Special Moment Frame – Connection Design

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).

(a) Design Parameters

(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, P_r-link , of the Yield-Link as:
• P_r-link = A_y-link x R_t x F_u-link
• Where
  • A_y-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 R_t and F_u for this calculation where other SMF connections typically use R_y, F_y and a C_pr factor that is less than or equal to 1.2. Using R_y of 1.1, R_t of 1.2, F_y of 50 ksi, Fu of 65 ksi and C_pr 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 P_r-link has been determined, design the rest of the connection to exceed this P_r-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)