Slipping can damage your health!

Dr Malcolm Bailey

The reason why slips occur

• The level of friction required by the person in walking, turning or running cannot be met by the floor/shoe interaction

• It is a relatively complex matter, so let us look at it in detail.

• Please do not be afraid to ask questions as we go along if you do not understand something.

When we walk on the floor we impose forces on the floor

• If the floor is level, then there are no problems with the vertical forces – the floor easily supports our weight.

• It is the horizontal forces which the floor /shoe interaction has to counter through friction

• If friction cannot counter these forces, the foot will slide, driven by the force we generate in our walking.

Diagram of the forces

Friction

• Friction is expressed as μ =H

V

_

Two Coefficients

• 1 The Limiting static coefficient

• 2 The Dynamic coefficient

• Static friction will be developed as much as is required to resist the imposed force providing it does not exceed the limit – if the imposed force is greater than the limit, sliding will occur.

Dynamic friction

• Dynamic friction can be constant but when rubber or plastic is involved can be dependant on the relative velocity of sliding.

• At low velocities, the Dynamic coefficient is usually lower than the Limiting Static coefficient.

The Friction needed

The GLC Criterion

• PTV Between 0 and 20 - Dangerous

• PTV Between 20 and 40 – Marginal

• PTV Above 40 – Safe

• This was based on 3500 floors using the Pendulum

How do we test a floor surface ?

• In theory we only have to measure H and V on a sliding object

It can be simple

Or more sophisticated

Testing Machines

• The previous examples are generally known as Tribometers or “Trundle” Tests

• Almost all are designed on the basis of Coulomb friction

• This type of friction is independent of contact pressure and thus one can use any value of V that is convenient

• However if Rubber or Plastic is involved this does not hold true !

The Pendulum

• Works by measuring H from the energy loss as the slider sweeps over a specified area of floor.

• V is applied by a spring loaded slider and over the centre 80% of the contact length is a constant 2.5kg.

• The Pendulum Test Value (PTV) is approximately 100 X μ

The Pendulum’s History

• Invented by Percy Sigler in the US in the 1930’s to measure flooring

• Brought to the UK by Barbara Sabey of TRRL to measure roads in the 1950’s

• Used by the GLC to measure floors in the 1960’s

• Adopted by BS8204 in 1980’s

• Was given its own BS7976 in 2002

The dilemma

• While most machines give reasonable agreement in Dry conditions, in Wet conditions there was no agreement

• Many machines suggest a floor to be Safe when clearly it is not !

• In the late 1990’s we realised that it was all to do with the water film

• Only if the machine develops the same uplift characteristics as a sliding heel can it give the right answer.

Hydrodynamic film theory

The uplift provided by the film is dependant on…

• the relative velocity

• the contact dimensions

• the contact shape

• the viscosity of the liquid

With the effect that…

• The uplift reduces V, the vertical reaction between the heel and the floor

• The value of H is similarly reduced

• Hence a machine needs to experience the same proportional uplift on its slider that a heel experiences during the slip.

• Most machines only experience a tiny uplift and give too high a result for μ

Which machines get it right ?

• By pure chance the Pendulum is almost correct – its velocity is equivalent to the latter stages of the heel slip

• SlipAlert was specifically designed to replicate the whole of the heel slip, from beginning to end. ( It just does it in reverse)

• It is thus no surprise that there is excellent between the two. ( r = 0.944 )

What about the Ramp ?

• Does not correlate with the Pendulum

• Uses Engine Oil not water

• The operator wears profiled soles/heels

• He places the whole foot rather than just the heel on the ramp surface in small half-steps

• R9 is the Lowest category (people think it must be OK as better than R8, R7, R6 etc)

What about Roughness ?

• No correlation with the Pendulum

• Tells you nothing more about the Slip Potential of the surface than you already know from the Pendulum

• Does not indicate change in Slip Resistance

• HSE’s SAT is likewise fatally flawed as it relies solely on roughness readings !

Pendulum Points to Ponder

• Most authorities recommend using only Four S / Slider 96 rubber. This only tells you the PTV for a good quality heel.

• TRL / Slider 55 rubber simulates Bare foot and Trainer heels

• There are shoes with far less grip than indicated by either of these.

• One cannot predict how these will behave from the Four S / Slider 96 result

How to advise your client

• Slip Resistance is only one factor, he/she has to consider…

• Aesthetics

• Ease of Cleaning

• Does it show the dirt quickly

• How well does it wear

• How much does it cost

Slip factors to consider

• The type of activity on the floor• How many people will use it• What contaminants will get on the floor• What footwear will people wear• How will the floor be cleaned• How well does the floor retain its slip resistance• Other environmental and ergonomic factors

The type of activity

• Straight walking e.g. in a corridor

• Turning and stopping suddenly

• Running e.g. to catch a train

• Pushing heavy trolleys

• Slopes

• Carrying dangerous objects ( in kitchens )

• Disabled people

The number of people

• Statistically the higher the number of people using the floor the greater the risk that one of them will be that one in a million person who needs the highest level of slip resistance

• Slip resistance of less than 40 PTV can be justified if only a few people use the floor

The Contaminants

• Dusts

• Polish overspray

• Water

• Grease

• Oil and Fats

• Cleaning residues

Footwear

• In a public place one must take account of the wide variety of footwear that will be worn. Some can be deemed unacceptable but as yet there is no properly defined and accepted minimum standard.

• In factories it is possible to control footwear and to issue staff with special high grip shoes.

• Beware – Safety shoes are usually not specifically high grip although they may imply this

Cleaning and Maintenance

• Critical in retaining the floor’s slip resistance• How is the floor to be cleaned – Mop & Bucket

or Mechanical Scrubbers• If Mechanical, is it the correct type for the floor• What Chemicals are to be used – will they really

do the job• Will the Slip Resistance be regularly monitored• Cleaning can radically change the slip

resistance of the floor in the short term without anyone being aware of it

Wear

• Each floor is different

• Accelerated wear tests can be indicative but should not be relied upon implicitly

• The only sure way is to monitor the floor regularly

Environmental

• Condensation – often virtually invisible but is ample to cause wet slipping

• Can users detect the presence of water or dampness on the floor surface

• Is the floor likely to be crowded with people so that one cannot see and avoid water on the floor

Floors can be managed

• Floors with lower than ideal slip resistance can still be acceptable if properly managed

• Take all necessary steps to ensure the floor remains uncontaminated e.g. by using barrier matting etc. and similar good design features

• Train staff to be vigilant and have well thought out procedures to ensure spills are cleared up quickly and completely

Case Study

• New bus station in UK• Long concourse (with slopes) with doors down

one side leading out to the bus stands• Tiled floor• Original tile rejected at design stage due to

perceived cleaning difficulty• New tile was smooth, easy to clean, and was

purported to have a reasonably high slip resistance. It was never tested to verify this.

Change of operation

• The original concept was for off-loading to take place in the road outside the terminal so that the buses were empty when they drew up to the stands.

• The Local Authority at the last minute refused to allow this

• Passengers thus entered the building through the doors which were originally designed for exit only ( no mats were provided )

The result

• The floor inevitably got wet

• There were a number of expensive slip accidents

• The floor when tested gave a low wet slip PTV

• The floor was chemically treated. This initially increased the wet PTV but it did not last long

Drastic action !

• The Client decides to rip up the floor and replace the tiles with a more slip resistant one

• This was done in stages working up the concourse – it involved replacing the screed and was very expensive

• Near completion the floor was tested (by me) and while recently laid tiles gave a wet reading of 45 – 50 PTV, this gradually decreased the further one went down the concourse. The longest laid tiles only managed a wet PTV of 18!

• In any one area the reading was similar no matter the level of pedestrian traffic

What caused it ?

• The answer was the cleaning• Essentially the cleaning methods were polishing

up the tiles to make them look nice, but was completely negating their slip resistant properties.

• We never found out exactly what the cleaners were doing but it was the only factor which could affect tiles equally in both heavily trafficked areas and virtually untrafficked areas.