EVALUATION OF SEISMIC FORCE PROVISIONS FOR ANCILLARY SYSTEMS IN PIERS AND WHARVES CONSIDERING EFFECTS OF NONLINEARITY

Much of the prior research on understanding seismic response of secondary systems and development of code provisions was conducted either for nuclear power plant facilities or buildings. Most marine structures differ from nuclear power plant facilities or buildings in that the marine structures can be idealized as one-story building type systems. Furthermore, most previous studies have been limited to linear elastic behavior of both primary and secondary systems. The objectives of this study are to (1) understand effects of nonlinearity, both in primary and secondary systems, on seismic forces in secondary systems, and (2) evaluate seismic force provisions proposed in MOTEMS that were based on linear elastic studies for secondary system considering effects nonlinearity. Since primary systems of concern in this investigation are marine structures such as piers, wharves, and marine oil terminals, which can be idealized as single-degree-of-freedom (SDOF) systems, this study utilized a simple model with two degrees of freedom, one representing the marine structure and the other representing the ancillary component. This investigation used the SAC ground motion set consisting of 20 ground motions from 10 sites for 10% probability of exceedance in 50 years for a site in Los Angeles, California. This investigation first studies the effects of damping in the secondary system on forces in the secondary systems. This investigation led to the conclusion that the damping has negligible effects on force in the secondary system for low values of the ratio of the secondary and primary system periods (say less than 0.5) regardless of the primary system period or ratio of the mass of the secondary and primary systems. For larger values of the period ratio, however, force may be 10% to 35% higher in secondary systems with 2% damping compared to systems with 5% damping. Based on this analysis, 2% damping in the secondary system was used in this investigation. Most previous investigation on nonlinear SDF systems have been focused on seismic displacement demands. It has been found that displacement of systems with non-zero postyield stiffness is lower than the corresponding elastic-perfectly-plastic SDF systems. However, the effects of non-zero post-yield stiffness is not clear. Therefore, this investigation next examined the effects of post-yield stiffness on system forces. It was found that very-short period system with strength lower than the strength required for it to remain elastic may experience force which may even be higher than force in the corresponding linear-elastic system. Therefore, it recommended that very-short period (or stiff) systems not be designed for strengths much lower than the elastic-level strength. While post-yield stiffness of primary system is often better known, e.g., nonlinear pushover analysis, such may not always be the case for secondary systems. Therefore, careful consideration must be given to force-based design of stiff (or short-period) secondary systems when post-yield stiffness is not known. The investigation next focused on the effects of nonlinearity in primary and secondary systems on seismic forces in secondary systems. This investigation led to the following conclusions:  Nonlinearity in the primary system alone leads to: (1) significant reduction of forces in the secondary systems with T T <1.5 because nonlinearity in the primary system results in p n lower acceleration transmitted to the base of the secondary system, which in turn leads to lower force in the secondary system, (2) minimal reduction of forces for systems with