Abstract: Sustained and transient load are two components acting on building floor. The probabilistic combination of these two load components is of great concern because of their time-varying randomness in nature. Therefore, a closed form solution and its simplification of combination of these two components were developed, and the validity of the solution was examined by the Monte Carlo simulation. Examples have shown that there is a great agreement between the closed-form solution and the simulation result and the simplified solution is a good approximation of the exact one especially for upper load probability. It is worth noting that the simplified solution is the same as that derived based on the simplified equally time interval load model which has been used in practice. Hence a conclusion can be drawn that, from practical point of view, the simplified equally time interval load model is acceptable for combination of sustained and transient load.