Basic guide to expansion anchors

Expanding anchors are used in hundreds of everyday M&E applications – but do you know how they work, what forces they apply to the substrate, and what to be aware of when selecting the anchor for the job?

Read our basic guide and expand your knowledge!

Expansion Fixings

When tightening, expansion anchors transmit forces which compress the substrate material locally, these forces are exerted at the point of expansion, not over the whole length of the fixing. When a load is applied to the Anchor additional forces are exerted showing the projected area or “The Concrete Cone”.

It is the projected area which relates to the anchor performance – deeper embedment gives a larger “Concrete Cone”. Care must be taken when installing anchors close to one another or near to the edge of the substrate as this reduces the projected area and results in reduced fixing performance.

Direction of loading

The most suitable Anchor for the application concerned can be selected after considering the direction of the applied load. The tension and shear values must be less than the safe working loads in the direction concerned. Other factors to be considered are centre-to-centre distances, centre-to-edge distances and proximity of reinforcement etc.

tensile loads

Common examples of Tensile Load situations include suspension of ductwork and pipework, mechanical services and suspended ceiling applications. These are loads which are applied along the axis of the fixing.

Shear loads

Shear Load performance is relative to the shear strength of the bolt material and also the compressive strength of the substrate in close-to-edge situations.

Cyclical Loads

A cyclical load is where the load on the anchor is not static but fluctuates from a load to a no-load situation. Over a period of time, the point of expansion of expanding anchors can cause hole wear and subsequent anchors failure. Chemical anchors offer significant advantages in this application and If repeated cyclical loads are envisaged, then expansion anchors are not recommended.

Oblique and Combined Loads

These are a combination of tension and shear loads. If the Oblique Loading is within 10° of pure tension or shear, then the safe working load of that direction may be assumed. However, if the Oblique Loading is greater than this, then the following equation must be satisfied.

Tightening Anchors

A clamping force is exerted on the substrate when an expansion anchor is tightened. This clamping force is relative to the tightening torque. Tightening an anchor to the recommended torque ensures that the clamping force is greater than the safe working loads published. It is important that these torques are not exceeded as overstressing of the bolt may occur.

types of expansion anchor

wedge anchors

Hugely popular and inexpensive anchor commonly used in overhead suspension applications. Must be fitted with relevant setting tool to ensure correct fitment and a secure fixing.

Sleeve anchors

Ideal expansion anchor for lighter weight fixing applications, such as sheet materials or handrails.

throughbolts

Heavy duty expansion anchor, ideal for higher load applications. Can be fitted through the fixture into a pre-drilled hole, making them ideal for bolting down heavy frames.

shield anchors

Heavy duty expansion anchor for use in applications where the fixture is offered to the fixing after it is inserted in a pre-drilled hole.

key points

It is essential that all Anchors are installed correctly in accordance with the manufacturer’s installation instructions. Essential points to remember are:

Ensure suitability of anchor for the type of substance.
Assess local factors eg proximity to the edge, reinforcement etc.
Use a drill bit in good condition.
Drill hole to correct diameter and depth.
Clean out the hole thoroughly – especially critical for chemically bonded anchors.
Ensure that the correct setting equipment and procedure are used.
Tighten to correct torques recommended by manufacturers.

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