L IST OF S YMBOLS
Chapter 4 E XPERIMENTAL S TUDY ON URM W ALLS
4.3. E XPERIMENTAL E VALUATION OF URM WALLS
4.3.1. Experimental Results
Lateral load response of all the three specimens was studied in terms of crack formation, lateral strength, energy dissipation, etc. All the models displayed different failure patterns and lateral load-carrying capacities, and the tests were continued until significant damage was observed. Although all the models finally failed in a brittle manner, each model started rocking motion after reaching a particular displacement level while sustaining the load till failure.
4.3.1.1. Wall 1 (Model 1)
Model 1 was the URM wall specimen without any opening. Under lateral loading, the first flexural crack occurred at the base of the model at 2 mm displacement level (Figure 4.9).
The crack propagated further with an increase in displacement level initiating the sliding shear failure at 8 mm displacement level. The test was terminated at 9 mm displacement level due to the out-of-plane movement of the wall. The lateral load-carrying capacity of Model 1 was found to be 27.5 kN. A small bed joint crack was also observed at the top of the wall specimen at 6 mm displacement (Figure 4.9a). The hysteresis response, capacity envelop curve, and cumulative energy dissipation obtained for Model 1 are shown in Figure 4.10 and Figure 4.11, respectively. The hysteresis response of Model 1 resembles more or less a bi-linear elasto- plastic behavior with limited ductility. The cumulative energy dissipation curve shows a steady increase in the energy dissipation capacity till failure.
(a) (b)
(c)
Figure 4.9. Lateral load testing of Wall 1: (a) load arrangement and crack formation and (b)(c) closer view of the crack propagation at the bottom of the wall.
Figure 4.10. Hysteresis response and capacity envelop curve obtained for Wall 1.
Figure 4.11. Cumulative energy dissipation curve obtained for Wall 1.
4.3.1.2. Wall 2 (Model 2)
Model 2 was the URM wall specimen consisting of a central door of size 0.9 m × 2 m. The cyclic test on the specimen resulted in a mixed failure mode of flexure and sliding shear- type (Figure 4.12). Small cracks appeared in the bottom of the specimen during the loading at 2 mm deformation level. At 4 mm deformation level, these cracks at the first bricklayer from the bottom started widening and extending. At 5 mm displacement level, cracks originated at the upper corners of the door opening, and these cracks subsequently extended towards wall edges till failure of the specimen at only 6 mm lateral displacement. The lateral load capacity of Model 2 is 19 kN, as shown in the hysteresis response in Figure 4.13. The strength envelops curve and cumulative energy dissipation curve also depict the sudden failure of the specimen at a low level of lateral deformability resulting in very low energy dissipation (Figure 4.14). The test was stopped at the first cycle of 6 mm displacement level to prevent complete disintegration of the wall. During the initial
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Lateral Load (kN)
Displacement (mm)
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0 2.5 5 7.5 10
Energy Dissipation (kNmm)
Displacement (mm)
displacement cycle up to 2 mm, the lateral load resistance of the model displayed a stiff rise, whereas during the later displacement cycles the capacity curve flattened until sudden failure. Clearly, there is a considerable influence of the presence of a door opening on the lateral load behavior of URM wall.
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(c) (d)
Figure 4.12. Lateral load testing of Model 2: (a) crack formation at 6 mm, (b) (c) closer view of crack at the base, and (c) closer view of crack near the top of the door opening.
Figure 4.13. Hysteresis and capacity curve obtained for Wall 2.
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Lateral Load (kN)
Displacement (mm)
Figure 4.14. Cumulative energy dissipation curve obtained for Wall 2.
4.3.1.3. Wall 3 (Model 3)
Model 3 was the URM wall specimen consisting of a central window opening of size 0.9 m × 1.2 m. The cyclic test on Model 3 also resulted in a mixed failure of flexure and sliding shear type (Figure 4.15). Unlike other walls, small cracks started appearing quite early in Model 3 at 1 mm displacement level in the bottom brick course. At 3 mm displacement level, the cracks at the bottom layer widened further extending towards the middle portion of the wall, and by 12 mm displacement level, the cracks passed throughout the length of the model. However, no visible cracks were observed in any other locations. Generally, cracks were expected to form at the corners of the opening due to stress concentration as observed in Wall 2. Therefore, it appears to be a failure due to flexure at the bottom, followed by sliding due to the presence of a weaker course of masonry at the base of the specimen. The hysteresis curve, strength envelop curve, and energy dissipation curve generated from the test results are shown in Figure 4.16 and Figure 4.17, respectively.
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Figure 4.15. Lateral load testing of Wall 3: (a) crack along the whole length of the wall at the bottom at 12 mm displacement level, and (b) closer view of the crack formed at the wall bottom.
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0 1 2 3 4 5 6
Energy Dissipation (kNmm)
Displacement (mm)
Similar to Wall 2, severe pinching was also observed in the hysteresis loops of Wall 3, resulting in lower energy dissipation capacity of Wall 3 compared to Wall 1. But, higher deformability of Wall 3 kept its energy dissipation capacity higher (more than two times) than that of Wall 2. In the case of Wall 1, Fig. 4.9 shows the formation of bed joint shear crack at the top of the wall for a partial length of the wall. The RC slab above the wall did not slide during the test. The crack formed much later than the flexural crack at the base of the wall. Whereas, in the case of Wall 3 (Fig. 4.15), the flexural cracks formed at the bottom of the specimen rather than near the corners of the window openings because of low axial load acting on the wall. It has been observed in several past studies (e.g., Javed et al. 2015, ASCE/SEI 41-13 2014; Magenes and Calvi 1997) that axial loads acting on the walls significantly influences the damage pattern of masonry walls under in-plane loading.
Figure 4.16. Hysteresis curve and strength envelop curve obtained for Wall 3.
Figure 4.17. Cumulative energy dissipation curve of Wall 3.
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Lateral Load (kN)
Displacement(mm)
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0 5 10 15
Energy Dissipation (kNmm)
Displacement (mm)
4.3.1.4. Comparison of Response of URM Walls
Figure 4.18 compares the experimental lateral load responses of all the three walls. It can be noted from Figure 4.18 and Figure 4.19 that the lateral load capacity of the wall without any opening and wall with window opening does not differ much, though a significant reduction in energy dissipation capacity is visible due to the presence of opening (Figure 4.20). As shown in the comparison (Fig. 4.19), the lateral strength of the solid wall and the wall with window opening was more of less identical, though the wall with a window opening (Wall 3) had a marginally higher lateral strength in the Push direction. The minor difference may occur in such large-scale testing due to variety of reasons, for example, variation in masonry tensile strength, presence of RC lintel above window opening, etc. It is evident from the figure that the presence of openings reduces the energy dissipation capacity of the walls drastically. The energy dissipation capacity of Wall 1 specimen was higher than that of the Wall 3 specimen because the hysteresis curves obtained for Wall 1 specimen were more stable with significantly lesser pinching compared to Wall 3 (Fig.
4.18). The higher pinching in Wall 3 was obviously a result of presence of the window opening that in turn resulted in discontinuity and change in the load/stress path in the masonry wall. Therefore, the window size can be optimized to reduce the negative influence of openings on energy dissipation capacity. IS 4326 (BIS 1993) suggests providing a smaller opening size for improved lateral load capacity. The location of the opening also significantly affects the lateral strength of the structure. But if located centrally, the present results show that a standard window size of 0.9 m × 1.2 m, will not result in a significant reduction in the strength of the wall.
Figure 4.18. Comparison of hysteresis curves of three walls.
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Lateral load (kN)
Displacement (mm)
Wall 1 Wall 2 Wall 3
Figure 4.19. Comparison of capacity envelop curves of three walls.
Figure 4.20. Comparison of cumulative energy dissipation curves.