Green Fire Retardants for Structural Timber

Pyrolysis of Structural Timber

When a timber section subject to fire undergoes the pyrolysis mechanism described here, then the section will become subject to two weakening conditions which will reduce the strength capacitance of the section. As it is presumed that the charred section of the timber carries no load, the lengthening pyrolysis process effectively reduces the area of timber cross-section actively carring loads. This means an increase in the stress carried by the uncharred section. Although it is almost impossible to accurately predict the charring rate of the timber in each place due to fluctuations in heat flux, the accepted value is 0.6mm/min.

The second process through which the section loses strength is the loss of load-carrying capacity of the uncharred timber due to prolongued exposure to heat. This has been investigated in depth by Kollman & Cote (41).


These graphs, from Fire Performance of Timber Construction (20) show the reduction in strength that a timber section undergoes with temperature.

Connection Failure

Another possible reason for failure in structural timber, or even collapse of an entire timber-frame building in fire is failure of the joints caused by a weakening of the embedment strength. It is recognised that metallic connections such as bolts and nails could be susceptible to changes in the lignin matrix even at low temperatures. The graph on the right (Moss P, Buchanan A, Fragiacomo M & Austruy C 42) shows the decrease of embedment strength with temperature.