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Floor Strength For Mobile Elevated Working Platforms

• Introduction
• Useful conversion factors
• Screed strengths
• Compressive strength of underfloor insulation
• Loads applied by mobile platforms

Introduction

It is known that there have been a number of unreported failures of floor systems, typically in areas such as atria and shopping centres, resulting from the loads applied by the mobile access equipment used for cleaning and maintenance. The failures have been related primarily to the use of mobile equipment with solid rubber wheels, which apply a highly concentrated load to the floor. Mobile equipment fitted with large pneumatic tyres, which spread the load over a larger area, causes fewer problems.

Investigation has revealed that the majority of failures have been caused by:

• Inadequate bedding of the floor finish.
• Poor workmanship of the screed below the floor finish.
• Early trafficking of screeds before they have reached their design strength (typically at 28 days).

A small number of the failures are attributable to inadequate specification of the insulation within the floor system build-up, but this problem may increase as floor insulation becomes more common and floor insulation gets thicker. There is also the potential for confusion caused by the use of different units of measurement for the different elements of the floor and the loads applied.

Useful conversion factors

102 kg/m² = 1 kPa = 1 kN/m² = 0.001 N/mm²

Screed strengths

Screeds have traditionally been specified prescriptively with testing restricted to soundness (now referred to as in-situ crushing resistance or ISCR) and with little or no reference made to the required compressive strength. This is probably because most screeds have a significant safety factor over the normal loads likely to be applied.

Typical compressive strengths are:

• Lightweight aggregate cementitious screeds: Between 10 and 15 N/mm².
• Normal aggregate cementitious screeds: Between 20 and 25+ N/mm² (typically specified as >25 N/mm²).
• Typical anhydrite and gypsum-based screeds: Between 25 and 30 N/mm².
• High strength cementitious screeds: Over 30 N/mm². Proprietary products are generally in the range 30 to 50 N/mm².

Compressive strength of underfloor insulation

There are many insulation board products suitable for underfloor use. Two of the better-known products are:

• Jablite Jabfloor, by Vencil Resil, available in five nominal strengths (70, 100, 150, 200, 250 kPa).
• Floormate, by The Dow Chemical Company, available in four nominal strengths (200, 300, 500, 700 kPa).

It is crucial to note these quoted nominal strengths (for both companies) are based on tests to BS EN 826 at 10% compression. 10% compression of the insulation under a screed is almost certain to cause the screed to fail. It is therefore essential to ignore the nominal compressive strength values and use, instead, the "Design Load" figures provided by both companies. (The two companies named above use different methods for calculating the design load: Jabfloor is tested at 1% strain to BS EN 826; Floormate is tested to a 2% maximum deflection over 50 years to BS EN 1606.) The two methods produce slightly different results, which make comparison difficult, but show a reduction to between 30% and 45% of the nominal compressive strength.

Loads applied by mobile platforms

The loads applied to the floor system (finish + screed + insulation) will depend on the type of equipment to be used. Large self-propelled scissor lifts with solid rubber tyres, intended for indoor use, generate the highest tyre contact pressures, up to 1400 kPa (= 1.4 N/mm²). This is well within the compressive strength of a properly-laid cementitious or anhydrite screed but it will still be necessary to check the compressive strength of the insulation.

Genie Industries [http://genielift.com/] supply a wide range of mobile access equipment and their website provides full specification details, including weights, tyre loads, and tyre contact pressures, for the various items of equipment supplied.

Example calculation

The typical calculation method is shown below:

• Maximum tyre load: 1150kg (A typical value for a large scissor lift with solid rubber tyres, taken from the manufacturer's specifications. The maximum tyre load is generally more than an even distribution of the total weight over all the wheels or supports.)
• Tyre contact area: 75 x 75mm (This is quite concentrated but is typical for solid rubber-tyred equipment.)
• Screed thickness: 75mm (A typical value for a floating screed over insulation.)
• Area of load as applied to insulation = 225mm x 225mm = 0.0506m² (This assumes a 45 degree angle of load spreading through the screed.)
• Pressure applied to insulation = 1150/0.0506 = 22727 kg/m² = 222 kPa.
• Nominal compressive strength of insulation = 222/0.35 = 636 kPa (A design load of 35% of the nominal compression strength is typical for the higher strength insulation products.)
• Insulation required: Dow Floormate 700-A (700 kPa) or equivalent.

Note that in this example the load generated by a large (but not extreme) piece of equipment applied to a typical screed has generated a requirement for one of the strongest insulation board products available on the market.

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