Special Moment Frame – Panel Zone Check
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 for the column. The capacity of the panel zone depends mostly on the thickness of the column web. When design limits are exceeded, 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 up to 75 plf (e.g., from a W14x74 to, say, a W14x145) can result in a less expensive frame due to the elimination of the welding cost and 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 D2.1). 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 D2.2) as well as reproduced in laboratory testing (Figure D2.3).
Figure D2.1 — 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)
Figure D2.2 — 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 Yield-Link® 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.
Figure D2.3 — 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)