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.
Figure 1 — Formation of Plastic Hinge at RBS Connection
(Reference: Gilton, Chi and Uang, UCSD SSRP-2000/03)
Figure 2 — Fracture of Beam Flange Plate Moment Connection
(Reference: Sato, Newell and Uang, UCSD SSRP-2007)
Figure 3 — End Plate Specimen at Failure
(Reference: Sumner et al. 2000)
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.
The highlighted green section illustrates the yielding area on the Strong Frame special moment frame connection, which is a patented system designed to yield in a seismic event.
(Protected by U.S. and foreign patents and other pending and granted foreign patents.)
Sample Yield-Link Compression and Elongation from Testing
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.
Prequaliﬁed moment connections are structural steel moment connection conﬁgurations and details that have been reviewed by the AISC Connection Prequaliﬁcation Review Panel (CPRP) and incorporated into the AISC 358-16 standard. The criteria for prequaliﬁcation are spelled out in the AISC seismic provisions, AISC 341. In short, AISC 341 contains performance and testing requirements that have been shown to produce robust moment connections, and AISC 358 includes connection details that meet those criteria. AISC 358-16 Prequalified Connections include the Simpson Strong-Tie Strong Frame moment connection in Chapter 12.
Strong Frame special moment frame and the Yield-Link structural fuse are included in ANSI/AISC 358-16, prequalified connections for Special and Intermediate Steel Moment Frames for Seismic Applications, Chapter 12.Read ANSI/AISC 358-16
Strong Frame moment frames are code listed under the 2009, 2012, 2015 and 2018 IRC/IBC and ESR-2802 with LABC Supplement.