Analysis and Design of a Complex Single⁃Layer Aluminum Alloy Daylighting Roof Grid Structure
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摘要: 位于长春影视文创孵化园区的影视研学基地采光顶,长方向跨度为85 m,短方向跨度为61 m。该采光顶形状奇特,且直接坐落于分布不规则的外围框架柱顶,从而导致该采光顶受力极为复杂。在进行采光顶结构选型时主要考虑了2种方案,分别为钢桁架结构和单层铝合金网壳结构。考虑到单层铝合金网壳具有自重轻、美观和施工难度低等优势,最终采用单层铝合金网壳采光顶结构,选用铝合金板式节点。由于单层铝合金网壳采光顶结构较为复杂,需采用MIDAS/Gen模型和RFEM模型分别进行结构计算,并通过计算结果对比来校核模型的准确性。对比结果表明:2个模型的前3阶振型和前3阶屈曲模态高度吻合,其自振周期和特征值误差极小,即2个模型的精度可以满足计算需求。另外,本采光顶网壳自振振型和屈曲模态均是整体振动和整体屈曲,即采光顶网壳的结构布置合理。采用MIDAS/Gen模型开展单层铝合金网壳弹性分析,结果表明杆件在荷载基本组合作用下的最大应力为112 MPa,大部分杆件的应力处于80 MPa以内,满足GB 50429—2007《铝合金结构设计规范》要求。在荷载标准组合(恒荷载+活荷载)作用下,本结构的最大位移为119 mm,该位移与跨度的比值为1/512,小于JGJ 7—2010《空间网格结构技术规范》限值1/400,表明本结构满足强度和刚度需求。采用RFEM模型对本结构开展双非线性稳定分析,对其稳定性承载力进行验算。初始缺陷的分布形态取各工况对应的最低阶屈曲模态,缺陷幅值取跨度的1/300。经计算各工况作用下的荷载临界系数最小值为2.5,大于GB 50009—2012《建筑结构荷载规范》限值2.0。为此,采用拆除构件法开展整体结构抗连续倒塌分析,倒塌工况选择双非线性稳定分析中率先失效的2根杆件作为初始失效杆件。由倒塌分析结果可知,当最先失稳的2根构件失效后,周围杆件的应力水平基本没有变化,即不会引起结构的连续性倒塌。提取关键杆件进行特征屈曲分析,根据欧拉公式推导出其计算长度系数为1.47,并输入至整体设计模型,验算关键杆件的强度。验算结果表明大部分杆件的应力比小于或等于0.5,结构具有足够的安全储备。建立典型节点有限元分析模型,得到该节点在荷载设计值作用下的应力状态,证实各部分应力均满足相关规范要求且具有较大安全储备,符合“强节点、弱杆件”的结构设计理念。Abstract: The film and television research base, located in the Changchun Film and Television Cultural Innovation Incubation Park, has a long span of 85 m and a short span of 61 m. The daylighting roof features a peculiar shape and is directly supported by irregularly distributed peripheral frame columns, resulting in highly complex stress conditions. Two schemes were mainly considered in the selection of daylighting roof structure, namely steel truss structure and single-layer aluminum alloy latticed shell structure. In view of the advantages of light weight, beautiful appearance, and low construction difficulty, the single-layer aluminum alloy latticed shell structure was ultimately adopted. Further structural analysis and design were carried out accordingly. Due to the complex structure of the single-layer aluminum alloy latticed daylighting roof, the MIDAS/Gen model and the RFEM model were used for structural calculations, and the accuracy of the models was verified by comparing the computation results. The calculation results showed that the first three modes of the two models were highly consistent with the first three buckling modes, and the errors in their natural vibration periods and eigenvalues were very small, indicating that the accuracy of both models met the calculation requirements. Further analysis revealed that both the natural vibration modes and the buckling modes of the daylighting roof latticed shell were characterized by global vibration and global buckling, indicating a reasonable structural arrangement. The MIDAS/Gen model was used to caary out an elastic analysis of single-layer aluminum alloy latticed shells. The results showed that the maximum stress in the members was 112 MPa, while the stress in most members remained below 80 MPa, meeting the requirements of the Code for Design of Aluminium Struectures(GB 50429‒2007). Under the standard load combination (dead load + live load), the maximum displacement of the structure was 119 mm, resulting in a displacement-to-span ratio of 1/512, which was less than the limit of 1/400 in the Technical Specification for Space Frame Structures(JGJ 7‒2010) . These analysis results demonstrated that the structure could satisfy both strength and stiffness requirements. The RFEM model was used to conduct a double-nonlinear stability analysis of the structure and to evaluate its stability bearing capacity. The distribution form of initial defects was based on the lowest buckling mode corresponding to each working condition, with a defect magnitude set at 1/300 of the span. After calculation, the minimum value of the load critical coefficient under each working condition was 2.5, which was greater than the limit of 2.0 in the Load Code for the Design of Building Structures(GB 50009‒2012). The progressive collapse resistance of the project was analyzed using the component removal method. The collapse condition selected the two members that had failed first in the double nonlinear stability analysis as the initially failed members. According to the collapse analysis results, when the first two members failed, the stress level of the overall grid structure remained largely unchanged, indicating that progressive collapse did not occur. Key members were extracted for characteristic buckling analysis. The effective length coefficient, calculated as 1.47 according to Euler’s formula, was input into the overall design model to check the strength of the key members. The calculation results showed that the stress ratio of most members was less than or equal to 0.5, and the structure had sufficient safety reserves. A finite element analysis model of a typical joint was established to obtain the stress state of the joint under the action of the load design value. It was confirmed that the stress in each part met the requirements of the code and had a large safety reserve. The structural design concept of "strong joint and weak member" was satisfied.
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