3.3 Nonlinear Static Pushover Based Methods
3.3.2 Influence of Central Opening Size in Masonry Infill
The PO curves obtained for 3B-4S OGS frames and FI frames with different Op are shown in Fig. 3.7(a) and Fig. 3.7(b), respectively, and are compared with the PO curve
0 0.2 0.4 0.6 0.8 1
0 0.01 0.02
Seismic Weight
Top Storey Drift
Bare FI OGS
0 0.2 0.4 0.6 0.8 1
0 0.01 0.02 0.03 0.04 0.05 0.06
SeismicWeight
Top Storey Drift
Bare FI OGS
obtained for 3B-4S bare frame in order to understand the influence of infill distribution on lateral load behavior. It is observed from the comparative PO curves that the peak lateral load carrying capacity of FI frames, even with Op up to about 50%, is much higher (nearly 4 to 5 times) than that of the OGS frames as well as the bare frames. In the case of FI frame, the presence of infills in the ground storey imparts tremendous stiffness and strength to the frame as seen from Fig. 3.7(b). Infills being brittle, fail after a certain level of deformation in the frame. Thus, the PO curve for any infilled frame initially shows very high lateral stiffness and strength that suddenly drops after reaching the ultimate capacity of the infills. This behavior of the FI frame is evaluated in the present study by increasing the opening sizes in the infill panels of all the stories. The sudden drop in strength and stiffness is observed in the FI frame until an opening of 50% is provided in the infill. When the opening size is further increased to about 60%, the stiffness and strength of FI frame under the action of lateral loads reduce gradually rather than exhibiting a sudden drop (Fig. 3.8). This is because of a balance achieved between the infill wall resistance and the RC frame resistance when 60% or more Op are provided in FI frame. Such a balancing behavior leads to sharing of lateral loads between the infill walls and the RC frame from the beginning. This, in turn, prevents the sudden drop in the lateral stiffness and strength as observed before.
(a) (b)
Figure 3.7 PO curves for 3B-4S: (a) OGS frame, and (b) FI frames with different central opening sizes.
Thus, globally, a relatively more ductile behavior is achieved for the considered frame as the opening size in infill is increased to about 60%. Therefore, it can be inferred that the performance of the FI frame with 60% Op is even better than the FI frame without Op as well as the bare frame. Two more examples of a 2B-2S and 6B-6S FI frame are shown in Fig. 3.9 to see the effect of opening size in infill. Though such a behavior is observed in
0 50 100 150 200 250
0 0.05 0.1 0.15 0.2
Base Shear (kN)
Top Displacement (m) OGS BARE 90%
0%
0 200 400 600 800 1,000 1,200
0 0.02 0.04 0.06 0.08 0.1
Base Shear (kN)
Top Displacement (m) FI BARE
90%
0%
3.3 Nonlinear Static Pushover Based Methods
all the frames analyzed in the present study, the observed optimal value of Op must be used with caution as it may change due to presence of non-structural members and other components, which are not considered in the analysis.
On the other hand, the effect of Op is found to be negligible in the case of OGS frames as compared to the FI frames. There is hardly any change in lateral strength and stiffness of OGS frames with a change in Op. Thus, it is implied that the upper storey infills do not contribute significantly to the lateral stiffness and strength of the OGS frame.
A similar observation is made for frames with other configuration of NB and NS. Since the study focuses primarily on OGS frames and the effect of openings is negligible on the behavior of OGS frames, further analyses are carried out only for 0% Op, i.e., no opening in infill, and 50% Op.
(a) (b)
Figure 3.8 Effect of increase in opening size in infill of a 3B-4S OGS and FI frame from: (a) 50% to (b) 60%.
(a) (b)
Figure 3.9 Effect of Op in infill on lateral load behavior of masonry infilled RC frames.
Fig. 3.10 shows response surface plot of performance points (PPs) in terms of spectral displacement (Sd) for 3B-4S OGS and FI frames with varying Op and PGA. The
0 200 400 600 800 1000
0 0.04 0.08 0.12 0.16 0.2
Base Shear (kN)
Roof displacement (m)
OGS-50%
FI-50%
0 200 400 600 800
0 0.04 0.08 0.12 0.16 0.2
Base Shear (kN)
Roof displacement (m)
OGS-60%
FI-60%
0 200 400 600 800
0 0.05 0.1 0.15 0.2
Base Shear (kN)
Roof Displacement (m)
2B-3S 0%
10%
40%
50%
60%
80%
0 400 800 1200 1600 2000 2400
0 0.05 0.1 0.15 0.2
Base Shear (kN)
Roof Displacement (m)
6B-6S 0%
10%
40%
50%
60%
80%
PPs are obtained by carrying out CSM for the frames with increasing seismic intensity (PGA) and Op. With an increase in Op in OGS frames, spectral displacement demand (Sd) decreases marginally for a given PGA, whereas, it increases for FI frames. It is, however, noteworthy that the Sd obtained for OGS frames (with any Op) is always more than that for FI frames with same Op for a given PGA. A comparison of ductility demand (µD) for OGS and FI frames for three different Op also verifies this observation (Table 3.2). It is interesting to note that most of the FI frames (with 50-60% Op) are not expected to undergo nonlinear behavior for PGA of 0.36g (maximum design basis earthquake as per the Indian seismic code (BIS 2016a) for seismic zone V) as the µD is more or less equal to one. On the other hand, large ductility demand is imposed on OGS frames, most of which is concentrated in ground storey columns. A similar trend is observed in Sd and ductility demand (µD) for other PGA levels also.
(a) (b)
Figure 3.10 Response surface plots of performance points in terms of Sd for: (a) OGS and (b) FI frames with different Op and PGA.
Table 3.2 Ductility demand (µD) on OGS and FI frames for a PGA of 0.36g.
Frames 0% Op 50% Op 60% Op
Sdy (m) Sdu (m) µD Sdy (m) Sdu (m) µD Sdy (m) Sdu (m) µD
FI 0.013 0.012 0.96 0.018 0.018 1.00 0.019 0.021 1.08 OGS 0.021 0.059 2.81 0.021 0.051 2.43 0.021 0.05 2.38 Note: Sdy = spectral yield displacement and Sdu = spectral ultimate displacement
Thus, from the study of opening size in masonry infill walls, it is inferred that the ground storey infill walls are the active participant in controlling the lateral load behavior of masonry infilled RC frames. Therefore, lateral load behavior of low-rise and mid-rise
3.3 Nonlinear Static Pushover Based Methods
OGS buildings remains unaffected by the amount of openings in infill walls. Also, the ductility demand imposed on columns of OGS frames for any level of seismic hazard is always much higher than that on columns of FI frames as well as BF with any opening size in the infill walls.