Study On The Earthquake-induced Collapse Of A Super-tall Mega-braced Frame-core Tube Building

Super-tall buildings have become one of the research focuses in civilengineering field as super-tall buildings are developing rapidly throughout theworld. Due to the complicated structural system of a super-tall building, which isquite different from a traditional tall building, the researches of the design method andthe numerical model of a super-tall building are not sufficient to meet the demandof engineering application. This paper carries out the research of a super-tallmega-braced frame-core tube building (H=550m), which is a real engineeringproject proposed to be built in China. Two preliminary design schemes arestudies, which are referred to as “half-braced scheme” and “fully-bracedscheme”. The main contents are as follows:(1) The numerical models for the mega-elements in this super-tall building,such as mega concrete filled steel tube (CFST) columns, composite shear walls,mega-braces, are discussed. Different numerical models are adopted to simulatethe mega-elements in this super-tall building. The multi-layer shell elements areadopted to simulate the shear walls and coupling beams. The fiber-beam modelprovided by the finite element software is adopted to simulate the mega-braces.The fiber-beam model that is developed based on the secondary developmentsubroutine of the finite element software is adopted to simulate the mega CFSTcolumns. The size effect of the axial strength of square CFST columns isinvestigated. The comparisons between the test results and the computationalresults by different design methods are conducted. It can be concluded fromthese comparisons that the design methods for the axial strength of square CFSTcolumns have obvious size effects. Therefore, the modified method consideringthe size effect to calculate the axial strength of square CFST columns isproposed.(2) A finite element (FE) model of this super-tall building is constructedbased on the fiber-beam and multi-layer shell models. The seismic elasto-plasticanalysis, the earthquake-induced collapse simulation and the incrementaldynamic analysis (IDA) are carried out. The elasto-plastic analysis shows thatthis super-tall building has a strong seismic-resistance capacity due to its strong structural design. The IDA results show that this super-tall building possesses ahigh collapse-resistance capacity. The collapse simulation subjected toextremely large earthquake reveals that the main collapse mode of this super-tallbuilding is of vertical “pancake” rather than lateral overturning. The“fully-braced scheme” has almost similar seismic performance as the“half-braced scheme”, while the consumption of concrete in “fully-bracedscheme” is significantly less than the “half-braced scheme”(3) Two types of simplified models of this super-tall building areconstructed, namely the bend-shear coupling model and the simplifiedbeam-spring element model. The bend-shear coupling is consisting of a flexuralcantilever beam and a shear cantilever beam, which considers the deformationcaused by bending and shearing. The dynamic characteristics of this super-tallbuilding can be obtained accurately and the calculation workload can be reducedby using the bend-shear coupling model with suitable parameter values. Thesimplified beam-spring element model simplifies a three-dimensional FE modelto a two-dimensional one. The simplified beam-spring element model cancalculate the dynamic characteristics and predict the nonlinear seismicresponses of this super-tall building. These two kinds of simplified models mayprovide a reference on comparing different structural schemes and structuralprimary designing.(4) The energy dissipation of this super-tall building under differentintensities of ground motions is investigated by using the simplifiedbeam-spring element model. The energy dissipation contribution of all kinds ofcomponents of this super-tall building can be evaluated and the energydissipation distribution in the structure can be predicted by means of the energydissipation calculation, which may provide a reference for theperformance-based design of super-tall buildings.