•The combined effect of duration and spectral shape on bridge response is investigated.•Ia &Sa(T1) are the best predictors of the response in shallow crustal ground motions.•Duration is insignificant in affecting the response in shallow crustal ground motions.•The duration of ground motions from subduction sources affects bridge response. The design practice of Box-Girder Seat-Type (BGST) bridges in the Western U.S. is continuously evolving based on the results of advanced modeling and analysis techniques. This is mainly to help engineers and researchers to better understand the behavior of BGST bridges during seismic excitations. Within this backdrop, this study fills the gaps in the current knowledge of assessing the combined effect of strong motion duration and spectral shape on the response of bridges using a comprehensive set of numerical simulations and statistical analyses. Three-dimensional finite element models of two real BGST bridges are analyzed using a large set of ground motions obtained from crustal sources and subduction sources. By means of Step-wise regression – and other statistical procedures – the sensitivity of bridge response parameters to various ground motion parameters including Arias Intensity (Ia), RotD50 spectral acceleration at the bridge’s first natural period (Sa(T1)), Significant Duration (D5-95), mid-frequency (f), the derivative of the mid-frequency (f’) and time at 30% of cumulated Arias Intensity (tmid) are evaluated. Results indicate that in the case of ground motions arising from shallow crustal sources, Ia and Sa(T1) are the best predictors of the bridge response, and strong motion duration (D5-95) has no statistically meaningful impact on the response of bridges. However, it is observed that the D5-95 of the ground motions ascending from the subduction sources highly affects the bridge response; utilizing D5-95 alongside Sa(T1), or Ia, can significantly increase the accuracy of bridge response estimates. Hence, it is concluded that D5-95 is not an important ground motion intensity measure for ground motion selection for bridges located in areas with crustal earthquakes. In contrast, D5-95 is important in subduction zone ground motions and must be given proper consideration in the design and analysis of BGST bridges.