This month (December) saw more changes to BS 8298 that give designers more responsibility for the thickness of stone cladding used on their buildings. The latest edition of Albion Stone’s technical manual includes an explanation written by Tim Yates of BRE how to determine the thickness of panels using BS 8298 Parts 2 and 4. The section is reproduced here with the permission of Albion and BRE.

Background

The British Standard for the design and installation of natural stone cladding and lining (BS 8298) has undergone a major revision over the last three or four years and the five parts of the new document will continue to be published in 2010.

Part two of the new code of practice, which covers the design and installation of traditional handset cladding, differs from the 1994 Code in that it encourages the designer to consider the thickness of stone required for different cladding projects on the basis of structural calculation supported by performance testing to determine the properties of the stone (particularly the flexural strength and the breaking load at the dowel).

The calculation requires knowledge of

• the expected load
• the required factor of safety
• the details of the panel and the stone.

Once this information is established, the calculation to determine the thickness of the stone can be completed using the properties of the stone (particularly flexural strength), the panel size, the imposed loads, and the fixing system.

It is necessary to consider both the flexural strength of the panel between fixing points and the failure at the fixing point so there are two separate calculations that need to be completed.

Given that there are six variables for the thickness calculations – the stone’s flexural strength, the panel size, the panel thickness, the imposed lateral load, the factor of safety and the fixing system – the designer can alter the calculation to start with different criteria.

The imposed lateral load and the factor of safety should be established first. Once the stone is selected, the flexural strength will be known, so the panel size, thickness and the fixing system become the variables. The fixing system is normally assumed to be dowels in the edges of the stone, although this can be varied to give more support if required.

The preferred panel size is often dictated by the designer leaving the thickness of the stone to be determined by the calculation.

Expected load

When stone is used as cladding and each panel is independently supported, the only significant load is wind loading. This should be calculated using the methods and factors given in BS 6399. Figures are shown in the classification in Table 1 in BS 8298 Part 2.

Factor of safety

The stone’s factor of safety should be set by the project engineer. It will generally be higher than for other building materials because stone is a natural material and is therefore not subject to the same level of control during its creation as a factory manufactured product.

Stone is also brittle and cannot yield to accommodate localised high stress. And it can be affected by other factors such as weathering, the moisture level and the stress structure.

A stone’s strength should be determined using BS EN 13161 using stone at the project thickness and in the same bedding orientation.

The test requires the sample to be dry when tested, but BS 8298 gives an additional method for the testing of saturated samples. A minimum of 10 samples are tested and the results are presented as a Mean and Lowest Expected Value (LEV).

In the case of Albion’s Portland stone, test results should be compared with, or be included with, the existing historical testing data recorded on the Albion Stone Technical Data Sheets to ensure that all the relevant information is incorporated into the Factor of Safety calculation.

The Factor of Safety to be used will depend on whether the samples are tested wet or dry or whether the mean or the LEV is used.

BS 8298 Part 2 for traditional cladding assumes that a Factor of Safety of six is used. Part 4 for rainscreen cladding provides some more specific guidance but it is important that the designer and the project engineer consider all factors, including impact resistance, when completing the calculation and selecting the correct figure to be used.

• If the stone is tested dry and the mean figure is used, the Factor of Safety should be 5.0, according to BS 8298 Part 4.
• If the stone is tested saturated and the mean figure is used, the Factor of Safety should be 4.3, according to BS 8298, Part 4.
• Probably the best method is to use a combination of dry and saturated samples (at least 10 of each) and calculate the LEV using the method stated in Appendix A of BS EN 13161. Ideally, the historical data for the stone should be compared or combined with any recent project test results. The Factor of Safety for this method should be 3.0, according to BS 8298, Part 4.

The panel

The surface area of the panel will affect the calculation for the stone thickness due to the wind loading on the stone. The increase in lateral load from a large surface area will also affect the stress strength around the fixings.

The stone’s strength around the fixings is normally the more critical value and will often determine the thickness of the stone and / or the panel size.

The calculation for the loading on each fixing is:

• Wind load (Nm-2) x Panel area (m2) x Factor of Safety = Lateral load on the stone panel
• Lateral load on the stone panel ÷ Number of fixings = Loading on each fixing

The loading on each fixing needs to be less than the ultimate strength as determined by the mean value from BS EN 13664 tested at the proposed project thickness.

The stone

Flexural Strength figures are determined in accordance with BS EN 13161 and saturated figures in accordance with the alteration stated in BS 8298.

To check the proposed stone thickness is adequate, it needs to be considered in association with with:

• the span between the fixings
• the wind loading
• the flexural strength.

The thickness calculation

There are three different complexities of the thickness calculation.

You can follow the actual calculation for the allowable design stress of a stone panel as set out in BS 6399 – and this calculation should be followed if wind load specific to the project is to be used. This calculation will determine most accurately the thickness needed for the cladding.

If a more generic value is acceptable then the tables reproduced on the left of this page from BS8298 should be acceptable and adequate to calculate the panel dimensions.

The calculation for both these methods should be completed by a competent person, normally an engineer with experience with stone or a suitably qualified stone designer or consultant.