V ALIDATION
3.6 F INITE E LEMENT M ODELLING
Design lateral force at each floor level is calculated as per Indian Seismic Code IS: 1893 (Part 1) (BIS, 2016b). The design vertical loads are obtained considering dead loads of members and code specified live loads on floor slabs. At each floor level, the lateral force in each of the two plan directions (X and Y) is shared equally by the two walls oriented in that particular direction. The walls are designed and detailed against combined vertical and lateral forces as per the relevant Indian Standard (BIS, 2016b).
In the junction region, the longitudinal reinforcement of the slab is extended up to the wall. 8 mm diameter bar spaced at 150 mm in the RC shear wall is provided in two layers along both horizontal and vertical directions. The reinforcement provided in the floor slab is 8 mm diameter bar placed at 300 mm spacing in both directions. The reinforcement details in the shear wall - floor slab junction are shown in Figure 3.14. Both the top and bottom bars of the slab are bent at 90º and extended up and down to the exterior face of the shear wall. The reinforcement ratios in vertical and horizontal directions are the same in case of shear wall.
3.6.2 Methodology
One five storied and another ten storied symmetrical frame shear wall building (FSWB) with or without embedded reinforcement are used for the simulations. To study the detailed behavior of the shear wall - slab junction, an exterior wall-slab assemblage (EWSC) is considered from the five storied building. 8-noded hexahedral (brick) elements (C3D8R) are used for concrete with reduced integration characteristics. 2-noded linear truss elements (T3D2) are used to model reinforcement.
The embedded method is adopted to simulate the bond between the concrete and the reinforcement, assuming perfect bond. Two brick elements were used through the thickness of the 120 mm slab with all concrete elements having the same mesh size of 60 mm. Restraints were introduced at the bottom edges of the specimens in the direction of the applied load. The translational and rotational Degrees of Freedom (DoFs) are restrained at the bottom nodes of all the specimens. The outer edges of slabs are supported on rollers in case of EWSC model. The out-of-plane bending of the shear wall and the vertical bending of slab are restrained along the roller supported edges. Generally, a shear wall is designed to resist shear forces and bending moments in its own plane. In the present study, displacement based pushover analysis is carried out in the in-plane direction of the wall to investigate the behavior of slab-wall junction. Hence, the out-of-plane movement of the wall is restrained in the present study. In finite element analysis, the in-plane displacement of the wall-slab assembly does not get mobilized properly in absence of the restraint of vertical displacement. Also, the intention of the study is to investigate the possible damages in the slab and the slab-wall junction due to the in-plane lateral displacement of the wall. The vertical displacement of the slab under the
applied vertical load does not influence the damage of the slab and the slab-wall junction significantly. Thus, the vertical restraint does not affect the slab-wall junction behavior.
Chapter 4 provides the details regarding the geometry and the boundary conditions of the specimens that are used for the simulations. FSWB specimens are analyzed using monotonic nonlinear static analysis in ABAQUS/Standard and ESWC specimen is analyzed using both monotonic nonlinear static and slow cyclic analysis in ABAQUS/Standard. In the monotonic static analysis, a displacement is applied in the plane of the shear wall at the top node of the wall. In case of slow cyclic analysis, the sequence of applied displacements consists of three cycles at each displacement of 2.5, 5, 10, 20, 40, 60, 80, and 100 mm. The displacement is increased with a smooth amplitude curve. Among the constitutive models for simulating the behavior of concrete, the CDP model is used and a detailed description of this model is already furnished earlier in this chapter.
Figure 3.14: Detailing of reinforcement at shear wall - slab junction.
3.6.3 Mesh Convergence Study
In the present study, the FSWB and EWSC models are analysed under combined vertical compression and lateral force at the top of the wall. Mesh convergence study is performed for the EWSC model. Three different mesh sizes (75 mm, 100 mm and 150 mm) are adopted in the analysis of shear wall-floor slab connection in order to investigate the influence of mesh sensitivity on its response. Although there is a marginal difference in the normalized lateral shear capacities for the specimens with 75 mm, 100 mm and 150 mm mesh sizes, the mesh size of 150 mm is adopted in
Development length =52 ø Shear wall
300 mm
120 mm
8mm ø @ 300mm c/c 8mm ø @ 150mm c/c
Slab
order to avoid the hourglassing numerical problem and the distortion associated with the C3D8R elements (Genikomsou and Polak, 2015) (Figure 3.15).
Figure 3.15: Variation of base shear with lateral drift for EWSC specimen with different mesh sizes.
The choice of adopting 150 mm mesh size is based not only on the load-deflection response but also on the comparison with the damage patterns. In the present study, the accuracy of the FE solution is assessed by comparing the tensile damage pattern. It is observed that the propagation of tensile damage does not depend on the mesh size. Similar damage pattern is observed considering different sizes of mesh. Also, the measure of tensile damage is similar considering all the mesh sizes (Figure 3.16). By adopting the mesh size of 150 mm, the number of elements gets reduced and the analysis takes less time to complete than by considering smaller mesh sizes. Since the 150 mm mesh size is not affecting the damage propagation and giving the maximum capacity, for all subsequent analyses, the mesh size of 150 mm is adopted.