A fleet, not a consignment

A scaffold hire company does not buy tube for a project. It buys tube for a fleet that will be loaded, stacked, dropped, cleaned and reloaded for fifteen years, and the tube that survives that is rarely the cheapest one on the quotation. The specification that matters is the one that keeps a standard in service: 48.3 mm outside diameter, 3.2 mm wall, 6 m long, 21.4 kg a length. Everything else follows from those four numbers.

The 48.3 mm is not arbitrary. It is the size the yard's couplers and swivel fittings are made for, and a tube a millimetre off changes the fit of every one. The wall is where arguments start, because it is the only dimension that can be reduced without anyone noticing at the unloading bay.

What the second number buys

A 48.3 x 3.2 mm tube carries 453 mm2 of steel and weighs 3.56 kg per metre, so a 6 m standard is 21.4 kg. Drop the wall to 2.75 mm and the weight falls to 3.09 kg per metre, 13 per cent less steel, which is a real number on a hundred-tonne order. The stiffness falls with it: the second moment of area drops from 115,857 to 102,433 mm4, 12 per cent, and the bending moment at yield from 1.13 to 1.00 kNm. The saving is visible on the invoice. The loss is spread across every bay designed on the heavier tube, and shows up as sway rather than failure.

A second penalty never appears in a calculation. Thin walls dent at the coupler, and a dent is a stress raiser at the exact point where the tube is gripped. A tube over-torqued against a 3.0 mm wall is scrap after one season; the same treatment leaves a mark on 3.2 mm.

Where Q195 stops

Q195 is a useful grade. It has a minimum yield strength of 195 MPa, it welds and forms easily, and the Q195 plate page names scaffolding among its applications, true for base plates and light formed parts. It is not the grade for a standard that spends its life in compression and bending. A standard at 2 m lift is a column, and needs yield strength with margin above the load, not the cheapest coil in the warehouse.

Q235B, at 235 MPa minimum yield, is the normal choice, and Q235 plate covers the base plates, cut from galvanized plate where they stand on wet ground, and the tie plates that go with them. Where the tube is bought as pipe rather than as scaffold tube, ASTM A53 is the nearest mill product: 1/2 to 36 inch nominal bore, walls of 3 to 12 mm in SCH 40, 80 and 160, lengths of 3 to 18 m, ends plain, bevelled or threaded, with a mill certificate to EN 10204 3.1B. The 1.5 inch SCH 40 size is 3.68 mm wall and 4.05 kg per metre, heavier than the scaffold standard, which is why hire yards buy purpose-made tube.

Galvanized inside, not just outside

Most scaffold tube starts as Z275 galvanized coil, or as A653 structural coil where the tube is formed in-house: 0.12 to 2.0 mm thick, 600 to 2000 mm wide, zinc from 30 to 275 g/m2. At 275 g/m2 the coating is about 38.5 microns per side, enough for a rural or general urban site, thin enough for a coupler to wear through. The coil is slit and formed, and the longitudinal weld is made after coating, so the seam is bare on both faces unless the tube is hot-dip galvanized after fabrication.

That difference decides the life of the tube. A pre-galvanized tube rusts first along the seam and at the cut ends, which is where water sits when tubes are stacked outdoors. A hot-dipped tube carries zinc inside the bore and over the ends, and the coating is thickest where the forming stresses are highest. The galvanizing process runs from cutting and bending through welding and polishing to the zinc bath and assembly, and it is that sequence, not the zinc weight, which decides whether the inside of a 6 m tube is protected at all.

Zinc has two enemies on a scaffold. Fresh mortar and concrete are alkaline and attack it before they set, so tubes used as props or stored against a wet slab should be washed before the mortar cures. Acidic cleaners and descalers do the same thing faster, and a tube cleaned with either loses coating at the bottom end where the water collects.

The load case is a bay, not a tube

A scaffolder does not design a tube; he designs a bay. Take a 1.5 m wide bay with a 2.0 m lift working at 2.0 kN/m2: the platform carries 6 kN, and with four standards that is 1.5 kN per leg before boards, wind and anything stored on the lift. The tube at 48.3 x 3.2 has a yield moment of 1.13 kNm, so at normal lift heights the standard is not the weak link. The coupler is.

A coupler is a friction joint. Its capacity comes from bolt torque, and torque is the one thing on a scaffold nobody measures after the first week. That is why the tube specification matters less than the inspection regime: a perfect tube in a loose coupler is a mechanism, and a slightly worn tube in a correctly torqued coupler is a structure.

What condemns a tube

Four things take a standard out of service: a wall loss of about 10 per cent, which is 3.2 mm down to 2.9 mm; a dent at a coupler position; a bow that shows against a string line, roughly 12 mm over 6 m; and a crushed or split end. Rust perforation is obvious; pitting at the seam is what passes a visual check and fails a wall thickness gauge.

The yard practice that pays for itself is separation. Painted tube, hot-dipped tube and pre-galvanized tube should not be mixed in a bundle, because the painted length rusts first and makes the rest look worse. Batch marking lets a bad coil be traced and pulled before it reaches a scaffold.

The order and the yard

The order is short: Q235B, 48.3 x 3.2 mm, 6 m, hot-dip galvanized, plain ends, certificate to EN 10204 3.1B. A bundle of 100 lengths is 2.1 tonnes, a normal crane lift and a normal truck load. Where the tube is cut and drilled in the yard, cutting and welding are better done before galvanizing than after, and any weld made later needs a zinc-rich repair rather than paint, which means surface preparation before either.

Storage is part of the specification. Tubes stacked on the ground draw water at the ends; tubes stacked on timbers with the ends clear drain. A fleet bought on price and stored on the ground will be back on the quotation in five years, and the second order will be for the same tonnage again.