Evaluation of seismic force provisions for nonstructural systems supported on marine structures considering effects of nonlinearity

Abstract

The investigation reported in this article focuses on evaluation of seismic force provisions for nonstructural systems proposed in Marine Oil Terminal Engineering and Maintenance Standards (MOTEMS) considering the effects nonlinearity in either primary (supporting), secondary (nonstructural), or both systems. The current provisions were developed based on prior research conducted using linear elastic behavior of both primary and secondary systems. This investigation evaluated effects of system nonlinearities on seismic forces in nonstructural systems by utilizing a 2 degrees-of-freedom model and a suite of 20 ground motions. It is found that the MOTEMS simplified and alternate formula may significantly underestimate the force in the secondary system for the ratios of secondary and primary system periods between 0.5 and 1.5. Therefore, it is recommended that engineers avoid systems within this period range. Outside this period ratio range, the MOTEMS simplified formula leads to significant overestimation, which can exceed 100% but the alternate formula generally provides forces that are very close to those from response history analysis. When the secondary systems are expected to respond beyond the linear elastic range, both the simplified and alternate MOTEMS formulae may lead to significant underestimation of forces in the secondary system for period ratios less than 0.5. Therefore, engineers are cautioned against designing secondary systems in this period range for forces much lower than those required to remain linear elastic. The MOTEMS alternate formula reduces overestimation or provides very good estimates of forces in the secondary systems compared to the simplified formula. Therefore, the MOTEMS alternate formula is preferable when information on period ratio is available.