Fixing types
Most sign include some sort of fixing, whether it be a humble screw of a large resin anchor. Simon Havard examines the most commons types of fixings, how loads can affect them and other issues to keep an eye on
Wednesday, 22 Aug 2012 04:24 GMT
Staying one step ahead of sign failure
Fixings play a major part of the safety aspect of a sign. In fact, it can often be the single most important component if it is the only thing stopping a large sign falling on people’s heads. There are several issues that sign-makers should consider when applying these fixing to ensure that their signs are safe.
The effect of load
Simply, this will generally consist of tensile load—stretching the fixing as the load tries to pull it out—or shear load, cutting it in half at right angles. For instance, the vertical weight of a flat panel secured to a wall will try to ‘shear’ the horizontal fixings.
A large projecting sign will again try to shear the fixings due to its weight. But as that weight is projecting out and not flat against the wall, it will also try to pull the fixings out horizontally—tensile load.
Choosing the right fixings is vital,
as small errors in the choice of size
and depth can result in gradual and
then total fixing failure
The same is true of a totem sign with bolt/rootcage fixed base plates. The horizontal wind load will try to shear the vertical bolts, but the leverage from the base plates will try to pull the bolts out as well.
The possibility of ‘leverage’ acting on fixings can be important and can work on a large or small scale.
In the example above, the bolts holding down the base plates will have a tensile load applied to them by the base plates trying to pivot on their back edge as the wind hits the sign face and pushes it backwards—like using a claw hammer to pull out a nail.
If weight or wind acts on the sign in a way that it is trying to twist the fixing or plate, this should be viewed as a shear load as well. But beware, as this can often produce very high loads.
Tek bolts and wall plugs
Such fixings are a great addition to the designer’s and installer’s toolbox, but they are less tolerant of the inevitable small mistakes. Over torquing a self-tapper, opening out a hole just a little too much or using a slightly smaller screw than recommended could have consequences out of all proportion to the minor error committed.
If one fixing fails, the remainder may become overloaded and the sign fails. I would urge caution when specifying these types of fixings, particularly for larger structures or signs fixed at height where failures may cause serious injury.
The following may help to reduce risk through installation error. The use of larger washers when securing thin materials will help prevent ‘pullover’ where the material tears around the fixing. The use of additional fixings in case one is installed incorrectly. Ideally use these fixings in conjunction with others—such as bolts and cavity style—which are more tolerant to minor errors.
Buying the right bolts
Good quality sign-fixings, such as those pictured from FK Moore,
are essential to bear the tensile loads of today’s often
complex sign systems
Nuts and bolts are very reliable, traditional fixings that are less open to error when installing. However, care should be taken not to miss-thread and in order to reduce this possibility I tend to recommend the use of larger diameter bolts even when smaller ones would suffice.
It is always advisable to use shake proof washers and/or nutlock fluid when the fixings may be affected by vibration. Again, good sized washers help spread the load and prevent ‘pull over’.
Resin and pressure anchors
These fixings are generally most used when fixing into concrete or brick. Their effectiveness is dependent on a number of technical issues which include spacing, distance from material edge and nature of the substrate into which they are fixed. Correct installation is also important and should always strictly follow the manufacturers instructions. For instance, when using resin/chemical anchors, the drilled hole must be cleaned of dust as this could reduce the effectiveness of the resin-to-concrete bond.
It is vital that you consider these points when installing signage that requires fixing, otherwise you could have a great deal of health and safety issues to deal with. If you follow the steps outlined in this article then your customers should be able to enjoy a safe sign for its duration, a claim that should surely help to improve your sales.
Simon Havard is a partner at Russel and Havard Consultants
Tel: 07900 247 904
E-mail:russellandhavard@googlemail.com
Web: www.russsellandhavard.co.uk
