Stud anchors are widely used. Tips to select the right one for a safe fastening

Anchor fastener: one of the most neglected construction detail in the design phase. It represents a small part in terms of the volume and cost of realization, but it can have a significant impact on the safety of the work overall.

However, we all know it is not just a "detail": the anchor (or anchor system) has the important role of connecting two different materials (i.e. an anchor plate to a concrete base) that were initially separate. And this may be the case in challenging conditions:

· It is not rare that the forces to be transmitted from the fixing to the base material are the maximum you see in the whole structure;

· The geometry is often not favorable, offering limited space and short edge distances, making it even harder to find the right anchor configuration (and affecting the load capacity of the base material itself).

There are many solutions available to successfully cover the needs of different applications. Over the next few months, we are going to cover the most common ones. Let's start now with one of the most widespread anchor technology: The stud anchor.

Stud anchors: a well-known fastening technology

The stud anchor is made up of a threaded bolt with a cone at the end, an expansion sleeve, a washer and a nut for tightening (as shown in the figure, for the stud anchor HST3).

To understand the working principle, it is useful to look at how the anchor itself is set. After drilling and cleaning the bore hole, the stud is introduced inside the hole by hammering (manually or using setting tools). The anchor plate of the element to be connected is then positioned and, eventually, the installation ends by tightening the nut to the torque value defined by the manufacturer (known as the installation torque): the torque causes the cone to be drawn into the expansion sleeve, which then deforms and expands against the concrete. Thanks to the radial pressure generated by the expansion and the friction between the sleeve and the concrete, the tensile strength of the dowel is activated.

Stress contour due to the expansion of the sleeve

Not all stud anchors are the same, especially in cracked concrete.

Under cracked concrete conditions, expansion fasteners without a specially designed sleeve may lose pretension and slip out. This might happen especially when cracks open and close throughout the service life of the concrete structure. In the most extreme conditions, it can even result in uncontrolled behavior or even failure.

How can you deal with this? To begin with, consider only using anchor systems that have been tested and approved by a qualification process for your application requirements. Otherwise, it will be difficult to predict the real fixing behavior in the design conditions.

Pragmatically, it means choosing only cracked-concrete-approved anchors, unless you cannot ensure that the concrete will never be tensioned under the applied loads. If your design is in a seismic area, look at using a seismic qualified system.

Simply refer to the European Technical Assessment document (ETA) of the system you intend to use for your application (click here to go to the HST3 product page and benefit from high performance in cracked and seismic conditions, thanks to the cutting-edge sleeve design). 

Then you need to complete your design accordingly using a reliable software like PROFIS Anchor.

Pay attention to the installation torque
When it comes to anchoring in concrete, applying the right torque is critical for ensuring that the cone is pulled into the sleeve and is stopped in the right position for generating the required expansion forces.
Under-torquing results in the cone being in the incorrect position i.e. it is not drawn far enough into the expansion sleeve to achieve the specified holding values. Only once the external load is applied does the anchor shift and the cone gets pulled in the right direction. In other words, the anchor was loose and relied on external loading to set it securely in place. The maximum load or resistance values can be achieved, but only once the anchor has been pulled slightly away from the intended position. The result is an installation with unforeseen displacement.

Over-torquing instead results in the cone being pulled too far into the expansion sleeve, resulting in some concerns when the installation occurs close to the edge or in the middle of the slab. When an over-torqued anchor is placed close to the edge, the extra expansion force generated could cause the concrete to break, even before loading the fixing.

Over-torquing close to the edge might result in the concrete breaking out

When an over-torqued anchor is away from the edge of the slab, the steel bolt can be damaged – or even broken apart – by this torsional moment. The result is a clear drop in the steel failure load under tension, shear, or combined tension and shear forces, up to the point of breakage.

Bear in mind that many installers may not be aware of the risk of not applying the right installation torque. To be on the safe side, always mention the right torque in your project documents and shop drawings or even document it during setting.
You can also specify the AT tool to ensure the correct tightening, record it throughout the execution and generate a report for inspections.

Application tips

Stud anchors are generally used in non-structural and structural applications where it is necessary to achieve a good resistance with a non-excessive embedment depth (about up to 150 mm) in medium-heavy fixings. They are preferred over adhesive anchors because the installation does not require waiting for the curing time (on the other hand, injection mortars are more suitable when deeper embedment is required).

If you are new to anchor design, after reading this article you perhaps cannot wait to start your first application with our stud anchor HST3: with PROFIS Anchor software you can do this very smoothly.