Influence of outrigger truss system on the seismic fragility of super high-rise braced mega frame-core tube structure

Abstract

The braced mega frame-core tube (BMFCT) structure is an efficient seismic structural system for super high-rise buildings of over 500 m high. The outrigger truss (OT) system has become a commonly adopted design option for the super high-rise BMFCT structure, however, its influence on the seismic performance is not clear. In this study, the seismic responses and collapse risks of the 590-m-high BMFCT structures with and without OT system were analyzed. An efficient elasto-plastic modeling method was introduced to establish the accurate FE models. Based on the incremental dynamic analysis-based (IDA) fragility analysis, the deformation responses, energy dissipations, collapse risks and collapse resistance capacities, etc. were researched. The results indicate the average top displacements under earthquakes of ‘one-degree-higher’ rare intensities are reduced by the OT from 1286.51 mm to 1348.31 mm by 4.6%. The OT system can effectively limit the story drift ratios of the BMFCT under earthquakes with the lower intensity. With the increasing seismic intensity, the OT presents an unfavorable effect on the lateral deformations. The OT system increases the proportion of damping energy dissipation from 68.28% to 70.24%, and decreases that of plastic deformation energy dissipation from 20.74% to 20.12%, both by less than 2%. The OT system causes the slope of IDA curves to rise first and then fall, which is the opposite of that without OT. The OT system is detrimental to the structural collapse resistance capacity, reducing the collapse margin ratio from 8.46 to 7.41 by about 14%. The OT's function can effectively reduce the cross-sectional dimensions of main components and ensure the architectural functionality and view-transparency. In the seismic design of BMFCT structures, the OT system should be given more attention, especially in the earthquakes stronger than rare intensity, and recommended to be designed to meet the basic structural stiffness requirement and not to provide excessive lateral stiffness resistance.