WOOD CHAR DEPTH: INTERPRETATION IN FIRE INVESTIGATIONS

Wood char depth patterns have been considered important by fire investigators in most countries where significant use is made of wood as a construction material. This is natural, since post-fire pat-terns found on wood members are generally more pronounced and extensive than those found on many other construction materials. But the present state of affairs has been such that there is not much agreement on what quantitative interpretation, if any, can be placed on such patterns. In fact, re-spected specialists have suggested that charring rates are meaningless with respect to fire investiga-tions [1][2]. The standard guide for fire investigators used in the US, NFPA 921 [3], states that: “The investigator is cautioned that no specific time of burning can be determined based solely on the depth of char.” Nonetheless, it then devotes two pages to a discussion on how to measure depth of char and what conceivable deductions might be made from these measurements, although it arrives at no defi-nite conclusions on the latter point. The situation arose, in the view of this author, because the experimental literature was not adequately surveyed and, especially, because persons attempting to use published charring rate have not consid-ered the factors that govern these rates. Instead, it has been common to take charring rate values pub-lished as guidance to structural engineers designing timber beams or columns, and attempt to apply them towards understanding the charring of wood floors, doors, or other members that are not mono-lithic, large-thickness structural members. In the present paper, we will endeavor to remedy this situation by examining the available data on (a) charring rate and char depth; (b) causation of deep or unusual patterns; (c) burn-through times of wood members; and (d) the characteristics of burn patterns due to use of ignitable liquids (liquid ac-celerants). The paper primarily examines three types of data: (1) Traditional data obtained in ASTM E 119 or ISO 834 fire endurance test furnaces, where exposure is controlled via the standard time-temperature curve. (2) Data obtained from Cone Calorimeter or other bench-scale tests where the test conditions are set by specifying a heat flux—normally invariant over time—imposed on the specimen. (3) Data from large-scale test programs where entire rooms or houses were burned. For any quantita-tive data to be reliable, the proper technique for measuring char depth must be used, but this will not be reviewed here since it is adequately covered in NFPA 921 [3].