Anchoring is the connection of an element to a concrete or masonry structure by anchors that transfer the element's loads into the base material. Typical applications include column baseplates on foundations, handrails, façade brackets, and pipe and sprinkler supports suspended from slabs. Anchors are also called fixings or anchor bolts; EN 1992-4 uses the term fastener.
The resistance of an anchorage depends on the anchor and on the conditions in which it acts. Anchor type, size, material and embedment determine its potential resistance. The strength and condition of the base material, edge distances and spacing, installation quality and the type and duration of loading determine how much of that resistance is available.
Fixture, anchor and base material
A fastening consists of three parts:
- The fixture is the attached element, such as a baseplate, bracket or rail.
- The anchor is the steel element, with any expansion part or bonding mortar, that connects the fixture to the structure.
- The base material is the concrete or masonry into which the anchor transfers the load.
Failure can occur in the anchor steel, at the interface between anchor and base material, or in the base material itself. Which failure mode governs depends on the anchor type and size, embedment, base material and the geometry of the anchorage. Where the concrete or masonry governs, higher-strength anchor steel does not increase resistance.
Cast-in and post-installed anchors
Cast-in anchors, such as headed studs and anchor channels, are positioned in the formwork before the concrete is placed. Their locations must be fixed at the design stage.
Post-installed anchors are installed in holes drilled in hardened concrete or masonry. They are used in existing structures, for design changes after casting and wherever the fixing position is not known before construction.
Load-transfer mechanisms of post-installed anchors
Post-installed anchors transfer tension into the base material by friction, mechanical interlock, bond or a combination of these:
| Mechanism | Load transfer | Typical anchors |
|---|---|---|
| Friction | An expansion sleeve, clip or shell is pressed against the hole wall | Throughbolts, sleeve anchors, drop-in anchors |
| Mechanical interlock | The anchor bears against a matching shape formed in the base material | Undercut anchors, concrete screws |
| Bond | A hardened mortar transfers load from the steel element to the hole wall | Resin bonded anchors |
Each mechanism depends on a different aspect of installation: friction on correct expansion, interlock on accurate hole geometry, and bond on hole cleaning and complete curing of the mortar. How anchors transfer loads into concrete describes each mechanism.
Base materials and post-installed rebar connections
Concrete is the base material for which anchor behaviour is best researched and for which assessment and design methods are most developed. Concrete strength and the presence of cracks at the anchor location both affect resistance. Cracking reduces the resistance of most anchor types.
Masonry varies far more. Units range from high-strength solid bricks to low-strength lightweight blocks, and perforated bricks and hollow blocks contain voids. An anchor suitable for solid units may have little or no resistance in a hollow unit. Mortar joints, particularly vertical joints, are often incompletely filled, and render can conceal joint positions. The type and condition of the masonry unit determine which anchors are suitable.
Because masonry units vary so widely, the resistance of an anchor in masonry can only be established by testing. A European Technical Assessment (ETA) for a masonry anchor gives resistances for the specific units tested for that ETA. For other units, or where the masonry is unknown, the resistance is determined by tests on site.
Post-installed rebar connections serve a different purpose. Reinforcing bars are bonded into drilled holes to connect new concrete to existing concrete, for example to extend a slab or add a wall. The bar continues the reinforcement, so the connection is designed as a reinforcement anchorage or lap to EN 1992-1-1.
Design of safety-relevant fastenings
Fastenings for lightweight, non-critical items, such as small ducts, lighting and wiring outside seismic areas, are generally selected on the basis of experience. Fastenings whose failure could endanger life or cause significant economic loss are designed by a structural engineer.
That design uses product-specific data. Anchors of similar appearance can have very different assessed resistances, minimum edge distances and installation conditions. In Europe, these data are given in the product's ETA, and EN 1992-4 provides the design method for fastenings in concrete.
The next guide describes the types of concrete anchors.