How anchors behave under blast loading

anchors under explosion and page load – Which is the safest for dynamic loads?

Anchors under explosion and blast loading — which is safest under dynamic loads?

It has long been known that anchors do not behave the same way under static loads and under dynamic loads (such as seismic action, explosion, or long-term load cycles). In the 1970s the problem was solved crudely — by applying a safety factor of 3 to any dynamic load. In other words, if the actual load was 1 kg, the design treated it as 3 kg, simply because the load was dynamic.

Since then we have moved on. Laboratory studies show that some anchors actually perform better under explosion loading than under static loading — and we see the same results in the field when we examine anti-blast systems fixed with anchors. A blanket safety factor of 3 applied to every type of anchor is simply inaccurate: some anchors perform well under shock, and some fail suddenly and dangerously.

In this article we review the four types of anchor common in construction, present real laboratory tests of their behaviour under dynamic (shock) loading, and explain which anchor is suitable — and which is not — for designing systems exposed to explosions.

Why does getting the design right matter so much?

The failure of an anchor under an explosion can be sudden and without warning, or gradual, with energy absorption. The difference between the two is sometimes the difference between a protection system that saves lives and one that fails at the precise moment it is needed most. Choosing the right anchor type is at least as important as calculating the load itself.

Which anchors are good under blast loading — and which are less so?

We looked at the four most common anchor types in construction, and compared their behaviour in laboratory tests under static loading against dynamic (explosion / shock) loading:

Anchor type	Change in pull-out capacity (dynamic vs static)	Failure mode	Suitability for explosions
Wedge anchor	Holds 30% less than static (sometimes only a third)	Sudden failure, with no energy absorption	✗ Not recommended
Heavy-duty (dynamic) anchor	Holds 0% to 80% more than the static load	Sudden failure under explosion	✓ A suitable option
Chemical anchor	Holds 0% to 80% more than the static load	Gradual failure, with some energy absorption	✓ Especially for weak or cracked concrete
Concrete screw	Holds 0% to 80% more than the static load	Very gradual failure, with high energy absorption	★ The most recommended solution

1. Wedge anchor

The wedge anchor is the most standard anchor in construction, and is found on almost every site. But under dynamic loading it turns out to be the weakest of the four.

Even the best wedge anchors in the world hold 30% less under dynamic loading than under static loading.
Ordinary wedge anchors hold, on average, only a third of their static load under dynamic loading.
There is no meaningful energy absorption before failure — the failure comes suddenly.

In light of these figures, the wedge anchor is not a good solution for explosions — particularly in systems where stability under shock is critical.

2. Heavy-duty (dynamic) anchor

The heavy-duty (dynamic) anchor — a heavy expansion anchor — is designed specifically to withstand shock and vibration. Under dynamic loading it holds between 0% and 80% more than its static load capacity, a significant improvement over the wedge anchor.

The anchor's heavy expansion sleeve allows the forces to be distributed better within the borehole.
Failure under explosion is still sudden, but it occurs at a significantly higher load than the static capacity.
Particularly suited to continuous vibration and fatigue loading — not only to one-off shock loads.

anchors under explosion and page load – Which is the safest for dynamic loads?

The heavy-duty (dynamic) anchor is certainly a suitable option for explosions, particularly where resistance to seismic action or prolonged vibration is required.

3. Chemical anchor

The chemical anchor is based on an adhesive (epoxy or vinylester) rather than on mechanical expansion, so its behaviour under load differs from that of the mechanical anchors.

Under dynamic loading, the chemical anchor holds between 0% and 80% more in pull-out than under static loading.
Failure is gradual, with some energy absorption before full collapse — a marked advantage over the wedge anchor.
A particularly recommended solution where the loads are very high, and where the concrete in the substrate is weak or cracked.

When is a chemical anchor the right choice?

The chemical anchor stands out particularly in cracked concrete, or in low-quality concrete, where expansion-based mechanical anchors (such as the wedge anchor) struggle to develop the required grip. The adhesive layer creates a continuous bond along the whole borehole, rather than acting at a single point.

4. Concrete screw

The concrete screw is threaded directly into the concrete, with no separate plug required. Under dynamic loading it shows the best behaviour of the four anchor types tested.

It holds between 0% and 80% more than its static load capacity under dynamic loading.
Failure is caused by a gradual increase in slip along the length of the thread, as a result of the pull-out force applied to the screw.
The higher the pull-out load, the more the slip increases, and the screw's behaviour changes accordingly. This process involves a partial release of the screw, which produces very large energy absorption and a very gradual failure.

Why is this exactly the behaviour we want?

This is precisely what is expected of an energy-absorbing element in a blast protection system: it should absorb a meaningful amount of energy before the anchor releases completely. On our analysis, the concrete screw is the best of the four anchor types for explosions.

The dynamic behaviour of the four anchors — summary

1. Concrete screw — gradual failure with maximum energy absorption. The most recommended solution.

2. Chemical anchor — gradual failure with some energy absorption. Recommended particularly for weak or cracked concrete.

3. Heavy-duty (dynamic) anchor — high pull-out capacity, but sudden failure. A suitable option, especially under seismic action and vibration.

4. Wedge anchor — holds less under dynamic loading, and fails suddenly. Not recommended for explosions.

A final word: an approval is not a guarantee of actual performance

The fact that an anchor is approved for dynamic loads under a European Technical Assessment means only that the anchor was tested for such loads. It says nothing about how well it actually performs under them. Most designers simply require the approval, and then design on the basis of static loads. There is no logic in that: it is like confirming that a candidate went to university, without checking whether they passed the exams.

Common questions

Is every anchor “approved for seismic” also suitable for explosions?

Not necessarily. A seismic approval tests cyclic behaviour, which is substantially different from a one-off shock such as an explosion. You should look at the specific test results for shock and dynamic loading, rather than assume that an approval for one transfers to the other.

Why does a wedge anchor fail more suddenly than a concrete screw?

The wedge anchor relies on a point mechanical expansion mechanism — once it reaches the failure threshold, its resistance drops away sharply. The concrete screw, by contrast, fails gradually through continuous slip along the thread, which allows energy to be absorbed before full failure.

When should a chemical anchor be chosen over a concrete screw?

When the concrete is weak or cracked, or when the embedment depth is very large and the load is particularly high. The chemical anchor forms a continuous bond along the whole borehole and performs well even in poor concrete, whereas the concrete screw depends on the quality of the concrete immediately surrounding the screw.

Can you rely on a generic safety factor instead of choosing a suitable anchor?

Not advisable. A blanket safety factor (such as 3× the dynamic load) ignores the fundamental differences between anchor types — some of which actually gain capacity under dynamic loading, while others weaken dramatically. Sound design is based on real test data for the specific anchor selected, not on a sweeping assumption.

Which anchor is best suited to blast protection systems?

Based on the behaviour measured in the laboratory, the concrete screw offers the combination of energy absorption and gradual failure that best suits the requirements of blast protection systems. The chemical anchor is another good option, particularly where the concrete is of low quality. In any case, consult a structural engineer specialising in dynamic loading before settling on the final solution.

How to choose the right chemical anchor

Why use a bad plug when you can use a good one?