I think it depends on the seismic zone of your bridge.

Response-spectrum analyses is to be performed for seismic actions represented by the uniform hazard spectrum. Separate response-spectrum analyses are to be carried out for the following two cases of seismic actions: (i) seismic actions in the longitudinal and vertical directions of the model, and (ii) seismic actions in the transverse and vertical directions. The horizontal and the vertical actions were applied simultaneously at the bases of the piers. The horizontal seismic actions were represented by the horizontal uniform hazard spectrum (UHS) , and the vertical actions were represented by a spectrum obtained by multiplying the horizontal UHS by 2/3. The factor of 2/3 is commonly used for defining vertical design spectra relative to horizontal spectra (Newmark et al. 1973). The analyses should included the first 100 modes, covering all natural periods above 0.02 s. It is necessary to mention that the mass participation of the modes considered in the analysis is larger than 90% in each of the three principal directions of the model. A modal damping of 5% is used for all the modes. The response maxima at each joint of the models are computed by combining the modal responses using the complete quadratic combination (CQC) rule.

Long span bridge should be checked economically first.

Check the following reference Please.. it might's assist you in your question … Regards…

https://pdfs.semanticscholar.org/7e95/5acbb93174ad779064c01aebb796ce4d14f8.pdf