| Light steel structure has formed a unique building system as it has been developed over half a century. Cold-rolled forming section Z&C are used as purlins or wall beams which serve as members among steel frame and metal roof or walls. Weight of purlins occupies about 55% in the whole weight of the light steel roof system. Therefore, an optimum design of the purlin becomes a decisive step in reducing the total weight of steel roof system. Because of the section characters of the cold-rolled forming section Z&C, stability design of purlins under wind suction turns into the controlling condition. It needs further study as there is less research on resistance to torsion of the Z&C purlins used in portal light steel frame and cold-bent thin-walled purlin system which uses rods and bar stays as its strut members.The twisting capacity of single purlin and purlin systems consisted of purlin bracings and rods with different spans and loads are researched in this dissertation using Finite Element Method. It is indicated that the application of bracings can remarkably improve the resistance to torsion and the stability of the purlin system, while the numbers of the bracings applied has little influence in changing torsional stiffness of the system. Useful data for future research is obtained through analysis. The validity and veracity of test result and code calculation are proved in the dissertation by applying the theoretic analysis on simple purlin under the wind suction force, reference to domestic relative testing results, and comparisons with different codes both domestically and internationally. Suggestions on stability calculation of purlins under wind suction force are given in this dissertation and the research will be propitious to improve and simplify the design method in the future. Further theoretical research of purlin bracing systems, the problems which should pay attention to and the research directions in the future test are also mentioned in this paper.
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