Load combinations for seismic and other loads are considered. To this aim the equiprobable sets of loads with the corresponding probabilities are analyzed. The combination of motor-car and seismic loads is considered in details. The log-normal distribution law was used as a distribution density functions for car load. The distribution of the earthquake event stream was taken according to Poisson's law, which makes it possible to estimate the design seismic intensity. In the frame of this intensity peak ground accelerations were estimated. The dependence of the combination coefficient of the seismic load on the combination coefficient of the motor-road load was obtained. The results obtained show, that combination coefficients in Performance Based Designing should be calculated separately for each input level. For the design earthquake and the maximum design earthquake the combination coefficients vary significantly. The values of the combination coefficients are determined mainly by the frequency of the design actions and to a lesser extent they depend on the seismic activity of the building site.
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