Tremo illbruck explores the use of movement joints

Simon Foy, specification manager from Tremco Illbruck, considers some of the issues in the selection of movement joints.

When engineers design a structure they usually consider Newtons per m2 and maximum loads.

More often than not, they are thinking of office equipment such as photocopiers, average and unmoving pedestrians and the dead weight of plant.

What interests them is the impact this weight can have on the loading on the structure.

When it comes to movement joints, such as Interspan or Superspan, there are different considerations.

To specify the right joint, the specifier needs to know about the sort of point loads that are liable to cross it.

If it is for pedestrian use there will be a cleaning machine crossing it every now and then and the major issue is probably going to be the loading on the nylon wheels of the machine, not foot traffic.

Fully loaded with water, these machines typically weigh half a tonne and may have only three or four wheels to carry it.

Wear and tear of nylon wheels can reduce the contact area by more than 20-30 per cent.

Joints should thus be gauged in terms of kilograms per millimetre of width of wheel.

Newtons per square metre do not always help when accessing the potential load for the joint.

Golf carts and electric vehicles used in airports or supermarkets to carry VIPs or people with disabilities, may weigh 1,200kg.

With pneumatic tyres, they have less of a point load impact than the lighter cleaning machine with its nylon wheels, yet building designers may have a different view.

They may tell you that when it comes to Newtons, the electric cart is a bigger load than the cleaning machine.

This is true if you are considering the whole floor but doesn't give all the information needed to work out point loadings.

An aggressive loading from nylon wheels going across day in, day out, is clearly going to cause a light duty movement joint to wear out quickly.

To get the right specification the design brief has to be carefully scrutinised.

If it's a hospital it needs to be known that a five-tonne CAT scanner is occasionally going to be taken across on machine skates.

Residential apartments may have garden areas but if they are wide enough to take a fire engine, movement joints may have to accommodate a 30-tonne load.

This may increase to 60 tonnes if the raised platform type of fire engine could pay a visit because this is a high rise block.

If there is no interrogation, the client is liable to end up with a product that is too light duty or in some cases one that is too heavy duty.

If the product is under specified the results can be costly.

If a failure occurs it is liable to be the most heavily trafficked areas that fail.

Good specification needs to factor in lifecycle costing.

But it cannot be done without asking a lot of questions.

The result may be that the client gets a less-expensive product than was expected as occasionally happens.

The critical issue is to make sure that the right movement joint is specified.

Back to top