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Special Moment Frame – Panel Zone Check

Strong Frame Special Moment Frame

Moment Frame Design Requirements and Assumptions

D2. Panel Zone Check

Other than drift check, the second limit state that governs the design of a moment frame is the connection panel zone shear capacity. The capacity of the panel zone depends mostly on the thickness of the column web. When the demand-to-capacity ratio is greater than 1, many engineers tend to increase the thickness of the column web by welding a doubler plate

to increase the shear capacity. However, many fabricators are aware that increasing the column web thickness by increasing column weight approximately 75 plf (e.g., from a W14x74 to, say, a W14x159) can result in a less expensive frame due to the elimination of the CJP welding cost and UT inspection cost of the doubler plate. If panel zone capacity is not checked, the consequence can be column kinking due to a weak panel zone (Figure 1). This can lead to column flange fracture just above and below the beam flanges connecting to the column. This phenomenon has been observed after a strong seismic event (Figure 2) as well as reproduced in laboratory testing (Figure 3).

Panel Zone Check, Figure 1 — Column Kinking Attributable to Weak Panel Zone

Figure 1 — Column Kinking Attributable to Weak Panel Zone
(Ref: Uang and Chi, SSRP-2001/05, Effect of Straightening Methods on the Cyclic Behavior of k Area in Steel Rolled Shapes)

Panel Zone Check, Figure 2 — Fracture of Welded Beam-to-Column Connection in Northridge Earthquake

Figure 2 — Fracture of Welded Beam-to-Column Connection in Northridge Earthquake
(Ref: NIST GCR 09-917-3, NEHRP Seismic Design Technical Brief No.2)

Strong Frame Special Moment Frame Panel Zone Check

For typical SMF connection design (e.g., RBS), the design shear demand on the panel zone is calculated from the summation of the moments at the face of the column by projecting the expected moment at the plastic hinge point to the column faces.

For the Strong Frame SMF, the panel zone demand is calculated from statics using the shear at the top and bottom of the beam from the link’s ultimate axial capacity (Pr_link). This demand is higher than that of a typical moment connection, where the expected moment is taken as, Mpe = Ry*Fy*Zx, where Ry = 1.1 and Fy = 50 ksi for A992 steel. For the Strong Frame, Pr_link is calculated using Rt = 1.2 and Fu = 65 ksi. On the capacity side, the Strong Frame panel zone’s shear capacity is calculated assuming a φ = 0.9, whereas φ = 1.0 is used in the typical moment connection design. Panel zone capacity check is required by AISC 341 and is provided in the calculations supplied by Simpson Strong-Tie.

Panel Zone Check, Figure 3 — Fracture of Welded Beam-to-Column Connection in a Laboratory Test

Figure 3 — Fracture of Welded Beam-to-Column Connection in a Laboratory Test
(Ref: Uang and Chi, SSRP-2001/05, Effect of Straightening Methods on the Cyclic Behavior of k Area in Steel Rolled Shapes)