Glue-In Anchors

U-bolts are by far the cheapest design of any glue-in bolt and, as the name implies, consist of a single length of metal bar bent into a U shape and fixed into two holes drilled parallel to one another. The cost-benefit alone would suggest that staples would be the default glue-in anchor choice, so why are they less prevalent than single-hole glue-in anchors? The principle issue, which concerns the requirement for two holes and the spacing between them, raises issues about strength and ease of placement in all types of placement. The close spacing between each leg requires very accurate drilling to ensure that upon installation, the adhesive is equally displaced around each leg and per placement, potentially more adhesive and drilling is needed. Staples also result in the quickdraw lying flat against the rock so require the rock surface around the staple to be free of any rock protrusions.

With holes drilled correctly i.e. parallel, staples have no means of hole retention, rendering their use problematic on steep rock. Certainly, climbers have found ways to prevent staples from slumping or dropping out, however, other glue-in designs are free of this issue. There is a higher risk of the quick draw unclipping itself from the bolt should it rotate around the placement when the rope puts tension on a quick draw (and not necessarily in a fall). The Fixe staple has a pronounced dropped nose for this reason.

As for any welded eyebolt, shaft preparation is crucial and staples cannot be rotated to improve the flow of adhesive over the legs.  Like stamped grooves or knurling employed for single-shaft designs, adhesive viscosity is particularly relevant in ensuring the adhesive flows in and around the legs. Epoxies have a clear advantage in this regard. Due to the low cost and simplicity of design, many staples are homemade thereby lacking the design features and assurance with a certified anchor.

Ultimately, the key issue often cited regarding staples is strength. However, this is very much subject to the anchor and rock in which it is being placed. In Australia, Steve Hawkshaw’s thesis demonstrated that staples are capable of sustaining acceptable loads; however, radial pull tests conducted by us on British Portland staples, a once popular homemade staple resulted in full extraction in soft limestone at loads no higher than 12kN which is well below either of the current standards (EN 959 / UIAA 123).

Whilst failure cones for climbing bolts originate closer to the rock surface than a comparison test in concrete (the matrix failure is different), there is the potential for stress cones to overlap for staple designs that feature closely spaced legs. In very soft rock, staples have failed because of this close distance. However, in most cases, the failure is the result of insufficient mechanical resistance due to the design (poor keying of adhesive to the legs) rather than all-out rock failure and staples typically fail at one leg rather than both. With sufficient leg spacing and adhesive of low viscosity (e.g. pure epoxy), this becomes less of an issue.

Our in-house tensile testing performed on the Titan Climbing staples we sell resulted in loads well above 30kN and this is partly due to the use of thicker rods, deeper leg embedment and wide leg spacing.

Staples combined with chains are particularly useful when requiring a theft-proof anchor configuration and a cheaper type of glue-in anchor when equipping routes that are not overhanging.