A steel workshop can fabricate the wrong thing without doing anything wrong. They cut, drill and weld exactly as the drawing says. The member arrives on site, the bolt holes miss the column, and nothing fits.
At that point the question is not who cut it wrong. The question is how those steel shop drawings came out of the 3D model. Most workshop errors begin at the modelling stage, not at the CNC cutting machine.
This article answers four questions. How do shop drawings differ from design drawings? What steps lead from a Tekla Structures model to a full fabrication set? Which modelling mistakes turn into defects on the shop floor? And how far can an automatic material schedule be trusted? If the software itself is unfamiliar, read What is Tekla Structures? first.
How detailed steel shop drawings have to be
Who produces shop drawings, how they differ from the approved design and what the submittal process involves — what is a shop drawing? covers that for every discipline. What follows is specific to structural steel, where the drawing feeds the cutting machine and the tolerance for error is far lower.
They give the cut length of every piece, the position and diameter of every hole, every gusset plate, every weld. Holes also carry a clearance Δd, and that clearance differs between ordinary bolts and high-strength bolts. Weld groove preparation belongs on these drawings too.
Every loose piece carries a unique mark — a part mark for a single part, an assembly mark for an assembly. Those marks drive fabrication tracking and tell the erection crew what goes where.
| Design drawings | Shop drawings | |
|---|---|---|
| Prepared by | Design consultant | Steel contractor, fabricator, sub-detailer |
| Level of detail (LOD) | LOD 300 to LOD 350 | LOD 400, ready for fabrication |
| Unit shown | Structural system and members overall | Assemblies and single parts |
| Purpose | Appraisal, load checking, cost estimating | Cutting, drilling, fit-up, welding, painting in the shop; erection on site |
| Content focus | Forces, sections, codes applied | Fabrication tolerances, erection clearances, shop and site welds, CNC cutting data |
| Read by | Client, checkers, contractor | Shop crew, erectors, quantity engineers |
| Approval | Design appraisal | Consultant approval before fabrication |
A complete shop drawing set has three layers, and these are exactly the three drawing types Tekla produces.
- General arrangement (Erection / GA) — plans, elevations and sections of the whole structure, marking each assembly and locating it against grids and foundation levels. The lifting crew works from this layer.
- Assembly drawings — one sheet per assembly. A truss, or a column with its gussets and stiffeners already welded on. The sheet carries overall dimensions, weight, centre of gravity and the welds made in the shop. It also gives straightening points and the tolerance checks required after welding. The welding shop works from this layer.
- Single part drawings — one sheet or one table row per loose piece. A gusset plate, a section not yet fitted up. The sheet carries cut dimensions and hole positions. Oxy-gas, plasma and laser cutting operators work from this layer, along with the punching and drilling machines.
Alongside those three sit the schedules. Sections by grade and length. Plate by thickness. Bolts split three ways: anchor bolts, structural bolts and high-strength bolts. Paint and galvanising by area, with dry and wet film thickness. Assembly weights for crane selection and transport.
Six steps from the Tekla model to fabrication drawings
These six steps are also the sequence of the 15-session Tekla Structures course at the Institute. RDSIC is a Trimble authorised training centre, so the programme follows how the software is used on real projects.
Step 1 — set up and read the design. Declare units, grids, levels, the steel and bolt standards in force, and the workshop's marking convention. Read the general notes carefully: steel grade, bolt type, welding and painting requirements. Anything unclear goes back to the consultant as a written request for information, straight away — never as a guess. Changing a connection principle after the model is built means starting again on much of it.
Step 2 — model the main frame. Columns, trusses, beams, purlins and bracing at the right sections and positions. For repeating frames, use Tekla's parametric components. Check the result two ways: compare the total weight against the designer's estimate, and cut a few sections against the design drawings.
Step 3 — model connections and details. Gussets, bolts, stiffeners, welds and holes for purlins. This step decides the quality of the whole set. It also demands the most understanding of how steel is actually built: erection clearances, edge distances, weld lengths, fabrication tolerances. Tekla has a clash check built in. Run it to catch bolts fouling plates or welds, and resolve every one of them before moving on.
Step 4 — numbering. Tekla recognises identical parts and gives them the same mark. Your job is to set the numbering rules and check what comes out. Rules usually follow member type, zone or delivery batch. Bad numbering has two consequences. The workshop fabricates two different members under one mark. Or the site cannot tell which member goes where.
Step 5 — produce the drawings. Generate GA, assembly and single part drawings using the company template set. Templates govern the title block, scales, dimensioning style, weld symbols and the material table on each sheet. Tekla creates hundreds of sheets automatically. Your job is to review each one: are dimensions overlapping, are the views sufficient, are the notes right. Fixes made in the template save work on the next run.
Step 6 — schedules and cross-checks. Export material schedules by assembly and in total, the bolt list, and machine data files if the shop runs numerically controlled equipment. NC files in DSTV format drive punching and drilling machines; DXF files drive plasma and laser cutters. Then cross-check three things. Schedule weight against the designer's estimate. Member count on the GA against the number of assembly sheets. A few members picked at random, re-measured on the model. Only then submit for approval, and fabricate only from approved drawings.
These six steps form a loop, not a straight line. Change a connection at step 3 and you renumber at step 4 and reissue at step 5. That loop is where modelling beats hand drafting: it takes hours rather than weeks. Provided the model was built properly in the first place.
Modelling errors that reach the shop floor
These are the errors a drawing checker at a fabrication plant catches most often, ordered by what they cost.
Erection clearances left out. The model is built perfectly tight, with the beam exactly as long as the gap between two column faces. On site it will not go in, because erected columns always lean or sit off the foundation by some amount within tolerance.
The cost is not just lost time. Someone brings a plasma cutter and trims the member in place. That cut destroys the protective coating and reduces the load capacity of the member. Clearances and tolerances belong in the model, following the workshop's convention.
Wrong bolts, or too short a grip length. The model uses the software's default bolt rather than the one specified for the project. Or the grip length is too short for the combined plate thickness.
It shows up at tightening: the nut runs out of thread before the plates are clamped. A correct bolt protrudes two to three thread pitches past the nut. The bolt schedule inherits the error, and the workshop buys the wrong bolts.
Numbering left unchecked. Two members differing by one hole carry the same mark, because the numbering setup ignores holes. The shop then makes a batch to that mark, and the crew installs a short member where a long one belongs.
The opposite is costly too: one identical detail split across five or six marks. Stock control gets confused, and so does fit-up. Renumber after every revision and compare against the previous run.
Asymmetric details shown from the wrong side. A gusset offset to one side, a stiffener on one face. The model is right, but the assembly sheet uses the default view direction and the welder tacks the plate on the opposite face. Check the main view direction for each member type in the template.
Fixing the drawing instead of the model. This is the worst habit in model-based work. Someone spots an error on a sheet and types over the dimension there for speed.
The displayed number is now right and everything generated from the model is still wrong: the NC file cuts the wrong blank on the CNC machine, and the quantity schedule is wrong. The rule that cannot be broken: every change goes through the model, then the drawings are reissued.
No frozen model after approval. Drawings have gone to the shop and cutting has started. A detailer touches the model because nothing was locked. The next issue no longer matches the members already being made. Every approval must correspond to a saved, locked model version.
What these six share: none of them is a missing software command. All of them are about understanding steel detailing and keeping to the process. That is why connections, tolerances and numbering take up a real share of the Institute's Tekla course, even though students usually arrive wanting only to learn how to produce drawings.
How far to trust the material schedule
Schedules are where modelling saves the most work. They are also where errors hide best, because the table always looks tidy even when the data underneath is wrong.
Tekla exports many kinds of schedule through custom report templates:
- Sections by grade, profile and cut length, used for ordering and for optimising cuts from stock lengths.
- Plate by thickness and blank size, used to nest parts for the cutting machine.
- Bolts, nuts and washers by type and diameter, delivered per assembly.
- Weight per assembly, used to select cranes, load trucks and split deliveries into batches.
- Painted or galvanised area per member, used for pricing and payment.
- Totals by zone and by batch, matching how payment is divided in the contract.
A Tekla schedule is mathematically exact. It is only trustworthy on a clean model, and a clean model needs three conditions.
Every part carries a correct material grade, with nothing left undefined or on the default. Classification attributes for zone, batch and phase are applied consistently, following how the work is packed and shipped. And numbering has been rerun since the last revision, with nothing left unnumbered.
One more condition sits outside the model. Tekla reports net weight. The quantity engineer has to add the waste factors on top: cutting waste is usually taken at 3 to 5 per cent, plus weld metal and rust loss, as the contract defines them. Taking the Tekla figure straight to a payment application understates the work.
A quick check: compare the schedule's total weight against the model's total weight. If they match, the data is clean. Whether the detailing is right is still a question for the model itself.
For a steel contractor's quantity engineer, this schedule is the basis for payment; What goes into a construction final account file? covers the paperwork around it. For an estimator it replaces counting steel by hand, and What is quantity take-off? explains why some hand checking is still worth doing.
Frequently asked questions
Do steel shop drawings have to be made in Tekla?
No. Advance Steel, SDS/2 and even hand drafting in AutoCAD all work. Tekla's strength is that numbering, schedules and machine data all come out of one model, so a single change carries through the whole package.
Who approves shop drawings?
Usually the design consultant checks that the drawings match the design, and the supervision consultant checks that they are ready for construction. The exact requirement is set out in the contract and technical specification for each project.
How long does a shop drawing package for one workshop building take?
It does not scale with floor area. It depends on how many different member types there are, how complex the connections get, and how many revision rounds the consultant asks for. A building repeating one frame many times goes far faster than one where every frame differs.
How does a model built for learning differ from one built for fabrication?
A fabrication model has to be buildable. Real bolt types, clearances wide enough to erect, welds a welder can reach, tolerances a machine can hold. Learning models tend to skip all of that, so they look correct while being impossible to produce.
What do you need before learning to produce shop drawings in Tekla?
You need to read structural steel drawings and understand how connections are put together: bolts, welds, gussets, tolerances. With that background, 15 sessions is enough to produce a full package. Without it you will still build a model that looks right, but the workshop will not be able to fabricate from it.