A guide to buying concrete anchors

Tuesday, August 21, 2012

No, this subject has nothing to do with sailing and boats!

It’s about the much more mundane DIY problem that arises if you’re trying to fix something load bearing into concrete or some other forms of masonry – i.e. the perennial issue of how to stop whatever it is pulling out as the concrete splits and erodes.

It’s possible to get into a lot of physics here. You can talk about tensile versus sheer loads (i.e. outward versus downward forces on the fixings) and static dynamic and impact loads (whether the force on the fixings is constant, vibrating or subject to sudden change etc.).

These things, though very important, can be complicated and perhaps it’s easier to think about matters in everyday language first then come back to them later.

If you’re fixing something into concrete, the most important thing to remember is that just putting some ordinary screws into rawlplugs, then hoping for the best, might well not work. You’re probably going to need some specialist anchors.

Prior to selecting which sort of anchor to purchase, there are a lot of things to be taken into account – including the exact nature of the material you’re trying to fix into.

The first key concept to grasp is that you’re probably going to look to be spreading the load on the fixings as far as possible. That means understanding the total likely load that the fixings are going to need to support and comparing that against the rating of the fixing.

This isn’t as complicated as it sounds. All you need to do is to look at the individual rating of anchor then divide by 4 to get a degree of safety margin.

Let’s say you’re putting up some shelves and you think that the total weight on them is likely to be 60kg. A particular type of anchor might have a rating of 25kg.  So, if you use 6 of them, you have (6x25kg = 150kg / 4 = total rating of 37.5kg).  In other words, you’re a little short and need to either increase the number of anchors or get some of higher rating, until such time as that sum results in 60kg or higher.

If that’s the basics of the calculation, you then need to think about what type of anchor is required.

There are many different types and space here doesn’t permit a full discussion.

Suffice it to say that the selection of concrete anchors depends a lot upon the exact environment you’re working in and the exact nature of the loads and forces involved – that takes us back to the sheer and tensile discussions at the outset.

Perhaps the best approach is to speak to a specialist supplier of concrete fixings and to ask their advice.  It might be safer that simply making guesses based upon informal generalist advice at your local