Welding requirements for steel structures under DSTU EN 1090-2: WPS, welder qualification and inspection by execution class EXC

Execution classes EXC1–EXC4: what they are

The execution class, EXC, is a set of requirements for how a structure is fabricated, welded and inspected. DSTU EN 1090-2:2019 defines four classes, with requirements rising from EXC1 to EXC4. EXC4 builds on EXC3 and adds special project requirements: for example, the project names the welds to inspect and the scope of that inspection weld by weld.

The class is stated in the execution specification, the document that goes with the design. It may be one class for the whole structure or different classes for individual members and joints; in the second case the DBN requires such members to be marked separately.

The class belongs neither to the steel nor to the welder. It determines what the contractor must demonstrate: the level of welding quality requirements, the qualification of the procedure and of the people, the scope of inspection and the criteria by which welds are accepted. This article deals only with welding (clause 7 and 12.4 of the standard); bolted connections under the same classes are covered in the article on bolted connections in steel structures.

How the execution class is chosen: table U.1 of the DBN

The execution standard itself does not choose the class: under cl. 4.1.2 the class must be stated in the execution specification. In Ukraine the rules for choosing it are given by Annex U to DBN V.2.6-198:2014 as amended by Amendment No. 1 (order of the Ministry of Regional Development of 30.12.2021 No. 365, in force from 01.09.2022). In the original 2014 edition Annex U dealt with corrosion protection; Amendment No. 1 replaced it completely, and it is now titled «Determination of the execution class». The annex has informative status, and the class is determined before execution starts.

The class depends on three factors: the consequences class of the object under DBN V.1.2-14, the category of the structure, member or detail under Annex A of the DBN, and the type of loading. The loading is one of two kinds: static, or dynamic loading that may lead to fatigue failure; structures under dynamic loading get a higher class. Table U.1 puts it together as follows.

Table U.1 of Annex U of DBN V.2.6-198:2014 with Amendment No. 1. The categories are written in Cyrillic letters А, Б, В. a) Class EXC4 may be applied to structures with extreme consequences of failure.

Categories А, Б, В, consequences class and Eurocode

The consequences class of the object is set by the project under DBN V.1.2-14:2018; how it is done is described in the article on consequences classes CC1, CC2, CC3. A member receives its category А, Б or В (table U.1 calls it the compliance category) under Annex A of the DBN: А means failure may make the structure or a considerable part of it unfit for use, Б means failure hampers normal use or leads to failure of other structures not in category А, В means failure does not disturb the work of other structures. Table A.1 gives a ready list of typical members; roof trusses and overhead line poles are placed in category А there, see how to choose a steel grade.

One project may contain different classes. If the class set for individual members or details differs from the class of the structure as a whole, those members and details are marked separately (cl. U.2.5).

The annex also has a note: in an EXC1 structure EXC2 applies to welded members of steel of class С355 and above, to welded members of category Б that are joined by welding on the building site, and to members that were hot formed or heat treated during manufacture.

A note to cl. 4.1.2 of the execution standard refers to Eurocode 3: EN 1993-1-1 with amendment A1:2014, Annex C (in Ukraine DSTU-N B EN 1993-1-1:2010 with Amendment No. 2:2018, in force from 1 February 2019). It is an international reference, while projects in Ukraine choose by table U.1. The Eurocode table is built differently: from the consequences class (or reliability class) and the type of loading, without categories А, Б, В. For example, for CC1 under static loading it gives EXC1, whereas the DBN gives EXC1 only for categories Б and В. Eurocode also advises against using a higher execution class as a reason to lower the partial factors for resistance in the design.

What changes in welding from EXC1 to EXC4

The main differences are in the table; each is explained in the sections below. A dash means the standard sets no requirement for that class at this point; the project may add its own.

WPS and welding procedure qualification

A WPS (welding procedure specification) is the written welding procedure. In the welding planning documents, under cl. 7.2.2, it fixes the requirements for welding consumables, preheat, the metal temperature before the next pass and post-weld heat treatment. The same documents record measures against distortion, the weld sequence and turning of the piece, requirements for intermediate checks, acceptance criteria under cl. 7.6, a cross-reference to the inspection plan and the requirements for weld marking. Tolerances for joint preparation and fit-up must be given in the WPS itself (cl. 7.5.1.1).

Under cl. 7.4.1.1 welding is done to a qualified procedure, with a WPS to the relevant part of EN ISO 15609. For the arc processes numbered 111, 114, 12, 13 and 14 in EN ISO 4063 the standard lists the qualification routes, and the set depends on the class.

Table 12 of DSTU EN 1090-2:2019. The table lists no routes for EXC1; the execution specification may require work instructions for EXC1 stating the process, materials and welding parameters (cl. 7.4.1.1). The EN ISO 17660 rows apply only to joints between reinforcing steel and other components.

What else the standard says about the WPS

For fillet welds on steel from S460 an additional tensile test on welded cruciform joints to EN ISO 9018 is required (three specimens in transverse tension). The execution specification may replace it with a test of the all-weld metal if the weld thickness is increased to compensate.

When welding through a shop primer, the procedure is tested at the greatest permitted coating thickness, and the primer’s suitability for welding is checked to EN ISO 17652. For EXC3 and EXC4 a primer is not left on the faces to be fused, unless the procedure has already been tested with such a primer (cl. 7.5.1.1).

Even with a ready qualification the WPS is checked in production conditions. The first five joints made to a new welding procedure qualification record (WPQR) are inspected to quality level B; the minimum length of welds submitted for this inspection is 900 mm (cl. 12.4.2.2). If the results are not acceptable, the cause is sought following EN ISO 17635.

Welders, operators and the welding coordinator

Welders are qualified to EN ISO 9606-1, welding operators to EN ISO 14732. Records of all qualification tests are kept (cl. 7.4.2.1). For EXC1 in a shop working to EN ISO 3834-4, welder certificates are confirmed under cl. 9.3 a) or b) of EN ISO 9606-1:2017, and operator qualification under cl. 5.3 a) or b) of EN ISO 14732:2013. Welders of hollow section joints meeting at angles below 60° are qualified separately on specimens typical for production (cl. 7.4.2.2).

For EXC1 adequate supervision of welding is enough, as stated in EN ISO 3834-4. For EXC2–EXC4 welding is coordinated by personnel with suitable qualification and experience under EN ISO 14731. Their technical knowledge level, B (basic), S (specific) or C (comprehensive), depends on the class, the steel group and the thickness. The coordinator is responsible for the qualification of welders and operators and may act as examiner; if external experts conduct the qualification, it follows the procedures of EN ISO/IEC 17024 or 17020.

Table 14 of DSTU EN 1090-2:2019, structural carbon steels; stainless steels have their own table 15. For column base plates and end plates the thickness limits in the table are 50 and 75 mm instead of 25 and 50 mm.

Preparation and execution of welding

The standard gives no preheat figure. Preheat is applied to the current WPS with reference to EN ISO 13916, EN 1011-2 and EN 1011-3 and maintained throughout welding, including tack welds and welds of temporary attachments (cl. 7.5.5). If the metal is colder than 5 °C, preheat to EN 1011-1 may be applied. The welder and the work area are protected from wind, rain and snow, and the surfaces are kept dry and free of condensation (cl. 7.5.3).

Surfaces to be welded must be dry and free of rust, organic matter and zinc coating. The prepared edge must show no visible cracks; for steels above S460 the edge zones are cleaned of scale by grinding after cutting and checked for cracks visually, by penetrant or by magnetic particle testing (cl. 7.5.1.1).

Tack welds for EXC2–EXC4 are made to a qualified procedure. A tack weld is at least four times the thickness of the thickest part joined, or 50 mm, unless testing justified a shorter length. Tack welds not incorporated into the final weld are removed; those left in the weld are made by qualified welders, cleaned before welding, and removed if they contain cracks (cl. 7.5.7).

Temporary attachments (stops, erection aids) are welded to a WPS, placed so that they can be removed without damaging the main structure, and the places where welding them is forbidden are marked by the project. After removal the surface is ground clean and inspected, and on steel from S355 checked by NDT; on steel from S460 and on parts under fatigue loading gouging and chipping are not permitted unless stated otherwise (cl. 7.5.6).

Fillet welds at the ends and sides of parts are carried continuously around the corner for at least two leg lengths if access and configuration allow. The minimum length of a fillet weld is four leg lengths. Intermittent welds are not used where capillary action may cause rusting. In lap joints the overlap is at least four times the thickness of the thinner part, and single fillet welds are not used unless the parts are restrained against opening of the joint under weld shrinkage. If the end of a part is attached only by longitudinal fillet welds, each is at least 75 % of the transverse distance between them (cl. 7.5.8.2). Where the gap exceeds the permitted value it is compensated by increasing the throat thickness: a = anom + 0.7h (cl. 7.5.8.1).

In butt welds for EXC3 and EXC4 (for EXC2 if specified) run-on and run-off plates are used on transverse full penetration welds, with weldability no worse than the parent metal. Steel backing left in place has a carbon equivalent of at most 0.43 % or the same as the parent metal with the best weldability; for EXC3 and EXC4 backing left in place is continuous (cl. 7.5.9). After each pass the weld is cleaned of slag and visible defects are removed before the next pass (cl. 7.5.16).

Weld quality levels to EN ISO 5817

The standard assesses permitted weld imperfections (lack of fusion, undercut, pores, overlap and so on) to EN ISO 5817:2014. There are three quality levels: B is the strictest, C is intermediate, D is the most lenient. For EXC1–EXC3, unless the project states otherwise, the requirements are as below; two imperfections, irregular geometry of the weld face edge (505) and micro lack of fusion (401), are disregarded.

Cl. 7.6.1 of DSTU EN 1090-2:2019. Welds in joints designed to EN 1993-1-8 normally need the level defined for EXC2.

Welds under fatigue loading

For welds designed to EN 1993-1-9 and loaded in fatigue, the project sets acceptance criteria taking account of the detail category DC for the particular location. In addition to the table above, for EXC2–EXC4 they may be defined by Annex C of EN ISO 5817:2014: for DC up to 63 level C63, above 63 up to 90 level B90, above 90 up to 125 level B125 (cl. 7.6.2). The execution requirements needed to comply with EN 1993-1-9 must be in the execution specification.

Weld inspection and its scope

The inspection and testing plan is drawn up to the relevant part of EN ISO 3834: it covers measures before, during and after welding. Non-destructive testing (NDT) methods are selected to EN ISO 17635. All methods other than visual are carried out by personnel qualified to EN ISO 9712 (cl. 12.4.1).

Visual inspection is mandatory for all welds along their full length. It is done after welding of an area is complete and before any other NDT: it checks that the welds are present and in the right place, their shape to EN ISO 17637, and stray arc strikes and spatter. If surface imperfections are found, penetrant or magnetic particle testing is carried out on that weld (cl. 12.4.2.3, 12.4.2.5).

Additional NDT is by sampling and is expressed as a percentage of weld length. For EXC1–EXC3 the scope is given in table 24; for EXC4 it is not less than for EXC3, and the project states the scope for each identified weld.

Table 24 of DSTU EN 1090-2:2019. a is the weld throat thickness, t the thickness of the thickest joined part. Longitudinal welds are those parallel to the member axis; all others count as transverse.

How the percentage is counted and what is used

The percentage is the minimum part of the length of each weld in the inspection lot; the area is chosen at random. If the total length of welds in a lot is under 900 mm, at least one weld is inspected over its full length regardless of the percentage. If a lot consists of identical welds shorter than 900 mm, randomly chosen welds with a total length of at least the set percentage are inspected in full. The sample must cover different joint types, product grades, welding equipment and welders.

The scope relates to weld production over a year. If routine tests at the company consistently show acceptable quality for a certain weld type, the coordinator may reduce the scope below the table percentages, provided a three-month audit programme of production and testing has been introduced and documented.

The coordinator chooses the methods from the list in cl. 12.4.2.6: penetrant (EN ISO 3452-1), magnetic particle (EN ISO 17638), ultrasonic (EN ISO 17640 with EN ISO 23279 or EN ISO 13588) and radiographic (the EN ISO 17636 series). The project may itself name specific joints, scope and method (cl. 12.4.2.4). The standard does not tie radiography to a particular class.

Hold time, repair and weld inspection classes WIC

Additional NDT does not start until the minimum hold time after welding has passed (table 23). For preheat by method A of EN 1011-2 and steels S275–S460 it runs from «cooling only» for the thinnest welds to 8–24 hours for larger ones; for steels above S460, 24–48 hours. The wait is there because some cracks in high-strength steels do not appear at once. If later work will block access to a weld, it is inspected before that work begins.

Weld defects in EXC2–EXC4 are repaired by welding to a qualified procedure and checked to the same requirements as the original welds (cl. 12.4.2.7). The general rule of cl. 12.1: each nonconformity is assessed individually, by the function of the part and by the type, size and location of the imperfection, and only then is it decided to accept or repair. Every weld in a zone where unacceptable distortion was corrected is re-inspected.

Informative Annex L suggests assigning welds weld inspection classes WIC: the class depends on the level of fatigue loading, the consequences of weld failure and the direction of stresses. It helps avoid demanding the same inspection for critical and non-critical welds. If the project introduces WIC, it states the class of each weld (cl. 12.4.2.4).

What to put in KM, KMD and the execution specification

The shop does not guess requirements that are not on the drawing. What the standard asks to be stated in the execution specification for the welding clause is collected below. How to mark welds on a drawing is covered in the KM and KMD drawing rules, and joint types and leg size in the article on weld joint types.

From tables A.1 and A.2 of Annex A of DSTU EN 1090-2:2019. Only the items that concern welding are listed.

Typical mistakes

• Not stating the execution class. The contractor then does not know which part of EN ISO 3834 to work to, which weld quality level to accept and how much to inspect.
• Putting quality level B on all welds «just in case». For non-critical welds such a criterion brings extra repairs and extra inspection; the standard itself advises distinguishing welds by criticality (Annex L). The opposite error, D on loaded EXC3 welds, contradicts the table in cl. 7.6.1.
• Writing «weld inspection per the specification» without scope and method. The contractor either inspects no more than the minimum or inspects everything, and price and schedule move away from what was expected.
• Expecting that a higher execution class will justify smaller margins in the design. Eurocode says not to do this.
• Leaving a shop primer on the edges for EXC3 and EXC4 without a WPS test with that primer. Or welding over zinc or rust: surfaces must be dry and clean.
• Designing a weld that neither the welder nor the inspector can reach. The standard requires welds to be easily accessible and clearly visible (cl. 7.5.4), and a weld that will be closed by the next operation is inspected before it starts.
• Mixing systems. Joint designations of the old system (С, У, Т, Н) and an EXC requirement may stand on the same drawing, but edge dimensions and fit-up tolerances are then set in the WPS and not copied from one system to the other.

What this means when ordering fabrication

We do the welding ourselves. We fabricate to the customer’s drawings and develop KM and KMD drawings; if the brief states the execution class, weld quality level and inspection scope, we build them into the process documents and drawings. We carry out visual inspection and ultrasonic testing of welds ourselves, while we do not carry out radiographic testing: if the project requires it, tell us in the enquiry. How welds are checked in the shop is described in the article on quality control of steel structures.

For an estimate we need: the execution class (for the whole structure or with members singled out), steel grades and thicknesses, the weld quality level, the NDT scope and methods by weld type, whether there are fatigue requirements and which documents you expect with the structures. If something is missing, say so and we will clarify it while preparing the offer. What else to send with the KMD is in the article on the document package for the fabricator; about our services see KM and KMD design and steel fabrication.

Standards and validity

DSTU EN 1090-2:2019 «Execution of steel structures and aluminium structures. Part 2. Technical requirements for steel structures» (EN 1090-2:2018, IDT): in force from 01.07.2020, it replaced DSTU B EN 1090-2:2014. The article rests on cl. 4.1.2, 4.2.1, clause 7, cl. 12.1 and 12.4, tables 12, 14, 23, 24 and Annexes A and L.

DSTU EN ISO 5817:2022 (weld quality levels; EN ISO 5817:2014, IDT): in force, effective from 31.12.2023, replaced DSTU ISO 5817:2016. A newer European edition, EN ISO 5817:2023, exists; whether it has been adopted in Ukraine needs checking by an engineer, and the execution standard refers to the 2014 edition.

DSTU EN ISO 3834-2:2022 and DSTU EN ISO 3834-3:2022 (EN ISO 3834-2:2021 and -3:2021, IDT): in force, effective from 31.12.2023. For part 4 we did not confirm that the 2022 edition is in force; the catalogue showed only DSTU EN ISO 3834-4:2019 (based on EN ISO 3834-4:2005), which should be checked before it is cited. DSTU EN ISO 14731:2019 (welding coordination): in force, effective from 01.11.2019.

DSTU EN ISO 15614-1:2019 with Amendment No. 1:2022 (EN ISO 15614-1:2017/A1:2019, IDT): in force, effective from 31.12.2023; the 2014 and 2016 editions are not in force. DSTU EN ISO 9606-1:2018 (EN ISO 9606-1:2017, IDT): in force, effective from 01.01.2019. The execution standard refers specifically to EN ISO 9606-1:2017.

The list of Ukrainian equivalents in Annex NA of DSTU EN 1090-2 itself is out of date: it includes DSTU ISO 3834-2:2008, DSTU ISO 5817:2016 and DSTU EN ISO 15614-1:2016, which have already been replaced by the editions named above.

The choice of execution class is Annex U to DBN V.2.6-198:2014 as amended by Amendment No. 1 (order of the Ministry of Regional Development of 30.12.2021 No. 365, in force from 01.09.2022; the text of the amendment was read). The consequences class is DBN V.1.2-14:2018, categories А, Б, В come from Annex A of DBN V.2.6-198. Eurocode 3 (EN 1993-1-1:2005/A1:2014, Annex C), to which the execution standard refers, is named only as a reference: in Ukraine it was adopted by amendment No. 2:2018 to DSTU-N B EN 1993-1-1:2010 (took effect on 01.02.2019), but the current status of that standard and a possible replacement by the EN 1993-1-1:2022 edition should be checked. Check the validity of each document at the date of the project. The article describes the requirements of the standard, not any manufacturer’s certificates.