Quality Control of Steel Structures: From Incoming Metal to Zinc Coating

Why every stage is checked

A mistake caught at the start is cheap to fix: replace a sheet or remake a part. The same mistake in a finished structure means reworking an assembly, and on site it means a halt in erection. That is why checks are spread across the whole fabrication cycle rather than reduced to a final inspection of the finished product.

Inspection does not replace design: it checks that the product matches the drawings and the order. What the product should be depends on the calculation, see Steel structure design. A general overview of the production is on the page Steel fabrication.

Incoming inspection of metal

Before the metal goes into production we check that exactly what was ordered has arrived:

• the steel grade, thickness and dimensions match the batch documents and the order;
• the surface condition: deep rust, laminations, dents and gouges beyond acceptable limits;
• the marking of sheets and rolled steel, so that at every stage it is clear which batch a part belongs to.

The quality and grade of the metal are confirmed by the manufacturer’s passports or certificates, or by laboratory test reports. The main steel grades we fabricate from are St3, S245, S345 and 09G2S.

Checking dimensions and geometry

Dimensions are checked on the parts and on the assembled product against the drawing: length, diameters and section sizes, the position and diameter of holes, the perpendicularity of flanges to the shaft axis, the straightness of the axis. Tape measures, calipers, squares and templates are used.

Such checks matter for poles and masts that are joined to the foundation and to each other: if the flange holes do not match the anchor cage or the neighbouring section, a problem appears on site that is hard to fix.

Welds: visual and ultrasonic inspection

Welds are checked by two methods that complement each other.

• Visual inspection — assessing the weld from outside: size and shape; absence of cracks, undercuts, surface lack of fusion, pores and overlaps; no slag or spatter left. All welds are inspected, including the longitudinal seam of the shaft and the welds attaching flanges.
• Ultrasonic testing — finds internal defects invisible on the surface: above all cracks and lack of fusion, as well as slag inclusions. An ultrasonic pulse travels into the metal and reflects from a defect. The method is non-destructive, so it suits load-bearing welds.

Welding is done by qualified welders.

We do not perform X-ray (radiographic) testing of welds. If the design requires exactly that, tell us at the enquiry stage so that we can clarify what is possible before a contract is signed.

If inspection finds a defect

A defective weld is not left as it is: the unacceptable section is ground out to sound metal, re-welded and checked again. Cracks in welds are not permitted at any size.

Zinc coating

After hot-dip galvanising we measure the coating thickness on the product and inspect the surface for continuity and appearance. What can go wrong and how it is fixed is described in the articles Zinc coating thickness and Hot-dip galvanising defects and Zinc Coating Defects.

Many coating problems are built in earlier, at the fabrication stage: closed cavities, missing vent holes, leftover slag and paint. See the article Designing for hot-dip galvanising.

Laboratory control

Laboratory tests are used when the documents for the metal are not enough or are in doubt, and when the design requires it.

What the customer should consider

• state in the enquiry what scope of control is needed: whether visual inspection of welds is enough or ultrasonic testing of particular welds is required;
• if the design requires X-ray testing, tell us in advance: we do not perform it ourselves;
• tell us which documents you need to come with the products, and we will agree the list before fabrication starts;
• send KM (structural) or KMD (shop-detail) drawings: we check the dimensions against them.

The standards we rely on, the warranty and the acceptance procedure are gathered on the page Standards and quality control.