We Have Moved On Since The Newton
Simon Foy on the selection of movement joints
HERE I will try to unravel some of the mysteries behind the selection of movement joints.
When engineers design a structure they usually consider Newtons per sq m and maximum loads. Typically they are thinking of office equipment like photocopiers, average and unmoving pedestrians and the dead weight
What interests them is the impact all this weight can have on the loading on the structure. But when it comes to movement joints, like Interspan or Superspan, it’s a whole different story: a moving story. To specify the right joint you need to know about the sort of point loads that are liable to cross it.
The first question is ‘will it be for pedestrian use only’ if the answer is ‘yes but there will be a cleaning machine crossing it every now and then’, the major issue is probably going to be the loading on the nylon wheels of the machine, not the 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 in excess of 20 – 30%.This is why joints should be gauged in terms of kg per mm of width of wheel. Newtons per sq m do not always provide a lot of help when accessing the potential load for the joint.
That’s usually what we get when we ask for information about traffic across the joint. I’m even told sometimes that it will be pedestrian traffic but we must allow for the occasional fire truck – a vehicle that can weigh up to
Golf carts, electric vehicles typically used in airports or supermarkets to carry VIPs or people with disabilities, may weigh 1,200 kg. Thanks to the 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 radically different view. They may tell you that when it comes to Newtons, the electric cart is a bigger load than the cleaning machine.
That’s true if you are considering the whole floor but it 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 going to be taken across on machine skates from time to time.
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.
In a small shopping centre the average mobile machine is probably going to be a push along one. Bigger malls, hospitals and substantial foyers may have a cherry picker that does things like put the Christmas
Specifying the right movement joint is all about taking the time to understand how the building is going to work, who is going to use it and what they are going to use it for.
The end result of all this questioning should be the specification of the right product for the right situation. 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. Sometimes joints fail in shopping centres shortly before Christmas, not the best time to shut retail units! But if a failure occurs it is liable to be the most heavily trafficked areas that fail.
Good specification is all about lifecycle costing. But it can’t be done without asking a lot of questions. They are not complicated questions.
There is no need for enormous equations. The result may be that the client gets a less expensive product than was expected. It does happen. The critical issue is to make sure that the right movement joint is specified.
Simon Foy is Tremco illbruck specification manager
T: 0191 419 0505
This article has been reproduced from the Contract Flooring Journal website. You can find them at www.contractflooringjournal.co.uk.