The approximate expression of function that describes the deflection surface of  the plate with two opposite end clamped while

other two opposite end fixed, which satisfies both geometric and force-related boundary condition, is proposed firstly based on small-

deflection thin plate theory. Then, the Rayleigh-Ritz method is  applied to obtain the bearing capacity of elastic buckling, and with

the aid of finite element analysis, the simplified formula of buckling coefficient is derived. Meanwhile, by taking the effect of

elastic-plastic buckling,  the reduced flexural stiffness of core plate and the discontinuous contact between the restrained plate and

the core plate into account, the calculation formula for buckling coefficient is modified. Finally, the design formula of calculating

the flexural stiffness required by buckling restrainer is determined.