Author(s): Rea Marzeddu; Xavier Gironella; Agustin Sanchez-Arcilla; James Sutherland
Keywords: Impact loads; Wave loads; Vertical breakwaters; Wave flume tests; Sample frequency
Abstract: Some small scale laboratory experiments were carried out in order to detect laboratory effects during the measurement of wave loads on rigid structures. Wave loads on rigid structures are divided into quasi static loads and impact loads. The procedure that will be demonstrated in this paper, the measure of quasi static loads is fairly easy and not affected by the sample frequency. Instead the measure of impact loads is very difficult and some laboratory effects can strongly influence the results and lead at an unsafe estimation of the total load. Particularly, the use of different equipment and sensors can lead to significant differences in the results, so, for this reason, a comparison between load cells and pressure transducers recorded under the same wave conditions were made. Impulsive wave loads are very fast and are characterized by a very short rise time (some milliseconds) and the results, also under regular waves attack, are very scattered. For that reason a higher frequency sampler (up to 19. 2 KHz) was used in order to study how the use of different sample frequencies can lead to very different results. In principle the utilization of a sample frequency that is too low can lead again to an unsafe estimation of the total load and affect the design process. Besides, due to the large scatter on the data, a deterministic approach is inadvisable and some repetitions of the same wave conditions are needed in order to characterize well the studied phenomena. The obtained results show the probabilistic performance inherent to the impact loads phenomena as a source of the scattered results.