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Special Moment Frame – Connection Design

Strong Frame Special Moment Frame

Moment Frame Design Requirements and Assumptions

Connection Design

The Strong Frame special moment frame using the Yield-Link® structural fuse incorporates the capacity-based design approach, wherein energy dissipation is confined predominantly within the reduced region of the Yield-Link structural fuse. Member and connection design is based on the maximum probable tensile strength, Pr-link, of the reduced region of the Yield-Link (see Figure 1).

SMF Connection Design, Figure 1(a) - Design Parameters

(a) Design Parameters

SMF Connection Design, Figure 1(b) Yield-Link Failure Mode

(b) Yield-Link Stretching and Shortening from Testing

Figure 1 — Yield-Link Design for Energy Dissipation

The following are steps for the Strong Frame connection design:

  1. Model and analyze moment frame with Yield-Link® moment connections to get demand loads (moment, shear and axial) using code level forces.
  2. Design Yield-Link yielding area to resist the maximum axial force from all the standard LRFD load combinations. This means our Yield-Links are designed to remain elastic under code force load combinations including lateral plus gravity loads.
  3. Once the yielding area is known, calculate the maximum rupture strength, Pr-link , of the Yield-Link as:
    • Pr-link = Ay-link x Rt x Fu-link
    • Where
      • Ay-link = area of reduced Yield-Link section, in.2
      • Rt = ratio of expected tensile rupture strength to minimum tensile stress of the link stem material, 1.2
      • Fu_link = specified minimum tensile strength of link stem material, 65 ksi
    • It is worthwhile to point out that we are using Rt and Fu for this calculation where other SMF connections typically use Ry, Fy and a Cpr factor that is less than or equal to 1.2. Using Ry of 1.1, Rt of 1.2, Fy of 50 ksi, Fu of 65 ksi and Cpr of 1.2. The difference in demand can be seen below:
      • Simpson Strong-Tie Strong Frame SMF Connection Design Demand: 1.2 x 65 ksi = 78 ksi
      • Standard SMF Connection Design Demand: 1.1 x 50 ksi x 1.2 = 66 ksi
    • The reason for this approach is to truly capture the ultimate strength of our Yield-Link structural fuse, since we want to make sure this is the only region where inelastic action occurs.
  4. After Pr-link has been determined, design the rest of the connection to exceed this Pr-link demand load:
    • a. Yield-Link stem-to-beam flange connection bolts
    • b. Yield-Link flange-to-column flange connection bolts
    • c. Yield-Link-flange thickness to prevent prying
    • d. Beam-to-column shear tab connection
    • e. Column panel zone
    • f. Column flange thickness
    • g. Stiffener/continuity plate (if required)