长期荷载作用下钢-竹组合构件受力性能与设计方法研究

The bamboo-steel composite members which are combined bamboo plywood with cold-formed thin-walled steel could take advantage of the strengths of bamboo and steel. However, due to the creep of bamboo plywood and the slippage at the bamboo-steel interface, the long term mechanical property of the composite members is a key factor affecting the practical applications. This project is to systematically investigate the creep of bamboo plywood, the time-dependent effect of the adhesive at the bamboo-steel interface, as well as the characteristics of deformation, the changing of the bearing capacity, and the evaluation method for mechanical properties of the composite members under long-term loading. Through experimental testing, theoretical analysis and numerical calculation, the creep model of the bamboo plywood in terms of stress level and environmental conditions will be built, and then the deterioration of the bonding property of the bamboo-steel interface under the long-term loading will be elaborated, based on which, the bonding strength model will be established. The relationship between the long-term mechanical property and the long-term loading level, steel ratio, axial loading ratio and slenderness ratio will be illustrated, The long-time deformation and the degradation of the bearing capacity of composite members will be revealed. This study will propose the calculation theory of the long-term bearing capacity, provide the accurate assessment methods, and establish a convenient and practical design for the bamboo-steel composite members based on the study of long-term properties of materials, interface and composite members. This study will provide the necessary theoretical basis to ensure the safety of bamboo-steel composite members and bamboo-steel structural system in the service period.

竹材人造板与冷弯薄壁型钢复合而成的钢-竹组合构件有利于充分发挥材料的高强度，由于竹材的蠕变和钢-竹界面的滑移，其长期受力性能是影响实际应用效果的关键。本项目系统研究竹材人造板的蠕变特性和钢-竹界面粘结性能的时随效应，以及长期荷载作用下钢-竹组合构件的变形特征、承载能力变化规律和力学性能评价方法。通过试验研究、理论分析和数值计算，构建考虑应力水平和环境条件影响的竹材人造板蠕变模型，阐明长期荷载作用下钢-竹界面粘结性能退化机制，建立其粘结强度模型；明确钢-竹组合构件长期受力性能与加荷水平、截面含钢率、轴压比、长细比之间的关系，揭示组合构件的长期变形规律和承载能力退化规律；基于材料、界面及构件的长期性能研究结果建立钢-竹组合构件长期变形和承载能力计算理论，实现构件长期力学性能的准确评价，提出便于操作的实用设计方法，为确保钢-竹组合构件及其结构体系在整个设计基准期的安全性提供必要的理论基础。

建筑物承受的荷载大部分为长期作用，而钢-竹组合构件中的竹材与木材相似，蠕变特征明显，合理评价其长期受力性能意义重大。通过本项目研究获得了如下结论：(1) 应力水平较低时竹材人造板蠕变应变-时间曲线具有比较明显的典型蠕变变形的前两个阶段，即瞬态蠕变阶段、稳态蠕变阶段，在高应力水平下，其蠕变变形可导致构件破坏，设计时应合理控制应力水平；用Burgers模型和Findley模型对重组竹长期拉伸和压缩蠕变试验结果进行了拟合，两种模型拟合曲线都与试验曲线吻合较好，能够很好地描述重组竹的蠕变行为。(2) 长期荷载作用后钢-竹界面完整性良好，复合胶结型界面试件的破坏具有显著的延性特征；2组单纯胶结型界面试件长期荷载作用后承载力发生衰减，衰减系数分别为0.91与0.81，界面最大剪应力约衰减10%和20%；复合胶结型界面试件长期荷载作用后，其界面承载力与相对滑移不发生显著变化；提出了钢-竹界面滑移及承载力计算方法，其计算结果与试验结果吻合较好。(3) 钢-竹组合构件在加载初期，蠕变速率较快，波动较大，三个月后趋于平缓，其最终蠕变变形与竹胶板厚度、钢板厚度、含钢率等有关；长期加载改变组合梁延性，在弹性阶段二次破坏试验与一次破坏试验的荷载-挠度曲线变化趋势基本一致，进入弹塑性阶段，二次破坏试验的曲线斜率明显减小，而组合柱的延性仍然大于2.0；长期荷载作用后，组合梁的承载力有一定的衰减，衰减程度基本小于9%，组合柱衰减值在3%～12%之间，仍符合构件长期使用的要求。(4) 提出了钢-竹组合梁蠕变变形计算公式，并根据有效惯性矩法、组合梁长期加载后变形模量的变化以及钢-竹界面滑移规律，建立了钢-竹组合梁挠度计算公式，其计算结果与试验值吻合较好；长期荷载作用下竹胶板厚度与承载力衰减呈正相关，钢板厚度呈负相关，含钢率对承载力衰减受竹胶板厚度影响，呈一定规律性变化；钢-竹组合柱的极限承载力受长期荷载作用的影响并不明显；Burgers四元件流变模型可以较准确地模拟组合柱的蠕变行为，拟合曲线和试验曲线的相关系数R2均在0.95以上；通过确定组合柱的蠕变与竹材蠕变间的关系以及竹材蠕变研究，确定了钢-竹组合柱蠕变变形-时间关系，并与试验值进行了对比，两者基本一致。