【作者】 李贤；

【导师】 肖岩；

【作者基本信息】 湖南大学 ， 结构工程， 2009， 博士

【摘要】 1994年的美国Northridge地震和1995年的日本Kobe地震中最为惨痛的教训就是现代钢结构焊接刚性节点的过早脆性破坏。两次地震的震害调查发现,钢结构焊接节点的连接破坏主要发生在下翼缘处,分析其破坏的主要原因为:一方面可能是混凝土楼板的组合效应导致下翼缘处应力增大;另一方面可能是下翼缘在腹板位置焊接的中断造成焊缝缺陷。因此迫切需要探求有效的措施(如采用工厂焊接方式和钢梁“狗骨式”削弱等)来保证节点的焊缝质量,同时有必要深入研究混凝土楼板组合效应的影响。钢—混凝土组合结构以其合理和有效地利用了钢梁和钢筋/钢管混凝土柱的力学性能而逐渐被视为中、高震区的纯钢筋混凝土结构和钢结构的有益补充,并且日益广泛的应用于工程实际。为确保钢—混凝土组合节点的施工方便性和良好的抗震性能,本文提出了新型的端板螺栓连接钢—混凝土组合节点型式。该节点型式的特点在于:钢梁首先在工厂焊接好端板,再在施工现场通过贯穿柱身的无粘结高强螺栓与钢筋/钢管混凝土柱连接。该类型节点无需现场焊接,从而能够减少现场焊接带来的焊缝质量问题,而且采用现场拼接的方法可以提高施工进度,降低施工成本。同时该类型节点可以通过贯穿柱的高强螺栓和端板的共同作用有效地调动节点域混凝土参与抗剪,从而提高节点的抗震性能。由于端板螺栓连接组合节点是一种新型的节点型式,本文探讨了端板螺栓连接组合节点的合理设计方法,并且采用试验和有限元相结合的方法研究了端板螺栓连接钢—混凝土组合结构的抗震性能及其抗震性能的影响参数,具体工作如下:1、通过4个足尺端板螺栓连接钢筋混凝土柱—钢梁节点(RCS节点)和3个足尺端板螺栓连接圆钢管混凝土柱—钢梁节点(CFT节点)的低周反复循环荷载试验评估了端板螺栓连接组合节点的抗震性能。试验考察了钢梁类型、钢梁“狗骨式”削弱以及混凝土楼板等因素对节点抗震性能的影响。本文试验结果表明:具有良好焊缝质量的端板螺栓连接钢—混凝土组合节点具有较好的延性和耗能能力;该类型节点在反复循环荷载作用下能够满足抗弯组合框架的强度和刚度要求;混凝土楼板的组合效应对节点的承载力的提高显著;同时,钢梁的“狗骨式”削弱可以有效地把钢梁的塑性发展控制在削弱区域从而保证节点的延性破坏模式。2、采用有限元软件ANSYS建立了考虑材料非线性、几何非线性和接触非线性的端板螺栓连接组合节点的三维有限元模型,并且通过大量的算例验证了有限元模型的有效性。通过验证好的有限元模型分析了高强螺栓的初始预紧力、端板螺栓连接构造、柱的轴压比以及楼板中的混凝土强度和配筋率等因素对节点受力性能的影响。通过对节点施加不同程度的螺栓预紧力分析表明高强螺栓的初始预紧力对节点的整体性能影响较小,但对节点的局部应力有一定影响;对采用了不同厚度的端板的节点研究表明端板厚度对节点的刚度有一定的影响,但是当端板抗弯强度大于翼缘强度后再增加端板厚度对节点的受力性能影响很小;通过改变节点楼板中混凝土强度和配筋率的对比研究表明:由于混凝土的抗拉强度较低,楼板中混凝土强度的变化对组合节点的负向承载力影响较小,但是混凝土楼板的存在使得其负向承载力高于纯钢梁节点。同时,随着楼板配筋率的增加,节点的负向承载力提高明显,但是楼板配筋率的改变对正向承载力的影响很小。混凝土的强度提高时,组合节点的正向承载力也相应的提高,混凝土楼板的存在可以使节点的正向承载力提高80%之多。因而在计算节点的“强柱弱梁”系数以及端板螺栓连接设计时都应该考虑混凝土楼板的影响。3、提出了端板螺栓连接节点的分析模型,并采用有限元软件OPENSEES对本文设计的共计12榀端板螺栓连接钢—混凝土组合框架进行了Pushover分析和非线性时程分析以评估端板螺栓连接钢—混凝土组合框架的抗震性能,并且重点考察了混凝土楼板的组合效应以及钢梁的“狗骨式”削弱对端板螺栓连接框架抗震性能的影响。通过有限元软件OPENSEES的分析表明:本文建议的端板螺栓连接组合节点的分析模型能够较好的模拟端板螺栓连接节点的受力性能,分析结果和试验结果吻合良好;端板螺栓连接钢—混凝土组合结构具有较好的抗震性能,能够满足抗震规范要求;考虑混凝土楼板的组合效应后,框架的承载力会有所提高。考虑了混凝土楼板强度贡献的框架的Pushover分析得到的最大基底反力为不考虑混凝土楼板强度框架的1.1倍。同时,考虑混凝土楼板的强度贡献使得框架的梁—柱—节点域的强弱可能会发生改变,导致柱端和节点域区可能出现较大的塑性变形;另外,钢—混凝土组合框架按照弹性侧移率控制时,其强度有一定的富余,而采用钢梁“狗骨式”削弱可以在有效减少钢梁强度富余的同时较小削弱整体框架的刚度,从而保证框架的梁塑性铰破坏模式,通过设计框架的对比分析表明按照FEMA350设计的钢梁“狗骨式”削弱对框架的整体刚度削弱约为5%~7%。综合上述试验和有限元分析结果可知,本文建议的端板螺栓连接组合节点和端板螺栓连接的钢—混凝土组合框架具有良好的抗震性能,能够满足地震区的抗震框架设计要求。本文建议的端板螺栓连接节点的设计方法和分析方法基本可靠,能为相关规范的编制和工程应用提供参考。更多还原

【Abstract】 One of the most significant lessons learned from the 1994 Northridge earthquake and the 1995 Kobe earthquake was the premature cracking and brittle failure of welded steel special moment resisting frame (SMRF) connections. Research after the earthquakes indicated that the possible reasons resulting in the premature fractures at the bottom flanges were mainly attributed to the presence of floor slabs and the potential weld defects due to the discontinuities of in-situ welding. Therefore, it is urgent to find improved details such as reduced beam sections in an effort to protect the welds and study the effect of composite action of concrete floor slabs. Composite steel and concrete moment resisting frame systems integrating structural steel beams with reinforced concrete columns or composite columns gradually gain acceptance as cost-effective and efficient alternatives to conventional steel and reinforced concrete structures in regions of middle or high seismic zones and are widely applied in the United States, Japan, China and etc in recent years. In order to improve the constructability and meanwhile ensure excellent seismic behavior, an innovative type of connection details for composite structures consisting of steel beams and reinforced concrete columns (RCS) or concrete filled circular steel tube columns (CFT) was proposed. The proposed composite connection details involve post-tensioning the shop-welded endplates of the steel beams to the reinforced concrete or concrete filled steel tube columns using high-strength steel rods. The proposed joint details require no field welding, eliminating the problems of the welded steel connections and improving the speed of construction. In addition, the contribution of concrete in panel zones on shear resistance could be mobilized by both endplates and through-column bolts. As a new type of connection details, a rational design procedure was suggested to assure a ductile behavior, and both experimental and analytical research was conducted to evaluate the seismic behavior of bolted endplate connections. The main achievements in the dissertation can be summarized as follows:1. To evaluate the seismic behavior of bolted endplate connections and investigate the effects of beam type, reduced beam sections and concrete floor slabs on joints, a total of seven full-scale exterior RCS and CFT joint models were designed and tested under simulated seismic loading. The experimental results indicated that composite RCS and CFT joints with good welding exhibited good ductility and energy- dissipation ability. The bolted endplate connections could provide sufficient strength and stiffness for moment resisting frames under inelastic cyclic loading. The proposed design method could result in a ductile failure pattern for the connections, particularly along with the use of a steel beam with reduced section near the critical moment end. The presence of floor slabs contributed to the strength of joints significantly and reduced beam sections were effective in moving the buckling zone away from the welds.2. Three-dimensional finite element models for proposed bolted endplate connections were developed using general-purpose finite element program ANSYS and the models were verified by comparing the behavior of models with experimental results. The nonlinearity of geometry, materials and surface-to-surface contacts was included in the models. Using the program ANSYS, some parameters such as initial pre-tension force of bolts, details of bolted endplate connections, axial force ratio on columns, and strength of concrete as well as ratios of steel bars in the floor slabs affecting the seismic behavior of joints were analyzed in detail. The analytical studies demonstrated that the initial pre-tension force had no apparent effect on the global performance of the joint specimens but local effect. The thickness of endplate has some influence on the stiffness of connections when the flexural strength of the endplate was not higher than that of the steel beam. Owing to the contribution of concrete floor slabs, both the sagging and hogging strength of composite joints increased. The varied strength of concrete had little effect on the sagging strength for the weakness of concrete in tension but the hogging strength of joints was enhanced significantly with the increase of concrete strength. The reinforcement ratio of floor slabs had obvious influence on the sagging strength of joints but it didn’t affect the hogging strength apparently. To ensure beam hinging according to the concept of“strong column–weak beam”, the effect of floor slab should be taken into account in calculation of the strength ratio between columns and beams.3. An analytical model for bolted endplate connections was proposed using the platform of OPENSEES and the seismic performance of composite frames with bolted endplate connections were investigated by nonlinear static and time-history analysis of twelve case-study frames. The influence of composite action of concrete floor slabs and reduced beam sections on the seismic behavior of composite frames was studied in detail. The analytical studies show that the proposed analytical model for bolted endplate connections can simulate the test results well. Composite steel and concrete frames with proper design exhibited good seismic behavior, meeting the requirements of current seismic codes. Comparative studies of frames with or without considering the contribution of concrete floor slabs to the strength of beams indicate that the composite action of concrete floor slabs enhance the strength of frames and may change the failure pattern of frames. The composite frames designed under control of elastic deformation result in over-strength of frames. The reduced beam sections can reduce the ratio of over-strength of frames with slight reduction on stiffness. The reduced beam sections designed according to FEMA350 can result in the reduction of stiffness of frames from about 5% to 7%.Based on the above-mentioned experimental and analytical results, the proposed bolted endplate connections and composite frames with bolted endplate connections exhibit good seismic performance, meeting the requirements of current codes for seismic resistant frames. The suggestions for the design and analysis of bolted endplate connections can be applied in engineering practice and revision of future codes.更多还原