Direct Answer
Seismic Design Category (SDC) is a classification assigned to a building by the structural engineer based on occupancy importance and seismic hazard (mapped ground motion at the site). SDC A and B have minimal seismic requirements. SDC C adds some ductility requirements. SDC D, E, and F require Special or Intermediate Moment Frames, Special Concentrically Braced Frames, or Eccentrically Braced Frames — all with significantly more demanding fabrication and inspection requirements.
Full Explanation
For fabricators, the SDC determines: which AISC 341 (Seismic Provisions) system applies, what connection prequalification is needed (AISC 358 for SMF), what weld requirements apply (AWS D1.8, Demand Critical welds), what material requirements apply (A992 for W-shapes in SDC D+, CVN toughness requirements), and what inspection level is required (special inspection with UT of Demand Critical welds). Projects in California, Pacific Northwest, and other high-seismic zones are almost always SDC D or E.
What This Means for Your Shop
If you are bidding seismic projects, verify your shop has the WPS, welder qualifications, and QC program for the SDC level. Seismic fabrication costs significantly more than standard construction — if your bid doesn't account for the seismic premium, you will lose money.
Common Mistakes
Bidding a high-seismic project without reading the structural drawings closely enough to identify the seismic system and its requirements. The words 'Special Moment Frame' or 'Special CBF' on the structural notes page mean a very different scope than standard construction.
Recommended Process
Before bidding any project, read the structural general notes for the seismic system type and SDC. If it's SDC D or higher, conduct a detailed scope assessment of seismic requirements before pricing.