Experimental Study of Beam Stability Factor of Sawn Lumber Subjected to Concentrated Bending Loads at Several Points

The beam stability factor (CL) is applied in construction practices to adjust the reference bending design value (Fb) of sawn lumber to consider the lateral-torsional buckling. Bending tests were carried out on 272 specimens of four wood species, namely, red meranti (Shorea sp.), mahogany (Swietenia sp.), pine (Pinus sp.), and agathis (Agathis sp.), to analyze a simply supported beam subjected to concentrated loads at several points. The empirical CL value is a ratio of the modulus of rupture (SR) of a specimen to the average SR of the standard-size specimens. The non-linear regression estimated the Euler buckling coefficient for sawn lumber beam (KbE) in this study as 0.413, with 5% lower and 5% upper values of 0.338 and 0.488. Applying the 2.74 factor, which represents an approximately 5% lower exclusion value on the pure bending modulus of elasticity (Eₘᵢₙ) and a factor of safety, the adjusted Euler buckling coefficient (KbE′) value for a timber beam was 1.13 (0.92–1.34), which is within the range approved by the NDS (KbE′ = 1.20). This study harmonizes the NDS design practices of CL computation with the empirical results. Because agathis has the lowest ductility (μ), most natural defects (smallest strength ratio, S), and highest E/SR ratio, the agathis beam did not twist during the bending test; instead, it failed before twisting could occur, indicating inelastic material failure. Meanwhile the other specimens (pinus, mahogany, and red meranti), which have smaller E/SR ratio, higher ductility, and less natural defects, tended to fail because of lesser beam stability. This phenomenon resulted in the CL curve of agathis being the highest among the others. The CL value is mathematically related to the beam slenderness ratio (RB) and the E/SR ratio. Because the strength ratio (S) and ductility ratio (μ) have significant inverse correlations with the E/SR ratio, they are correlated with the CL value. Applying the CL value to adjust the characteristic bending strength is safe and reliable, as less than 5% of the specimens’ SR data points lie below the curve of the adjusted characteristics values.