Tube and Clamp Scaffolding Explained: Tube and Fitting Parts, Names and Assembly

Tube and clamp scaffolding is the oldest form of steel access scaffold still in daily use: loose lengths of 48.3 mm steel tube, joined wherever they cross by a clamp. Nothing is prefabricated, so the scaffold can follow any building shape, and every joint is a separate purchase decision. Americans say tube and clamp; the OSHA regulation that governs it says tube and coupler; British scaffolders say tube and fitting or tube and coupler. This guide is written for buyers on either side of that vocabulary. It covers what the system is, what each part is called in each market, which coupler joins which members, and the rules that decide how much a finished scaffold can carry.

One system, three names

OSHA’s fact sheet on the subject defines it in one sentence: “A tube and coupler scaffold has a platform(s) supported by tubing, and is erected with coupling devices connecting uprights, braces, bearers, and runners.” The regulation itself, 29 CFR 1926.452(b), is titled Tube and coupler scaffolds. Trade speech in the US shortens coupler to clamp, which is why searches and supplier catalogues say tube and clamp.

In the UK the same scaffold is tube and fitting: the fittings are the couplers, and scaffolding fittings is the stock category name at every UK supplier. The European product standard for the couplers, EN 74-1 (published in the UK as BS EN 74-1), uses coupler throughout, as does AS/NZS 1576.2 in Australia and New Zealand.

The rest of this guide uses the UK member names, with the OSHA name in brackets where a regulation is quoted.

What the package consists of

A tube and fitting scaffold is bought as four groups of parts. The couplers are the group with the most variants and the one this site exists for; the other three are listed so a purchase order is complete.

Tube. Steel tube of 48.3 mm outside diameter. EN 74:1988, the first European coupler standard, was written for “steel tubes of 48,3 mm outside diameter and of at least 3,2 mm nominal wall thickness”; the couplers are sized to that tube and to nothing else. In the UK the tube itself is made to BS EN 39. One UK supplier lists galvanised 4 mm wall tube as “BS EN 39:2001, S235GT TYPE 4” at “approximately 4.5kg per meter” (accessed 16 September 2026). Couplers for other tube diameters and for mixed-diameter joints exist but are different products and must be ordered as such — if your tubes are not 48.3 mm, confirm the size with the supplier before ordering.

Couplers (fittings, clamps). The fittings discussed here include the double coupler, the swivel coupler, the sleeve coupler, the joint pin, the putlog coupler, the beam or girder coupler and the board retaining coupler. Each is described, with its UK and US names and the standards’ figures where our source ledger records them, in the coupler types guide.

Platform. Timber scaffold boards or steel or aluminium battens laid on the transoms, held by board retaining couplers or toe boards. Board specifications are outside the scope of this site.

Base. A base plate under every standard, spreading the load into a timber sole board on soft ground. Base plates are within the scope of the EN 74 family of standards, alongside couplers and spigot pins.

Which coupler joins what

The rule that makes tube and fitting scaffolding work is that each type of joint has its own coupler, chosen for the direction of the load that passes through it.

OSHA writes several of these choices into the regulation. Bearers (transoms) “shall be installed transversely between posts, and when coupled to the posts, shall have the inboard coupler bear directly on the runner coupler”, and when coupled to the runners “the couplers shall be as close to the posts as possible”. Bearers “shall extend beyond the posts and runners, and shall provide full contact with the coupler”. Both clauses are about the same thing: the coupler must sit on tube, not on air, and the load path from bearer to runner to post must pass through couplers that are stacked, not offset.

Bracing and ties: what OSHA requires

The US regulation is the most specific public text on how a tube and coupler scaffold is braced, and it is free to read, so it is quoted here. UK practice follows NASC guidance and BS EN 12811-1, which are not reproduced on this site.

Transverse bracing “forming an ‘X’ across the width of the scaffold shall be installed at the scaffold ends and at least at every third set of posts horizontally (measured from only one end) and every fourth runner vertically”. On straight-run scaffolds, longitudinal bracing “shall be installed diagonally in both directions, and shall extend from the base of the end posts upward to the top of the scaffold at approximately a 45 degree angle”; on scaffolds longer than they are high it is repeated “beginning at least at every fifth post”. Bracing is to be installed “as close as possible to the intersection of the bearer and post or runner and post”, and where it cannot be attached to posts it “shall be attached to the runners as close to the post as possible”. Building ties go in “at the bearer levels between the transverse bracing”.

Every one of those braces ends in a swivel coupler, which is why a tube and fitting scaffold uses far more swivels than a first count of the diagonals suggests.

How much it can carry

There is no single load figure for a tube and fitting scaffold. The capacity comes from the design: the bay length, the lift height, the number of boarded lifts, the tie pattern and the coupler class all enter it. What the public rules fix are the margins and the thresholds.

In the US, 29 CFR 1926.451(a)(1) requires that “each scaffold and scaffold component shall be capable of supporting, without failure, its own weight and at least 4 times the maximum intended load applied or transmitted to it”. OSHA’s fact sheet FS-3760 classifies scaffolds by duty: “Light-duty scaffolds can support a limited number of workers and hand tools (25 lbs. per sq. ft.). Medium-duty scaffolds must be able to safely hold workers, hand tools, and the construction materials being installed (50 lbs. per sq. ft.). Heavy-duty scaffolds must support workers, tools and the weight of stored materials (75 lbs. per sq. ft.).” Scaffolds “must be designed by a qualified person”, and “tube and coupler scaffolds over 125 feet (38 m) in height must be designed by a registered professional engineer”. The fact sheet itself states that it “does not impose any new compliance requirements”; the CFR is the rule.

The coupler’s contribution to that design is its class. The current edition, BS EN 74-1:2022+A1:2025, requires a Class B right-angle coupler to reach a characteristic slipping force of 15,0 kN in the type test against 10,0 kN for Class A, measured as the lowest force between 1 mm and 2 mm of displacement (Δ2); the superseded EN 74:1988 required the same two figures. Those are type-test characteristic values, not working loads: the UK distributor TRAD lists working loads of 9.10 kN and 6.10 kN for the same two classes (accessed 16 September 2026). The full table for both editions, and the difference between a test value and a working load, are set out in the coupler types guide.

Tube and fitting versus system scaffold

The alternative to tube and fitting is a system scaffold: standards with fixed connection points (cups, rosettes or wedge pockets) that take ledgers and transoms with built-in ends, at fixed lift heights. System scaffold is faster to erect on a regular building and needs fewer separate fittings; tube and fitting is slower, needs a scaffolder who can set every joint, and can be made to fit anything. In practice the two are combined: system scaffold for the main run, tube and fitting for the awkward corners, the cantilevers, the ties and the bridging over doorways. That is why a system-scaffold contractor still buys couplers by the pallet, and why a coupler order from an exporter usually lists double, swivel and sleeve couplers together.

What to specify when buying the package

A tube and fitting order from a Chinese cluster factory goes through cleanly when each of the four groups is specified separately.

1. Tube: 48.3 mm outside diameter; wall thickness (3.2 mm Type 3 or 4.0 mm Type 4 to BS EN 39); steel grade; length range; galvanised or black.
2. Couplers, by type: quantity of each of double, swivel, sleeve, putlog, joint pin, girder and board retaining; forged or pressed; the standard edition and class for each type the cited standard classifies, following the eight-point list in the coupler types guide.
3. Base plates and sole boards: base plate size and thickness, and whether the plate is fixed or has a spigot.
4. Market name: state whether the order is for a US or a UK market so that the packing list and the test reports use the names the site inspector expects. A US inspector reads “coupler” in the CFR; a UK client reads “fitting” and expects the EN 74-1 mark with class letter on each piece.

Specifying by type and class is also the fastest way to get a comparable quote from more than one factory. A request for “scaffolding fittings, 5 tonnes” produces five different answers; a request for “3,000 double couplers EN 74-1 Class B drop forged, 1,200 swivel couplers EN 74-1, 400 sleeve couplers EN 74-1 Class B” produces five prices for the same thing.

For a mixed scaffold package, list the coupler quantities separately in your enquiry.

Sources

1. 29 CFR 1926.452(b) Tube and coupler scaffolds (Cornell LII mirror of the CFR; osha.gov is the authoritative host) — accessed 2026-09-16
2. 29 CFR 1926.451(a)(1) Scaffolds — General requirements, Capacity (eCFR) — accessed 2026-09-16
3. OSHA Fact Sheet FS-3760, Tube and Coupler Scaffolds — Planning and Design (11/2014) — accessed 2026-09-16
4. BS 1139-2.1:1991 (EN 74:1988) Metal scaffolding — Part 2: Couplers. Superseded; cited for its scope wording. Current edition sold by BSI — accessed 2026-09-16
5. 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, BSI product page) — accessed 2026-09-16
6. Scaffolding Direct — 48.3 mm x 4 mm galvanised scaffold tube, BS EN 39:2001, S235GT Type 4 — accessed 2026-09-16
7. TRAD UK — Double coupler product listings (drop forged Class A 6.10 kN, pressed steel Class B 9.10 kN) — accessed 2026-09-16
8. Altrad Generation — Scaffold fittings catalogue (codes, weights, standards) — accessed 2026-09-16