Went to a talk about this (among other things). Short answer: They size the structural members making an allowance on depth from the surface for char (zero strength) and a further depth of affected material assumed to be weakened. The unaffected wood inside is what supports the structure.

For a dramatic demonstration of how shallow a fire actually penetrates wood over a short period of time and how well it insulates, see here:

https://www.youtube.com/watch?v=YU48ypPTH1E

(Promotional, but informative nonetheless)