S Steel Moment Frame Buildings

Buildings that use a system of steel beam and columns, with connections capable of transferring bending moments, for the primary lateral force resisting system are called steel moment frame buildings. Steel moment frames are those frames that develop their seismic resistance through bending of beams and columns and shearing of the beam-column panel zones (ATC, 1997). Steel moment frames classified as special moment resisting frames (SMRF) by the 1997 AJSC Seismic Provisions for Structural Steel Buildings (AISC, 1997) are designed to provide ductile performance during strong earthquake ground motions. SMRF's designed according to the 1997 AISC Seismic Provisions must satisfy strong column-weak beam requirements at all of the beam-column intersections which is intended to ensure ductile building performance. By designing the beams to yield before other frame members, the input energy from earthquake ground motions is dissipated through inelastic bending of the beams. In order to ensure that the beams will yield first, the columns and beam-to-column connections are designed to develop the full strength of the beams. After the 1994 Northridge Earthquake, it was discovered that many of the buildings in the Los Angeles metropolitan area suffered severe damage to their welded beam-column moment connections. Therefore, the beams were not able to yield as intended and dissipate energy through inelastic deformations. The significance of this discovery is that the connection fractures occurred in buildings that otherwise, showed no signs of distress. This discovery sparked the organization of the SAC Joint Venture, a partnership between SEAOC, ATC, and the California Universities for Research in Earthquake Engineering (CUREe). The purpose of the SAC Joint Venture is to investigate the causes of these premature connection fractures, to develop methods to predict the probable earthquake performance, and to develop methods to design and construct more reliable steel moment frame buildings (FEMA, 1995).

FEMA 273 was published after the Northridge Earthquake. Therefore, it recognizes the deficiencies in current steel moment frame building design. Table 2.9 provides a description of steel moment frame building damage that is expected to occur for each performance level defined by FEMA 273. Connection fractures are expected to occur only for the Life Safety and Collapse Prevention performance levels, which for the Basic Safety Objective, correspond to earthquake hazard levels of 10% in 50 years and 2% in 50 years, respectively. These damage descriptions only apply to steel moment frame buildings constructed using pre-Northridge connections details. Beam-to-column connections prior to the Northridge Earthquake were typically constructed with full penetration groove welds between the beam flanges and the column flanges, and a shear plate welded to the column flange and bolted to the beam web. Since the Northridge earthquake, several connection details have been proposed (FEMA, 1995) that effectively force the inelastic bending of the beam away from the vulnerable beam-to-column connection, thereby reducing the demands on the connection. Table 2.9 also provides guidelines for acceptable performance in terms of building response quantities, such as building drift, connection plastic rotation, and panel zone plastic rotation.

The SAC Joint Venture is now in the final stages of developing a seismic design criteria (FEMA, 2000) for steel moment frame buildings. During these final stages, part of the research described in this dissertation will be used in the development of the seismic design criteria. The SAC design criteria is cast in the same framework as the 1997 NEHRP Provisions with the intent of supplementing the requirements of the locally applicable building codes. The seismic design criteria will be the first building design criteria that includes guidelines that will enable the structural engineer to evaluate the probability that a new or existing WSMF building design will meet specified performance objectives. These reliability evaluation guidelines are intended to satisfy the purpose of the Design Verification steps contained in the Vision 2000 PBSD framework.

Establish Site Suitability and Design Ground Motions z

Initial

Design

Phase

Conceptual Design: Select Structural System and Configuration

Conceptual Design: Select Structural System and Configuration

Seismic Eurocode

Steps Not Traditionaly Found in Building Codes

Design

Steps Not Traditionaly Found in Building Codes

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