Incremental Dynamic Analysis (IDA) is a promising approach that has recently risen to meet the needs of performance-based seismic analysis. The first important phase of IDA is the choice of IM and DM. IM increased progressively with respect to the scale factor for each GM record. IMs of seismic activity are measured in terms of Peak Ground Acceleration (PGA), Peak Ground Velocity (PGV) or 1st mode spectral acceleration at 5% damping Sa(T1,5%). The Sa(T1,5%) accord more consistent outcomes than any other IM. [R.P. Dhakal, et al. (2007)]. The seismic response of the smaller fundamental period structure is controlled by Sa(T1)which reveals both ground and structural response [N. Luco and C.A. Cornell (2007)], hence it is used as an IM in the present research. The structural responses subjected to horizontal loads like base shear, extreme drift value, and extreme inter-story drift can be indicated with reference to Damage Measure (DM). The selection of DM is governed by the objective of the analysis, for example, lateral deformations and inter-story drifts are the utmost general for dynamic analysis [T.L. Kravasilis, et al. (2006), X. Ramao, et al. (2013)]. Here, extreme inter-story drift is utilized as the DM as many of the design codes have prescribed different limit states based on extreme inter-story drift values.
To execute IDA and obtain consistent results, a set of GM records must be chosen. The amount of GM records to employ in nonlinear time history analysis (NLTHA) is a topic of debate. The sort of structure responses i.e. distribution or mean value of responses are essential, the accuracy of the response, the probable degree of inelastic response, and the expected estimation of collapse response are the influencing factors to decide the number of GMs. As a result, the quantity of GMs in each research differs. According to Vamvatsikos and Cornell (2002), 10 to 20 GMs are generally adequate to predict seismic performance with acceptable precision for medium-rise buildings, whereas Haselton, et al. (2011) recommended at least 7 GMs for the IDA. In this research, 10 near-field GMs are selected as per FEMA 695P criteria.
Table 6
Set of ten near-field ground motion records
GM
|
RSN
|
Event
|
Magnitude
|
Rjb(km)
|
PGA (g)
|
1
|
77
|
San Fernando
|
6.61
|
0
|
1.22
|
2
|
126
|
Gazli USSR
|
6.8
|
3.92
|
0.86
|
3
|
143
|
Tabas Iran
|
7.35
|
1.79
|
0.85
|
4
|
495
|
Nahani Canada
|
6.76
|
2.48
|
1.10
|
5
|
825
|
Cape Mendocino
|
7.01
|
0
|
1.49
|
6
|
864
|
Lander
|
7.28
|
11.03
|
0.27
|
7
|
1077
|
Northridge
|
6.69
|
7.28
|
0.75
|
8
|
1111
|
Kobe Japan
|
6.9
|
7.08
|
0.48
|
9
|
1602
|
Duzec Turkey
|
7.14
|
12.02
|
0.80
|
10
|
4895
|
Chuetsu Oki
|
6.8
|
0
|
1.12
|
The actual GMs at different places have variations in forms of PGA, frequency content and period. The next step in IDA is scaling the chosen ground motion records for definite parameters to reduce variability in capacity estimates. Many researchers [D. Vamvatsikos and C.A. Cornell (2002b), H. Shakib and M. Pirizadeh (2014)], and different codes of developed countries (ASCE/SEI 7-10-2007, FEMA P695-2009) have specified different techniques for scaling of GMs. In current research work, Vamvatsikos and Cornell (2002b) prescribed a ‘simple technique’ for the direct scaling of GMs. In this technique, normalised spectral acceleration for the fundamental mode at 5% damping is implemented. Here, designated GMs are scaled to the fundamental natural period of the buildings individually to decrease the variation between the design response spectrum and the GM response spectrum. Here, the IM values of GMs are increased in intermissions of 0.1 for IM values.
In IDA, nonlinear time history analysis is performed using the selected set of scaled GMs for the nonlinear structural model. The scale factor is progressively increased until the structure develops a collapse mechanism.
In the present work, the IM magnitudes of accelerograms are increased in the spell of 0.1 for IM values. By plotting the results of DM and IM for each GM, IDA curves along the x-direction are developed for all three building frames. The ensuing IDA curves for maximum responses are depicted in Fig. 4.
For the BF model as shown in Fig. 4(a), IDA curves are comparatively yielded at lower values of IM [Sa(T1,5%)g < 3.5] indicating low resistance to seismic loading. The plots for BMIF and OGS models as represented in Fig. 4(b) and (c) are comparatively yielded at higher values of IM [Sa(T1,5%)g < 5 and 4 respectively] in the linear range. With a further increase in IM, the structure loses stiffness rapidly. This behaviour indicates that due inclusion of infill, the strength and stiffness of the structure considerably increased in the linear range. For the OGS model, because of the soft story mechanism, the stiffness and strength of columns on the parking floor reduce and thus show poor performance as compared to the BMIF model.
The variation of the upper and lower limit of DM or IM for a selected value of IM or DM is known as dispersion. The dispersion is estimated for all three structures in the X−direction for both DM at the specified IM (= 1.0 g) and IM at the specified DM (= 4% inter−story drift), with the results shown in table 7.
Table 7
DM and IM dispersions for the three building models
Model
|
DM-Dispersion (%)
|
IM-Dispersion(g)
|
BF
|
2.81
|
2.25
|
BMIF
|
0.31
|
1.9
|
OGS
|
0.47
|
2.0
|
From Table 7, the dispersion of damage measures (DM) are less than 88.97% and 83.27% respectively in the case of BMIF and OGS as compared to BF models. The dispersion of intensity measures (IM) are less by 15.56% and 11.11% respectively for the BMIF and OGS as compared to BF models. Thus the provision of infill improves the robustness of the building responses and hence causes predictability in design.