Abstract: An analytical method of calculating the power spectral density (PSD) of traffic-induced force acting on the ground through the viaduct systems is presented. The vehicle is modeled as a two-DOF system with linear suspensions and tire flexibility, and the bridge is modeled as a continuous Euler-Bernoulli beam simply supported at both ends. The effect of road surface roughness is also considered. The theory of stationary random process is used to obtain expressions for the PSD of traffic-induced force given the PSD of road surface roughness. The presented numerical results show that roughness class and vehicle velocity have considerable influence on the PSD. The PSD of traffic-induced force has three peak values, the vehicle model determines the first two peak values and the vertical stiffness of pier determines the third one. Dominant frequencies of PSD mainly range from 0 Hz to 120 Hz.