Podium and transfer structure interference on seismic shear demands of tower walls in buildings

© 2018 Dr Mehair YacoubianThe preference for the mixed-use of buildings has led to a paradigm shift in the way buildings are designed, and their geometries are determined. A podium-tower building configuration that caters for both commercial and residential functionalities has become a popular form of building construction in many metropolitan cities around the world. The tower portion of a building with this configuration may simply be extruded from part of the podium (i.e. a setback) or in some cases is planted on top of a transfer plate. In the event of an earthquake, the podium (lower) storeys of the building will function to redistribute the loads applied onto the tower. The interferences of the podium structure on the lateral resistance of the building are central to ensuring that an adequate load path is maintained up the height of the building. Nevertheless, the consequences of these interferences on the seismic performance of the building are not well understood and often require sufficiently detailed numerical simulations to model their effects. Even then, some aspects of these interferences (by the podium) may be hidden from the user. The aim of this study has been to investigate the different elements of the podium interferences on the response behaviour of the cores and walls that support the tower structure above. The two forms of podium-tower buildings (featuring a setback and a transfer plate) have been thoroughly investigated to develop an understanding of the complex podium-tower interactions in a building under lateral loads. A systematic investigation has been carried out by way of detailed numerical analyses on representative building models to highlight the key contributors to these interferences and to develop analytical tools to quantify their effects on the response behaviour of the building in seismic conditions. In a podium-tower building featuring a setback (in the floor area), the interferences by the podium structure are believed to only occur in the lower (podium) storeys of the building. These interferences are manifested by the shear force reversals in the tower walls and cores at the level of the podium interface (commonly referred to as the backstay effect). This study has revealed that significant redistributions of shear forces in the tower walls above the podium interface level are attributed to the interferences by the podium structure when the building is subjected to lateral loads. Furthermore, these interferences were shown to be most pronounced in a building where the tower structure is not centred about the supporting podium. The direct result of these redistributions was the high shear force concentration that has been revealed in some of the tower walls above the interface level. As such, shear-critical conditions have been demonstrated to occur in some tower walls which have reduced the capacity of the building to develop the required lateral displacements and strengths when subjected to earthquake loads. More importantly, the study emphasised that the described interferences (by the podium) would not have been necessarily signalled by the engineer should some (common) modelling assumptions were made in the early stages of the design. Analytical models have been developed to quantify the extent of these interferences and the amount of the additional shear forces that are locally developed in some tower walls above the level of the interface. These additional shear forces, which have originated in the floor slabs connecting the tower walls (and cores), were proportional to the ratio of the relative lateral stiffnesses of the tower and the podium. Accordingly, a wider podium at only a few storeys in the lower storeys of the building pertains to a higher interference and a higher shear force concentration in the tower walls. In podium-tower building featuring a transfer plate, the interferences by the lower (podium) structure mostly occur at the level of the transfer (which in this case is the podium interface level). The redistributions of the lateral forces from the tower to the supporting podium are manifested by the out-of-plane distortions (i.e. bending) of the transfer plate. Similar to a building with a setback feature, these interferences by the podium (represented by the plate) also resulted in high shear force concentrations in the tower storeys above the level of the transfer plate. An analytical framework has been developed to estimate the extent of interferences by the transfer plate and to model the anomalous increase in the shear force demand on the tower walls. In the developed model, the in-plane strains (and forces) in the floor slabs have been found to be linearly proportional to the difference in the transfer plate rotations evaluated at the base of the tower wall (which is the primary parameter controlling the extent of the interference). The complexity of estimating these deformations motivated the development of a simple displacement-based solution to estimate the extent of the interferences by the transfer plate by the use of only a few parameters that can be conveniently computed in a structural design office. As such, local deformations of the plate have been formulated proportionally to the resulting rotations (drifts) of the tower structure evaluated at its centre of mass. The study revealed that both the transfer plate deformations and the consequent additional shear force demand on the tower walls displayed displacement-controlled features which ultimately led to the development of a closed-form analytical solution for quantifying these interferences. The applicability of the framework has been tested on different case study buildings of different heights and plate thicknesses. In the absence of detailed guidelines in modelling such effects, the introduced analytical technique (which can be conveniently programmed on a single Excel spreadsheet) can potentially assist the design engineer in accurately estimating the shear force demands on the tower walls which would have been misrepresented in certain numerical simulations of the building. A thorough assessment of the seismic performance has been conducted on the two types of buildings examined in this study. The main objective has been to evaluate and quantify the response modification factors (the force reduction, ductility and overstrength factors) used in the design of the building. The results of a large number of incremental dynamic analyses (IDA) using ground motions with different intensities have been employed to assess the damage limit states sustained by the building. Seismic fragility curves were constructed to identify the limiting ground motion demands corresponding to the different performance levels. The results highlighted the high propensity for the building to develop local concentrations of inelastic demands in the storeys immediately above the podium level. These concentrations were attributed to the unfavourable interferences by the podium which has ultimately restricted the building from developing the required displacement capacities in seismic conditions. While the computed values of the force reduction factors were (generally) in good agreement with the code recommended values, the limited-ductile response of the tower walls above the interface resulted in lower values of the ductility factors than those assumed in the design. Thus, the assumed trade-off between strength and deformation may be unconservative for the types of podium-tower buildings examined in this study. A design framework has been proposed to mitigate the risks of developing the potential unfavourable mechanisms above the podium level by adjusting the design seismic force distributions up the height of the building