## 3 Results and Discussion
### Seismic Dynamic Analyses of Unidirectional Eccentric Structures
#### 3.2.1 Response of the Unidirectional Eccentric Structure under Unidirectional Mainshock
The influences of aftershock and the eccentricity of structures on the seismic responses are discussed. Figure 13 shows the seismic responses of structures under mainshock-aftershock sequences with different mainshock PGAs, including maximum displacement (as shown in Figure 13a,c,e) and IDR (as shown in Figure 13b,d,f). For each mainshock PGA, the PGA of the aftershock is scaled to 0.6, 0.8, 0.9 and 1.0 times that of the mainshock. Figure 11: Relative torsional effects of the structural models. Figure 6:0 Maximum IDR of structural models in the Y direction. Figure 6:1 Seismic responses of structures under mainshock–aftershock sequences with different mainshock PGAs (case UUS): (**a**) Top floor maximum displacement with mainshock PGA at 0.15 g; (**b**) IDR with mainshock PGA at 0.15 g; (**c**) top floor maximum displacement with mainshock PGA at 0.25 g; (**d**) IDR with mainshock PGA at 0.25 g; (**e**) top floor maximum displacement with mainshock PGA at 0.32 g.  
Moreover, the maximum displacements during the MSAS sequence and the mainshock are extracted. The ratios of structural maximum displacement during the MSAS sequence to that during the mainshock are presented in Figure 14a,c,e, corresponding to the mainshock PGAs which are 0.15 g, 0.25 g, and 0.32 g, respectively. Similarly, Figure 14b,d,f show the ratios of structural IDR during MSAS sequences to that during the mainshock when the mainshock PGAs are 0.15 g, 0.25 g, and 0.32 g, respectively. Figure 6:2 Ratios of maximum displacement and IDR under mainshock–aftershock sequences with different mainshock PGAs (case UUS): (**a**) Ratio of maximum displacement of aftershock to that of  
mainshock with mainshock PGA at 0.15 g; (**b**) ratio of IDR of aftershock to that of mainshock with mainshock PGA at 0.15 g; (**c**) ratio of maximum displacement of aftershock to that of mainshock with mainshock PGA at 0.25 g; (**d**) ratio of IDR of aftershock to that of mainshock with mainshock PGA at 0.25 g; (**e**) ratio of maximum displacement of aftershock to that of mainshock with mainshock PGA at 0.32 g; (**f**) ratio of IDR of aftershock to that of mainshock with mainshock PGA at 0.32 g.  
As illustrated in Figure 14a, at the same eccentricity, the ratio of aftershock displacement to mainshock displacement exceeds 1.0 and increases gradually with an increase in aftershock magnitude. It shows that the displacement during the aftershock is larger than that during the mainshock. When the scaling factor of aftershock amplitude is equal to 0.8, the displacement ratios increase significantly, except for the structural models with eccentricities of 25% and 30%. While the scaling factor is 1, the displacement ratios of almost all structures are increased, and the maximum ratio is increased by about 1.4 times. This means that if an aftershock with the same intensity as the mainshock (PGA 0.15 g) is encountered, there is a high probability that the displacement of the eccentric structure during the aftershock will exceed the maximum displacement during the mainshock. In addition, the amplification effect is more prominent in high eccentric structures, up to about 1.4 times more prominent. Figure 14b indicates that the effect of aftershock on the 15% eccentricity structural model is the most significant. When the magnitude of aftershock is equal to that of the mainshock, the maximum IDR of the aftershocks is 1.5 times that of the mainshock. Compared with the displacement, the effect of scaled aftershocks can be more clearly reflected in the maximum IDR. When the mainshock PGA is 0.25 g, as shown in Figure 14c, the ratios of the maximum displacement response show an upward trend with the increase in aftershock amplitude in each eccentricity. All the models' displacement response ratios significantly increase when the aftershock to mainshock ratio is 0.9, with the exception of the structural model with a 30% eccentricity. All structural models have significantly improved displacement response ratios when the mainshock to aftershock PGA ratio is 1.