In order to analyze the out-of-plane stability behavior of circular arch structures, according to the deformation correlation of circular arch with circular curved beam, the equilibrium equations of circular curved beam were firstly established by taking secondary moment effect into account. Combined with the geometric equation and physical equation of circular curved beam, the deflection control equation and torsion angle control equation of circular curved beam with free torsion were derived by considering large displacement. Both the general formats and corresponding simplified formats of analytical solution for circular curved beam deflection and torsion angle were obtained. Meanwhile, the expressions for deformation and internal force of circular curved beam were also derived. On this basis, the methods to analyze the out-of-plane bifurcation instability and extreme point instability of circular arch structure were presented. Critical load coefficients and their instability modes of four kinds of circular arch structure were calculated when out-of-plane bifurcation instability occurs, and the calculation results between the proposed model and the models from the literature were discussed; their load-displacement curves were calculated, and the extreme point instability of circular arch structures was analyzed. The results show that the critical load coefficient of the circular arch with two ends simply supported can be calculated by this model and has no difference with other models. In addition, this model is also useful to calculate the critical load coefficient of that with single hinge at mid-span or that with two ends inserted supported which are rarely seen in research. The out-of-plane bifurcation instability modes of all kinds of circular arch under in-plane uniformly distributed radial load are in the form of single symmetric wave. The radial load does not change the linear character of the out-of-plane load vs. displacement curve, but reduces the out-of-plane flexural rigidity. When the radial load reaches a certain value, the out-of-plane flexural rigidity becomes 0, and then the out-of-plane instability occurs. Copyright ©2021 JOURNAL OF SOUTHWEST JIAOTONG UNIVERSITY. All rights reserved.
