of the pine logs and the rotation periods have changed. This investigation was undertaken to verify the visual grade 5 grading rules of SANS 1783-2 (1997) on 36 x 111 mm specimen sizes. Qualified timber graders identified forestry areas with low-density SA pine. Four sawmills were randomly chosen as being representative of the forestry in those areas. Bending, tension and compression tests were done in accordance with the ISO standard test methods. It was found that only one of the sawmills came close to making the grade strength. The combined strength from all the sawmills indicated that these would only make a grade 4. It is thus suggested that sawmills that do only visual grading, in accordance with SANS 1783-2 (1997), draw random specimens that can be tested in-house for strength or by an independent testing laboratory to ensure that their timber meets the requirements of a grade 5.

The Effect of Timber Density and Type of Adhesive on the Strength of Finger-joints in S.A. Pine and Eucalyptus grandis

SYNOPSIS It is universally accepted that a finger-joint is a strength reducing feature in timber members that are subjected to tensile stresses. This weakening effect is most pronounced in high density timber such as high grade S.A. Pine and the Eucalyptus species. This study compares the strength of finger-joints manufactured with phenol resorcinol formaldehyde (p.r.f.) to those manufactured with two different epoxy mixtures. The epoxy resins had to provide a stronger finger-joint without sacrificing any of the positive qualities of p.r.f. such as stiffness, durability and minimal creep. All the test specimens were manufactured according to the glue manufacturers instruction sheet on a normal production line as it was felt that test specimens manufactured under laboratory conditions would give a false impression of the strengths that could be expected from commercially prepared finger-joints.

Finger Joint Strength. A Laminated Beam Strength Predictor

SYNOPSIS The new South African Timber Design Code, SABS 0163, no longer specifies strength values for laminated products in South Africa. The onus now rests on the manufacturers to determine the strength of their laminated beams. This could lead to unnecessary costs, if every possible size of member has to be either tested or proof loaded. Proof loading at the incorrect level could lead to a large number of members being broken unnecessarily. In this paper the author illustrates that the finger joint strength is one of a number of factors that influences the strength of a laminated beam. These factors are explained and values for them are given. The author describes how, based on the strength of the finger joints, the strength of any possible beam size can be predicted. The author compares theoretical strengths with some test values and shows that there is good correlation between theoretical and test values. This method will help the manufacturers design their members correctly for strength. Costs can be...

Round-end cut truss webs — the South African Omni-web® experience

Abstract This paper discusses the concept of round-end cut webs for timber trusses. Patented punched-nail plates or punched-steel gusset plates that are nailed by means of individual nails may be used to assemble trusses with round-end webs. Full-scale truss test results are presented for both methods of fixing. Ways of improving the performance of nailed gusset plates are also suggested. The author points out that this method of truss assembly has advantages for the owner–builder and the small rural builder. In underdeveloped (Third World) countries, this method is particularly suitable due to its job-creation possibilities, which are referred to as the reconstruction and development programme (RDP) in South Africa.

Torsional Grading of Eucalyptus grandis Sawn Timber

SYNOPSIS The correlation between the local modulus of elasticity and the strength has been used to grade South African pine for some time. This method does not work all that well when grading Eucalyptus grandis sawn timber. This paper describes a method, using torsional stiffness changes along the length, which was used in an attempt to establish a better method for grading this material. Boards were placed under a torsional moment and the angle of twist was measured every seven millimetres along the length. The twist was correlated with the tensile strength of the board. From the test results it is apparent that there is very little correlation between torsional stiffness and tensile strength and that the amount of brittle wood in the board plays a far greater role than the presence of knots or even finger joints. Unfortunately, brittle wood does not seem to affect either torsional stiffness or modulus of elasticity to any noticeable degree.

Number of Strength Reducing Features—How does it Affect the Strength of Laminated Beams?

SYNOPSIS Test done by the author and others have shown that the strength of laminated beams is usually governed by the strength of the finger joints in the extreme tension laminate. The author illustrates how the influence of the number of finger joints in a laminate can be quantified. He also points out that the variability of the finger joint strength can have a big influence on the strength, especially at the lower end of the strength distribution.

Stiffness and buckling strength of SA pine compression members

SYNOPSIS In this paper, the author investigates the buckling strength of S A pine compression members. Serviceability and strength failures of compression members in trussed roofs lead the author to believe that both the stiffness and the strength were being over-estimated. The actual compression strength, of slender members in buckling, is compared to the Euler buckling strength, the stress values and the modulus of elasticity given in SABS 0163-2:1994. Different buckling modes were found and the reason for these is explained. A theoretical method is shown to explain the load-deflection curves for members with initial curvature. This method can be used to determine theoretical buckling strengths. Although the average modulus of elasticity was found to be higher than the values given in SABS 0163-2:1994, the fifth percentile value was lower than the SABS 0163 values. This has a direct impact on the strength of slender compression members. The author suggests that SABS 0163-1 and SABS 0163-2 reflect the lower fifth percentile modulus of elasticity so that the compression strength of slender members may be calculated correctly.

The Applicability of the South African Visual Grading Rules to Malawian Pinus patula Timber

SYNOPSIS This paper investigates whether the new South African visual grading rules can be used to grade Malawian Pinus patula. Malawian Pinus patula was visually graded to the revised South African softwood visual grading rules for V5, V7 and V10. Three different sizes of timber, namely 50 x 75,50 x 100 and 50 x 150 mm were sampled. The visually graded timber was examined for correct grading by an inspector from the South African Bureau of Standards. All test specimens were trucked to the University of Pretoria where they were tested for bending, compression and tension strength. The results of the investigation are presented and discussed. Malawian Pinus patula appears to be superior to South African pine when visually graded to the South African visual grading rules. Although the number of specimens was limited, the authors feel that it would be safe to adopt the South African rules even though they penalise the Malawian pine.

S A Pine—Load Sharing. Fact or Fiction?

SYNOPSIS Load sharing amongst timber members, that are restrained to the same deflection, has been accepted as a given fact for some time. This paper discusses a simulation method that can be used to determine whether load sharing does in fact exist when S A Pine members are used. Results of tests as well as simulations are presented and discussed. It is concluded that load sharing will not occur when the material behaviour is elastic-brittle. Some form of ductility is required for load sharing to take place.

Derivation of Grade Stresses for South African Pine Plywood

SYNOPSIS Very little use has been made of plywood in engineered timber structures in South Africa. One of the reasons could be that little or no information was available regarding the mechanical properties of the currently produced S.A. pine plywood. This paper compares theoretical values to test values for the bending strength, modulus of inertia and modulus of elasticity. The derivation of the strength values, as published in the addendum to the South African timber design code SABS0163, is discussed. The shortcomings of these values are pointed out. Methods are described that could be used to design plywood for specific strength properties.