Preliminary anchor tests: working out the test load

A preliminary test is a site test on 5 anchors that shows whether an anchor can carry the characteristic action N_Ek (the unfactored load) in a base material for which the manufacturer has no data. BS 8539:2012+A1:2021 calls it the simplified approach to tests for allowable resistance, in Annex B.2.3.1.

It is used when no anchor with a relevant European Technical Assessment (ETA) is available for the base material, which in practice usually means masonry, or when the condition of the concrete casts doubt on the ETA's figures. The anchor must still be a type its manufacturer approves for that category of base material. The 5 anchors are loaded only as far as the test load, not to failure, so the tests cause less damage than tests to failure. See when you need to site test.

The anchors for a preliminary test are installed specially for it and are never used on the job. Put them in base material that matches the job, at a spot where any damage does no harm to the structure and can be repaired. Keep them well away from anchors that will be used, and never closer than 3 × h_ef where space is tight.

The test load is the characteristic action multiplied by a test factor, then divided by the reduction factor α_dist and the condition factor Ω:

N_test = N_Ek × ν_test ÷ (α_dist × Ω) (equation B.4 of BS 8539)

• N_Ek, the characteristic action, is the load the design applies to one anchor, without safety factors. The specifier supplies it.
• ν_test, the test factor, depends on the anchor material, the base material and whether the load is long-term or short-term. See the table below.
• α_dist, the reduction factor, allows for bridge supports set so close to the anchor that they could hold the base material together and raise the result. It is 1.0 when the legs are far enough apart. See choosing a bridge.
• Ω, the condition factor, allows for differences between the test and the real use, such as testing in dry conditions for an anchor that will be wet. The specifier decides Ω.

Our test-load calculator works through these steps for you.

Nylon anchors are given higher factors because of creep. 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.

The specifier chooses Ω by asking how the anchor's real use will differ from the test. Each answer gives a value:

• How long will the load act? For short-term loads, such as scaffold or steeplejack anchoring, Ω = 1.0. For a sustained tensile load, Ω = 0.9.
• Will the anchor be wetter or hotter in use than in the test? If it will be wet in use but is tested dry, Ω = 0.8. If it will run hotter than the manufacturer's normal service temperature range but is tested at normal temperature, Ω = 0.8.
• Could working anchors land somewhere weaker than the test anchors? On a rendered or plastered wall, where the joints are hidden or the anchors cannot be kept inside the bricks, and the test anchors are in solid brick, Ω = 0.75.
• Will the concrete be cracked? For an anchor designed for concrete in tension, Ω = 0.7.

Where more than one condition applies, the specifier gives a single Ω for the case. This guide gives no rule for combining them; ask the specifier.

αdist depends on the support spacing adist and the effective embedment depth hef. adist is measured to the axis of point supports, or as the clear width between continuous supports. Staht's figures, 127 mm for the t25 and t60 standard bridges and 375 mm and 575 mm for the wider bridges, are the clear distance between the feet along the bridge (Staht datasheets), the basis the industry's published bridge table uses. That errs on the safe side: the feet of a four-leg bridge actually sit further from the anchor, so the clear spacing gives a slightly higher test load, never a lower one.

Up to h_ef = 150 mm. If adist is at least 3 × hef, αdist = 1.0. If adist is less than 3 × hef but more than 1.5 × hef, αdist = 0.4 + adist ÷ (5 × hef) (equation B.12 of BS 8539). BS 8539 also caps the spacing needed at 450 mm, but up to hef = 150 mm, 3 × hef never exceeds 450 mm, so the cap does not change the answer. If adist is 1.5 × hef or less, the bridge is too narrow for this method: use a wider one. In concrete, a preliminary test also needs the closest support more than 1.0 × hef from the anchor, as worked example B checks.

Deeper than 150 mm. Equation B.12 does not apply. In concrete, keep the closest support at least 1.5 × hef from the anchor, which means adist of at least 3 × hef. In masonry, the rules for bricks and blocks apply. EOTA TR 053 needs no reduction in multi-leaf walls of small solid bricks, such as solid clay bricks, provided the supports are more than 1.5 × hef apart and leave room for the failure to form. See choosing a bridge.

hef is the depth to the lowest point at which the anchor grips the base material. For nylon anchors, take hef as 50% of the nominal embedment depth.

A bonded anchor in solid brick carries a sustained tensile load. The specifier gives NEk = 0.9 kN, hef = 60 mm and a single Ω of 0.9 for the sustained load. ν_test for a non-nylon anchor in masonry under long-term loading is 4.

With the Staht t25 standard bridge (support spacing a_dist = 127 mm, the clear distance between the feet along the bridge (Staht datasheet)):

• 3 × hef = 180 mm, which is more than 127 mm, so αdist applies. 127 mm is more than 1.5 × h_ef (90 mm), so equation B.12 holds.
• α_dist = 0.4 + 127 ÷ (5 × 60) = 0.4 + 0.423 = 0.823.
• N_test = 0.9 × 4 ÷ (0.823 × 0.9) = 3.6 ÷ 0.741 = 4.86 kN.

With the Staht Bridge 375-25 (a_dist = 375 mm):

• 375 mm is more than 3 × hef (180 mm), so αdist = 1.0.
• N_test = 3.6 ÷ (1.0 × 0.9) = 4.00 kN.

The wider bridge removes the reduction, so the anchors are tested to 4.00 kN instead of 4.86 kN. In brickwork, set the bridge feet on the bricks next to the one holding the anchor. See testing in masonry.

Anchors for steeplejack work go into older concrete whose condition is in doubt, so the specifier asks for preliminary tests. NEk = 3.0 kN, hef = 50 mm, short-term loading, so Ω = 1.0. ν_test for a non-nylon anchor in concrete under short-term loading is 2.

• With the Staht t60 standard bridge, adist = 127 mm, the clear distance between the feet along the bridge (Staht datasheet), which is less than 3 × hef (150 mm) and more than 1.5 × h_ef (75 mm), so equation B.12 applies.
• The closest support, at 63.5 mm, is more than 1.0 × h_ef (50 mm), as preliminary tests in concrete need while loading to the test load.
• α_dist = 0.4 + 127 ÷ (5 × 50) = 0.4 + 0.508 = 0.908.
• N_test = 3.0 × 2 ÷ (0.908 × 1.0) = 6.0 ÷ 0.908 = 6.61 kN.

Nylon anchors will hold a temporary hoarding to older concrete whose condition is in doubt, so the specifier asks for preliminary tests. NEk = 0.4 kN, short-term loading, so Ω = 1.0. The nominal embedment is 70 mm, so hef = 35 mm. ν_test for nylon under short-term loading is 3.5.

• With the Staht t25 standard bridge, adist = 127 mm, the clear distance between the feet along the bridge (Staht datasheet), which is more than 3 × hef (105 mm), so α_dist = 1.0.
• The closest support, at 63.5 mm, is more than 1.0 × h_ef (35 mm).
• N_test = 0.4 × 3.5 ÷ (1.0 × 1.0) = 1.40 kN.

When all 5 anchors hold the test load, the characteristic action may be taken as the allowable resistance in that base material: NR,all = NEk (equation B.5 of BS 8539). The anchor can then be used at that load.

If an anchor falls short of the test load, the specifier decides what happens next, and two questions settle it. Could the job simply take more anchors, for example by setting them closer together? And did the anchor fail near the test load, for example above 0.8 × N_test?

• Yes to both: keep the anchor and use more of them, at a lower allowable resistance worked out as below.
• No to either: change the specification, to a bigger diameter, deeper embedment or another type of anchor, and run a fresh set of preliminary tests on it.

Working out the lower allowable resistance:

1. Take all 5 test anchors to failure, slowly, noting the load at first movement on each. To spare the structure, you can stop at about 1.0 mm of movement and count that load as the failure load.
2. From the average failure load Nu,ave: NR,all = Nu,ave × αdist × Ω ÷ ν_ave (equation B.6).
3. From the lowest failure load Nu,low: NR,all = Nu,low × αdist × Ω ÷ ν_low (equation B.7).
4. Use the lower of the two, capped at the mean first-movement load N_1st,m.
5. Scale up the anchor count: n' = n0 × NEk ÷ NR,all (equation B.8), where n0 is the number first planned. Round up.

In example A with the t25 bridge, the test load was 4.86 kN. One anchor fails at 4.2 kN, which is above 0.8 × 4.86 = 3.89 kN, and the job can take more anchors. All 5 are loaded to failure: 4.2, 5.6, 6.1, 5.3 and 6.4 kN. The mean load at first movement is 2.3 kN.

• Nu,ave = 27.6 ÷ 5 = 5.52 kN, and Nu,low = 4.2 kN.
• α_dist × Ω = 0.823 × 0.9 = 0.741.
• From the average: 5.52 × 0.741 ÷ 5 = 0.818 kN.
• From the lowest: 4.2 × 0.741 ÷ 4 = 0.778 kN.
• NR,all = 0.778 kN, the lower value. It is below N1st,m (2.3 kN), so it stands.
• The job planned 24 anchors: n' = 24 × 0.9 ÷ 0.778 = 27.8, so 28 anchors are needed.

BS 8539 gives three other routes, each based on tests to failure.

• No ETA, statistical approach (Annex B.2.3.2): 5 to 15 anchors are loaded to failure. NRk1 = αdist × NRu,m × (1 − K × v) × Ω, where NRu,m is the mean failure load, v the coefficient of variation (no more than 30%) and K a statistical factor: 3.40 for 5 tests, 2.57 for 10 and 2.33 for 15. NR,all = NRk1 ÷ ν, no more than N_1st,m. The specifier chooses ν, for example 3 to 4 for static loads, or 5 for nylon anchors.
• ETA anchor in masonry outside the ETA, 5 to 15 tests (Annex B.2.2.1): NRk1 = αdist × NRu,m × (1 − K × v) × β, with β from the ETA (1 if none is given), no more than the ETA's characteristic resistance. Here β takes the place of Ω. Then NR,all = NRk1 ÷ γM ÷ γF, with γM from the ETA and γF 1.35 for permanent actions, 1.5 for variable actions or 1.4 where they are unknown or mixed, and no more than N1st,m.
• ETA anchor, 15 or more tests (Annex B.2.2): the job-site test method of the relevant European Assessment Document (EAD) and its EOTA Technical Report.

The more anchors tested to failure, the higher the allowable resistance is likely to be, but tests to failure do more damage.