Historical experience from real fires shows that masonry walls perform extremely well. The occurrence of wall collapses is generally due to the following factors:

• the surrounding structure placing eccentric or lateral loads on the walls
• the large thermal bowing of some tall walls not fixed at the top
• the large vertical deflection or collapse of supporting members to the walls

Masonry walls made from clay and concrete units have a low thermal conductivity and so the temperature gradient across the section will be markedly steep when the walls are subjected to one side heating. The thermal bowing resulting from the differential thermal expansion will have detrimental effect on the stability of the walls, in particular for tall walls not fixed at the top. A rule of thumb is if the wall deflection is less the wall thickness, the resulting eccentricity is unlikely to promote failure.

In the UK, the current prescriptive rules for the fire resistance of masonsry structures are based on either:

• The use of tabulated data which provides the fire resistance of wall for various forms of construction. These design values are normally based on wall test results from standard fire resistance tests extending over 50 years.
• Test results from standard fire tests on a particular wall construction satisfying the performance criteria in accordance with BS476. This approach is normally applied for other forms of masonry construction not covered by the current tabulated data.

In general, the fire resistance of masonry walls is defined by a minimum wall thickness for a specified period of fire resistance, ranging from 30 minutes up to 6 hours. The codes of practice and reference providing the tabulated data for fire resistance of masonry walls include:

• BS5628-3 (2001)
• EN1996-1.2 (2005)
• BRE Report 128 (1988)

The tabulated data provided in the BRE Report 128 (1988) have been based on BS5628-3 (1985), but presented in a different manner. As a result, this approach with the BRE report will not be discussed in details.