Strength, lumber

Because modulus of elasticity, E, is a less than perfect predictor of strength, lumber sorted solely by average E falls into one of four accept-reject categories (Figure 10.16a) ... 

Fig. 3. Effect of moisture content on bending strength of stmctural lumber, MOR = modulus of mpture.

Fig. 10. Permanent effect of heating in air on bending strength of spmce—pine—fir lumber.

Tiemann, H.D. (1915). The effect of different methods of drying on the strength of wood. Lumber World Review, 28(7), 19-20. 

Many examples of bonded wood constructions have been known from ancient times for their usefulness in the building of boats and ships. However, the important technology of combining small pieces of wood to form large timber sections, with the strength and durability of intact lumber, is certainly a more recent product of the last forty years. The early stages of this modem development have been reviewed by Selbo and Knauss (1). 

From about 1930 to the present, casein glues have been used successfully for bonding high-strength softwood lumber into glued laminated beams and arches for interior or covered exterior service. Even earlier, until perhaps 1900, casein... 

At the time the end-joint test unit was being built, the industry used both horizontal and vertical end-joint profiles. This question/dilemma was eliminated by designing the unit for a lumber cross-section of 1.5 x 1.5 inches. Restriction to this dimension was also necessary to keep the power requirements within reasonable limits. At present, the unit is limited to approximately 3,000 psi in tension, at which point the hydraulically-driven grips begin to slip. The unit thus allows one to determine the strength development in the joint during the... 

Large knots drastically reduce strength and are a major cause of downgrade in lumber. They are undesirable in fibre products. Knots are very dense, some 2-3 times as dense as stem wood and are frequently above 1000 kg m . This is a consequence of compression wood formation and heavy resinification after mortality or pruning resin can account for 30% or more by weight of knot wood. Knots are hard to penetrate with chemicals and are inadequately pulped, while in mechanical pulping knots are resistant to defibration. 

Species sueh as oak and beech check quite readily, and to avoid this problem the humidity is kept high early in the kiln schedule. Also the temperature is kept low in order to maintain the timber s strength. Only as the lumber dries and becomes stronger can the humidity be lowered and the temperature raised to provide more rapid drying eonditions. Surfaee ehecks forming early in the kiln schedule may close up later when the surfaee fibres go into compression and the core into tension, although the failure plane remains in the tissue. Such checks can be revealed subsequently as hairline streaks if the wood is stained. 

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