Double Coupler (Right-Angle Clamp): EN 74-1 Classes A/AA/B/BB and What They Require

A double coupler holds two 48.3 mm outside-diameter scaffold tubes at a fixed right angle. It is a principal load-bearing joint in a tube-and-fitting scaffold — ledger to standard, transom to ledger.

It is also the only coupler type in the current European standard with four classes rather than two, and the only one that standard lets you use in touching pairs to raise the slipping requirement. Where the double sits among the other types is in the coupler types guide; the fitting for crossings that are not 90° is the swivel coupler.

What a double coupler is, and what it is called

Two coupler bodies fixed back to back at 90°, each a flap closed by a T-bolt and nut — or, less commonly, a wedge — around one tube. Nothing rotates: the angle is built in and cannot be set wrong on site. BS EN 74-1:2022+A1:2025 lists four coupler types in its Table 1 — right angle (RA), swivel (SW), parallel (PA) and sleeve (SF) — and the right angle coupler is the type defined for tubes crossing at a right angle. Its scope covers couplers for 48,3 mm outside-diameter steel and aluminium tubes meeting EN 12811-1, EN 12811-2 and EN 12810-1; what the other three types do is in the coupler types guide.

Because suppliers use “fixed clamp” inconsistently, confirm the crossing angle and whether a screw- or wedge-tightened coupler is required before quoting. For an EN specification, use type RA for a right-angle crossing; if the crossing is not 90°, the fitting wanted is a swivel.

What BS EN 74-1:2022+A1:2025 requires of a right-angle coupler

This is the current edition — EN 74-1:2022 as amended by A1:2025. Its Table 8 holds the right-angle coupler’s numbers.

Transcribed from Table 8 of BS EN 74-1:2022+A1:2025. A dash means the standard sets no requirement for that combination. Δ1 is the slip displacement of no more than 7 mm; Δ2 is the 1–2 mm band; Δ10 is the indentation limit.

Classes B and BB carry two further cruciform bending stiffnesses the table leaves out for width: on steel tube, 15,0 kNm/rad at MB/1,65 = 0,48 kNm and 6,0 kNm/rad at MB = 0,8 kNm; on aluminium tube, 13,0 kNm/rad and 5,0 kNm/rad at the same two moments. Those, the rotational moment and the rotational stiffness are all marked in the standard as applying to screw couplers only.

The conditions attached to the numbers matter as much as the numbers:

• They are characteristic values. Clause 7.2.1.5 requires the test loads to be analysed statistically to the 5 % quantile, which must not fall below the specified value; clause 6.2.1 sends the reader to EN 12811-1 and EN 12812 for the values and safety coefficients to be applied in use. The Table 8 figures bind a prototype in a type test.
• Δ2 is defined by the A1:2025 amendment as the lowest force measured between 1 mm and 2 mm of displacement — the one substantive change that amendment made.
• Different tests run on different tubes. Slipping is tested on reference tube RTS1 (48,3 mm outside diameter, 3,2 mm wall, ReH ≥ 235 N/mm²), ten tests, plus five on aluminium reference tube RTA; failure and pull-apart on a solid steel reference bar RB; indentation on reference tube RTS2.
• Tightening is part of the test. Screw couplers are tightened to 50 Nm with a torque wrench (clause 7.1.7), wedge couplers with a hammer of at least 500 g until the jarring blow is reached.
• The assessment is third-party. Clause 12 requires it to be carried out by a person or organisation different from the one that designed the coupler — not a CE mark and not a per-batch certificate.

Classes A, AA, B and BB, and what “used touching” means

In BS EN 74-1:2022+A1:2025, the right-angle coupler has four classes; swivel, parallel and sleeve couplers have Classes A and B only. Read them in three steps.

1. A against B. The two classes differ in the internal forces and moments they must transmit and in their load-bearing and stiffness values. Only B and BB carry the rotational-moment, rotational-stiffness and cruciform-bending requirements, and those apply to screw couplers only.
2. AA and BB used singly. The standard says couplers of classes AA and BB, used as single couplers, have the same characteristics as A and B respectively — but Table 8 makes one exception that is easy to miss: the single-coupler failure force is 30,0 kN for AA against 20,0 kN for A, and 45,0 kN for BB against 30,0 kN for B. Clause 6.2.5 also requires an AA or BB coupler to meet the performance requirements of A and B couplers.
3. AA+AA and BB+BB. The doubled letter is for the pairing rule: two identical couplers positioned touching each other may be used to increase slipping capacity. Table 8 gives that configuration its own row — AA+AA must reach 10,0 kN at Δ1 and 15,0 kN at 1–2 mm; BB+BB, 15,0 kN and 25,0 kN — and sets no separate failure or pull-apart value for the pair.

Two consequences for an order: the pairing is described for two identical couplers of the same doubled class touching each other, not as a general “add a second coupler” rule, and the class letter has to be AA or BB from the start — an A or B coupler carries no paired requirement at all.

Marking, designation and what to look for on a sample

Clause 9 of BS EN 74-1:2022+A1:2025 requires every coupler to be marked by impressing or embossing on the flap or the body, legible after coating, and the class is one of the items in that mark; the full character sequence and the character sizes are in the coupler types guide. What belongs on a specification is the clause 8 designation, EN 74-1 <type> <class> <corrosion class> <inspection level>: a Class BB right-angle coupler with electrolytic zinc and supervised production control is EN 74-1 RA BB C2 M, not “EN 74 double”.

Forged or pressed, and what it weighs

Both constructions are sold against EN 74-1 and neither is the “standard” one. What the standard says about material, coating and bolt is generic to every coupler type: clause 6.1 allows only steel or cast iron, with cast irons reaching a minimum elongation of 5 %; an electrolytic zinc coating of 15 µm average thickness is what conforms to corrosion class C2; clause 6.2.3 sets the bolt at M12 or larger, strength class 5.6 or better, proof load not less than 32,2 kN; and 29 CFR 1926.452(b)(9) requires a structural metal and prohibits grey cast iron. The process comparison is in the coupler types guide.

Nor does the process decide the class: TRAD UK’s catalogue lists its drop-forged double coupler as Class A and its pressed-steel double coupler as Class B (accessed 16 September 2026) — a concrete example showing that the construction process does not determine the EN class.

Vendor catalogue weights for 48.3 mm couplers, accessed 16 September 2026. When a container is weight-limited rather than volume-limited, estimate freight from the catalogue weight of the SKU quoted, not from the range.

Where the double coupler sits in the scaffold

The double carries the principal right-angle load path: ledger to standard, and load-bearing transom to ledger. US rules are specific about how those joints are made: 29 CFR 1926.452(b)(5) requires the inboard coupler of a bearer coupled to the posts to bear directly on the runner coupler, and (b)(6) requires bearers to extend beyond the posts and runners and to provide full contact with the coupler. Which fitting joins which member is in tube and clamp scaffolding explained.

Earlier and regional editions

The editions must be kept separate. What the older table said, edition-labelled:

Those are prototype type-test limits from a superseded edition — that standard states its requirements are for the assessment of prototypes — and they are not safe working loads. The slipping forces are the same numbers in both editions, which is why they travel so freely; the displacement rule is not. The 1988 edition measured the slipping load at under 0,5 mm of movement, while the current edition takes the lowest force between 1 mm and 2 mm. The 1988 table also has Class A and Class B only, and EN 74:1988 listed the shear-pin sleeve coupler (SS) and the loose spigot (LS), which Table 1 of BS EN 74-1:2022+A1:2025 does not list. The rest of that table, type by type, is in the coupler types guide.

Some fittings and catalogues still carry the 2005 edition’s marking, which we have not read — no figure on this site is attributed to it. Where a product page says “BS EN 74-1:2005”, treat the class letter as a description of the marking, and ask for the type-test report behind it.

China. GB 15831-2006, the withdrawn Chinese national standard, came from ISO 4054 rather than from EN 74 and covered the 直角扣件 (code Z) with its own numbers: a 抗滑 (slip) test at 7.0 kN with Δ₁ ≤ 7.00 mm and 10.0 kN with Δ₂ ≤ 0.50 mm, a 抗破坏 (failure) test at 25.0 kN, and a 扭转刚度 (torsional rigidity) test in which a 900 N·m moment must not produce a displacement over 70.0 mm — all at 40 N·m bolt torque on a 48.3 × 3.5 mm red-lead-painted tube, so none of them is interchangeable with BS EN 74-1:2022+A1:2025, whose slipping tests run at 50 Nm on reference tube RTS1 (48,3 × 3,2 mm, ReH ≥ 235 N/mm²) and whose failure test runs on a solid steel bar. Two further differences matter when a Chinese quotation lands. The 2006 edition permitted only 可锻铸铁 KTH 330-08 or 铸钢 ZG 230-450, so drop-forged steel is not among its listed materials. And its 出厂检验 (factory inspection) table marked the mechanical tests for both type inspection and factory inspection; by comparison, EN 74:1988 described its requirements as the assessment of prototypes, while the current BS EN 74-1:2022+A1:2025 gives informative ongoing-production-control recommendations in its Annex B. GB 15831-2006 was withdrawn on 1 December 2023 and replaced by GB/T 15831-2023, which we have not read, so nothing here is attributed to the 2023 edition; note only that GB (mandatory) became GB/T (recommended). Ask a Chinese supplier which edition its test sheets are written to.

Australia and New Zealand. AS/NZS 1576.2:2016 tests the right-angle coupler differently again. Its Appendix A beds the coupler in at 6 kN, then applies a vertical force of 12.5 kN progressively over 30 s and holds it for a minute; under clause 4.2.2.2 the coupler must not slip more than 6 mm at its contact face. In the Appendix B rotation test — 30 kN applied progressively over 90 s for a right-angle coupler — the vertical rotation must not exceed 125 mm on the pointer. At least six couplers are tested per appendix. Its torque rules (54 Nm sufficient under clause 3.2.3(a); at least 80 Nm withstood without permanent distortion under clause 3.2.4) are generic to screw-tightened couplers and are summarised in the coupler types guide; neither is a site torque for a coupler type-tested to BS EN 74-1:2022+A1:2025, which tightens screw couplers to 50 Nm for its own tests. Two rules are worth carrying into a quotation: Appendix A states that where a screw-tightened coupler is tested for a higher slip resistance, the test load shall be twice the intended working load limit — a ratio the European standard does not print — and clause 1.6 offers a conformity route to the European standard written against an edition we have not read, so ask which edition a supplier’s type-test report is actually written to.

AS/NZS check couplers are a separate rule

This is not an EN 74-1 requirement and it is unrelated to the AA+AA / BB+BB classification. The UK trade uses check and supplementary couplers — a second fitting added beside a load-bearing one. NASC publishes a four-page guidance note on them, TG14:20 Supplementary Couplers and Check Couplers (July 2020); we have not obtained it and publish no figure from it. The one primary source in our ledger that quantifies a check coupler is AS/NZS 1576.2:2016: clause 4.2.2.7 limits the load rating of a loadbearing coupler backed by a check coupler to half the load at which a slip of not more than 6 mm occurs, and requires the check coupler itself to have a working slip resistance of at least 9 kN.

Double versus swivel

Within a class, BS EN 74-1:2022+A1:2025 asks the same characteristic slipping forces of a swivel coupler as of a right-angle coupler; what the swivel does not have to reach is the same failure force — 14,0 kN for Class A and 20,0 kN for Class B, against 20,0 / 30,0 / 30,0 / 45,0 kN for right-angle classes A, AA, B and BB — and the standard asks nothing of it in pull-apart or rotational stiffness and gives it no AA or BB classes. That is why a swivel is not a substitute for a double at a load-bearing right-angle joint: not because it slips sooner. The full side-by-side table is in the swivel coupler guide.

What UK and Chinese suppliers quote

UK and Chinese supplier pages may quote different kinds of number. UK distributors publish a working-load figure: TRAD UK’s listings state a slip resistance of 6.10 kN for its drop-forged double coupler (code 026001) and 9.10 kN for its pressed-steel one (code 026013), accessed 16 September 2026 — the supplier’s stated working-load figures, which no standard in our source ledger prints. The same pair circulates as trade convention, with practitioners in a Scaffolders Forum thread quoting 6.1 kN for Class A doubles and 9.1 kN for Class B (accessed 16 September 2026) — convention, not a published rating.

Chinese exporters quote test-table figures instead: Wellmade states a slipping force of “more than 15KN” and a failure load of “more than 30KN” for its Class B forged double coupler (accessed 16 September 2026). Dura Scaffold ties its claim to EN 74-1:2005, an edition we have not read, so we do not reproduce or rely on that claim. The two markets may therefore quote different kinds of figures for similarly described Class B doubles — one repeating a type-test requirement, the other publishing its own working-load figure. The comparison across fitting types is in the coupler types guide.

What to put on the purchase order

Four lines are specific to the double; the rest of the order is common to every fitting on it.

1. Type, edition and class, together. “Double coupler (right-angle clamp), BS EN 74-1:2022+A1:2025, type RA, Class B” — or the full designation, EN 74-1 RA B C2 M. “EN 74” or “BS 1139” alone identifies nothing.
2. The tube wall you actually buy. 48.3 mm outside diameter, plus the wall thickness the couplers will see. Do not copy 3,2 mm across: that is the wall of reference test tube RTS1 (48,3 × 3,2 mm, ReH ≥ 235 N/mm²) on which the current edition’s slipping tests run, and it was the superseded EN 74:1988 that wrote 3,2 mm into its own scope wording.
3. AA or BB, if the design relies on a touching pair. Specify the doubled letter from the start; the pairing requirement does not exist for Class A or Class B.
4. A type-test report for the right-angle coupler. To the edition you named, from a named laboratory, with the clause 12 assessment by an assessor independent of the designer — not the factory’s swivel report. Check the torque it was run at: 50 Nm under BS EN 74-1:2022+A1:2025, not the 40 N·m test torque of the withdrawn GB 15831-2006.

Finish, bolt, marking and production-control level are generic to the whole order, and are covered in the eight-point purchase-order list in the coupler types guide.

Request double couplers with the class specified by your design and a note on any intended touching-pair configuration.

Sources

1. BS EN 74-1:2022+A1:2025 Couplers, spigot pins and baseplates for use in falsework and scaffolds — Part 1: Couplers for tubes (current edition; Table 8, clauses 4.2.1, 6.1, 6.2.1, 6.2.3, 6.2.5, 7.1.7, 7.2.1.5, 8, 9, 12, Annex B). BSI product page — accessed 2026-09-17
2. BS 1139-2.1:1991 (EN 74:1988) Metal scaffolding — Part 2: Couplers. Superseded; cited for its Table 1 right-angle coupler type-test limits. Current edition sold by BSI — accessed 2026-09-16
3. AS/NZS 1576.2:2016 Scaffolding — Couplers and accessories (clauses 1.6, 3.2.3, 3.2.4, 4.2.1, 4.2.2.2, 4.2.2.7, Appendices A and B). Standards New Zealand product page — accessed 2026-09-16
4. 29 CFR 1926.452(b) Tube and coupler scaffolds — bearer, runner and coupler material clauses (Cornell LII mirror; osha.gov is the authoritative host) — accessed 2026-09-16
5. GB 15831-2006 《钢管脚手架扣件》 Steel tube scaffold couplers — withdrawn 2023-12-01, replaced by GB/T 15831-2023. Official catalogue record, 全国标准信息公共服务平台 — accessed 2026-09-16
6. NASC TG14:20 Supplementary Couplers and Check Couplers (July 2020) — cited for metadata and existence only: the document has not been obtained and no figure from it is published here. NASC official website — accessed 2026-09-16
7. TRAD UK — Double coupler product listings (drop forged code 026001, pressed steel code 026013, with the supplier's stated slip resistance, class and weights) — accessed 2026-09-16
8. Gilray — Double couplers, drop forged, 0.91 kg — accessed 2026-09-16
9. Altrad Generation — Scaffold fittings catalogue (drop forged double coupler 26001, pressed steel double coupler 26124) — accessed 2026-09-16
10. Wellmade — Forged double coupler product page (supplier's stated slipping and failure figures) — accessed 2026-09-16
11. Dura Scaffold — EN74 British forged scaffolding couplers (double coupler weights by SKU) — accessed 2026-09-16
12. Scaffolders Forum — 'Class A & B fittings' thread (practitioner convention on double coupler working loads, and on class segregation in yards) — accessed 2026-09-16