"Cracking Repair, and Ductility Enhancement of Large-Size Steel Beam-to-Column Connections Subjected to Dynamic CyclicLoading"

9416266 Lu北岭地震带来的一个重要问题是，在地面动力震动下，钢框架中的一些焊接弯矩连接的脆弱性。 据报道，50多座低层和高层建筑的梁柱连接处发现裂缝。 两种不同类型的解理裂纹被观察到形成在梁底部翼缘坡口焊接连接到柱翼缘面。 这两种类型的裂缝都起源于天然裂缝状条件，这是由留在原位的钢支撑杆造成的。 初步调查表明，熔敷焊缝金属具有异常低的断裂韧性。 但是，结构钢的机械和化学性能一般满足AISC和ASTM标准要求。 值得关注的是，如果连接没有可检测到的变化，是否应该对其进行改造。 提出了如何“保护”关键连接的问题，以及减少对这些连接的强度和变形要求的方法，以及减少对这些连接的强度和变形要求的方法。 尽快建立可靠的连接设计和制造方法至关重要，以便将来建造的结构不会遇到类似的故障。 本项目下的研究有以下目标：（1）确定开裂的可能原因，（2）评估推荐的修复方法的适当性，（3）找到合理且具有成本效益的方法来改善连接性能和/或减少对连接的需求，以及（4）开发焊接连接的可靠设计和制造程序。 研究计划包括六项任务：（1）焊接工艺和焊缝金属质量的研究（包括对用于修复的电极进行程序鉴定试验，以确保足够的断裂韧性），（2）对大尺寸连接的动态实验，（3）对修复的连接的动态实验，（4）对具有增强的延展性的改进连接的动态实验，（5）分析研究;（6）提出建议。 将对8个大尺寸连接试件进行共计16次试验，并施加循环动荷载。 实验将在利哈伊大学的ATLSS实验室进行。 实验将在利哈伊大学的ATLSS实验室进行。 一个包括结构工程师和焊接专家在内的跨学科团队将进行这项研究。 结果将允许开发有关的明确建议：焊接金属和焊接程序，裂纹连接的修复，和连接的性能增强。 该研究还将确定连接开裂的可能原因。 ***

9416266 Lu One of the significant issues that were brought forth, quite unexpectedly, by the Northridge earthquake is the vulnerability of some welded moment connections in steel frames subjected to dynamic ground shaking. It has been reported that in more than 50 low- and high-rise buildings cracks were found in the beam-to column connections. Two distinct types of cleavage cracks were observed to form at the beam bottom flange groove welded connection to the column flange face. Both types of cracks had their origin at the natural crack-like condition that results with a left- in-place steel backing bar. Preliminary investigations indicates that the deposited weld metal has unusually low fracture toughness. However, the mechanical and chemical properties of the structural steel generally meet the AISC and ASTM standard requirements. Of concern is whether a connection should be retrofitted if it has not detectable change. The issue of how to "protect" critical connections has been raised as well as approaches to lessen strength and deformation demand on these connections, has been raised as well as approaches to lessen strength and deformation demand on these connections. It is vitally important that reliable connection design and fabrication methodologies be established as soon as possible so that structures constructed in the future would not experience similar failures. The research under this project has the following objectives: (1) to determine the probable causes of cracking, (2) to evaluate the adequacy of the recommended repair methods, (3) to find rational and cost-effective ways to improve connection performance and/or to reduce demand on connections, and (4) to develop reliable design and fabrication procedures for welded connections. The research plan includes six tasks: (1) studies of welding procedure and quality of weld metal (including performing procedure qualification tests on electrodes to be used for repairs to insure adequate fracture toughness), (2) dynamic experiments on large-size connections, (3) dynamic experiments on repaired connections, (4) dynamic experiments on modified connections with enhanced ductility, (5) analytical studies, and (6) development of recommendations. A total of 16 tests will be conducted on 8 large-size connection specimens with cyclically applied dynamic load. The experiments will be performed in the ATLSS Laboratory at Lehigh University. The experiments will be performed in the ATLSS Laboratory at Lehigh University. An interdisciplinary team including structural engineers and welding metallurgist will carry out the study. The results will permit the development of definitive recommendations related to: weld metal and welding procedure, repair of cracked connections, and performance enhancement of connections. The study will also establish probable causes of connection cracking. ***