Elsevier

Engineering Structures

Volume 148, 1 October 2017, Pages 639-659
Engineering Structures

Effects of podium interference on shear force distributions in tower walls supporting tall buildings

https://doi.org/10.1016/j.engstruct.2017.06.075Get rights and content

Abstract

High rise constructions featuring a podium surrounding tower walls are often favoured for the versatile functionality of the building. It is shown in this paper that the podium can impose significant differential restraint on coupled tower walls. Incompatible tower wall displacements under lateral loads were found to be the main contributor to the generation of in-plane strutting forces in floors above and below the podium-tower interface level. Shear force localisations in the interior tower wall immediately above the interface was found to be the direct consequence of these actions. Key parameters contributing to this detrimental shear force localisation in a tower wall were analysed by way of parameter studies on representative models of the building and sub-assemblages. It is revealed that the in-plane rigid diaphragm assumption commonly adopted in practice can significantly suppress compatibility forces generated within the building floor leading to unconservative design of the tower walls. Elaborate nonlinear model has been examined to showcase the consequences of understating the shear demands on these walls.

Introduction

Podiums are augmented floor area at the lower level of a high rise building which are common in metropolitan areas in regions of low-to-moderate seismicity. The lateral load resisting system for such building configurations comprises moment resisting frames and shear (or core) walls. As the tower walls of the building is offset from the centre of the podium, high torsional moments can be imposed on the podium [1], [2]. High shear forces can also be induced on the structural walls thereby jeopardising their structural integrity when subject to severe earthquake ground shaking. Recommendations against this form of construction have not been mandated in many design codes of practices in spite of potential undesirable behaviour in a rare seismic event [3], [4].

At the podium-tower interface, horizontal forces are transferred from the tower to the podium. Reactive forces are developed at the podium-tower interface to resist the overturning actions (Fig. 1). The reacting mechanism is synonymous to the back span of a cantilever. Intuitively, the described backstay mechanism can induce high intensity shear force in the structural (tower) wall within the podium. The amplitude of the induced shear force is dependent on the in-plane flexibility of the floor structure connecting the pair of walls. This was first investigated in the early works of Bevan-Pitchard et al. [5] by means of linear analysis of the tower walls. A quarter of a century later, Rad and Adebar [6] extended their work into the inelastic domain and concluded that stiff sub-grade diaphragms and perimeter walls can lead to shear-critical conditions occurring in tower walls below their base.

The backstay phenomenon as described is well known [7]. However, shear anomalies generated by differential restraints on a tower wall (which has an offset from the centre of the podium) is not well understood. The structural wall which is closer to the centre of the podium (referred herein as the interior wall) is subject to higher moment restraints from the podium structure than the exterior wall. As a result, high strutting forces are developed in the connecting floor structure (beam and slab) to maintain compatibility. This strutting action can only be modelled accurately if the horizontal in-plane deformation of the floor diaphragm has been incorporated into the modelling. Thus, the extent of such actions can be misrepresented by analyses in which the (usual) rigid floor diaphragm assumption has been made. Effects of diaphragm flexibility on the lateral response behaviour of the wall were examined by Pantazopoulou and Imran [8] Shin et al. [9] and Su et al. [10]. It was found that diaphragm flexibility in buildings (featuring vertical or on-plan irregularity) can adversely affect dual wall-frame interaction. This issue has been highlighted in the PEER/ATC 72-1 document [7] inciting practitioners to use explicit floor models at the podium-tower interface, and particularly so in situations where there are high transfer forces.

Rutenberg and Bayer et al. [11], [12] studied the prevalence of incompatibility (strutting) forces in slabs and beams connecting structural walls of different base dimensions. They concluded that these in-plane forces were the result of incompatible inelastic deformations within the structural wall. Gardiner et al. [13] and Bull [14] further examined incompatibility issues resulted from abrupt stiffness variations up the height of the building and dual frame-wall interaction. Their work highlighted the detrimental increase in the transfer (in-plane) forces when the structure undergoes inelastic response behaviour. Diaphragm-wall interaction issues is further highlighted in the New Zealand earthquake loading standard, NZS 1170.5 [15] in which more detailed analytical models and procedures are mandated to encapsulate the effects of diaphragms interference on the seismic behaviour of the structure.

The implications of podium-tower interactions as described have not been thoroughly covered in the research literature or in code provisions.

In this paper, parametric studies based on quasi-static analyses were first conducted on two-dimensional (2D) planar podium-tower sub-assemblages to demonstrate shear anomalies that can be developed in structural walls that are resulted from interferences from the podium structure. The cause of significant increase in the shear intensity of the structural wall above the podium level is to be illustrated. Results from the parametric studies have been used to track the trends of the increase in the shear force intensities in order that conditions deserving special attention in design can be identified (Section 2). Findings from parametric studies of the sub-assemblages have been verified by static and dynamic analyses of example 3D finite element (FE) models of existing buildings assuming linear elastic behaviour (Section 3), and non-linear inelastic behaviour (Section 4).

Section snippets

Analyses on 2D planar podium-tower sub assemblages

The first part of the study to be reported in this paper is based on 2D planar sub-assemblages of a podium-tower building in which tower walls are offset from the podium (Fig. 2a). Results obtained from the analysis of the sub-assemblages are compared with that of centrally configured sub-assemblages as control (Fig. 2b). For both building models, the podium structure was simplified to a series of walls that were connected by floor slabs. The later methodology was chosen such that it can

Verification studies on 3D FE model of buildings

A 3D FE model of an existing building was employed in static and dynamic analyses to verify findings reported in Section 2. The case study building is a podium-tower structure wherein the tower is offset from the centre of stiffness of the podium (refer to Fig. 16). The medium rise ten-storey reinforced concrete structure has been designed and detailed for gravity and wind loads, but without taking into account the potential occurrence of seismic actions. The building features vertical

Inelastic shear behaviour of coupled tower walls

In view of limitations with analyses based on the assumption of linear elastic behaviour, non-linear time history (NLTH) analysis of case study building 3 (refer to Appendix A) and inelastic push over analyses have also been employed. Results from non-linear analyses confirm the findings of the parametric studies in terms of the increased likelihood of the interior shear walls experiencing higher shear demands (Section 4.1) and shear critical behaviour in the interior tower walls (Section 4.2).

Conclusion and recommendations

This paper sheds light on the unfavourable interference of the podium structure on twin tower walls. It was found that podium structures can impose significant differential restraints on the walls. Diaphragm-wall interaction in the form of (strutting) compatibility forces were mobilised in the stories immediately above, and below, the podium level. These forces are shown to redistribute internal actions between the interior and the exterior wall, and consequently offsetting the distribution of

Funding

The support of the Commonwealth of Australia through the Cooperative Research Centre program is acknowledged.

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