Evaluation of Seismic Performance for Outrigger System and Ladder System in High Rise Buildings

Excellent seismic performance of high-rise building is a continuous destination in structural engineering, that highly relies on the lateral load resisting system. As one of the most common lateral load resisting systems, the outrigger system is widely applied to skyscrapers. However, its mechanical properties are affected by the truss location, the topological form as well as construction process, in addition, the irregular vertical stiffness of the entire building occurs. Therefore, the actual seismic behavior and optimization methods are worthy paying attention to. The ladder system is a novel lateral load resisting system, in which the horizontal beams are set to connect the mega columns at each floor, resulting in a more uniform vertical stiffness compared with the outrigger system.
In order to compare the seismic performance between the above two systems, the numerical models of 80-story building were established by means of the finite element program ETABS, and the performance-based seismic design method is adopted to carry out calculation and analysis. For the outrigger system, four levels of steel outrigger trusses were set along the height, and the specific locations that floor 17-18, floor 32-33, floor 46-47, and floor 62-63, respectively, were determined by considering the multiple factors including structural weight, base moment, base shear, inter-story drift, and core moment. In addition, the optimum topology of the outrigger truss was selected out of four different trusses' topologies with the same steel consumption according to the favorable natural period and the global response of the structure, such as base shear, bending moment and inter-story drift under the action of frequent and moderate earthquakes. Subsequently, the ladder system was generated based on the principle of the equivalent lateral stiffness with the outrigger model, and the cross section as well as the design parameters were presented.
Nonlinear dynamic time history analysis was carried out to compare the global and components' performance between the outrigger, ladder, and stand-alone core systems under the FEMA356 guidelines requirements, where 7 sets of natural ground motions and 2 sets of artificial ground motions for different intensities were selected as a seismic input. The results showed that:1) comparing with the stand-alone core system, the inter-story drift under the frequent earthquake level was reduced by 60% and 47% in outrigger and ladder systems, respectively. While the modification factors of both systems equal to 1, denoting that outrigger system is more efficient in the elastic stage. However, under the rare earthquake, the inter-story drift reduction was 56% and 70% for outrigger and ladder systems, respectively. While the modification factors of two systems equal to 3 and 6, indicating that the ladder system has better ductility and energy dissipating performance in the elastoplastic stage. Besides, the top displacement response of both systems reflect the same structural deformation feature. This indicates that the outrigger system is more effective in the elastic stage, while the ladder system is more ductile in the elastoplastic stage dissipating more input energy. 2) The same trend was for the internal forces, where the outrigger system reduced the core bending moment under the frequent earthquake level, while the ladder system showed a superiority under the moderate and rare earthquake evaluation. 3)In terms of the damage state evaluation, all the structural components in both outrigger and ladder systems were under Immediate Occupancy (IO) performance level for the frequent earthquake; while for the rare earthquake, most of the coupling beams reached the Collapse Prevention (CP) performance level in the ladder system, while only some of them reached the Life Safety (LS) and CP performance levels in the outrigger system. At the same time, the damage state of the shear walls was CP in both systems under the rare earthquake, and the damage state grew severer from lower to higher floors in the ladder system, however, the global plastic damage distribution has similarity between two systems. It can be concluded that the principle of the equivalent stiffness is more adequate under the frequent earthquake evaluation than the high earthquake intensity, where the calculation errors on energy dissipating and damage distribution would occur.