Structural steel fabrication is the shop work that turns plain mill lengths of beams, columns, plate and tube into finished, numbered pieces that bolt together on site: cutting to length, making holes, welding on connection plates, cleaning, coating and shipping. A fabricator works from the engineer’s design drawings, but what it actually builds from are its own shop drawings, which show every cut, hole and weld on every piece.
Fabrication sits between design and erection. The engineer decides what the frame must do, the fabricator makes the parts, and the erector assembles them on site. This guide walks through the shop process in order, explains what the paperwork means, and lists what to check when you are buying fabricated steel.
Step 1: Shop Drawings and Detailing
Design drawings show the frame: member sizes, grades, and the loads or connection types the engineer requires. They do not show where every hole goes. A steel detailer, working for the fabricator, turns them into two sets of drawings:
- Erection drawings show where each piece goes in the building, by its piece mark.
- Shop drawings (piece drawings) show one member at a time with exact lengths, cuts, copes, hole sizes and positions, attached plates, and weld sizes in standard weld symbols.
The shop drawings normally go back to the engineer of record and the architect for review before cutting starts. That review is the last cheap chance to catch a mistake. Reading member callouts on these drawings is covered in how to read steel beam sizes, and the weld notation in our weld symbols chart.
Most shops now detail in 3D modelling software that exports cutting and drilling files straight to CNC machines. SteelConstruction.info notes that this link between detailing software and CNC machinery improves accuracy and reduces the need for tape-measure work, which also speeds up erection on site (SteelConstruction.info, Fabrication, checked October 2026).
Step 2: Material and Mill Certificates
The fabricator buys sections and plate from a mill or a service center. Wide flange shapes in US buildings are usually ASTM A992, the specification for rolled structural shapes for building framing and bridges (ASTM A992/A992M-22 scope, checked October 2026). Every heat of steel arrives with a mill test report listing its chemistry and tested strength, and good shops keep those reports traceable to the pieces cut from that steel.
Ordering is planned around standard lengths. Shapes are rolled in set depth and weight increments, listed in our wide flange beam sizes chart, and a fabricator nests the pieces it needs into stock lengths to keep offcuts down.
Step 3: Cutting and Holing
Members are cut to length, coped where they meet other members, and given their bolt holes. The common methods:
| Process | Used for | Notes |
|---|---|---|
| Band or circular saw | Cutting beams, columns and tube to length | Square, clean ends; the standard first operation on most members |
| Oxy-fuel (flame) cutting | Thick plate, copes, profiles | Steel is preheated, then cut with an oxygen jet; handles very thick sections |
| Plasma cutting | Plate and profiles, often CNC | Faster than flame on thinner plate, with a cleaner edge |
| Drilling | Bolt holes in flanges, webs and plates | CNC drill lines drill several faces at once from the detail file |
| Punching | Holes in thinner plate and angles | Fast; where fatigue matters holes are punched undersize and reamed |
Accurate holes matter more than accurate lengths. A beam a little short can usually be shimmed; holes that do not line up mean reaming or re-drilling in the air, which slows erection and may need the engineer’s approval.
Step 4: Fit-Up, Welding and Inspection
Fitters lay out and tack-weld the attachments onto each main member: shear tabs, clip angles, stiffeners, base plates, cap plates and splice plates. Welders then complete the welds shown on the shop drawings. SteelConstruction.info identifies assembly and welding as generally the largest cost element in fabrication, which is why engineers who favor bolted site connections and simple shop welds tend to get cheaper steel.
In the US, structural welding is normally done to AWS D1.1, the Structural Welding Code for steel, which the American Welding Society describes as one of the most-referenced codes in the world (AWS Codes and Standards, checked October 2026). D1.1 covers welding procedures, welder qualification and inspection. Welds are checked visually, and where the project requires it, by non-destructive methods such as magnetic particle or ultrasonic testing.
Heat from welding distorts steel. Shops plan the welding sequence to limit that, and straighten members afterwards where needed.
Step 5: Cleaning and Coating
Many shops shot-blast steel before or after fabrication to remove mill scale and rust and give the surface a profile that paint can grip. Then the specified coating goes on: a shop primer, a full paint system, or a trip to a galvanizer. Some steel gets no coating at all. Steel that will be enclosed in a dry, heated building often does not need one, and surfaces that will be fireproofed or that form part of slip-critical bolted joints have their own rules about what may be applied. The specification decides; the shop does not guess.
If the pieces will be galvanized, the fabricator has to plan for it, adding vent and drain holes in closed sections and keeping the pieces within the size of the galvanizer’s kettle. Field welding on galvanized steel brings its own steps, covered in can you weld galvanized steel.
Step 6: Marking, Loading and Shipping
Every finished piece is stamped or painted with its piece mark so the erector can match it to the erection drawings. Pieces are loaded in the order the erector needs them, because a truck that arrives with the roof beams on the bottom costs crane time. Long or wide pieces may need special permits and escorts, which affects how the engineer and fabricator split members and where splices go. What happens after delivery is covered in our guide to structural steel erection.
Fabrication vs Erection vs Miscellaneous Metals
| Scope | Where it happens | Typical items |
|---|---|---|
| Structural fabrication | Fabrication shop | Beams, columns, bracing, connection plates, base plates |
| Erection | Job site | Lifting, bolting, field welding, plumbing the frame |
| Miscellaneous metals | Often a separate shop | Stairs, railings, ladders, lintels, embeds, grating |
One company may do all three, but on larger projects they are often separate contracts, so check whose scope covers items such as lintels, embed plates and stair stringers.
What to Check When Buying Fabricated Steel
- Quality certification. Many project specifications require a fabricator certified under the AISC Certification program. Building codes generally also require special inspection of fabricated structural items unless the shop is approved by the building official.
- Who does the detailing and how long shop drawing review takes. This is often the longest lead item.
- What the price includes: connection design, detailing, coating, delivery, and whether erection is included or separate.
- Coating specification, written down: none, primer, paint system or galvanizing.
- Mill test reports for the steel used.
- Tolerances and a punch list process for fixing pieces that do not fit.
None of this replaces design. Member sizes, connections and welds have to come from a licensed structural engineer; a fabricator builds what the approved drawings show.
Structural Steel Fabrication FAQ
What is the difference between steel fabrication and steel erection?
Fabrication is shop work: cutting, drilling, welding and coating the individual pieces. Erection is site work: lifting those pieces into place and bolting or welding them together into a frame.
What are shop drawings in steel fabrication?
Shop drawings are the fabricator’s piece-by-piece drawings showing exact lengths, holes, plates and welds for each member. They are produced from the engineer’s design drawings and normally reviewed by the engineer before fabrication starts.
What welding code applies to structural steel?
In the US, AWS D1.1 Structural Welding Code – Steel is the usual code for welded building steel. It covers procedures, welder qualification and inspection. The project specification states which code applies.
Why are most connections bolted on site and welded in the shop?
Shop welding is done under controlled conditions, in good positions and with easy inspection. Field welding is slower, weather-dependent and harder to inspect, so frames are usually detailed with shop-welded plates that bolt together on site.
How long does steel fabrication take?
It depends on the job size, the shop’s backlog and how quickly shop drawings are approved. Detailing and drawing review often take longer than the cutting and welding itself, so ask the fabricator for a schedule that shows both.
Reference information only, not engineering advice. Disclaimer.