Seismic behaviour and design of a tall mixed steel-reinforced concrete structure supporting an oil refinery reactor

Abstract This study investigates the seismic behaviour of a special mixed steel-reinforced concrete structure that supports an oil refinery reactor. The structure is 64.90 m tall and consists of three parts: (a) a concrete frame basement; (b) a steel braced frame that supports the oil reactor and (c) the steel reactor itself. A three-dimensional model of the structure is created to perform static non-linear (pushover) analyses in order to obtain the capacity curves and understand the overall inelastic behavior of the structure. The results of the pushover analysis reveal that the structure exhibits similar inelastic behavior in both horizontal directions and satisfy the capacity design principles. The structure shows limited ductility considering the fact that has been designed with a behavior factor of q = 1.5 and primary damages are expected mainly in concrete members. Subsequently, dynamic non-linear time-history (NLTH) analyses are performed utilizing the three translational components of three seismic motions recorded during past earthquakes. These results involve: i) the maximum values for displacements, accelerations and base shears; ii) the maximum stresses at critical points of the oil refining reactor and iii) the formation of plastic hinges at columns, beams and braces of the structure. Opposite to the pushover analysis, NLTH analyses revealed that strong ground motions can lead to the development of plastic hinges that do not follow the desirable failure pattern and is likely to be related to the absence of adequate ductility-based design requirements. In case of a significant seismic event, accelerations and displacements observed are expected to cause failure of the piping and mechanical equipment, while local failures at high-stress areas of the oil reactor is possible. Localized strengthening might be necessary to avoid repair works and downtime after strong seismic events.