Building Materials & Fire Part 1

Building fires, which normally reach temperatures of about 1000 ºC, can affect the loadbearing capacity of structural bearing elements in a number of ways. Apart from such obvious effects as charring and spalling, there can be a permanent loss of strength in the remaining material and thermal expansion may cause damage in parts of the building not directly affected by the fire. In assessing fire’s effects, the main emphasis should be placed on estimating the residual load-carrying capacity of the structure and then determining the remedial measures, if any, needed to restore the building to its original design for fire resistance and other requirements. Obviously, if weaknesses in the original design are exposed, these should be corrected.

Making an analysis of the damage and assessment of the necessary repairs may be possible within a reasonable degree of accuracy, but final acceptance may depend on proof by a load test, where performance is generally judged in terms of the recovery of deflection after load removal.

Timber

Timber browns at about 120 to 150 ºC, blackens around 200 to 250 ºC, and emits combustible vapors at about 300 ºC. Above a temperature of 400 to 450 ºC (or 300 ºC if a flame is present), the surface of the timber will ignite and char at a steady rate. Table A-2 shows the rate of charring.

Analysis and Repair
Generally, any wood that is not charred should be considered to have full strength. It may be possible to show by calculation that a timber section or structural element subjected to fire still has adequate strength once the char is removed. Where additional strength is required, it may be possible to add strengthening pieces. Joints that may have opened and metal connections that may have conducted heat to the interior are points of weakness that should be carefully examined.

Masonry

The physical properties and mechanisms of failure in masonry walls exposed to fire have never been analyzed in detail. Behavior is influenced by edge conditions and there is a loss of compressive strength as well as unequal thermal expansion of the two faces. For solid bricks, resistance to the effects of fire is directly proportional to thickness. Perforated bricks and hollow clay units are more sensitive to thermal shock. There can be cracking of the connecting webs and a tendency for the wythes to separate. In cavity walls, the inner wythe carries the major part of the load. Exterior walls can be subjected to more severe forces than internal walls by heated and expanding floor slabs. All types of brick give much better performance if plaster is applied, which improves insulation and reduces thermal shock.

Analysis and Repair
As with concrete, it is possible to determine the degree of heating of the wall from the color change of the mortar and bricks. For solid brick walls without undue distortion, the portion beyond the pink or red boundary may be considered serviceable and calculations should be made accordingly. Perforated and hollow brick walls should be inspected for the effects of cracks indicating thermal shock. Plastered bricks sometimes suffer little damage and may need repairs only to the plaster surfaces.