Effects of Horizontal and Vertical Mass-Asymmetric Distributions on the Seismic Response of a High-Rack Steel Structure

Abstract

The paper examines the seismic response of an existing externally braced steel frame high-rack structure and analyses the effects of mass eccentricities that can be realistically achieved by asymmetric positioning of the stored payload. Apart from the symmetric, three different extreme payload distributions with varying occupancy levels have been considered, with the payload mass concentrated: (i) at the topmost part of the structure, (ii) at the upper corner of the structure and (iii) at the outermost edge of the structure. The seismic performance has been analysed by using unidirectional non-linear dynamic analyses as well as by non-linear static analyses, with the structure's response observed in the cross-aisle direction in which it is possible to account for the effects of torsional twist. The results showed that most unfavourable payload eccentricities might increase the seismic risk leading to local instability of the rack columns. From a seismic point of view, a fully occupied structure does not present the most critical condition. It is obtained at an intermediate occupancy level, which allows more space to produce horizontal and/or vertical eccentricities. Very small occupancy levels in turn produce smaller induced seismic forces and might therefore not be of critical concern. The paper concludes that the payload distribution concentrated symmetrically at the topmost part is the most critical for the central part of the rack structure and that concentrating the payload eccentrically at the outermost edge is the most critical for the flexible side of the structure, while shifting the payload to one of the upper corners is generally not of critical concern.