All you need to know about Anchor Working Principles

How many solutions are out there to fix an anchor plate? Stud and sleeve dowels, epoxy and hybrid injection mortar, torque-controlled bonded anchor, capsule adhesive, screw, undercut fastening: these are all fixing technologies offering different benefits, depending on the application and installation condition requirements (performance, flexibility, speed, suitability).

How to pick the right one? The first step is to identify a short list of suitable anchor solutions by looking at a third-party qualification (aka ETA approval while designing according to Eurocodes, or ICC-ES according to ACI 348). What does this mean? Let’s assume you are currently dealing with a seismic application: you just need to consider anchor solutions that offer the seismic suitability and performance assessed in the above-mentioned documents.

The second step is to run a proper design to filter out all the solutions that do not comply with the load requirements. Thirdly, you need to consider aspects like productivity, ease of use and value for money. Finally, the installer is provided with guidance to ensure a proper setting, for example indicating the torque moment to be applied for a stud anchor or stressing the need for cleaning the hole for a chemical fixing. In this respect, you can recommend systems that automatically mitigate the risks concerning the execution (such as Safeset for chemical anchors and AIT for mechanical anchors). 

If you are new to anchor design, we are happy to accompany you on your long journey, starting with the very basics of anchor design and selection.

The first thing you need to know is that, despite many differences in quality, performance and application coverage, all post-installed anchor technologies have a common ground: the way these systems transfer the load into a concrete building material is mainly based on one or a combination of three basic working principles that will cover in this article.

Friction

The tensile load N is transferred to the base material by friction, R. The expansion force Fexp is neces­sary for this to take place and can be generated by applying a controlled force or displacement.
In the case of controlled force, the lateral stress (aka expansion) is produced when a tightening torque is applied to the end. This is the case for anchors such as a HST3 stud anchor and HSL-3 sleeve anchor.

Keying
An interlock occurs by undercutting the concrete and creating an interlock with the fixing. The tensile load N is in equilibrium with the supporting forces R, acting on the base material, as per a HDA undercut anchor.

​Bonding
An adhesive bond is produced between the anchor rod and the hole wall by an adhesive, such as HIT-RE 500 V3 or HIT-HY 200-R with anchor rods.

Combination of working principles
Many anchors obtain their holding power from a combination of the above-mentioned working principles.
For example, an anchor exerts an expansion force against the wall of its hole because of the displacement of a cone relative to a sleeve. This permits the longitudinal force to be transferred to the anchor by friction. At the same time, this expansion force causes permanent local deformation of the base material: a keying action results, which enables the longitudinal force in the anchor to be transferred additionally to the base material.

To find out more, you have a number of options: leave a comment or post your questions in our Community, join our Webinars, or contact your local Hilti Enginner.