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A simple formula was proposed to calculate the equivalent fire exposure based on time- heat flux area. Given a fire severity and duration of design fires, the heat flux absorbed by building element was described by an analytical formula for heat conduction in a semi-infinite medium. Using the formula, the time to give the same amount of heat to building elements under design fires and under standard (lSO 834 fire) is derived. Assuming that the behavior of building element is identical if the total amount of heat is equivalent, then the formula can be used as a representation of equivalent fire duration for structural fire resistance design. Numerical examinations and several experimental results show that the assumption holds fairly well for practical range of application.