Design of special moment frames (SMF) and intermediate moment frames (IMF) in accordance with the AISC Seismic Provisions for Structural Steel Buildings (AISC, 2022a), hereafter referred to as the AISC Seismic Provisions, and applicable building codes includes an expectation that they will experience substantial inelastic deformations when subjected to design-level earthquake ground shaking, generally concentrated at the moment-resisting beam-to-column connections. In the 1994 Northridge earthquake, a number of steel moment frame buildings were found to have experienced brittle fractures that initiated at the welded beam flange-to-column flange joints of moment connections. These brittle fractures were unexpected and were quite different from the anticipated behavior of ductile beam flexural yielding in plastic hinge zones. Where they occurred, these brittle fractures prevented the formation of ductile plastic hinge zones and resulted in frame behavior substantially different from that upon which the design requirements for these systems were based.
Following this discovery, the Federal Emergency Management Agency (FEMA) provided funding to a coalition of universities and professional associations, known as the SAC Joint Venture. Over a period of six years, the SAC Joint Venture, with participation from AISC, AISI, AWS, and other industry groups, conducted extensive research into the causes of the damage that had occurred in the Northridge earthquake and effective means of reducing the possibility of such damage in future earthquakes.
Numerous issues were identified in the SAC studies as contributing causes of these brittle fractures. This standard specifically addresses the following four causes that were identified in the SAC study:
- (1) Connection geometries that resulted in large stress concentrations in regions of high triaxiality and limited ability to yield.
- (2) Use of weld filler metals with low inherent notch toughness and limited ductility.
- (3) High variability in the yield strengths of beams and columns resulting in unanticipated zones of weakness in connection assemblies.
- (4) Welding practice and workmanship that fell outside the acceptable parameters of the AWS D1.1/D1.1M, Structural Welding Code, at that time.
A more complete listing of the causes of damage sustained in the Northridge earth-quake may be found in a series of publications [FEMA 350 (FEMA, 2000b), FEMA 351 (FEMA, 2000c), FEMA 352 (FEMA, 2000a), FEMA 353 (FEMA, 2000d), FEMA 355C (FEMA, 2000f), and FEMA 355D (FEMA, 2000e)] published in 2000 by the SAC Joint Venture that presented recommendations for design and construction of moment resisting frames designed to experience substantial inelastic deformation during design ground shaking. These recommendations included changes to material specifications for base metals and weld filler metals, improved quality assurance procedures during construction, and the use of connection geometries that had been demonstrated by testing and analysis to be capable of resisting appropriate levels of inelastic deformation without fracture. Most of these recommendations have been incorporated into the AISC Seismic Provisions as well as into AWS D1.8/D1.8M, Structural Welding Code—Seismic Supplement (AWS, 2021).
Following the SAC Joint Venture recommendations, the AISC Seismic Provisions required that moment connections used in special or intermediate steel moment frames be demonstrated by testing to be capable of providing the necessary ductility. Two means of demonstration are acceptable. One means consists of project-specific testing in which a limited number of full-scale specimens, representing the connections to be used in a structure, are constructed and tested in accordance with a protocol prescribed in AISC Seismic Provisions Section K2. Recognizing that it is costly and time consuming to perform such tests, the AISC Seismic Provisions also provide for prequalification of connections. The specific criteria for prequalification of beam-to-column connections are prescribed in AISC Seismic Provisions Section K1 and include a rigorous program of testing, analytical evaluation, and review by an independent body, the Connection Prequalification Review Panel (CPRP). Connections contained in this standard have met these criteria for prequalification when applied to framing that complies with the limitations contained herein and when designed and detailed in accordance with this standard.