Abstract: In this paper,from the differential equations of stability for every brace,a group of monolithic differential equations of stability,in which bending and torsion are coupled,has been berived.The Galerkin method has been employed in solving the equations and a minimum monolithic critical load has been obtained.Then it is easy to gain the loads which would be borne by every brace when monolithic instability happened.These figures will be significative of correctly selecting the plane-form and the ratio of the height and the width of the structure,of reasonably adjusting the positions of various braces in plane and of rationaly determining the size of cross-sction for every brace.