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BS 7976: 1- 3 2002; "The Pendulum Method" describes the specification, operation and calibration of the TRL (Transport Research Laboratory) Pendulum for slip resistance testing. The BS 7976 Pendulum forms the basis of our on site slip risk assessment. Originally designed to replicate the actions of a slipping foot, the pendulum skid tester was subsequently adopted extensively for the testing of road surfaces.

The BS 7976 pendulum slip testing method remains the preferred slip test method of the HSE and UK Slip Resistance Group. This is in part due to its extensive history in slip risk assessment and strong correlation to slip incidents. The pendulum slip test remains one of the few methods to accurately recreate the hydrodynamic squeeze film generated in a slip in wet conditions. Though the pendulum slip tester is somewhat awkward to use and requires an experienced operator to avoid misleading/erroneous results, it is considered the best method of pedestrian slip testing and is used extensively in slip and fall legal proceedings.

Image 1. Pendulum slip tester in-situ testing an outdoor decked area with water based contamination. More recent guidelines require anti-slip footings and an increase in the weight of the rear stabiliser.

The device swings a rubber slider mounted on a weighted 'foot' from horizontal, striking the floor surface near vertical and subsequently providing a reading as the foot swings back up. The dynamic coefficient of friction of the surface has a measurable effect on the Pendulum Test Value (PTV), previously known as the Slip Resistance Value (SRV).

Image 2. The foot is released from horizontal, swings down, strikes the surface and produces a CoDF/PTV reading on the over swing.

BS 7976 has 3 parts, BS 7976-1 describes specification. BS 7976-2 describes preparation of sliders prior to testing, pre-test checks, testing procedure, temperature correction and essential information for reporting. BS 7976-3 describes the calibration method.

Sliders are prepared in advance of a site visit/test by swinging across a sheet of 400 grade silicon carbide resin bonded paper mounted on a glass plate. Slider edges are further smoothed by swinging across a sheet of wet 3M 261X Imperial Lapping Film Grade 3MIC mounted in a similar manner. In line with UKSRG guidelines, Grip Potential's calibration procedure sees tests performed on wet 3M 261X Imperial Lapping Film Grade 3MIC of an expected PTV as tested by a UKAS accredited laboratory. In addition, tests are conducted on wet float glass and a wet ceramic tile of known PTV. If any result is outside calibration tolerances the preparation process is repeated as necessary. Results for slider/pendulum on site calibration tests are included in all our reports.

Image 3.

The pendulum 'foot' is weighted to negate mechanical and air resistance and apply an appropriate force on the test surface. One of the few limitations of this test method is that the force applied is not commensurate with that of a pedestrian. On surfaces that deform excessively this can produce misleading results.

Prior to pendulum slip testing being conducted, full check testing is carried out on three surfaces of known PTV whilst on site. In addition to check testing, before and after every direction/condition measured, the pendulum foot is swung 3 times without striking the surface. The zero can be adjusted as necessary before testing and any discrepancy can be investigated.

Image 4. Mounted TRL (Slider #55) rubber slider. The slider is spring loaded vertically and allowed to pivot horizontally to ensure uniform contact. Sliders have a detailed preparation procedure and must be disposed of after a year or when the edge becomes too worn/damaged.

Once the pendulum slip tester is assembled, check tested, leveled and zeroed, testing can commence. The pendulum height is adjusted as necessary to ensure a slider contact length of 126±1mm along the floor surface. Tests of the floor surface are conducted parallel, at 45 degrees and perpendicular to the direction of foot traffic in dry and then contaminated conditions. The contaminant is often water, however it is advised that the contaminants likely to be found in normal use are used. In the past we have conducted tests with icing sugar, oil, MDF dust, mayonnaise, cement, flour, soap, ketchup, even chocolate.

Image 5. The pendulum slip tester must be on a secure stable footing and operated in a vertical position in order to achieve reliable results. It features a built in spirit level and adjustable legs to achieve this. Inaccurate leveling will be highlighted when conducting zeroing swings prior and post testing.

8 measurements are taken in each direction and each condition. The first 3 measurements of each run are discarded as the slider is mounted freely and requires adjustment. The median of the remaining 5 readings is calculated and a median dry and wet/contaminated result produced.

The HSE classify PTV's as follows;

PTV Range
Slip Potential
0 - 24
25 - 35

It should be noted that it is not as simple as generating a slip potential from PTV and signing the floor off as a 'low risk of slip'. Many external factors can significantly affect the frictional demands of pedestrians using the surface. For this reason Grip Potential conduct a full, holistic "Slips Potential Model" risk assessment, details of which can be found here.

Image 6. The contact distance of the slider is set at 126±1mm as measured with a calibrated rule. A small change in contact distance will have a significant impact on results and extra care should be taken on macro-profiled surfaces.

There are various slider materials available for use with the pendulum slip test method. Most commonly used are Slider #96, also known as 4S (standard simulated shoe sole) rubber and Slider #55, also known as TRL rubber (representative of bare feet). There is tremendous versatility in this regard with materials including leather and a wide range of rubber being mounted on the standard pendulum slider plate. The need for a standardised test however means that the 4S and TRL sliders are by far the most frequently utilised.

Image 7. After striking the test surface the foot continues its swing, pushing an indicator needle along the scale. The friction of the test surface reduces the kinetic energy of the foot and consequently the height the needle reaches on the scale.

A video demonstration of pendulum slip testing can be found as part of the HSE's STEP tool below.

In conclusion, BS 7976 pendulum slip testing is a robust test method backed up with an extensive history. Tests performed accurately by experienced operators produce an indisputable measurement of slip risk. Grip Potential conduct this service independently on a daily basis, often for personal injury cases, using externally calibrated equipment to ensure clients are fully aware of and/or protected from potential slip incidents and associated costs. To discuss your requirements get in touch today.

It should be noted that the Pendulum Skid Tester requires a skilled operator in order to produce valid results and it is rare that the purchase of test equipment is cheaper than employing our services as independent experts, even for large or repeated projects. Further information.

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