Module: Concrete - variable embedment depth: anchor length optimization

Module: Concrete – variable embedment depth: anchor length optimization

Throughbolts are widely used for fast mounting various components to concrete substrate. The application range of this type of anchor includes:

fixing base plates of load-bearing structures,
fixing substructures of elevations,
fixing installations in construction infrastructure (e.g. tunnels, bridges, noise barriers)
fixing installations in industrial infrastructure.

However, throughbolts have shown limitations in terms of design flexibility compared to chemical anchors.

The embedment depth in concrete determines both the load transfer process and the failure pattern of the anchor. As a rule of thumb, the deeper the anchorage is, the higher load carrying capacity of the anchor. However it should be remembered that deeper embedment results in longer drilling time and also a higher risk of hitting rebar.

Most mechanical anchors, also throughbolts, are approved for invariable embedment depths, usually between 1 and 2 values. This limits the flexibility and potential for optimization in anchor selection and design, because the designer has to choose from a predefined set of anchor lengths and embedment depths.

The new European Assessment Document EAD 330232-01-0601 v03, covers the use of variable embedment depth for mechanical anchors for use in concrete. This means that the embedment depth can be freely selected within certain ranges (as specified in the ETA, European Technical Assessment document). With variable embedment depth, the designer or site engineer can choose the optimum anchor length and embedment depth for his needs, getting the best balance between value and money, which helps save on material and reduce installation costs.

As an example of optimization, let us use the R-HPTIII-ZP anchor with variable embedment depth.

In the basic example, we have a 200 x 200 mm base plate with a thickness of 10 mm, loaded with a force of 40 kN. Four M12 anchors in the case without variable embedment depth have h_ef value of 125 mm, which in our case gives us an R-HPTIII M12 anchor with a length of 180 mm and a 74.3% utilisation.

The usage of variable embedment depth allows us to optimize design. The M12 anchor with a length of 100 mm and an embedment depth of 64 mm gives an otimal utilization of load capacity of 98.4%.

Due to the variable embedment depth, we can also optimize the base plate, reducing the diameter and spacing of the anchors. As a consequence we get a 165X165 mm plate for 4 R-HPTIII anchors with a length of 115 mm anchored to a depth of 81 mm to take full utilization of its load-bearing capacity.