R EVIEW OF L ITERATURE
2.5 S TRUCTURAL W ALL -S LAB J UNCTION
Figure 2.18: Relative displacements of floors (Schwaighofer and Collins, 1977).
(a) (b) (c)
Figure 2.19: Crack patterns at the top of the slab in slab-coupled wall system: (a) first visible crack, (b) initial crack spread back along the wall and (c) flexural crack across the entire width of slab (Schwaighofer and Collins, 1977).
slab connection was recommended to be designed considering stress concentration (Pantazopoulou and Imran, 1992). It was observed that the design of slab-wall connection is governed by the same requirements as shear walls, although it is common practice to omit shear design consideration due to the large extent of connection boundary between slabs and walls. A simple plane stress model was used to develop expressions for estimating the nominal shear resistance at the slab-wall junction. The governing code requirements were evaluated based on the experimental evidence and indicated that the existing codal equations are unconservative in case of lightly reinforced slab diaphragms. An expression for the nominal shear resistance of the slab-wall connection region of typical slab was also proposed.
Greeshma and Jaya (2013) carried out experimental and analytical studies of the floor slab-shear wall connection by considering two different reinforcement details, namely (i) conventional joint detailing with the provision of U-shaped hooks connecting shear wall and slab and (ii) extension of slab reinforcement into the shear wall as 90° bent bars at the core region with the provision of shear reinforcement in the slab. Good energy dissipation capacities were observed with the provision of shear reinforcement for an effective width of the slab as compared to the conventional joint reinforcement details. The experimental results were compared with numerical simulation results obtained using the finite element software ANSYS and it was found that the experimental results are in good agreement with the numerical results. Also, the specimens detailed with 90° bent slab bars with slab shear reinforcement exhibited higher ultimate strength as compared to the other specimens. They concluded that the exterior shear wall-slab joint with slab shear reinforcement and 90° bent bars at the joint can be effective in regions with the possibility of moderate to high intensity of earthquake shaking.
In another study, the behaviour of a core wall-slab connection was investigated experimentally under earthquake induced deformation to identify alternative details for improving the performance of that connection under large rotational demands (Klemencic et al., 2006). The main objective was to study the performance of those connections when subjected to gravity load combined with slab- wall rotation consistent with deformation compatibility of building lateral drifts. Two full-scale specimens were constructed and subjected to design gravity loads and increasing lateral deformations. In this study, the slab-wall connection was made through reinforcing dowels placed near the top and bottom of the slab, with mechanical couplers at the slab-wall interface, often supplemented with intermittent shear keys. The test successfully demonstrated that connection details proposed by the researchers achieved the collapse prevention performance objective of 2%
interstory drift as required by common building codes.
Rad and Adebar (2009) studied the interaction between tower walls and foundation walls due to
diaphragm action of concrete floor slabs. The concrete slabs and columns are assumed to be part of the gravity load system, not the seismic force resisting system, and the interaction between these two systems is usually ignored by designers for simplicity. The core walls extend from the top of the tower down to the foundation, and are supported near the base by a surrounding structure that may be partially or entirely below grade (Figure 2.20). The foundation walls import significant rigidity to the base structure. Nonlinear response history analysis was carried out to understand the reverse shear phenomenon. Depending on the stiffness of floor diaphragms, and on the shear rigidity and flexural rigidity of the highrise concrete walls, the reverse shear force below the flexural hinge may be much larger than the base shear above the flexural hinge. Nonlinear dynamic analyses indicated that the maximum reverse shear force is proportional to the bending moment capacity of the wall and inversely proportional to the accompanying base shear force. The conclusions from this study were summarized in terms of a complete design procedure that made use of a series of linear static analyses with appropriate reduced effective flexure and shear stiffness to estimate the reverse shear force.
Figure 2.20: Seismic shear force demand below base level (Rad, 2009).
In another study, comparative analyses of experimental and numerical research were carried out for RC connections between the precast slab and monolithic wall and both monolithic slab and wall elements (Zenunović and Folić, 2012). In total six specimens (3 precast and 3 monolithic) were tested under quasi-static loading. Mathematical models were proposed in order to analyse both types of connection, based on the exact method of displacement. Furthermore, the stiffness matrix was modified by introducing the stiffness parameter for the semi-rigid connection. An approximate strut- and-tie model was proposed according to the stress field analysis obtained by FEM was proposed.
From the experiments, it was proved that the designed precast connection enable dislocation of joints opening on the outside of the wall and dissipation of seismic energy comparable with similar monolithic connection.
Mercer (2009) examined the nonlinear shear response of walls including the effect of wall-slab system through state of the art nonlinear finite element analysis. The objective of the research was to quantify the shear capacity and response from the wall-slab system. A state-of-the-art finite element analysis was performed studying the behaviour of shear-critical walls. The analysis was carried out by varying the parameters such as horizontal reinforcement ratio, axial load and different slab spacings. The existing nonlinear shear models to describe the wall behaviour were first validated. Then, the failure of the wall without slabs was analysed and the shear behaviour of walls was compared with the behaviour of equivalent membranes. A simple shear model was developed to predict the shear capacity of the wall-slab system as well as the shear strain response of isolated systems. Also, some guidelines were provided for potential future development of a wall-slab system shear model. Although the past studies on wall-slab junction focused on the overall behaviour of the building system including different failure modes, none of the studies investigated in detail the influence of floor slab on wall behaviour and the possible change in the design strategy of the structural wall.