Preliminary tests on scaffold ties, and using more anchors

Run preliminary tests whenever the maker's data can't show that the anchor will carry the tie duty in the wall you are fixing to. On scaffolds that mostly means brick, stone and timber with no published figures for that material, and any case where you would otherwise be leaning on a concrete rating: masonry, old concrete, or concrete with cracks (NASC TG4:25 §6.2 and §9.1).

Five anchors taken to a set load show whether there is a safe margin, without breaking anything.

If none of the 5 slipped as much as 1 mm at the test load, you can use that anchor in that base material at the planned tie duty. Every job still needs proof tests on the installed anchors. See proof testing scaffold ties.

When any test anchor falls short, the results go to an engineer, who chooses one of three routes (TG4:25 §9.3 lists them in full):

• Change the anchor: go deeper, go bigger in diameter, or switch to another type. A new anchor means a fresh set of 5 preliminary tests.
• Lower the load: pick a scaffold arrangement with a lighter tie duty, for example by swapping sheeting for netting, taking off cladding or adding ties; the TG20 and TG30 eGuides model this. Or have the scaffold redesigned.
• Share the load: keep the anchor but use more of them, as worked out below.

The number needed is the original number × tie duty ÷ allowable tie load, rounded up; the allowable tie load comes from loading the 5 test anchors to failure.

1. Take each of the 5 test anchors to failure, carefully, stopping when it has slipped 1 mm so the wall is not damaged; that load counts as its failure load.
2. Allowable tie load: the smaller of (average failure load ÷ 3) and (lowest failure load ÷ 2).
3. Nylon anchors use ÷ 5 and ÷ 3 instead, unless the maker says creep is not significant for its PA6 or PA66 anchor, in which case ÷ 3 and ÷ 2 apply.
4. Anchors needed: original number × tie duty ÷ allowable tie load, rounded up.

TG4:25's divisors are for scaffold anchors only.

A scaffold design gives a tie duty of 4.8 kN, so the preliminary test load is 9.6 kN. Four anchors hold it, but one slips 1 mm at 9.1 kN. The engineer chooses to use more anchors. The 5 anchors are then loaded to failure, stopping at 1 mm of slip: 9.1, 11.4, 10.6, 12.0 and 10.2 kN.

• Average failure load: 53.3 ÷ 5 = 10.66 kN. Divided by 3: 3.553 kN.
• Lowest failure load: 9.1 kN. Divided by 2: 4.55 kN.
• Allowable tie load: the lower value, 3.553 kN.
• The design had 30 anchors: 30 × 4.8 ÷ 3.553 = 40.5, so 41 anchors are needed.

One way to add anchors without adding ties is to fix each tie with two anchors, such as a plate held by two screws.

Two anchors on one tie should be spaced as the manufacturer recommends. TG4:25's welded-plate detail uses two hex-bolt screws 150 mm apart vertically in 75 mm brick courses, which keeps one clear course of bricks between them. Without the maker's guidance, TG4:25 §8.4 rules out two anchors in one brick and, where you can, in bricks next to each other. See scaffold anchor types.

A BS 8539:2012+A1:2021 preliminary test uses different factors. Its test load is NEk × νtest ÷ (αdist × Ω), where νtest for short-term loading such as scaffold anchoring is 2 in concrete, 3 in masonry and 3.5 for nylon. After a failure, its divisors also depend on the base material: 3 and 2 in concrete, 4 and 3 in masonry, 5 and 3.5 for nylon, again with α_dist and Ω. Under BS 8539, the "more anchors" route should only be considered when the failure was close to the test load, for example above 0.8 × the test load.

Follow TG4:25 for scaffold ties unless the job specification calls up BS 8539. The scaffold designer or responsible engineer decides. See preliminary tests and our test-load calculator.