Effect of box beam wall thickness on the stiffness of the camphor wood timber bridge truss

Abstract

Bridges are vital for community development, and wood is a primary material due to its environmental benefits. However, wood’s moisture absorption can cause swelling and shrinkage, and low-density wood tends to have lower strength. The moment of inertia of a box beam is related to wall thickness and stiffness. Insufficient wall thickness can lead to plastic retention before peak load, reducing structural integrity. Thin walls can cause buckling under compressive loads, leading to failure. Truss bridge failures can also result from design errors. This study aims to analyze the effect of box beam wall thickness on the stiffness of camphor wood timber bridge trusses. Camphor wood with a cross-sectional area of 1,600 mm2 was used, with box beams of dimensions 45×45 mm (12 mm wall), 50×50 mm (10 mm wall), 58×58 mm (8 mm wall), and a solid beam of 40×40 mm (20 mm wall). Physical tests showed the wood’s specific gravity at 0.506 g/cm3 and moisture content at 12.47 %. The highest peak load was 19.613 kN for the BB.58.58.8 variation, which also had the greatest stiffness at 3.502 kN/mm. The BB.58.58.5 variation had the largest moment of inertia at 683,733 mm4 compared to the solid beam SB.40.40.20 at 213,333 mm4. The BB.45.45.12 sample had a t/D ratio 1.93 times larger than BB.58.58.8, indicating a more flexible structure with lower stiffness. This is confirmed by experimental results, showing that BB.45.45.12 had a stiffness 1.73 times lower than BB.58.58.8. Theoretical calculations also showed that BB.45.45.12 had a stiffness 2.03 times smaller than BB.58.58.8. Thus, the t/D ratio is inversely proportional to stiffness. This research contributes valuable insights for developing engineered wood products in construction and bridge design, particularly for village bridges in Indonesia