Torsional behaviour of curved composite beams in construction stage and diaphragm effects

• Torsional behaviour of curved composite beams is investigated analytically.
• Closed form analytical solutions are derived for assessing such behaviour.
• Warping stress ratios are sensitive to diaphragms, span-to-radius ratios and spans.
• Formulae for predicting warping stress ratios are developed by nonlinear fitting.
• Design guidance for diaphragms of such beams under construction is proposed.

During the concrete-casting process of curved steel–concrete composite beams in construction stage, all loads including the wet concrete weight are carried by the curved open thin-walled steel girder. In order to determine the required diaphragm spacing, firstly the curved girder is simplified to straight one by using the M/r method, and its torsional behaviour is subsequently derived by solving the restrained torsional differential equation with the initial parameter method, where the discrete diaphragms are treated to be equivalent to a continuous elastic thin plate. Taking the maximum stress ratio of the warping normal stress to the bending normal stress as the control index, effects of the number of diaphragms, the span of the curved girder, the span-to-radius ratio, the girder height and the flange width on the stress ratio are studied. The parametric analysis results indicate that with the increase of the number of diaphragms, the stress ratio decreases exponentially; and with the increase of the span-to-radius ratio, the warping stress ratio increases linearly; additionally, longer span leads to reduced stress ratios, but the variation of the girder height and the flange width has little influence on the stress ratio. Calculation formulae for determining the required diaphragm spacing are obtained eventually by regression analyses. Based on the research outcomes herein, given the limit value of the warping stress ratio in the practical design of curved composite beams under construction, the maximum diaphragm spacing of the open thin-walled steel girder can be determined by using the formulae proposed in this paper.