Effects of the structural mass on the waterborne debris impact force

In flood- and tsunamis-prone areas, waterborne debris impacts on structures cannot be neglected in structural design and assessment. For these purposes, the current practice is to represent these loads with their force-time (F-t) diagram and use them in structural analyses. Many existing models to compute such diagrams, among which the design formulas by ASCE/SEI 7–22, assume the structure to be massless. However, the accuracy of this assumption lacks extensive investigation. This is studied here for the first time using Finite Element (FE) simulations conducted on a plate structure under debris impact loads. Different cases of structure-to-debris mass and stiffness ratios are investigated. It is found that if the mass ratio is higher than a critical value, the structural mass significantly affects the F-t diagrams by causing an abrupt rise of the impact force and force values proportional to the mass ratio. These loads substantially differ from those computed under the massless structure assumption. It is also found that the existing analytical models become significantly inaccurate in predicting the F-t diagrams under the effects of the structural mass. It is finally pointed out that the scenarios where the structural mass is crucial include many realistic cases of impacts on masonry, concrete and solid cross-layered wooden panels (XLAM) walls. These results, together with post-disaster evidence of significant structural damage due to debris impacts and the growing risk of extreme floods due to climate change, highlight the significant practical interest of this study.