## 4 Seismic Response of CSBs Under Uniform Excitations
### Seismic Responses of Towers and Priers  
#### 4.2.1 Accelerations Responses  
In the tests, the accelerometers are assembled on the key nodes of tower, such as on the lower-beam (T2), middle cross-beam (T3) and top node (T4). Figure 12a,b show the longitudinal acceleration responses of tower when subjected to uniform excitation of ScEL and ScTCU. As shown, the acceleration responses time histories of the different nodes are various. However, the peak time seem the same for all nodes. When subjected to NF excitations with same PGA (2.08 m/s\({}^{2}\)), maximal accelerations  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Test cases**} & \multicolumn{2}{c}{**Uniform Excitation**} & \multicolumn{4}{c}{**Non-Uniform Excitation**} \\ \cline{2-9}  & **Case1** & **Case2** & **Case3** & **Case4** & **Case5** & **Case6** & **Case7** & **Case8** \\ \hline \hline \multirow{2}{*}{
\begin{tabular}{c} Excitation \\ delay time (\(dt\)) \\ \end{tabular} } & ScEL & ScTCU & ScEL & ScEL & ScEL & ScTCU & ScTCU & ScTCUU \\  & 0 s & 0 s & 0.1 s & 0.2 s & 0.4 s & 0.1 s & 0.2 s & 0.4 s \\ \hline \hline \end{tabular}
\end{table}
Table 4: The shake table test cases.  
Figure 11: Measured accelerations of two tables subjected to uniform ScEL (Case1) (**a**) Time history (**b**) Fourier Amplitude.  
subjected to NNF motion (ScEL) seems a little larger than the PNF motion (ScTCU). The phenomenon can be explained by the response Fourier spectrum, such as Figure 13a,b. As operation frequency limitation of the test system, the frequency range of 0-50 Hz are shown in the figures. It is obviously the energy of tower top node (T4) are mainly at 6.9 Hz, 13.8 Hz, 27 Hz, which are close to the frequencies of the 2 nd, 4 th, 5 th modes. Among them, the 2 nd mode dominate the acceleration response of tower top node. However, the response of mid cross beam (T3) has more energy than top node (T4). For \(f=6.9\) Hz, the peak amplitude is 0.05 m/s\({}^{2}\) for tower top node, while that is 0.15 m/s\({}^{2}\) for mid cross beam. In addition, there are an energy crest around 20 Hz, which is close to the frequency of the 5 th mode. In the result, the acceleration response of tower mid cross beam is larger than top node. As shown in Figure 13b, the very similar phenomenon can be found in the tower response subjected to ScTCU.  
Figure 14 shows the maximum tower acceleration distribution of the 3# tower and 4# tower. For the both towers, the maximum acceleration happens on middle cross beam (T3), rather than the tower top node (T4). The reason is the tower top node is effectively restricted by stayed-cables. In the result, the vibration on tower top node always smaller than mid cross beam under the longitudinal excitation. In general, from the point of view of longitudinal seismic response, the tower can be equivalent to a cantilever column with limited restriction on top node. When subjected to the ScEI, the acceleration responses seem a little larger than ScTCU. It should be noted that the acceleration for the same node of the two towers seems a little different. This may come from the manufacturing differences between the two towers or bearings.  
Figure 12: Accelerations time history of 3# tower (**a**) subjected to ScEL; (**b**) subjected to ScTCU.  
Figure 13: Accelerations Fourier amplitude of 3# tower (**a**) subjected to ScEL; (**b**) subjected to ScTCU.