Why a fabrication drawing is not a general arrangement

A general arrangement tells you where the steel goes. A fabrication drawing tells the workshop how to make it. That distinction matters more than most drawing offices admit, and it is where the gap between a tidy model and a buildable piece of steel usually opens up.

A GA might sit at 1:100 and show a frame in outline. A fabrication drawing comes in at 1:20, 1:10 or 1:5, covering a single member, a plate or a small assembly. If a fabricator has to ring you to ask the plate thickness, the drawing has failed.

Every fabrication sheet should carry:

• a unique piece mark tied to the model and to the GA
• section size, material grade and cut length
• hole diameters, positions, bolt grades and edge distances
• weld symbols, sizes and lengths
• surface treatment, primer or galvanising requirements
• the execution class and the tolerances that go with it

Under BS EN 1090-2, execution class drives weld quality, NDT and dimensional tolerance. If EXC2 is stated on the GA but EXC3 is needed at one connection, say so on the fabrication drawing.

Weld symbols the workshop can actually read

Weld symbols follow BS EN ISO 2553, and most errors are simple ones. A fillet weld size sits to the left of the symbol; length and pitch sit on the right. The circle at the junction means weld all round. A flag in the tail means site weld — and if something is welded on site, the workshop needs to know before it paints.

Four habits prevent most weld queries:

• State a default fillet size in the notes, so unmarked welds are not guesswork. "All welds 6mm fillet unless noted" is far more use than a blank sheet.
• Give intermittent welds both length and pitch — 6-50, for example — rather than leaving anyone to scale it off the screen.
• Use the dashed line correctly for the far side of the joint. A mirrored symbol welds the wrong face.
• Note any NDT requirement, and its extent, not just the technique.

Material grades, sections and cut lengths

Grade matters as much as size. S275JR will not always substitute for S355J2, particularly where impact toughness at low temperature is the reason for the J2. Weathering grades and galvanising are another trap: silicon and phosphorus levels affect the finish, so a small specification change can mean a visibly different coating.

Cut length is where the expensive errors hide. The length in a 3D model is often the setting-out length between node points, not the steel you need to buy and cut. If a beam is mitred at a corner, or coped to receive a secondary member, the cut length is longer than the centre-to-centre dimension. State plainly which number you have given: setting-out, cut or finished length.

Also allow for:

• saw kerf and any end machining allowance
• square, mitred, bevelled or drilled ends
• the mass of each piece, so the workshop can plan lifting
• camber, where required, including value and direction

Dimensions, marks and assembly logic

Dimension from a single datum rather than chaining across the whole member. Chains accumulate tolerances, and a 12-metre beam with eight chained dimensions can finish 5mm out at the far end. Dimension holes to centres and state diameter and bolt grade: M20 8.8 bolts normally take a 22mm hole, but check whether standard clearance or an oversize hole applies, particularly on galvanised work.

Marks do more than label. Use assembly marks for sub-assemblies and erection marks that match the GA, so workshop and site speak the same language. If a sub-assembly is welded and then drilled as a unit, say so — otherwise it may be built in the wrong order.

Revision clouds, notes and change control

Revision clouds are not decoration. They tell a fabricator holding a printed sheet on the bench exactly what has moved since the last issue.

A workable routine looks like this:

• cloud the changed geometry, and only the changed geometry
• add a revision triangle with the letter beside the cloud
• complete the revision block with date, description and initials
• state the reason — client change, clash resolution, fabrication query
• add "supersedes revision C" in the notes, and reissue any NC or DSTV files if a cut length has changed

Keep the previous revision on file. When someone asks why a beam finished 40mm short, the answer needs to be traceable.

Checks before the drawings are issued

Run the same list every time, because it is the boring items that cause the costly rework:

• every part has a unique mark, and every mark appears in the GA
• material grade stated on each fabrication drawing, not just in the general notes
• default weld note present, with size and process
• cut lengths consistent with the end preparation
• hole diameter, bolt grade and position checked against the connection design
• surface treatment and dry film thickness specified
• title block, drawing number and revision consistent across all sheets
• a test print at true scale — details that read well at 1:5 on A1 often vanish when squeezed onto A3

A little extra care at the checking stage buys a great deal of goodwill on the workshop floor, and it is always cheaper than cutting a replacement member.