Examples of Triggers for Combined Seismic Analysis of Stacked Industrial Structures

Abstract

Petrochemical facilities frequently have large equipment supported by equally large nonbuilding structures. Examples of stacked nonbuilding structures in petroleum refineries include coker structures, vacuum tower structures, and fluidized catalyst cracking reactor and regenerator structures. Some of these stacked nonbuilding structures have a supported structure with a fundamental period that is larger than the fundamental period of the supporting structure. Examples are provided that show that when the ratio of fundamental periods is greater than two, then the supported structure can be decoupled and analyzed as if it were supported on a rigid base. In other stacked nonbuilding structures, such as elevated compressors supported by a reinforced concrete tabletop structure, the fundamental period of the supported structure is low compared to the fundamental period of the supporting structure. An established trigger for combined analysis of stacked structures is when the ratio of the supported structure weight to the combined weight of both structures is greater than twenty percent. Examples are provided that show that when the ratio of fundamental period is less than one half, then the supporting structure can be decoupled and analyzed as if there were a rigid mass on top of the supporting structure. The provided examples illustrate that decoupling triggers based upon natural frequency ratios are reliable for stacked structures.