Inelastic tangential stiffness for 2-D frames

A general instantaneous or tangential stiffness matrix for a plane frame member is presented. Expressions in the form of integral equations are developed for members having axial forces and arbitrary material properties, distributed loads and moment-axial force-curvature relations. The expressions are used for w sections of elasto-plastic material and several examples of plane frames are loaded to failure. Experimental results reported in the literature for plane frames tested to inelastic failure are closely matched by the analytical technique.