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

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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. ***