Seismic Design of Steel Special Moment Frames

Structural steel special moment frames often are used as part of the seismic force-resisting systems in buildings designed to resist earthquakes with substantial inelastic energy dissipation. They are one of a few select systems that U.S. building codes permit without restriction in buildings exceeding 160 ft in height, even in the most critical occupancies and in areas
mapped as having the highest ground motions. Beams, columns, and beam-column connections in steel special moment frames are proportioned and detailed to resist flexural, axial, and shearing actions that result as a building sways through multiple inelastic displacement cycles during strong earthquake ground shaking. Special proportioning and detailing requirements are therefore essential in resisting strong earthquake shaking with substantial inelastic behavior. These
moment-resisting frames are called Special Moment Frames because of these additional requirements, which improve the inelastic response characteristics of these frames in comparison with less stringently detailed Intermediate and Ordinary Moment Frames.

Design requirements for steel special moment frames are contained in a series of standards. ASCE/SEI 7-05, Minimum Design Loads for Buildings and Other Structures (ASCE 2006), referred hereafter as ASCE 7, sets the basic loading criteria for steel special moment frames together with associated lateral drift limits. ANSI/AISC 341-05, Seismic Provisions for Structural Steel Buildings (AISC 2005a) provides detailed design requirements relating to materials, framing members (beams, columns, and beam-column joints), connections, and construction quality assurance and quality control. In addition, AISC 341 presents requirements for columns that are not designated as part of the seismic force-resisting system. The numerous interrelated requirements for steel special moment frames are covered in several sections of AISC 341, with the primary requirements covered in Section 9 of Part I of the document. Section 9 of AISC 341 references ANSI/AISC 358- 05, Prequalified Connections for Special and Intermediate Steel Moment Frames for Seismic Applications including Supplement No.1 (AISC 2005b), which is written to facilitate and standardize the design of steel special moment frame connections to allow their use without the need for projectspecific testing. A series of different moment connection details is presented in AISC 358-05, and additional connections are being added for the 2010 edition of this document.