Analytical simulation of seismic collapse of RC frame buildings

Abstract

Application of a fibre-element nonlinear modelling technique for seismic collapse capacity assessment of RC frame buildings in comparison with conventional lumped plasticity models is investigated in this paper. Constitutive material models of concrete and steel for fibre elements are adopted to enable simulation of the loss in vertical load carrying capacity of structural columns. Inclusion of the nonlinear second order P−Δ effects accelerated by degrading behaviour of steel and concrete materials in the fibre model allows prediction of the sidesway mode of collapse. The model is compared with nonlinear lumped plasticity models in which stiffness and strength degradation is replicated through degrading parameters in structural components. Static cyclic analyses of an example cantilever column and a portal frame indicate that the variation of axial loads in columns may result in accelerated degradation and failure of structural components which is not taken into account by lumped plasticity models. Moreover, incremental dynamic analysis of a ten-storey RC frame shows that the lumped plasticity model may overestimate building collapse capacity when vertical failure of structural components occurs prior to sidesway instability.

Effect of ground motion selection methods on seismic collapse fragility of RC frame buildings

“…Modelling of the frame members consists of nonlinear fiber elements justified in [17] for reliable building collapse analysis. As elaborated in [17], the collapse predicted by this model is either the sidesway mode of collapse due to P-Δ effects at large inelastic displacement or when one of the vertical load carrying elements loses capacity due to crushing of the concrete in compression for the whole section or buckling of the bars due to excessive compression or low cycle fatigue. To mimic the behaviour of steel bars, the generic model proposed by Kunnath and Heo [26] is utilized, which accounts for the strength degradation due to fatigue in cyclic loading as well as buckling of reinforcing bars in compression.…”

“…The degrading behaviour of RC sections is affected by buckling of longitudinal steel bars in compression, which is modelled based on [27,28]. Further details of the structural model as well as verifications can be found in [17]. Apart from vertical load carrying capacity and sidesway modes of collapse, other modes of collapse such as shear failure of structural components and punching shear failure in slab-column joints were neglected.…”

“…A two-dimensional nonlinear frame model is created in OpenSEES structural analysis platform [25]. Modelling of the frame members consists of nonlinear fiber elements justified in [17] for reliable building collapse analysis. As elaborated in [17], the collapse predicted by this model is either the sidesway mode of collapse due to P-Δ effects at large inelastic displacement or when one of the vertical load carrying elements loses capacity due to crushing of the concrete in compression for the whole section or buckling of the bars due to excessive compression or low cycle fatigue.…”

“…Significant among them is that the loss of vertical load carrying capacity of one or more components in the structure, such as loss in axial capacity of columns, is not generally modelled. The authors [17] have shown that use of fiber elements equipped with path-dependent cyclic nonlinear material models that account for concrete confinement and crushing as well as reinforcement buckling and low cycle fatigue can reliably simulate the sidesway mode of collapse due to destabilizing P-Δ effects at large inelastic deflections plus the loss in vertical load carrying capacity of buildings due to inability of columns to carry axial compressive stresses due to permanently opened cracks. Moreover, lumped plasticity models have to be calibrated for individual sections based on experimental results or predetermined generic quantities (e.g.…”

“…To overcome these limitations, adds-on lumped plasticity models (e.g. in [14][15][16]) or application of fiber-element-based nonlinear modelling [17] have been used by some researchers for collapse analysis. The authors [17] have shown that use of fiber elements equipped with path-dependent cyclic nonlinear material models that account for concrete confinement and crushing as well as reinforcement buckling and low cycle fatigue can reliably simulate the sidesway mode of collapse due to destabilizing P-Δ effects at large inelastic deflections plus the loss in vertical load carrying capacity of buildings due to inability of columns to carry axial compressive stresses due to permanently opened cracks.…”