Effect of increased charring on the narrow side of rectangular timber cross‐sections exposed to fire on three or four sides

Owing to the influence of two‐dimensional heat transfer, fire tests and heat transfer calculations show that charring on the narrow side of rectangular timber cross‐sections may be considerably greater than charring on the wide side. For timber members unprotected throughout the time of fire exposure as well as members initially protected from fire exposure, it is shown by calculation, that the relationship between charring on the narrow side and time is nonlinear. In order to take into account increased charring on the narrow side of timber members a section factor k s was determined. By calculation it is shown that bending moment resistance is only insignificantly influenced by the effect of two‐dimensional heat transfer and the factor k s can therefore be neglected, making the fire design of timber members easier.