Experimental and numerical study of the torsional response of a modular hybrid FRP-aluminum triangular deck-truss beam

A hybrid fiber-reinforced polymer (FRP) - aluminum modular space truss emergency bridge has been designed that consists of two triangular deck-truss beams incorporating a new structural form and advanced pultruded FRP profiles. As a fundamental investigation, this paper explores the detailed torsional behaviors of the new triangular deck-truss beam. A full-scale specimen was prepared and tested under pure torsion to evaluate the representative structural performances. A three-dimensional finite element model (FEM) was constructed and validated with experimental results showing good comparisons. Furthermore, the torsional behavior of the triangular deck-truss beam was numerically investigated by removing individual primary truss members to clearly understand the load transfer mechanism. It was found that the beam exhibited a specific and unfavorable mechanical behavior prohibiting full utilization of the FRP material' potential. The external torsional moment is confirmed to be primarily resisted by the torsion of the longitudinal profiles and the in-plane bending of the bilateral Vierendeel trusses rather than the out-of-plane bending of the web diagonals. Possible improvement and optimization procedures are further suggested for this beam when solely used as a structure subjected to off-axis load or pure torsion.