Cyclic Testing of RBS Moment Connections with Built-Up Box Columns for San Diego Central Courthouse Facility

Built-up box columns are used from time to time for the construction of steel Special Moment Frames (SMF) to provide effective bi-axial column strength and stiffness especially in taller SMF structures where seismic drift demands and impact on architectural column sizes are particularly important. When used with prequalified “reduced beam section” (RBS) moment connection, AISC 358 permits built-up columns but with a limit on the column width or depth to 24 in. Note that the design requirements for these prequalified connections were mainly based on cyclic testing of beam-column subassemblies with wide-flange columns. But using built-up box columns raises some questions.
First, the force transfer mechanism is very different from that when the beam is framed into an I-shaped column in the strong direction as the stiff column web, and hence the stress concentration location, is shifted from the middle to the edges of the beam flange width. Second, continuity plates are almost always required when box columns are used. For built-up box column fabrication, one side of the continuity plate is typically connected to the closing column plate by the electro-slag welding (ESW) process, which requires a higher input energy. In the US, little attention has been paid to the effect of ESW process on the CJP joints in a welded SMF moment connection. A testing program has been developed to evaluate the issues mentioned above. A total of three full-scale RBS moment connections have been fabricated for cyclic testing in accordance with AISC 341. The 24-in wide and 36-in deep built-up columns were fabricated with 2 in plates of A572 Gr. 50 steel. A W36x302 section of A992 steel was specified for the beam.
Key testing parameters of interest to improve the performance and reliability of the RBS moment connection include (1) ESW detail, (2) RBS cut width (i.e., “c” dimension), (3) beam top flange CJP weld steel backing treatment, and (4) impact of WF beam weld access hole configurations. Tests conducted so far showed that brittle weld fracture could occur prematurely. A detailed analysis of the tested CJP welded joints showed that the hardness in the ESW and the heat-affected zones are not significantly higher than that in the base metal. But a notch condition inside the column at the ESW location was created due to the need to use containment plates for welding. Such notch condition-initiated fracture, which then propagated into the column flange plate. Additional testing was conducted with a modified ESW process in order to minimize the effect of notch condition.