Direct Answer
Column stiffener plates (continuity plates) are horizontal plates welded inside the column flanges at the level of the beam flanges in a moment connection. They prevent local flange bending and web crippling under the concentrated beam flange forces. Stiffeners are required when the beam flange force exceeds the column's local flange bending and web crippling capacity — specified by the engineer and called out in the connection design.
Full Explanation
Two types of column stiffeners at moment connections: (1) Continuity plates (or transverse stiffeners) — full-width plates spanning between flanges, aligned with the beam flanges. (2) Doubler plates — plates welded to the column web to increase shear resistance in the panel zone. Continuity plates are typically 1/2"–1" thick, A572 Gr 50, and are welded to both flanges and the web. AISC Design Guide 13 provides detailed design guidance. Stiffener plate geometry must fit within the column's clear dimension (depth between flanges minus fillet radius), which the detailer must verify from section tables.
What This Means for Your Shop
Continuity plates are among the most labor-intensive items in a column assembly. Each plate requires precise fitting within the column (often 1/32" clearance on each side), multiple welds in a confined space, and special inspection for the flange-to-plate CJP welds (which are Demand Critical in SMF).
Common Mistakes
Not accounting for the k-zone restrictions on column flange welding. AWS D1.8 requires that CJP welds start away from the k-zone (the fillet region of the column flange). If the stiffener plate is too wide, the weld start falls in the k-zone — a code violation.
Recommended Process
Verify stiffener plate dimensions against the actual column section's flange-to-web clearance and k-distance from AISC section tables before ordering. Cut stiffeners to fit precisely — undersized stiffeners with excessive gaps require more weld fill and don't meet fit-up requirements.