Pull tester bridge leg spacing and the reduction factor

The support spacing matters because supports close to an anchor can press on the base material that would otherwise break away, so the anchor seems to hold more than it would in service. In concrete, an anchor that fails by pulling out a cone of concrete gets help from any support standing inside that cone.

In concrete, the effect only shows when the anchor is loaded near its failure load and a concrete cone is the likely failure, so the rules are strict for tests to failure and preliminary tests, and relaxed for proof tests. In masonry, the spacing rules apply to every test except proof tests.

hef is the effective embedment depth, L the distance from the anchor to the nearest support, and adist the support spacing.

• h_ef, the effective embedment depth, is the depth from the surface to the lowest point at which the anchor grips the base material. It is not the hole depth.
• L, the reaction spacing, is the distance from the anchor's centre line to the closest bridge support.
• a_dist, the support spacing, is measured to the axis of point supports, or as the clear width between continuous supports. For a bridge set symmetrically over the anchor, a_dist = 2 × L. 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 × h_ef or less, the bridge is too narrow for this method: use a wider one.

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.

αdist divides the test load, so a value below 1.0 raises it. In tests to failure, αdist multiplies the result, so it lowers the allowable resistance. Either way, close supports cannot make an anchor look stronger than it is. See preliminary tests.

EOTA TR 053, Section 3.2.1, drops the reduction where the failure stays local: anchors deeper than 150 mm in multi-leaf walls of small solid bricks, such as solid clay bricks, anchors in mortar joints, and anchors in hollow or perforated bricks. The supports must still be more than 1.5 × h_ef apart and leave room for the failure to form.

The right Staht bridge is the narrowest one whose support spacing reaches 3 × hef for a test to failure or a preliminary test. A narrower bridge can still be used with αdist while its spacing is more than 1.5 × hef, with two extra conditions in concrete: a preliminary test also needs the closest support more than 1.0 × hef from the anchor, and a test to failure with the closest support under 1.5 × hef must be noted in the report and repeated with a wider bridge, unless the cone resistance is checked another way. Proof tests can use a bridge with deeper anchors than the table shows. In masonry, the brick and block rules above apply; the reduction is dropped only in the EOTA TR 053 cases described under αdist.

For the 575 bridge, equation B.12 stops at hef = 150 mm. Deeper than that, in concrete, keep L at 1.5 × hef or more: the 575 bridge does this up to h_ef = 191 mm.

Worked example: h_ef = 80 mm, a preliminary test in concrete blockwork.

• t60 standard bridge: 127 mm is less than 3 × 80 = 240 mm, and more than 1.5 × 80 = 120 mm, so α_dist applies: 0.4 + 127 ÷ (5 × 80) = 0.4 + 0.318 = 0.718. The test load rises by a factor of 1 ÷ 0.718 = 1.39.
• Bridge 375-60: 375 mm is more than 240 mm, so α_dist = 1.0 and the test load is not raised.

In concrete, the standard bridge would not do at this depth: L must be more than 1.0 × h_ef (80 mm) while testing to the test load, and the standard bridge's L is 63.5 mm.

Worked example: h_ef = 150 mm. 3 × hef = 450 mm. The Bridge 375-25 (375 mm, more than 1.5 × 150 = 225 mm) gives αdist = 0.4 + 375 ÷ 750 = 0.900. That suits masonry, or a preliminary test in concrete, where L = 187.5 mm is more than 150 mm. For a test to failure in concrete, L is under 1.5 × 150 = 225 mm, so use the 575-25. The Bridge 575-25 is wider than 450 mm, so α_dist = 1.0.

Never load a bridge above its maximum load: the 375-25 and 575-25 take a t60 load cell but are limited to 25 kN.

On masonry, put the feet where the test will load the wall around the anchor as well as the anchor itself. How depends on what you are fixing into:

• Solid brick: stand the feet on the bricks around the anchor's brick, never on it. Then a weak joint, or a brick shaken loose when the hole was drilled, shows up in the result. Standing the bridge upright usually achieves this (NASC TG4:25 §11.1).
• Blocks: keep the closest support at least 1.5 × hef from the anchor, or apply αdist as set out in the table above.
• Perforated or hollow bricks: turn the feet at an angle so that neither lands along a row of holes.
• Proof tests: use the neighbouring units where you can; on large units the closest support may come in to 0.5 × h_ef.

See testing in masonry.

Before relying on a bridge for a particular test, run three checks:

1. Security: on a wall or soffit, it is fixed so that nothing falls if the anchor lets go.
2. Aim: you can set it so the pull is square to the surface, or follows the anchor's own line if the anchor went in at an angle. Adjustable legs make that possible on uneven work.
3. Rating: its maximum load is above the test load. A t60 load cell on a 25 kN bridge is still limited to 25 kN. For very high loads, a stiff fixed-leg beam pulling through a hollow ram flexes less than a bridge with adjustable legs.

Staht's bridge spreader plates fit the feet of the t25, t60, 375-25 and 575-25 bridges and spread the load on internal walls and roofs. The 3 leg adaptor turns a t25 or t60 bridge into a three-legged one for tight spaces. It replaces one pair of legs with a single Ø35 mm foot, centred where that pair stood (Staht datasheet, part 600-085), so no foot comes closer to the anchor than on the four-leg bridge: the 127 mm figure still applies.