Dynamic response of caisson breakwaters: suggestions for a static equivalent analysis of a single caisson in the array

The dynamic response of a vertical breakwater to breaking waves has been studied by the authors with prototype tests on two breakwaters in Genoa Voltri and one in Brindisi. The tests were interpreted by Mass Spring Dash-pot elements connected among them in order to represent the Array structure typical of a breakwater (MSDA model). The model is similar to the classical MSD one, except that it also considers the force exchanged by adjacent caissons through the foundation. The objective of this note is to present an analysis of the caisson interaction and to study and compare the different responses to breaker impacts on average European and Japanese caisson breakwaters. These represent typical section shapes using two areas with scaling rules relating caisson dimensions to the incident design significant wave height. Inertia, foundation stiffness and added masses are quite different for the two types of structures and their effect is analyzed. The dynamic response is presented in terms of a static equivalent response factor given as a function of the ratio between impact load duration and the main structure oscillation period, of the ratio between caisson length and width and of the fraction of the total mass represented by foundation added mass (the latter being the essential difference between European and Japanese design). Although the modeling of the array structure is essential to interpret the oscillations measured in the prototype, and is obviously important to study the effects of a load that varies along the breakwater, the prediction of the classical approach is similar in terms of maximum load on the foundations whenever the applied forcing is longitudinally homogeneous.