Seismic performance of step back, step back set back and set back buildings in sloping ground base

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This paper presents the structural behavior of buildings located in the sloping ground level subjected to seismic load. Three different categories of building from three to five storey are considered for the numerical modelling namely set back building (SB), step back building (SBB) and step back set back building (SBSB). The dynamic response of different buildings are analyzed and compared to assess the seismic vulnerability associated with each buildings. The seismic vulnerability is accessed by comparing the base shear, drift, displacement and torsion factor values. Linear static method is used for the calculation of earthquake load using ETABS. It is observed that SBB and SBSB are highly affected by torsion compared to the SB building. It is desirable to use shear wall in periphery of the bottom storey to improve seismic performance of building. The results in this paper can be employed for construction of buildings with seismically active sloping ground.

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Introduction During earthquake the seismic waves are more catastrophic to the building constructed with sloping ground foundation. Due to the terrain and geography, it is most likely that the buildings are constructed on the sloping ground with foundations at different levels. The buildings constructed in hilly regions are broadly classified as: (a) step back building (SBB), where the buildings of more than one storey height are constructed in the terraced land; (b) step back set back building (SBSB), where buildings are constructed in the pure sloping ground; (c) set back building (SB), where the buildings are constructed on the plane surface prepared by cutting the hill slope. Figure 1 shows SBB, SBSB and SB type of building constructed in sloping ground. When subjected to ground motion, such buildings constructed in masonry with mud mortar/cement mortar without conforming to code provisions have proved unsafe and resulted in loss of life and property [1]. Field reconnaissance of 3500 buildings of various types, after 2015 Gorkha earthquake, it was found that RC buildings failures were more attributed to soft story, pounding, shear failure and lack of symmetry in buildings [2]. Such buildings are more at risk, because the column of the building rest at different levels of the slope, causing irregularities in the structure. Dynamic characteristics of the buildings on flat ground differ to that of buildings on slope ground as the geometrical configurations of the building differ horizontally as well as vertically. The natural period of building decreases as the slope of the ground increases and short column resists almost all the storey shear as long columns are flexible and cannot resists the loads [3]. Also, the irregular building has the higher time period in linear static analysis [4]. The buildings in the sloping foundation produce the torsional effect as the center of mass and center of stiffness does not coincide with each other [5]. In addition to the torsion, building in the sloping ground generally experiences the short column effect which increases the vulnerability of the structures. Similarly with the increase in the column stiffness the axial force and base shear also increases in the building [6]. The increase in the storey height and number of bays will also have impact in the shear and longitudinal reinforcement of the structure [7]. a b c Figure 1. Different types of building in slope ground: a - SBB; b - SBSB; c - SB Table 1 SBB, SBSB and SB for different storey building Building type SBB SBSB SB Plan for three storey Elevation for three storey Plan for four storey Elevation for four storey Plan for five storey Elevation for five storey Earthquake impact have amplified the problem of landslide and erosion in the hilly regions thus all residential, educational, hospital and commercial buildings in the hilly regions must be analyzed for the seismic loads. The buildings must be designed to resist the seismic waves to prevent the loss of life and property. The upcoming section of the paper discuss the seismic behavior of the differently configured buildings on hill slopes followed by the comparison of seismic behavior of hill buildings with regular buildings on the plane slope. In this study a RC framed residential building having regular rectangular shape in plan is considered for analysis. SBB, SBSB and SB building type with three, four and five storey is consider in the modelling. For the regularity of the structure, the three-storey building consists of three bays in x and y direction, similarly four storey building consists of four bays in x and y direction and five storey building consists of five bays in x and y direction. Center to center distance between the columns in each bay is 3.9 m in both x and y direction, for all the models consider in the analysis. The plan and elevation of the building considered for the analysis is shown in Table 1. The RC beams and columns are model as three-dimensions frame elements with centerline dimensions. The rigid zone factor for beam and column joint are assign as one. The area loads is applied on slabs (model as rigid diaphragm in each storey), and non-uniform soil pressure is applied to shear walls, which are assign with pier label in model. Bracing width and thickness is taken same to wall thickness of 230 mm. Foundation is model as isolated footing in fixed condition at the base, as soil foundation interaction is not considered in present study. Regular building model was design as per1, with torsion consideration, using different load combinations. Normal static and dynamic load combinations consist of 13 load combinations, to study the torsional effect additional 12 load combinations are adopted by considering the eccentric load combinations, making up of total 25 load combinations. Numerical modelling Numerical simulation of the buildings is performed by using ETABS software. Figure 2 shows the 3D diagram of the building considered for the design. Elevation geometry for SBB and SBSB are considered assuming the 30° slope to the natural level of ground. The buildings are modelled as RC frame structure. M20 grade of concrete and Fe 500 reinforcement bar is considered in the modelling. The properties of concrete and reinforcement bars used in modelling are as shown in Tables 2 and 3 respectively. Basic parameters of building models for different storey is shown in Table 4. a b c Figure 2. 3D model of the buildings considered in the modelling: a - SBB; b - SBSB; c - SB Table 2 Table 3 Material properties of M20 grade of concrete Material properties of Fe 500 grade of rebar Weight per unit volume ρ 25 kN/m3 Weight per unit volume ρ 76.9729 kN/m3 Modulus of elasticity E 22360.68 N/mm2 Modulus of elasticity E 2×105 N/mm2 Shear modulus G 9316.95 N/mm2 Shear modulus G 76923.08 N/mm2 Poisson ratio ν 0.2 Poisson ratio ν 0.3 Coefficient of thermal expansion α 5.5×10-6 Coefficient of thermal expansion α 1.17×10-5 Minimum yield stress fy 500 N/mm2 Minimum tensile stress fu 545 N/mm2 In the present study, building sole purpose is residential. Table 5 shows the loading and its pattern used in the analysis as per IS 875 (Part 2): 1987 and IS 1893 (Part1): 2016. In the analysis the soil pressure is applied as non-uniform loads in the shear wall. The soil pressure on the building demands the shear wall which is considered in the modelling of the building. Additional analysis is performed in five storey building for the cases with and without shear wall. The results for the same are discussed in the next section. The load in the shear wall is applied as per considerations made in Figure 3. 1 IS 1893 (Part 1). Criteria for earthquake resistant design of structures. New Delhi: Bureau of Indian Standards; 2016. Table 4 Basic parameters of building models for different storey Parameters Values Unit Remarks Number of storey 3, 4,5 - Storey height 3.3 m For all models consider Column size for three-storey and four storey building 300×300 mm×mm Column size for fivestorey building 450×450 mm×mm Beam size for three-storey and four storey building 300×225 (D×b) mm×mm Beam size for five-storey building 500×300 (D×b) mm×mm Slab depth 125 mm For all models Shear wall thickness 200 mm For all models consider Seismic zone Z V - Zone factor = 0.36 (IS 1893 (Part 1): 2016) Importance factor I 1 - AS per IS 1893 (Part 1): 2016 Frame system SMRF - Response reduction factor = 5, as per IS 1893 (Part 1): 2016 Soil type Medium - Angle of friction 30° and unit weight 20 kN/m3 [8] Table 5 Loads considered in the analysis of buildings Parameters Value Unit Mass source for analysis, % Remarks Imposed load on floor 2 kN/m2 25 For all cases of buildings Imposed load on roof 1.5 kN/m2 0 For all cases of buildings Floor finish 1.0 kN/m2 100 For all cases of buildings URM infill wall load 10 kN/m 100 External wall load URM infill wall load 6 kN/m 100 Internal wall load Soil pressure Applied as non-uniform loads in shear wall Figure 3. Soil pressure in shear wall The seismic coefficient method (Lateral static method) is one of the static procedures for earthquake resistant design of structures. Horizontal forces are calculated as products of the seismic coefficients and weight of the structures. Design parameters depends upon the shear computation, which again depends upon the seismic weight and fundamental time period of the structure. Response reduction factor R accounts for both damping and ductility of structure. The fundamental time period is calculated based on code-based formula. This method is recommend and specified in various seismic design code, including IS 1893 (Part1): 2016, which is detail below. The design base shear Vb along any principal direction of a building shall be determined by Vb = Ah×W, where Ah - design horizontal acceleration coefficient value using approximate fundamental natural period; W - seismic weight of building. Also,

About the authors

Subhash Chhetri

Tribhuvan University

M.Sc. student, Pashchimanchal Campus, Institute of Engineering PO Box 46, Lamachaur Pokhara, Federal Democratic Republic of Nepal

Sailesh Adhikari

Tribhuvan University

Author for correspondence.
ORCID iD: 0000-0002-8368-7770

Assistant Professor, Head of the Department of Civil Engineering, Pashchimanchal Campus, Institute of Engineering

PO Box 46, Lamachaur Pokhara, Federal Democratic Republic of Nepal


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