Scaffold tie test load: tie duty, proof and preliminary

The tie duty comes from the TG20 or TG30 compliance sheet that covers the scaffold, or from the scaffold designer's drawing for a bespoke design. It is the working load in tension that each tie must carry.

TG20 and TG30 base their tie patterns on three tie classes, by working load in pure tension: light duty at 3.5 kN, standard duty at 6.1 kN and heavy duty at 12.2 kN. NASC TG4:25 also lists a standard-duty tie at 9.1 kN, for Class B couplers. A design may give a different figure.

Pass if the tie duty is no more than the anchor's recommended load (maker's data) or allowable load (preliminary tests). If it is more, scale up the anchors: the number planned × tie duty ÷ rating, rounded up. That can mean more ties, or two anchors on each tie, such as a plate with two screws (TG4:25 §5.1).

The recommended load is already the maker's safe working figure, so the check is simply tie duty ≤ recommended or allowable load. Do not round the tie duty down.

Multiply the tie duty by the factor for the test: 1.25 for a proof test, 2 for a preliminary test, or 3 for a preliminary test on nylon anchors. Work from the exact tie duty for your scaffold, not a rounded figure.

Worked case 1: a 6.1 kN standard-duty tie.

• Proof test: 1.25 × 6.1 = 7.625 kN.
• Preliminary test, non-nylon anchor: 2 × 6.1 = 12.2 kN.
• Preliminary test, nylon anchor: 3 × 6.1 = 18.3 kN.

Worked case 2: a bespoke tie duty of 4.8 kN, from a design drawing, for anchors in brickwork.

• Preliminary test, non-nylon anchor: 2 × 4.8 = 9.6 kN, on 5 anchors.
• Preliminary test, nylon anchor: 3 × 4.8 = 14.4 kN.
• Proof test: 1.25 × 4.8 = 6.0 kN, on at least 5% of the anchors.

TG4:25 Appendix A tabulates the loads for all four standard tie classes. For jobs specified to BS 8539 instead, our test-load calculator works out the preliminary test load.

TG4:25 gives its proof loads to one decimal place, and not always upwards: for a 6.1 kN tie, 1.25 × 6.1 = 7.625 kN, which TG4:25's table lists as 7.6 kN. Which figure to test to is for the specifier to decide.

By capacity, a 25 kN tester such as the Staht t25 covers proof tests on all four standard tie classes, the highest being 1.25 × 12.2 = 15.25 kN. The light-duty proof load, 4.375 kN, is 17.5% of 25 kN, so check on the tester's calibration certificate that its accuracy at that load meets the 5% TG4:25 asks for. It also covers preliminary tests on non-nylon anchors up to heavy-duty ties, at 2 × 12.2 = 24.4 kN. Preliminary tests on nylon anchors for 9.1 kN and 12.2 kN ties need 27.3 kN and 36.6 kN, so they need a tester such as the 60 kN Staht t60.

Wind pushes and pulls the scaffold at right angles to the building, so the main load on a tie anchor runs along the tie. The other directions are normally taken by something else, and the table shows what to do if they reach the anchor (TG4:25 §5.4 to §5.7).

Yes. If structural fittings form part of the tie, the tie can only carry the safe working load of those fittings, whatever the anchor can hold.

Where an anchor carries tension and shear together, check the two together, not just each on its own. One check that NASC TG4:25 shows is:

Napp ÷ Nrec + Vapp ÷ Vrec ≤ 1.2

where Napp and Vapp are the working tension and shear loads, and Nrec and Vrec the recommended tension and shear loads. Combine shear from sideways and vertical loads into one resultant first. Ask the anchor manufacturer for the check that applies to its anchor. See shear testing.

If a job specification calls up BS 8539:2012+A1:2021 rather than TG4:25, use its factors. For short-term loading such as scaffold anchoring, BS 8539's preliminary test factor νtest is 2 in concrete, 3 in masonry and 3.5 for nylon anchors, and the test load is also divided by αdist and Ω. In masonry, this gives a higher test load than TG4:25's 2 × the tie duty. The scaffold designer or responsible engineer decides which applies. See preliminary tests.