Strong Frame^® Special Moment Frame Introduction

History of Special Moment Frame Development

Traditionally, special steel moment frames are designed so that the beam will yield under large displacement. The yielding of the beam section provides energy dissipation and is designed to ensure the beam-to-column connection is not compromised. The current design philosophy is the product of extensive testing of SMF connections from the findings of the 1994 Northridge and 1989 Loma Prieta earthquakes in California. Figures 1, 2 and 3 are test specimens showing yielding at designated areas of the beam.

Introduction to Simpson Strong-Tie Strong Frame® Special Moment Frames

Traditional prequalified moment frames most often require a welded connection with either a weakened beam or a stiffened connection in order to allow the beam to yield as necessary during a seismic event so as to dissipate energy. These types of connections require that the beam be braced to resist the lateral torsional buckling per code. However, it is difficult to meet the bracing stiffness requirements with the use of light-framed wood members. Because of concerns about beam bracing and welding in wood structures, Simpson Strong-Tie designed the Strong Frame special moment frame (SMF) with a fieldbolted moment connection that is a partially restrained (Type PR) connection that uses the Yield-Link® structural fuse for moment transfer.

The yielding during a major seismic event has been moved from the beams to the Yield-Links, and the connection follows a capacity-based design approach. This allows the connection to remain elastic under factored load combinations, and seismic inelastic rotation demand is confined within the connection when yielding is experienced from severe events. With the yielding confined predominantly to the replaceable Yield-Link moment connection, inelastic behavior is not expected from the members and the beam can be designed without beam bracing.

Special Moment Frame Applications

There are several benefits to using the Simpson Strong-Tie Yield-Link® moment connection for new and retrofit projects. In new construction, the frame can be incorporated into the early stages of design. Simpson Strong-Tie can provide design options for the customer without charge. The field-bolted connections allow for quicker frame erection and installation. In retrofit designs, the bolted connection means the frame can be erected in the interior conditions of light-frame construction without the risk of fire. The beam and columns can be erected in parts, making the SMF much easier to handle than a fully welded frame.