Influence of the boundary constraint condition on torsional effects of thin-walled beams

Abstract To reveal the influence of boundary constraints on the torsional effects of open and closed thin-walled beams, the governing differential equation in the classical thin-walled beam torsional theory is used. Initial parameter solutions are provided, and calculation formulas for the torsional effects of thin-walled beams under uniform torque load is developed for various boundary conditions. Through numerical examples, the influence of boundary constraints on the internal forces and displacements of open and closed thin-walled beams is analyzed, and the analytical theoretical framework of thin-walled beams under different boundary constraint systems is improved. The results indicate that the analytical theory presented in this paper aligns well with finite element analysis, confirming its validity. Open thin-walled beams are sensitive to changes in boundary constraints. Compared to the simply supported open thin-walled beam, the bimoment at the mid-span section of a beam with start end fixed and another end simply supported, and a fixed beam at both ends, decreased by 39.04% and 55.80%, respectively. The torsional angle decreased by 52.36% and 72.70%, respectively. For closed thin-walled beams, changes in boundary constraints significantly affect the internal forces near the beam end, while the torsional effects in regions 1/8 of the span length from the beam end is almost unaffected. This suggests that in practice, the torsional effect of open thin-walled beams can be reduced by reinforcing their boundary constraints, and the local design of fixed ends in continuous rigid bridges should be strengthened to mitigate significant warping effects.