Designing Buildings at 07:40, 23 October 2026

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Designing Buildings at 07:40, 23 October 2026

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CanglongSteel: Created page with "Introduction Structural steelwork is normally fabricated away from the construction site, in a workshop where sections and plate are cut, drilled, welded and coated under contro..."

2026-10-16T08:45:17Z

Created page with "Introduction Structural steelwork is normally fabricated away from the construction site, in a workshop where sections and plate are cut, drilled, welded and coated under contro..."

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Introduction

Structural steelwork is normally fabricated away from the construction site, in a workshop where sections and plate are cut, drilled, welded and coated under controlled conditions. Factory quality control is the set of checks carried out between the delivery of raw material and the dispatch of finished members, so that the steelwork arriving on site matches the approved design and can be erected without rework.

The case for controlling quality at this stage is largely economic. Once components have been fabricated, coated and packed for shipment, correcting a member that is too short, a hole that is in the wrong position, or a coating that has failed is far more expensive than correcting the same fault in the workshop. Rework on site also disrupts the erection sequence, and on a project where material has to be manufactured and shipped across an ocean, a replacement member can add weeks to the programme.

Factory quality control is also a contractual matter on many projects. Fabricated components are commonly required to comply with a fabrication standard such as BS EN 1090-2, and where structural steel components are placed on the market they fall under the harmonised standard BS EN 1090-1 and the CE marking regime. Both place obligations on the fabricator's own factory production control, not only on the finished product.

The checks made in the workshop are usually grouped into the following stages: material identification and traceability; cutting and drilling; dimensional control and tolerances; welding; surface preparation and coating; and the compilation of the documentation that accompanies the delivery.

Material identification and traceability

Sections and plate are supplied with inspection documents, most commonly mill certificates to BS EN 10204 type 3.1, which record the cast or heat number, the steel grade, and the results of mechanical and chemical testing. The first factory check is to confirm that the material received is the material specified, typically S275 or S355 to BS EN 10025 for hot rolled structural sections.

Traceability is the ability to relate a finished item back to the material it was made from. It is the point at which fabrication quality control most often breaks down, usually for mundane reasons:

* Different grades being stored together, so that a member is cut from the wrong stock without anyone noticing. The two grades look identical.

* Identification markings being lost during blasting, cutting or handling, so that the identity of a piece can no longer be established.

* Material taken from stock without a record of the heat it came from.

Requirements increase with the execution class. BS EN 1090-2 sets out four execution classes, EXC1 to EXC4, according to the consequences of failure, and documented traceability is required for the higher classes. Practical controls include marking each piece before cutting, transferring markings onto offcuts, segregating grades, and relating material certificates to the member list. Where a component cannot be traced, the usual remedies are testing or replacement, both of which are costly and both of which delay the project.

Cutting, drilling and dimensional control

Cutting is carried out by sawing, flame cutting or plasma cutting. The method matters where a cut edge is subject to tension, because thermal cutting affects the material at the edge and thicker material may need additional preparation. Cut edges are normally dressed to remove dross and sharp irregularities before welding.

Holes are usually drilled under numerical control, either from a digital model or from approved shop drawings. The aspects that matter are:

* Hole diameter and position, so that bolted connections assemble on site. Holes are drilled to the clearance required for the bolt type; oversize and slotted holes are used only where they are specified, because they change the behaviour of the connection.

* The relationship between holes on different members, rather than the accuracy of any single hole. It is the hole pattern that determines whether a connection closes.

* The fit-up gap before welding. Components are tack welded in jigs or fixtures, and the gap between the parts should sit within the range assumed by the welding procedure.

* Camber or pre-set. Members are sometimes fabricated with a designed camber to offset deflection, and it must not be lost during handling or storage.

Tolerances

A tolerance is the permitted deviation from a specified dimension. In fabrication, tolerances matter because they accumulate: a member at the limit of its permitted length, connected to another member also at its limit, produces a frame that cannot be closed with standard bolts. This is why tolerance is a design decision as much as a fabrication one.

BS EN 1090-2 distinguishes between essential tolerances, for which the standard provides default values, and functional tolerances, which are established for each project according to what the structure actually has to do. Fabricators normally work to the essential tolerances unless the project specification is more demanding.

Dimensional checks in the workshop typically cover:

* The length of individual members and the distance between end connections.

* Straightness, sweep, bow and twist of members.

* The position of holes and welded attachments relative to the member datum.

* The overall dimensions of pre-assembled frames or sub-assemblies.

* The flatness and squareness of base plates and end plates.

Measurement is normally made against the approved shop drawing or model, using tapes, jigs and templates and, increasingly, total stations or laser scanning for complex assemblies. Where members are pre-assembled in the workshop, that assembly is the most reliable check available, because it tests the cumulative effect of every individual tolerance at once rather than each one in isolation.

Welding quality control

Welding is the stage at which quality control is most heavily regulated. Its main elements are:

* Welding coordination. European practice requires fabrication to be coordinated by a responsible welding coordinator whose competence is assessed against the levels in BS EN ISO 14731, with the level required depending on the execution class.

* Welding procedures. Each weld type is made to a welding procedure specification (WPS), supported by a welding procedure qualification record (WPQR) produced by testing to BS EN ISO 15614. The procedure sets out the consumables, the joint preparation, the range of current and voltage, and the preheat and interpass temperatures.

* Welder qualification. Welders are qualified to BS EN ISO 9606 for the position and joint type concerned, and certificates are checked for validity, since a qualification lapses after a defined period if it is not confirmed.

* Consumable control. Electrodes and filler wire are stored, and where necessary dried, in accordance with the manufacturer's instructions. Consumables must suit the base material, and hydrogen controlled consumables are used where hydrogen cracking is a risk.

* Preparation before welding. Joint gap, bevel angle and cleanliness are checked before welding begins, including removal of primer and mill scale from the weld zone.

* Thermal control. Where thick sections or highly restrained joints are welded, preheat and interpass temperatures are monitored with contact thermometers or thermal crayons and recorded.

* In-process inspection. Weld size, length and profile are checked visually as welding proceeds, and the work is sequenced to control distortion.

* Non-destructive testing. Visual examination is carried out on all welds. Surface and volumetric testing, using magnetic particle, dye penetrant, ultrasonic or radiographic methods, is applied to a proportion of welds determined by the execution class and, more often, by the project specification. Ultrasonic testing has become more common than radiography for structural steelwork.

Welds that fail inspection are repaired to a documented procedure, re-inspected and recorded. Repeated repair of the same location is generally restricted, because the accumulated heat input and restraint increase the risk of further cracking.

Surface preparation and coating

Corrosion protection applied in the workshop determines much of the durability of the finished building, and the quality of the coating depends more on surface preparation than on the paint itself. Mill scale, rust, salts and oil have to be removed before the first coat is applied, and the surface needs the roughness for the coating to key to the steel. Abrasive blast cleaning to a specified preparation grade, commonly Sa 2 1/2 in accordance with BS EN ISO 8501-1, is the usual requirement, with surface profile checked against BS EN ISO 8503.

Conditions at the time of application matter as much as the preparation:

* The steel temperature should exceed the dew point, commonly by at least 3 degrees C, so that condensation does not form beneath the coating.

* Ambient temperature, humidity and steel temperature are recorded, and work stops when they fall outside the coating manufacturer's limits.

* Salt contamination is tested and removed where specified, because salts draw moisture through the coating film and cause early blistering.

Checks after application typically include dry film thickness measured with an electronic gauge at a defined number of readings per area, coating adhesion where required, continuity of the coating where pinholes would allow early corrosion, and the condition of surfaces that will later be welded or left bare, such as the faying surfaces of slip resistant connections.

For hot dip galvanising to BS EN ISO 1461, the design of the component affects quality as much as the process does. Vent and drain holes must allow air to escape and molten zinc to reach internal surfaces, distortion in thin or asymmetric members has to be anticipated, and threaded holes may need re-tapping after galvanising. Coating thickness and appearance are checked against the standard, and handling damage is normally touched up with a compatible repair coating.

Documentation and inspection records

Quality control is only as good as its records, because they are the evidence the client and the building control process rely on. A delivery documentation package typically includes:

* An inspection and test plan (ITP) setting out what is checked, at which stage, by whom, and which checks are hold points that require approval before work continues.

* Material certificates related to members by heat number.

* Welding procedure specifications, qualification records and welder certificates.

* Welding inspection and non-destructive testing reports.

* Dimensional inspection records and, where applicable, pre-assembly reports.

* Coating inspection records, including surface preparation grade and dry film thickness readings.

* Non-conformance reports and a record of the corrective action taken.

* Where required, the declaration of performance and CE marking documentation under BS EN 1090-1.

Limitations of factory quality control

Factory quality control cannot guarantee the quality of the completed structure, and it is worth being clear about what it does not cover:

* Handling and storage. Coated steelwork can be damaged in transit, in storage or during erection, and no factory check can prevent that.

* Foundation interfaces. The position of holding-down bolts and the level of base plates cast into the foundations are site responsibilities, and the most accurate workshop member will not fit a misplaced bolt.

* Erection tolerance. Workshop checks confirm that members meet their own tolerances; they cannot absorb an accumulation of error between the foundations and the frame.

* Design detail. Details that assume perfect fit-up transfer avoidable cost to the fabricator. It is usually more economical to detail connections for the tolerance that fabrication can reasonably achieve.

* Cost and programme. Inspection beyond the specified level adds cost and lead time without necessarily improving the safety of the finished structure.

Its purpose is therefore not perfect workmanship, but demonstrable compliance: documented evidence that the steelwork supplied is the steelwork that was specified.

Quality control in structural steel fabrication - Revision history

Designing Buildings at 07:40, 23 October 2026

Designing Buildings at 07:40, 23 October 2026

CanglongSteel: Created page with "Introduction Structural steelwork is normally fabricated away from the construction site, in a workshop where sections and plate are cut, drilled, welded and coated under contro..."