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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 incorporates the capacity-based design approach, wherein energy dissipation is con ned predominantly within the reduced region of the Yield-Link® structural fuse. Member and connection design is based on the maximum rupture strength, Pr_link, of the reduced region of the 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 Failure Mode

Figure 1 — Link Tested to Failure at 6% Story Drift

The following are steps for the Strong Frame connection design:

  1. Model and analyze moment frame to get demand loads (moment, shear and axial) using code level forces.
  2. Design link yielding area to resist the maximum axial force from all the load combinations. This means our 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 link as:
    • Pr_link = Ay_link x Rt x Fu_link
    • Where
      • Ay_link = specified area of the reduced link area, 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. Link stem-to-beam flange connection bolts
    • b. Link stem-to-link flange welds
    • c. Link flange-to-column flange connection bolts
    • d. Link-flange thickness to prevent prying
    • e. Beam-to-column shear tab connection
    • f. Column panel zone
    • g. Column flange thickness
    • h. Stiffener/continuity plate (if required)