Strength Requirements On Knee-braces Of Rafters In Portal Frames

First this article introduces the principle of FEM. The method of potential energy and increment variation are used to deduce the stiffness matrixes. It is the most important thing to deduce the additional stiffness matrix including the torsional braces, the lateral braces, the vertial braces, the axial braces and the warping braces in the nodes. So completes the theory of FEM.Then, the article adopts the program to get the formula's coefficients of the bending members' ultimate moments with different loads, and tries to anlysise the ultimate stiffness of more lateral-brace system.The following, the article considers the axial force's design requirement of the knee-brace in 《Technical specification for steel structure of light-weight buildings with gabled frames》 (CECS 102:2002) is conservative. Then investigated the buckling of beams braced by purlin-knee-bracing system. The bracing stiffness of the purlin-knee bracing system is analysed. Relation of buckling moment to bracing stiffness is analysed by FEM. Finally the initial out-of-plane deflection and initial twisting of the beam is considered to find the bracing forces when the buckling moment reaches the yield moment of the beam. It is found that the maximum bracing forces is less than 1% of the yield force of the beam bottom flange, far less than the value required by the specification CECS 102:2002.At last, it uses the ANSYS software to analyse the model including the beam, the purlins and the knee-braces. Then adopts a few examples to proceed to the elastic analysis. And gets the braces' axial forces. The analysis is assistant for the finite element program's analysis.