A Study on Damping Ratio in the Design Code of Vertical Vessels Supported by Skirt

The vertical vessels supported by skirt are sensitive to dynamic loads, that might oscillate intensely under earthquake and wind loads. Therefore, calculating the dynamic loads and response accurately, especially the damping ratio, influences the amplitude of vessels vibration greatly and is vital for safe operation of the vertical vessels. In Chinese design code, the damping ratio is 0.01 for steel structure, and in American code, it consists of structural damping β_s and aerodynamic damping β_a, while in European code, the additional damping β_d is also considered. With full-scale field tests, the damping ratios of vertical vessels, through a thorough data collection, is recommended as 0.002∼0.005. The damping ratio is 2/3 less than the design value 0.01 in empty condition, that makes the vessel in a crucial state. A new design concept is proposed that the damping ratio of vertical vessels should be determined based on various conditions. Fatigue checking is required in empty condition besides designing by code in operating condition. In addition, the allowable placement time [T_p] is calculated in empty condition. The actual placement time T_p should satisfy T_p < [T_p] and the damper should be installed on the vertical vessel to provide additional damping ratio in the case of T_p > [T_p]. An experiment was carried out to illustrate the new design concept that the damping ratio was enlarged by installing dampers. The damping ratio with other research could provide fundamental parameters for the amendment of the design code.