| Steel materials will be obviously hardened after the cold-formed process. And as kind of most economical formed material, cold-formed steel members are used in practical engineering more and more widely. In the existing code-Technical Code of Cold-Formed Thin-Walled Steel Structures, after considering the hardening of the cold-formed process, the strength design formula is only used for cold-formed thin-walled steel members, which means that the thickness of steel members is no larger than 6mm. However, the relational formula for thick-walled steel members has not been brought forward. Also, investigations of cold-formed effect on thick-walled steel members have been seldom reported.In this thesis, based on many experiments on wattles, stub columns and relational raw material, cold-formed effect of thick-walled steel members on the mechanical properties including strength, ductility, and so on, are studied respectively. The wattles and stub columns are all cut from cold-formed square (or rectangular) steel tubes. The wattles include flats and corners. Each kind of test samples has three thickness, they are 8mm, 10mm and 12mm respectively. Firstly, according to the experiments of wattles, the hardened strength of flats and corners are gained by considering the cold-formed effect. Then, based on the data results of flats and corners, correlative codes used overseas and in China are applied to calculate yield stress of the overall cross section of square or rectangular tubes. Also, the results of each calculation method and the results of the stub columns' tests are compared and analyzed. Lastly, in order to gain a proper design formula in which the cold-formed effect on thick-walled steel members is considered, a modify function for the existing design formula in Technical Code of Cold-Formed Thin-Walled Steel Structures is brought forward. Furthermore, so many back calculations are made, which explains that the modified formula can be applied in rather wide areas. Also, veracity and reliability of the modified formula are also proved.Cold-formed residual stress resulted from the cold-formed process is a disadvantageous factor for stiffness and stability of steel members. In this thesis, the cold-formed process is divided into two stages: the being formed (or member's loading) stage and the elastic restoration (or member's unloading) stage. Based on the experimental results of the cold-formed effect, considering the hardening of steel members, stress fieldof the being formed stage is obtained. Then, using the ANSYS-finite element analysis software to imitate the elastic restoration stage, stress field of this stage is obtained, too. The reasonable and comprehensive cold-formed residual stress field of thick-walled steel members is achieved by adding the stresses of those two stages.After considering the effect of the cold-formed process, the strength design formula for cold-formed thick-walled steel members brought forward in this thesis provides advice for steel structure code to add relational content. Meanwhile, by thinking about the hardening, the residual stress field put forward in this thesis is also a foundation for further study on the disadvantageous effect of cold-formed residual stress to structures' stability and stiffness. |
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