Direct Answer
Erection drawings are plan and elevation views showing how all the fabricated pieces fit together in the field — they show gridlines, elevations, member locations, and piece mark callouts. Piece mark drawings (also called detail drawings) show how to fabricate each individual member. The erector uses erection drawings to know where each piece goes; the fabricator uses piece mark drawings to make each piece.
Full Explanation
An erection drawing for a typical floor level shows the structural grid, beam and column locations, and a callout for every member (e.g., 'B-7' at a specific bay). The erector references this plan to sequence installation. Piece mark B-7's detail drawing, meanwhile, shows its exact length, end connections, bolt holes, and any camber requirements. Both drawing types are produced by the detailer and submitted together as part of the shop drawing package.
What This Means for Your Shop
Fabricators must ensure that every piece mark called out on the erection drawings has a corresponding detail drawing. A mismatch — a piece mark on erection drawings with no detail — is a detailing error that must be flagged as an RFI before fabrication.
Common Mistakes
Erectors improvising the location of a member because the erection drawing callout didn't match what was fabricated. This is always a detailing or fabrication error that needs a formal resolution, not a field workaround.
Recommended Process
Before releasing the IFC package to the shop, cross-check that every piece mark on every erection drawing has a corresponding piece mark detail sheet and a BOM entry. This QC step takes an hour and prevents days of field delays.