What an Assembly Drawing Contains
An assembly drawing shows all the components that make up the finished assembly — their relative positions, the welds that join them, and the overall assembled dimensions. It includes: a bill of material listing each component piece mark, weld symbols between components, overall assembly dimensions for quality control, and notes for any special requirements (heat treatment, galvanizing, pre-assembled in sections for transport). The assembly piece mark — the mark that identifies the finished unit — appears in the title block.
Common Assembly Types
The most frequently detailed assembly types in structural steel include: built-up plate girders (flanges and web welded together), built-up column sections (plates welded into box or I-shapes), column base plate assemblies (plate, stiffeners, and anchor bolt hardware), stairway assemblies (stringers, platforms, and landings), and equipment support platforms (beams, grating support angles, and handrail posts).
The Relationship Between Assembly and Erection Drawings
Do not confuse assembly drawings with erection drawings. Assembly drawings live in the shop — they show fabrication of an individual multi-component unit. Erection drawings live on the jobsite — they show where assembled members go in the structure. A complex column assembly might have an assembly drawing showing how the base plate, column shaft, and cap plate are welded together, and a separate erection drawing showing that assembled column at its specific grid location.
Weld Symbols on Assembly Drawings
Assembly drawings carry complete AWS A2.4 weld symbols for every joint between components. The weld symbol specifies the weld type (fillet, complete joint penetration groove, partial joint penetration groove, plug), the weld size in sixteenths of an inch for fillets or the throat dimension for groove welds, the weld length and pitch for intermittent welds, the side of joint, and any field weld or all-around designations. For built-up members like plate girders, the assembly drawing must show the flange-to-web fillet welds with the exact size required per AWS D1.1 calculations — not just a generic callout. If the structural drawings specify CJP groove welds at flange-to-web joints on seismic members per AISC 341, the assembly drawing must reflect that requirement explicitly.
Quality Control Dimensions on Assembly Drawings
Assembly drawings should include overall quality control dimensions that allow the fabricated assembly to be checked before painting or shipping. For a built-up column, QC dimensions include: overall height, flange-to-flange width at top and bottom, cap plate squareness tolerance, and base plate hole pattern verification dimensions. For a plate girder, QC dimensions include: overall length, camber (if pre-cambered), web flatness, and bearing stiffener locations. These dimensions are distinct from fabrication dimensions — they are check dimensions that confirm the assembled geometry meets tolerances before the assembly is moved to the next production stage.
Assembly Drawing BOMs and Sub-Component Linking
An assembly drawing's bill of material lists every sub-component by its own piece mark. The assembly mark (e.g., A-201) is the finished assembly; it contains sub-components like P-201 (base plate), P-202 (stiffener plates, qty 2), and C-201 (column shaft). Each sub-component has its own single-part drawing where its individual dimensions are fully detailed. The assembly drawing should cross-reference the single-part drawing numbers for each sub-component, so the fitter can pull the plate drawings alongside the assembly drawing when setting up the fit-up jig. In Tekla Structures, these links are maintained automatically — the assembly's BOM is generated from the model objects within the assembly container, and cross-references are consistent.
When Assembly Drawings Are Generated from Tekla
In Tekla Structures, assembly drawings are generated from the model's assembly containers — groups of objects that Tekla treats as a single fabricated unit. The assembly drawing includes all objects in the container: the main part and all secondary parts welded to it. Connection components (bolts, welds between separate assemblies) appear on the assembly drawing for the member that receives them, not for both members. Detailers must confirm that Tekla's assembly container boundaries match the fabrication intent — a column assembly that includes its shear tabs is correct; an assembly that accidentally includes a beam framing to it will produce a drawing showing both members as a single assembly, which is incorrect for the fitter.
Special Inspection Requirements Referenced on Assembly Drawings
Many assembly drawing sets trigger mandatory special inspection requirements under IBC Chapter 17 and the project's statement of special inspections. The assembly drawing is the right place to document these requirements so the shop inspection agency and the fabricator's QC team know what is required at each production step. Common inspection callouts on assembly drawings include:
- CJP groove welds requiring UT per AWS D1.1 Table 9.1 for seismic-force-resisting system members per AISC 341
- PJP groove welds requiring visual and MT or PT inspection per project specification Section 05 12 00
- High-strength bolts requiring installation verification per AISC Specification Section J3 and RCSC Specification
- Built-up members requiring camber verification before and after welding to confirm the as-fabricated geometry meets AISC CoSP tolerance
- Certified mill test reports (CMTRs) required for all A992 wide-flange material per the structural specification
Shipping and Handling Notes on Assembly Drawings
For large or irregular assemblies, the assembly drawing should include notes governing how the fabricated unit is to be handled, stored, and shipped. A plate girder may require dedicated shipping saddles to prevent lateral buckling of the web during transport. A stairway assembly may need to be shipped in sections and spliced at the site. A column with a projecting bracket may require a temporary brace to protect the bracket during handling. These notes are typically derived from the fabricator's own experience and the EOR's guidance on erection stability. When shipping notes are absent, the receiving party at the jobsite defaults to common practice — which may not be appropriate for an unusually large or fragile assembly.