In designing a smoke control system of a building, it is of course indispensable to assume various conditions under which the system may be operated. Of these the main factors to be defined are the positions where a fire may break out, fire severity, the external ambient air conditions, space composition, temperatures and ventilating conditions of the building etc. However we have never been able to deal appropriately with them up to now, because each of them has too many probable states or conditions. It is extremely difficult, and perhaps impossible to take into account the enormous number of combinations of different probabilities of the many factors to be considered in design. We are therefore obliged to study the methodology of how to determine the conditions to be considered in designing.

From such a point of view, this paper proposes a method for evaluating effectiveness of a smoke control system with taking account of probability in which an environmental condition may be occurred in a fire, and a technique for simplifing the evaluation process. The technique is to estimate the critical conditions for smoke safety in order to decrease the number of the environmental conditions to be examined on smoke safety or predicted on the smoke movement. This is based on the reason that it is not necessary to examine smoke safety for the conditions that are regarded to be evidently safer than a certain environmental condition for which it has already been judged to be safe. These are discribed through an example appling them to a fifty storied sky scraper.