Prequalified Connections for Special and Intermediate Steel Moment Frames for Seismic Applications, ANSI/AISC 358-05

Building code provisions for the seismic resistant design of structures incorporate two basic types of requirements. The first of these is very similar to the code requirements for most other types of loading and comprises specification of minimum permissible strength and structural stiffness. The second is unique to design for seismic resistance and consists of prescriptive criteria on the detailing practice for the structural elements. While strength and stiffness requirements have been part of building codes for nearly 100 years, these detailing practice requirements, which include such things as prescription of the volumetric ratio and spacing of reinforcing in concrete structures, and permissible width/thickness ratios for elements of members in steel structures are a recent addition to the code. They were first introduced into the codes in the late 1960s and primarily affected the design of reinforced concrete structures. However, as researchers have continued to understand the importance of detailing to seismic performance and actual earthquakes have made clear to the profession that poor detailing practice directly leads to adverse structural behavior, the volume and complexity of these detailing requirements has steadily increased. Extensive detailing requirements for reinforced concrete structures were introduced into the building codes following the 1971 San Fernando earthquake. Requirements for timber and masonry structures were also added throughout the 1970s and 1980s as relatively modest earthquakes, such as the 1979 Imperial Valley, California; 1983 Coalinga, California and 1984 Morgan Hill, California earthquakes indicated problems associated with improperly detailed structures of this construction type. However, relatively few requirements for detailing of steel structures were placed in the codes during this period, largely because there were few examples of poor performance of steel structures. This began to change with the 1985 Mexico City, Mexico earthquake in which several large steel buildings in the lake bed region of Mexico City collapsed. Additional requirements for detailing of steel structures were introduced following the 1987 Whittier Narrows, California earthquake. Most of these requirements pertained to the detailing of braced steel frames. The 1994 Northridge, California earthquake resulted in the introduction of extensive code detailing requirements for moment-resisting steel frames. Immediately following the earthquake, brittle fractures were discovered in the beam to column connections of several buildings in the San Fernando Valley (Figures 1, 2). This damage was