En resumen
Drop-in sockets with M16 ring bolts are for structural concrete and hard natural stone only. Concrete screws and nylon plugs with screw-in eyes cover concrete and sound brick, stone and dense block. Resin anchors suit weak or inconsistent masonry, positions near edges, mortar joints and, with sleeves, hollow units. Masonry usually needs preliminary tests.
Esta guía explica cómo se planifican y realizan los ensayos in situ conforme a BS 8539:2012+A1:2021 y NASC TG4:25. No sustituye a dichos documentos, a los datos del fabricante del anclaje ni a las instrucciones del diseñador, prescriptor o ingeniero del andamio responsable de la obra, quien decide el plan de ensayos, las cargas y los coeficientes. Un ensayo registra lo ocurrido ese día; no aprueba una fijación para su uso.
Key figures
| M16 drop-in anchors, recommended loads | 9.0 to 12.6 kN, depending on the maker |
|---|---|
| M16 drop-in anchors, typical edge distance in concrete | 220 to 230 mm |
| Ring bolt engagement in a socket | At least six full turns |
| Distance from a vertical edge in masonry, without maker's data | One full unit, at least 280 mm in brickwork |
| Distance from a horizontal edge in masonry, without maker's data | At least 2.0 m |
| Nylon plugs | Many for light-duty ties (3.5 kN) only; standard-duty versions up to 6.1 kN exist |
Which anchor suits which base material?
| Base material | Anchors that suit it |
|---|---|
| Structural concrete | Drop-in expanding sockets; self-tapping screws; nylon plugs with screw-in eyes; resin anchors |
| Hard natural stone | Drop-in expanding sockets |
| Brickwork and stonework | Self-tapping screws; nylon plugs with screw-in eyes; resin anchors; self-tapping screws with resin |
| Concrete blockwork | Self-tapping screws; nylon plugs with screw-in eyes; resin anchors |
| Timber | Screw-in eyes; self-tapping screws or nylon plugs where the manufacturer approves |
| Steelwork | Bolts; self-tapping screws |
The four main scaffold anchors compared
| Anchor | Where it works | What limits it | What to check when fitting | Can it be reused? |
|---|---|---|---|---|
| Drop-in socket with ring bolt | Structural concrete, hard natural stone | Never brick or block; keep well back from edges | Expanded fully with the setting tool; ring bolt in at least six full turns | No: it stays in the wall, so cap it unless stainless |
| Concrete screw | Concrete, sound brick, stone and dense block; timber if approved | Avoid lightweight, insulating or hollow blocks; sizes vary by maker | The maker's socket size, never a worn one | Not unless the maker allows it |
| Nylon plug with screw-in eye | Concrete, brick, stone, concrete block; timber if permitted | Load class; creep (tested at 3 × unless the maker says otherwise) | Eye not overtightened | Plug: capped and reused only with double the proof testing. Eye: after inspection |
| Resin anchor | Concrete, block, brick, stone, mortar joints; hollow units with sleeves | Full curing time before any load | Hole cleaned to the maker's method; installation torque with a calibrated wrench | Only as the maker allows |
Which anchor should I use in concrete?
In structural concrete you can use any of the four, so choose by load and by what is near the anchor.
- Drop-in socket and ring bolt: the usual partner for an M16 ring bolt. Mark the hole clear of the rebar and well back from column faces and slab edges, because the socket is hammered home and can crack concrete near an edge. The setting tool expands it; screwing the ring bolt in will not. Give the ring bolt six or more full turns, then back it off to line up with the tube, never wound hard into the end of the thread, which can break the socket. For heavy-duty ties, confirm the maker's rating first: M16 drop-ins vary from 9.0 to 12.6 kN.
- Concrete screw: quick to fit and to take out. Hex-head versions up to 20 mm suit band-and-plate couplers.
- Resin anchor: worth it close to an edge, because it puts no expansion force on the concrete.
TG4:25 §7.1 to §7.3 and §7.6 give the detail.
Which anchor should I use in solid brick or stone?
In sound brick or stone, use a concrete screw, a nylon plug with a screw-in eye, or a resin anchor, and expect to run preliminary tests unless the maker gives data for that wall. Never a drop-in socket: it can split the brick and may not expand properly.
Where the anchor goes matters as much as which one you pick. Drill into the body of the brick, not the joint. If render hides the courses, find them first, by stripping a patch or test drilling, and agree the method with the client before the scaffold goes up. The table below gives TG4:25's placing rules. Where the maker gives its own edge distances and spacings, use those instead; the depth, header-brick and render rules apply either way.
Placing anchors in brickwork
| Situation | Rule | TG4:25 |
|---|---|---|
| Cavity or collar-jointed wall, fixing in the front leaf only | Embedment no deeper than 75 mm | §8.3 |
| Header brick, used because the far leaf or infill is weak | Double the proof-test rate, as drilling can loosen a header | §8.3 |
| Solid or collar-jointed wall, fixing through to the back leaf (the strongest option) | Hole depth as the maker says, but stopping at least 30 mm short of the far face | §8.3 |
| Rendered wall | The whole embedment in the brick behind the render | §8.3 |
| Two anchors close together, where the maker gives no spacing | Never both in one brick; where you can, not in bricks next to each other | §8.4 |
| Near a horizontal edge, where the maker gives no edge distance | 2.0 m or more from it | §8.4 |
| Near the end of a wall or an opening, where the maker gives no edge distance | One full unit in from the vertical edge, so 280 mm or more in brickwork; more where ties carry large loads along the wall, on an engineer's advice | §8.4 |
| Plate held by two screws, where the maker gives no spacing | TG4:25's welded-plate detail spaces them 150 mm apart vertically in 75 mm courses | §7.3 |
What if the masonry is weak, perforated or hollow?
For weak or inconsistent masonry, a resin anchor is often the better choice, since it does not push against the wall as it is set. For perforated or hollow bricks and blocks, use resin with a plastic injection sleeve, and get an engineer to check that cavity construction can carry the tie load. Nylon plugs suit weak bricks better than metal expanders, because the softer plug spreads its grip.
With resin, timing and temperature matter: keep the cartridges within the maker's storage range, never warm them to speed things up, and wait the full curing time for the wall temperature before loading or testing. TG4:25 §7.4 and §7.6 give the detail.
What if I can't drill the bricks?
Only as a last resort, and only with sign-off from the anchor's maker or a structural engineer. The usual reason is a conservation order that keeps you out of the bricks themselves. Resin anchors are usually the best choice. Expect more testing: preliminary tests are compulsory, and proof tests rise to 1 anchor in 10 over the whole job. Set each anchor where a bed joint meets a vertical joint, and choose one clearly wider than the joint: TG4:25 §8.5 gives 14 mm in a 10 mm joint as its example.
What about timber and steel?
Timber takes screw-in eyes, and concrete screws or nylon plugs where their makers approve it. Steelwork takes bolts or self-tapping screws.
Why are nylon plugs tested harder?
Held under load for a long time, nylon slowly deforms, so an anchor that survives the test load for a short time can still fail at a lower load in service. Preliminary tests therefore use 3 × the tie duty for nylon, unless the manufacturer says otherwise; PA 6 nylon is the recommended type. See preliminary tests on scaffold ties, and our test-load calculator for jobs specified to BS 8539.
Can scaffold anchors be reused?
Bin any anchor that is bent, rusty or in doubt, and if anchors or ties come back bent, find out why. Beyond that, reuse only what the maker allows, inspect every one, and keep a count so none goes back more often than the maker permits. Screw-in eyes usually go round again after a check; capped nylon plugs can be reused with double the proof testing; concrete screws normally cannot. See TG4:25 §12.
How do I test each type with Staht kit?
Test every type by pulling on the anchor itself. The Staht clevis is made for safety eyebolts, lifting eyes and load-bearing connections (Staht datasheet, part 601-010), and threaded washers in the tool adaptor jaw take projecting studs. See connecting the tester to the anchor and proof testing scaffold ties.
More questions
No. Drop-in sockets are for structural concrete and hard natural stone. In brick or block they can crack the units and may not expand properly.
Six full turns or more, with no forcing. Then back it off to line up with the tie tube, and never jam it against the end of the thread.
Concrete screws, nylon plugs with screw-in eyes and resin anchors can all suit brickwork, subject to the maker's guidance. Brickwork usually needs preliminary tests, unless the manufacturer gives data for that brickwork. For weak masonry, resin anchors put no expansion stress on the wall.
Not normally. Their cutting edges wear as they go in, so a second fitting holds less well. Reuse them only if the maker explicitly allows it.
The scaffold designer, or whoever is responsible for the ties, chooses the anchor for the wall and the tie duty, following the maker's guidance. The scaffold designer or responsible engineer decides the test plan; testing checks the choice and the installation, and the report records results without approving the anchor.
Fuentes
- NASC TG4:25 Anchorage systems for scaffolding, Sections 6.4, 7, 8 and 12, NASC
Test every anchor type
Staht's clevis connects the t25 and t60 to eyebolts and lifting eyes, and its threaded washers to studs.