Effect of Pipe Mass Vertical Distribution on Seismic Demands in Industrial Steel Pipe Racks

Abstract

Industrial pipe rack structures support piping systems carrying hazardous fluids at multiple elevations. In current seismic design practice, pipe mass is typically idealised as uniformly distributed across all levels, yet the actual distribution varies considerably depending on the operational configuration. The structural consequences of this idealisation on nonlinear seismic demands have not been systematically investigated. This study examines the effect of vertical pipe mass distribution on the nonlinear seismic response of a representative multi-level steel pipe rack structure. A finite element model is developed in SAP2000 with fiber-section hinges and link elements for bracing. Four pipe mass distribution cases—uniform, top-heavy, bottom-heavy, and mid-concentrated—are defined with constant total pipe mass. Nonlinear time-history analyses are performed under eleven spectrally matched ground motion pairs applied separately in the longitudinal and transverse directions, yielding eighty-eight analysis runs. The uniform distribution produces the highest seismic demands in both directions, with a maximum interstory drift ratio of 7.19% (longitudinal) and 3.10% (transverse). The non-uniform distributions reduce the maximum interstory drift ratio by 45% - 59%, peak roof displacement by 34% - 59%, and normalised base shear by 64% - 71% relative to the uniform baseline. The demand reductions are driven by a fundamental period shift exceeding a factor of 2.5, repositioning the structure across distinct spectral regions. When assessed against performance limit states, the uniform case exceeds collapse prevention in the braced direction, while non-uniform cases remain within life safety in the transverse direction. The uniform pipe mass assumption is shown to be inherently conservative; accounting for the actual distribution could shift the performance classification by one or more levels and enable more economical designs.