Performance-Based Seismic Design of Concentrically Braced Frames

基于性能的同心支撑框架抗震设计基于性能的同心支撑框架抗震设计基于性能的抗震设计要求结构能够满足多种性能目标和足够精确的工程模型来评估系统性能。同心支撑钢框架（CBFs）可以设计成满足PBSD要求的目标。CBF具有刚性和强度，这是达到使用极限状态所需的属性。此外，还开发了特殊的同心支撑框架（SCBFs），以保证稳定的循环非弹性抗震性能，使SCBFs能够满足生命安全和防倒塌性能水平。最近的研究导致了无键支撑CBFs （UBCBFs）的发展。在这些框架系统中，无粘结支撑可以承受拉伸和压缩，而不会发生屈曲，从而提供卓越的抗震性能，是PBSD的理想选择。目前，SCBFs和UBCBFs的抗震性能都受到连接性能或连接设计要求的限制。目前scbf的设计规定试图确保足够的连接阻力，以避免连接过早失效，同时允许由于支撑屈曲而导致的严重非弹性连接旋转。ubcbf的连接必须设计成防止屈曲或过度变形，然后才能开发出完全粘合支撑的延展性。虽然这些设计要求在概念上很简单，但使用当前的设计条款很难实现这些目标，因为工程师必须使用近似和经验规则来实现这些设计目标。例如，对于SCBFs，设计规定要求阻力系数大于支撑的预期抗拉屈服强度。虽然这些规定试图实现合理的收益等级，但目前的应用存在缺陷。电阻系数的使用严重影响了连接设计，并可能导致不经济的大连接，从而对系统的抗震性能产生不利影响。即使是比例连接，系统的抗震性能也是未知的，因此，支撑断裂可能在连接屈服之前发生，从而大大降低了系统的承载力。为了克服当前设计的缺点，将开发一种合理的PBSD程序，以改进UBCBF和SCBF系统的连接设计和抗震性能。该研究将以现有的CBF设计要求为基础，并使用SAC钢铁项目期间为抗弯矩钢框架开发的创新技术。在该项目中，开发了设计程序来平衡理想的屈服机制，并限制不良的破坏模式，以达到理想的抗震性能。在这里，将开发类似的设计程序来平衡支撑和连接行为，以实现多个性能目标。该研究项目将评估和改进现有的工程模型，为SCBF和UBCBF系统开发合理的PBSD程序，利用实验研究结果评估、修改和验证设计程序。最后，该程序将用于设计几个框架。将开发一个概念性测试框架，用于拟议的未来测试，以展示使用明尼苏达大学未来NEES设施的完整系统的性能。结构工程专业认识到，cbf的连接设计规定代表了当前设计规定中存在的一个重大差距。提出的研究计划将产生稳健的设计程序和精确的工程模型。这些工具将允许工程师设计高效的SCBF系统，以满足当前和未来的地震工程需求。为了将研究成果转化为实践，研究小组将与著名结构工程师和结构工程学会合作（信件见补充文件）。除了培养今天的工程师，研究团队还将利用研究过程和结果来培养未来的工程师，包括研究生和本科生。学生将观察测试以熟悉实验研究过程。在课堂上，学生将把测试结果与文献中使用代码标准（本科生）和工程模型（研究生）估计的连接强度进行比较。学生将使用提出的设计方程，并将结果与规范标准进行比较。这个练习将为学生提供一个讨论和洞察替代设计方法的基础。

Performance-Based Seismic Design of Concentrically Braced Frames 0301792PI: Charles Roeder, University of WashingtonPerformance-based seismic design requires structures that can be designed to meet multiple performanceobjectives and engineering models that are accurate enough to assess the system performance. Steel concentrically braced frames (CBFs) can be designed to meet the required objectives of PBSD. CBF are stiff and strong, which are attributes need for serviceability limit states. In addition, special concentrically braced frames (SCBFs) have been developed to assure stable cyclic inelastic seismic performance, which enables SCBF to satisfy life safety and collapse prevention performance levels as well. More recent research has resulted in the development of unbonded brace CBFs (UBCBFs). In these framing systems, the unbonded braces yield in both tension and compression without brace buckling to provide a superior level of seismic resistance and are ideal candidates for PBSD. Currently, the seismic performance of both SCBFs and UBCBFs is limited by the connection performance or the connection design requirements. Current design provisions for SCBFs attempt to ensure adequate connection resistance to avoid premature connection failure, while permitting significant inelastic connection rotation due to brace buckling. Connections of UBCBFs must be designed to prevent buckling or excessive deformation prior to development of the full bonded brace ductility. Although these design requirements are conceptually simple, the objectives are difficult to achieve using current design provisions since engineers must use approximate and empirical rules to accomplish these design objectives. For example, for SCBFs, design provisions required that the factored resistance is larger than the expected tensile yield strength of the brace. Although the provisions attempt to achieve a rational yielding hierarchy, the present application is flawed. The use of the resistance factor severely penalizes the connection design and may result in uneconomically large connections that adversely affect the seismic performance of the system. Even for proportional connections, the seismic performance of the system is unknown, and therefore, brace fracture may precede connection yielding, significantly decreasing the system capacity.To overcome the shortcomings of current design, a rational, PBSD procedure will be developed to improvethe connection design and seismic performance of both UBCBF and SCBF systems. The research will build upon the existing CBF design requirements and use innovative techniques developed for moment-resisting steel frames during the SAC Steel Project. In that project, design procedures were developed to balance desirable yield mechanisms and to restrict undesirable failure modes to achieve the desired seismic performance. Here, similar design procedures will be developed to balance the brace and connection behavior to achieve multiple performance objectives. The research program will evaluate and improve existing engineering models, develop a rational PBSD procedure for both SCBF and UBCBF systems, evaluate, modify and validate the design procedure using experimental research results. Finally, the procedure will be used to design several frames. A conceptual test frame will be developed for proposedfuture testing to demonstrate the performance of the complete system using the future University of Minnesota NEES facility.The structural engineering profession recognizes that the connection design provisions for CBFs represent asignificant gap present in current design provisions. The proposed research program will result in robust design procedures and accurate engineering models. These tools will permit engineers to design efficient and effective SCBF systems to fulfill current and future seismic engineering needs. To facilitate the transfer of the research result to practice, the research team will partner with prominent structural engineers and structural engineering societies (letters provided in Supplementary Documents). In addition to educating today's engineers, the researcher team will use the research process and results to educate future engineers, including graduate and undergraduate students. Students will observe the testing to familiarize them with the experimental research process. In the classroom, the student will compare the test results to estimates of the connection strength using code standards (undergraduate) and engineering models (graduate) found in the literature. The students will use the proposed design equations and compare the results with the code standards. This exercise will provide the students with a basis for discussion of and insight into alternative design methods.