高温下冷弯型钢组合截面轴压构件的失稳机理和设计方法研究

Cold-formed steel structural members can obtain a significant improvement on loading capacities by connecting two or more single section together. However, the application of cold-formed steel built-up sections is limited by some unfavorable factors such as its various failure modes under an axial compressive load, the complicated design calculations, the insufficient of relevant design provisions，etc. Furthermore, the fire resistance analysis of cold-formed steel built-up sections is at a very early stage. Therefore, a comprehensive investigation will be conducted experimentally and numerically on the instability mechanism of cold-formed steel built-up compression members at elevated temperatures, simple and reliable design rules will be proposed...Firstly, the reduced material properties of cold-formed steel due to the elevated temperatures and the temperature distribution in a built-up section are considered in this study. The investigation will focus on the failure modes of a built-up section, especially the distortional buckling, the effect of the interaction of distortional and local buckling. The study will also identify the trigger condition and development of the distortional buckling of a built-up section. Secondly, a quantitative analysis will focus on the effect of connector spacing, the coupling of the connector spacing and the half-wavelength of local or distortional buckling to the structural behavior of a built-up section, and a modified slenderness ratio will be developed by considering the aforementioned factors for built-up sections. Finally, based on the direct strength method, this project will develop the local and distortional buckling equations at elevated temperatures. The strength calculation and design rules, involving the strength reduction coefficient due to temperatures, will be proposed for cold-formed steel built-up compression members. The outcome of this project will improve the understanding and design of cold-formed steel structural members at fire conditions.

冷弯型钢通过组合拼接方式能够显著地提高构件承载力。然而，冷弯型钢组合截面的推广使用受到诸多不利因素如轴压破坏形式多样、极限承载力计算复杂、设计规范匮乏等的限制。而冷弯型钢组合截面抗火性能的研究更是刚刚起步。因此，本项目将结合试验与数值仿真，系统地研究高温下冷弯型钢组合截面轴压构件的失稳机理，提出简明可靠的设计方法。.首先，本项目以冷弯型钢高温材料特性和组合截面内温度分布规律为起点，研究组合截面高温下的失稳模式，特别是畸变屈曲、及畸变屈曲与局部屈曲的耦合效应，揭示组合截面畸变屈曲的触发条件和发展规律。其次，本项目量化连接件间距、及连接件间距与失稳屈曲半波长耦合效应对结构承载力的影响，提出组合截面修正长细比。最终，本项目在直接强度法的基础上，给出高温下组合截面轴压构件畸变屈曲、局部屈曲曲线方程；提出包含温度效应的组合截面承载力计算方法和截面设计准则，为提升我国冷弯型钢抗火设计水平提供有力支撑。

组合拼接方式能够显著提高冷弯型钢轴压构件的承载力，但此类截面破坏形式多样，承载力计算复杂，且冷弯型钢的力学行为极易受到温度的影响。本项目结合试验与数值仿真，系统地研究高温及高温后的冷弯型钢组合截面轴压构件的力学行为，提出简明可靠的设计方法。..在试验测试的基础上，本项目给出了高强度冷弯型钢在高温/高温后的材料衰减预测方法和高温下的材料本构关系模型。通过常温组合截面轴压测试，明确了组合截面承载力和连接件间距的关系。在温度场下的开口截面短柱测试中，本项目考察了纵向加劲对开口截面承载力和失稳模态的影响，试验结果发现高温下的构件承载力下降趋势与材料强度的下降并不保持一致。在试验的基础上，本项目建立了冷弯型钢组合截面的有限元模型。模型数据可靠。结合试验数据和参数分析，本项目提出了开口/闭口组合截面的承载力计算方法和修正长细比。给出了高温下的开口/闭口截面失稳模态强度曲线和基于直接强度法的修正承载力计算方法。