Dynamic Ship-Impact Load on Bridge Structures Emphasizing Shock Spectrum Approximation

In current design specifications, a static procedure using code-prescribed loads has been commonly used to design bridges against ship collisions. However, the collision-induced response may be greatly underestimated as a result of the absence of the dynamic amplification effect in the static procedure. Hence, the safety of bridge structures exposed to ship collisions is still a concern, although they have been designed according to the current specifications. To this end, this paper aims to determine the dynamic ship-impact load on bridge structures and to evaluate its rationality. A simple approach to determining the dynamic ship-impact load was first developed based on the ship-bow force–deformation relationship (i.e., P–a curve) and the analytical model considering the structural stiffness. The equations for estimating the load duration and the time history of ship deformation were analytically derived. To evaluate the accuracy of the developed ship-impact loads and some other impact loads in current studies, the shock spectra corresponding to these dynamic loads were compared with the desired spectra. It was found that the shock spectra using the developed ship-impact loads are generally in good agreement with the expected results, indicating the rationality of the developed approach. The results obtained using the bilinear P–a curve and the sine pulse are often unconservative. In contrast, the trapezoidal pulse generally leads to a conservative spectrum. Finally, case studies of ship–bridge collisions further confirm the aforementioned observation from the shock spectrum comparison. The importance of shock spectrum approximation is clearly identified for the development of the dynamic ship-impact loads on bridge structures.