Seismic evaluation
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page 19 4. Analytical Investigations on Seismic Performance Observations on load bearing calculations for masonry carried out by Mr. V.V. Ranga Rao, Structural Consortium, Hyderabad: 1. The basic compressive strength of masonry of 0.5 N/mm 2 has been determined based on the allowable strength approach of IS: 1905 (1987): Code of practice for structural use of unreinforced masonry. While this code does not address compressed earth blocks for structural use, this is the only guideline structural design document in the Indian context. 2. The calculations are based purely on gravity and imposed load analysis for a two- storied structure, and the stresses at the base of the CSEB wall is 0.314 N/mm 2 , which is lesser than the allowable compressive strength. These calculations do not consider seismic loads, which is essential for zones 3, 4 and 5, as per IS: 1893-1 (2002). 3. As per Clause 5.4.1. of IS: 1905 (1987), the basic compressive strength of masonry can be estimated as 0.25f ’ m , where the compressive strength of masonry (f ’ m ) has been estimated from prism testing in the laboratory. Compressive strength of masonry estimated in the current experimental investigations with grout proportion 1:2:½ is 1.09 MPa, hence the basic compressive strength works out to be 0.273 MPa, which is lesser than the stresses estimated at the base of the ground storey CSEB wall (0.314 N/mm 2 ) for a two-storied structure. Therefore, based on the results of the experimental investigation and IS 1905 (1987) recommendations, D-300 blocks with 1:2:½ grout is safe only for a single-storied structure. The behaviour of CSEB reinforced masonry walls under the action of lateral loads (seismic) has been investigated, in light of the results of the experimental tests characterising the strength of CSEB masonry. Calculations have been based on the configuration of a proposed single-storied building at Porur, Chennai, designed by EPCMR, Chennai for WorldHaus Inc., USA. Three different wall configurations (with D-300 blocks) are identified in Table 10: Table 10: Reinforced CSEB wall configurations in single-storied prototype Wall no. Length (mm) Height (mm) Thickness (mm) Vertical reinforcement bars Horizontal reinforcement bars Wall 1 6000 2400 145 8 nos. 12 mm 1 no. 10 mm above door. 1 no. 10 mm bar @ 985 mm and 1 no. 10 mm bar @ 2025 mm (walls) Wall 2 2550 2400 145 4 nos. 12 mm Wall 3 1200 2400 145 2 nos. 12 mm Wall 1 is the rear wall of the structure; wall 2 refers to the two masonry walls on either sides of the door opening in the front of the structure and wall 3 refers to the two masonry piers on either sides of the window opening on the two sides of the structure. Calculations have been performed on unreinforced CSEB masonry walls to identify the in- plane shear mechanism at failure for the estimated level of axial load. Table 11 reports the estimated lateral load capacity of the three wall configurations and the expected in-plane mechanism. It is observed that the squat walls (l > h), wall1 and walls 2, are controlled by shear sliding mechanism, whereas the slender wall (l < h), walls 3, is controlled by flexural mechanism.
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Seismic evaluation
Transcript of Seismic evaluation
page 19
4. Analytical Investigations on Seismic Performance Observations on load bearing calculations for masonry carried out by Mr. V.V.
Ranga Rao, Structural Consortium, Hyderabad: 1. The basic compressive strength of masonry of 0.5 N/mm2 has been determined
based on the allowable strength approach of IS: 1905 (1987): Code of practice for structural use of unreinforced masonry. While this code does not address compressed earth blocks for structural use, this is the only guideline structural design document in the Indian context.
2. The calculations are based purely on gravity and imposed load analysis for a two-storied structure, and the stresses at the base of the CSEB wall is 0.314 N/mm2, which is lesser than the allowable compressive strength. These calculations do not consider seismic loads, which is essential for zones 3, 4 and 5, as per IS: 1893-1 (2002).
3. As per Clause 5.4.1. of IS: 1905 (1987), the basic compressive strength of masonry can be estimated as 0.25f’m, where the compressive strength of masonry (f’m) has been estimated from prism testing in the laboratory. Compressive strength of masonry estimated in the current experimental investigations with grout proportion 1:2:½ is 1.09 MPa, hence the basic compressive strength works out to be 0.273 MPa, which is lesser than the stresses estimated at the base of the ground storey CSEB wall (0.314 N/mm2) for a two-storied structure. Therefore, based on the results of the experimental investigation and IS 1905 (1987) recommendations, D-300 blocks with 1:2:½ grout is safe only for a single-storied structure.
The behaviour of CSEB reinforced masonry walls under the action of lateral loads (seismic) has been investigated, in light of the results of the experimental tests characterising the strength of CSEB masonry. Calculations have been based on the configuration of a proposed single-storied building at Porur, Chennai, designed by EPCMR, Chennai for WorldHaus Inc., USA. Three different wall configurations (with D-300 blocks) are identified in Table 10:
Table 10: Reinforced CSEB wall configurations in single-storied prototype Wall no.
Length (mm)
Height (mm)
Thickness (mm)
Vertical reinforcement bars
Horizontal reinforcement bars
Wall 1 6000 2400 145 8 nos. 12 mm 1 no. 10 mm above door. 1 no. 10 mm bar @ 985 mm and 1 no. 10 mm bar @ 2025 mm (walls)
Wall 2 2550 2400 145 4 nos. 12 mm Wall 3 1200 2400 145 2 nos. 12 mm
Wall 1 is the rear wall of the structure; wall 2 refers to the two masonry walls on either sides of the door opening in the front of the structure and wall 3 refers to the two masonry piers on either sides of the window opening on the two sides of the structure. Calculations have been performed on unreinforced CSEB masonry walls to identify the in-plane shear mechanism at failure for the estimated level of axial load. Table 11 reports the estimated lateral load capacity of the three wall configurations and the expected in-plane mechanism. It is observed that the squat walls (l > h), wall1 and walls 2, are controlled by shear sliding mechanism, whereas the slender wall (l < h), walls 3, is controlled by flexural mechanism.
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Table 11: Lateral load capacity of the masonry walls Wall no. Axial load
(kN) Axial stress at wall
base (N/mm2) Lateral load
capacity (kN) Governing in-plane mechanism
Wall 1 91.26 0.15 146.04 Shear-sliding Wall 2 38.79 0.15 41.46 Shear sliding Wall 3 18.25 0.15 11.11 Flexure
The presence of the 12 mm vertical steel reinforcement will provide additional resistance against shear sliding to the walls 1 and 2. This is however dependent on the anchorage available for the vertical steel in the RC foundation slab below and RC roof above. In the case of the slender wall, controlled by a flexural failure mechanism the flexural moment corresponding to the lateral load capacity (13.33 kN.m), would impose an axial stress of approximately 0.38 N/mm2 and in addition to the stress due to axial loading of 0.15 N/mm2, would result in a state of compressive stress of 0.53 N/mm2 on the compressed toe of the slender wall and a tensile stress of 0.23 N/mm2. The vertical steel will participate in resisting additional lateral force, however before the vertical steel yields, compressive failure at the compressed toe is expected (compressive strength from prism tests is 1.09 N/mm2).
