三种屋面系统的冷弯薄壁型钢檩条的稳定性及设计方法

Cold-formed steel C- and Z-purlins are frequently adopted in the roof systems of the light steel constructions. The behaviour of a typical cold-formed steel purlin under gravity and wind suction loads depends on the restraints by the steel sheeting and sag rods, besides its material and geometric parameters. The behaviour of the steel purlin of different roof systems may be very different, as the restraints provided by the steel sheeting rely on the connections between the steel sheeting and purlins. There are three types of connections between sheeting and purlins:self-drilling screws,brackets with movable slices and clips, and the related provisions in current design codes do not cover all these roof systems. Sag rods is an effective way to improve the lateral stiffness of the steel purlins of long span, while, according to preliminary study by the PI of this project, it is found that the stiffness of the sag rods, based on the axial tensile stiffness of the sag rod, normally used in existing studies may be dramatically overestimated. The continuous purlins, using lapped connections over the internal supports, are very popular in practice as the better performance can be achieved compared to the common simply supported purlins. While the previous studies on the continuous purlins have been mainly on their flexural strength, and thus the research on the buckling behaviour of such continuous purlins is still necessary, as their strengths are frequently governed by buckling failures. Therefore, a systematic study will be conducted in this project on the behaviour of cold-formed steel C- and Z-purlins in these roof systems, via theoretical development, numerical simulation and experimental tests. In this study, the restrains by the steel sheeting, the effective stiffness of sag rod, the behaviours of simply supported purlins with/without sag rods and the behaviours of continues purlins with/without sag rods will be carefully investigated. Based on the study of this project and by considering the related solutions in previous studies and design codes, the solutions for predicting the behaviour of the steel purlins in the standing seam roof systems will be proposed. The work of this project is believed to be meaningful to the prediction of the structural behaviour and to the design of the steel purlins in these widely adopted roof systems.

冷弯薄壁型钢檩条是轻钢屋面系统的主要受力构件。除了自身刚度参数外，屋面檩条的受力性能还取决于屋面板和拉条的约束作用，屋面的约束作用与屋面系统采用的连接方式相关。工程中常用自攻螺钉、可滑动支架和暗扣连接等三种屋面系统，对于这三种屋面系统的檩条的受力性能计算，目前还存在：现行规范的计算方法不完全适用；对拉条真实的有效刚度认识不足，已有的方法常简单地将拉条作为檩条的侧向刚性支座；缺乏针对常用的嵌套连接连续檩条稳定性的可靠的计算方法等问题。为此，本项目拟针对三种屋面系统中使用的冷弯薄壁型钢檩条的受力性能开展研究，采用试验研究、理论分析和数值模拟相结合的手段，对屋面板的约束作用、拉条的有效刚度、无/有拉条简支檩条及连续檩条的受力性能进行系统、深入研究，提出实用计算和设计方法。项目的研究可为这三种屋面系统使用的冷弯薄壁型钢檩条提供实用的计算和设计方法，同时为我国相关规范的修订提供理论和试验研究基础。

本项目按照计划开展了三种屋面体系对檩条的扭转约束的试验研究。通过理论分析，提出了自攻螺钉屋面、360度卷边滑动座屋面和暗扣座屋面的扭转约束的分项抗扭刚度计算公式，并采用个分项的串联模型得到最终的扭转约束刚度公式。其中后两种屋面进行了试验研究，公式与试验结果进行了对比,符合良好。自攻螺钉屋面则与欧洲的试验进行了对比，偏于安全。. 提出了研究C形和Z形檩条受压翼缘畸变屈曲和弯扭屈曲的统一模型，新模型采用力和弯矩模拟檩条腹板对檩条翼缘的约束作用，从而能够引入相互作用的交叉项。同时在刚度性质的计算上，考虑了一部分腹板高度的侧向抗弯刚度的贡献，使得模型更为合理。通过大量计算确定了模型后，对檩条在绕强轴、绕弱轴的弯矩和双力矩作用下的畸变屈曲进行了系统的研究，对各个工况都提出了屈曲系数的计算公式，尤其是研究了Z形檩条卷边斜角和宽度的影响，提出了公式。研究了两种内力联合作用下的畸变屈曲。. 对檩条体系中拉条的作用，尤其是平行檩条体系中拉条的作用进行了深入的研究。研究分两个方面：对线性的应力分布和位移的影响和对稳定性的影响。因为考虑了拉条拉力导致檩条腹板的局部变形，使得拉条拉力松弛了，而在平行檩条体系中拉条在相邻檩距的错位布置，使拉条的侧向约束效率进一步下降，因此在应力分布和变形方面的结论是，拉条无法阻止檩条的侧向弯曲和扭转，平行檩条体系中离开拉条的锚固端越远的檩条，其内力分布越接近于无拉条檩条。在稳定性方面，拉条刚度的松弛同样有非常不利的影响，拉条的有效性随檩条根数的增加急剧下降。对自攻螺钉屋面和滑动座屋面体系中的研究发现，拉条的作用不如屋面体系的扭转约束对檩条稳定性的贡献。. 对滑动座下的平行檩条体系在风吸力作用下的稳定性进行了试验研究，四个檩条跨度7.5m,檩距1.5m, 拉条分别是0,1,2道。试验同样证实了拉条对檩条在风吸力作用下的稳定性影响很小。. 本项目系统的研究证实，现有实际工程中的檩条体系中的拉条，对提高稳定性是基本无效的，需要根本性的改进。提出了改进措施：例如拉条必须设置两层，下层应设置在檩条的下翼缘上。