The question of how many modes to include in the dynamic analyses of multiply-supported structures subjected to spatially varying ground motions is considered in this paper. The issue is investigated within the framework of the Multiple Support Response Spectrum (MSRS) method developed by Der Kiureghian and Neuenhofer (1992). The original MSRS rule is extended to approximately account for the contribution of the truncated highfrequency modes. A bridge structure designed by the California Department of Transportation (Caltrans) is used as an example application. The modal contributions to various response quantities are examined in conjunction with measures of the participating modal mass and the improvement achieved with the extended MSRS rule.