Numerical Evaluation of the Structural Response of a Balsa Wood Truss Bridge under Eccentric Loading Using the Finite Element Method

Abstract

Eccentric loading is important in bridge design because it can cause asymmetric internal forces and combined bending and torsion. This study evaluates a lightweight balsa wood truss bridge under eccentric loading using the Finite Element Method (FEM). Designed for the 2025 Bridge Design Competition (BDC) at Nanyang Technological University (NTU), the bridge was modeled in SAP2000 with three-dimensional frame elements. Loads of 225, 450, 675, and 900 N were applied two-thirds along the span. Parameters included vertical deflection, axial forces, stress distribution, and structural efficiency, defined as maximum load divided by self-weight. Deflection increased from 0.0580 mm at 225 N to 0.2322 mm at 900 N, showing a linear load–deflection relationship. Differences among main, longitudinal, diagonal, and transverse members reflected their load-transfer contributions. Layered main girders and lateral bracing supported stiffness and stability. In competition, the bridge reportedly sustained 176 N and weighed 18.37 g, producing a load-to-weight ratio of approximately 970–977. These findings demonstrate numerical analysis’s value before testing. Experimental validation and torsional assessment remain necessary to confirm predictions and bracing effectiveness.