In order to investigate the effect of ground motion frequency contents on the seismic performance of flexible bridges

sine waves were used to deduce the relationship between the ground motion energy and the ratio of ground motion peak acceleration and peak velocity A

p

/V

p

.A process was proposed for seismic performance assessment of bridges based on capacity to demand ratio（C/D ratio） and the calculation methods were formulated to derive C/D ratios. In the background of a typical rigid frame bridge with high piers

22 sine waves and 7 ground motion records with a same round motion parameter A

p

=0. 4 g were applied to examine the tendency of C/D ratios in both longitudinal and transverse directions with respect to the parameter of A

p

/V

p

.The results show that in the same ground motion parameter A

p

the C/D ratios for critical components of the illustrative bridge increases with an increasing A

p

/V

p

in both longitudinal and transverse directions. The maximum growth rate of C/D ratios reaches 1292% in the sine waves and 231% under the earthquake records

respectively. The C/D ratios obtained from the sine waves are in general larger than those from the earthquake records

and the growth trend of C/D ratios is more remarkable under the sine waves. It is found that the use of ground motion parameter A

p

as a single indicator for excitation input is insufficient to ensure the seismic safety of bridges and tends to produce divergent seismic designs. The parameter of A

p

/V

p

can be used as a supplementary indicator for seismic design of flexible bridges.