PEB Detailing Errors: Where They Start and How to Catch Them

Most detailing leads know the phone call. A rafter reaches site and its end plate does not line up with the column haunch. A set of purlin cleats has been welded to the wrong face. Two mirrored frame halves were given the same mark, so the shop made two left-hand pieces. The drawing was released and probably checked. The mistake took a few minutes to make in the drawing office. Fixing it costs far more, in the shop, in transport and at the crane.

This post covers what research says about the cost of error, where detailing errors in pre-engineered building (PEB) work usually start, and a stage-gate checklist that a detailing team can use before anything goes to fabrication.

What errors cost, and why the number is hard to pin down

There is little published data on detailing error costs specific to Indian PEB projects. The best available evidence comes from wider construction research, and it is still worth reading.

• The UK's Get It Right Initiative estimated the total cost of error at around 21% of construction spend. Recorded direct costs made up only about a quarter of that. The rest was indirect costs, unrecorded process waste and latent defects.
• The US Construction Industry Institute reports mean field rework above 3% of construction-phase cost on heavy industrial projects. Two of the five variables most closely linked to field rework were design rework and interdisciplinary design coordination.

Neither study is about PEB detailing, and you should not quote either as a figure for your own projects. The lesson that does carry over is this: most of the cost of an error never shows up as an error. It shows up as re-cutting, extra site hours, a truck held back, or a checker's week lost to revisions. If you only count what gets formally logged, you will underestimate the problem.

Where detailing errors come from

1. Inputs that were wrong, incomplete or out of date

Detailers work from design output, approved general arrangement (GA) drawings and client comments, and these often change after modelling has started. The Get It Right research found poor design, communication failures and "action commencing before there is sufficient understanding of the requirements" among the strongest causes of error. In his AISC paper on detailing practice, Hugh Dobbie makes a related point: the first quality control step is an adequate scope list, job standards, and the fabricator's and erector's preferences, all given to the detailer before work begins.

2. Settings decided too late

Some errors are really process errors. Tekla's own documentation notes that contour mark and pop-mark settings "should be fixed before numbering and drawing creation, because both affect numbering." The same applies more widely. Numbering prefixes, drawing templates, bolt and hole standards and NC settings should all be decided at project set-up. If you change them halfway through, drawings that were already correct can quietly stop matching the model.

3. The familiar drawing-level mistakes

Dobbie lists the errors checkers find most often on shop and erection drawings. Several apply directly to PEB work:

• Bolt hole counts in a member that do not match the supporting connection, or holes of the wrong diameter
• Connections left out altogether, or pieces with no shop drawing at all
• Bills of material where weights, sizes or quantities do not match the detail drawing
• Reversed slopes, and confusion over right-hand and left-hand or "as shown" and "opposite hand" pieces
• Wrong shipping marks on erection drawings
• Required stiffeners left out, and welding symbols shown incorrectly

In a typical PEB building, these turn up as mono-slope rafters detailed the wrong way round, mirrored frame halves given one mark, flange brace holes missing from a rafter, or purlin cleat quantities in the BOM that do not match the drawings.

4. Checking that is only looking

Dobbie describes checking as the basic quality control function, done by someone who has "advanced successfully from a beginning detailer" through experience and ability. He also describes back-checking. When a checked drawing comes back, the detailer reviews every comment, "the detailer checks the checker," and remains as responsible for accuracy as the checker. In busy offices this second step is often the first thing to go. Juniors hand over drawings they have not checked themselves, and checkers become the only line of defence.

What a model-based workflow fixes, and what it does not

A single 3D model that produces GA drawings, assembly and single-part drawings, BOMs and NC files removes a whole class of transcription errors. Trimble's case study of Zamil Steel's Bangalore Metro Rail depot (about 1,600 MT) describes NC and DXF exports for CNC fabrication of gusset plates and separate assembly and single-part drawings, with clashes and departmental coordination resolved at the engineering stage. PHENIX Construction Technologies reports producing 68,900 assembly drawings on a project of more than 6,700 MT.

The limit is just as important. A model spreads whatever is in it. A wrong plate thickness entered once will appear, consistently, on every drawing, BOM line and NC file. Model-based errors are systematic rather than random. That makes them easier to catch if someone checks the model itself, and easier to miss if the team only checks drawings.

A stage-gate QA checklist for PEB detailing

Adapt the gates below to your own office. The principle is that each gate has an owner and a sign-off, and nothing moves to the next stage until the current gate is closed.

Gate 1: Project set-up (before modelling)

• Design revision register created and current revisions confirmed
• Scope list, job standards, and fabricator and erector preferences issued to the detailer
• Numbering prefixes, drawing templates, bolt standards and NC settings (including pop-marks and contour marks) fixed

Gate 2: Model check (before numbering)

• Grids, eave heights, ridge positions and roof slopes checked against the approved GA
• Frame member sizes and plate thicknesses checked against design output
• A connection present at every node, with bracing and flange braces placed
• Secondary member laps, sag rods and openings checked, and a clash check run

Gate 3: Numbering freeze

• Numbering run and reviewed for mirrored pieces that should differ and identical pieces that should share a mark
• Numbering locked, with any later change recorded as a revision

Gate 4: Drawing check

• Dimension chains add up to overall dimensions
• Hole counts and diameters match the mating part
• Weld symbols, shipping marks and orientation (grid and north) checked on erection drawings

Gate 5: Reconciliation

• BOM quantities and weights reconciled against model reports
• NC file list reconciled against single-part drawings
• Material ordered confirmed as adequate for the pieces billed

Gate 6: Back-check and release

• Detailer has closed every checker comment
• Revisions clouded and logged, and transmittal issued

Gate 7: Feedback

• Every shop query and site fit-up problem logged against the gate that should have caught it

Measuring improvement without inventing a benchmark

You do not need an industry benchmark to improve. Track a few internal measures month by month: drawings revised after release, shop queries per project, and site fit-up issues by category (holes, marks, missing pieces, BOM). The Gate 7 log shows which gate is leaking. That tells you whether the fix is a set-up standard, a checking habit or a training gap.

The people side

Software enforces consistency. It does not teach a junior detailer why a hole gauge matters, how a haunch connection is actually bolted up at site, or why back-checking is not optional. Offices that make checking part of training from the first week, instead of saving it for senior staff, tend to get juniors who find their own errors before the checker does. Seeded-error exercises help here: a drawing set with known mistakes planted in it.

Where STEEL fits

STEEL is a Tekla Authorized Training Centre. Its 12-week PEB Steel Detailing & Tekla Structures course covers connections, numbering, GA, assembly and single-part drawings, BOMs and reports, NC/DSTV workflow concepts, and model and drawing QA/QC, ending with a final PEB project. Companies that want new hires trained in this workflow can look at corporate training. Teams hiring trained freshers can share a hiring requirement.

Sources

1. Get It Right Initiative – Research Report: Observations and Conclusions
2. Construction Industry Institute – The Field Rework Index: Early Warning for Field Rework and Cost Growth
3. Hugh Dobbie, Structural Steel Detailing Practices—Good and Bad, AISC Engineering Journal (Q3 2004)
4. Tekla User Assistance – NC files
5. Trimble – Zamil Steel relied only on Tekla to deliver the Bangalore Metro Rail Depot
6. Trimble – Tekla: the right choice for a project with zero error tolerance (PHENIX)