Effects of static air gap and pontoon height on breaking wave impact loads on a fixed surface-piercing square column structure

Breaking wave impacts are of particular concern in the field of coastal and marine engineering, due to their potential threat to structural safety. A good understanding of the characteristics and influencing factors of breaking wave impact loads is essential for designing cost-effective structures with the ability to withstand extreme marine environments. This study focuses on the effects of static air gaps and pontoon heights on the impact pressure, pressure impulse, and total impact force generated by various types of breakers on a square column with an overhanging deck. Breaking wave impact tests were carried out on the column with four static air gaps and six pontoon heights under six focused waves. The wavelet-based method was used to analyze the time-frequency characteristics of the breaking wave impact pressure, as well as the vertical variations in peak impact pressures and pressure impulses. The influences of static air gaps and pontoon heights on the breaking wave behavior and the impact forces by four types of breaking waves were discussed. It was found that the presence of columns and pontoons increases the local wave steepness of focused waves, and the pontoons alter the maximum wave height, acting in a similar way to the shoaling effect. The results showed that the increase in static air gaps reduces both the magnitude and position of maximum impact pressure, as well as the maximum pressure impulse and horizontal impact forces. A higher pontoon can cause the maximum wave crest to exceed the deck, directly producing intense impacts. Regardless of the type of breaking waves, pontoon heights close to the wave trough are adverse to decreasing horizontal impact forces of the column structure. In conclusion, the appropriate increase of the static air gap and the rational design of the pontoon height can effectively reduce breaking wave impact loads, lowering the risk of local structural damage.