3.1 Test phenomenon
The the PGA of 0.113g in Z direction: In the fortification earthquake of 6 degree stage, El-Centro wave, Taft wave and Artificial wave were input according to loading sequence. In the test process, the vibration of the whole model was very small, and no visible crack was found on the surface of the concrete cylinder, all the members of the copper structure did not buckle and no obvious deformation. The experimental phenomena showed that the model structure was in the elastic stage under fortification earthquake of 6 degree.
The the PGA of 0.158g in Z direction: In the frequent earthquake of 8 degree stage, El-Centro wave, Taft wave and Artificial wave were input according to loading sequence. The overall vibration of the model structure was obvious, some micro-cracks were formed on the surface of the concrete cylinder, and all the cracks appeared at the height of 0-2m, as shown in Fig. 12. Most of them were horizontal and several were diagonal, the length and width of every micro-crack was short and narrow, and all of the cracks were discontinuous. In the meanwhile, no visible local buckling or damage occurred in the copper structure. The white noise test showed that the vertical natural frequency of the model structure was attenuated slightly, but the reduction was small, which indicated the model structure was still in elastic stage, or may reach the edge of the elastic stage.
The the PGA of 0.225g in Z direction: In the fortification earthquake of 7 degree stage, El-Centro wave, Taft wave and Artificial wave were input according to loading sequence. The vertical amplitude of the model structure increased significantly, the vertical cracks appeared near the opening at the bottom of the model, and a series of cracks developed near 6-7m of the concrete cylinder, some of them extended to 300mm, as shown in Fig. 13. There was no obvious local buckling or damage occurred in the copper structure, which indicated the copper structure still kept good performance. The vertical natural frequency of the structure decreased further after white noise test, the results showed that the concrete cylinder may enter the elastic-plastic stage.
The the PGA of 0.900g in three directions (X: Y: Z= 1.00: 0.85: 065): In the rare earthquake of 8 degree stage, El-Centro wave, Taft wave and Artificial wave were input according to loading sequence. Under the action of three-way seismic waves, the overall model shaked violently and the seismic responses were strong. The cracks at the 3m and 7m of the concrete cylinder continued to develop into horizontal circumferential cracks, the length and the width of the horizontal circumferential cracks were almost up to 3.5m and 0.8mm respectively (Fig. 14). Many new cracks appeared on the surface of the concrete cylinder due to the action of horizontal earthquake. In the copper structure, some columns and diagonal braces buckled, the overall stability reduced because of the buckling of members. But no visible deformation appeared in the horizontal beam, which meant the seismic responses of beam was small. By the white noise test, the natural frequency of model structure attenuated greatly, which indicated the stiffness of the model structure decreased at this time. Furthermore, the model did not collapse under high intensity earthquakes, it still remained bearing capacity.
3.2 Dynamic characteristics
Before and after each test stage, white noise test was carried out on the model structure to obtained the natural frequencies and damping ratios (Table 7) calculated by the transfer function and the half-power bandwidth method (Papagiannopoulos et al. 2011). To quantitatively analyze the change of stiffness, the stiffness degradation coefficient η can be defined based on Equation (1) and (2). The first order mode factor can be obtained by normalizing the maximum displacement vector. Based on the mode factor, the first-order mode shape in vertical is shown in Fig. 15.
(1)
(2)
Where: k0 is the initial stiffness of the model, and k is the stiffness at the end of each loading condition; m is the total mass ; f0 is the initial natural frequency, and f is the natural frequency at the end of each loading condition.
The first-order natural frequency in vertical of the model structure is relatively large, which is also in line with the general characteristics of high-rise structures: the vertical stiffness is significantly large, and the vertical vibration is close to high-frequency vibration (Li et al. 2011). With the increase of PGA, the vertical natural frequencies of the model structure gradually decreases. Before the PGA of 0.225g was input, the natural frequencies decreased slowly, and it begun to reduce quickly after the fortification earthquake of 7 degree stage. The reason is that no damage occurred on the model structure or the damage was not severe before the PGA of 0.225g, while the damage became serious after the PGA of 0.225g was input. After the test, the vertical natural frequencies of first three orders of the model structure reduced to 16.04 Hz, 17.38 Hz and 18.33 Hz respectively, which were decreased by 13.5%, 13.6% and 17.2% compared with the frequency before the test. The damping ratio increased gradually with the increase of PGA, especially after the input of the three dimensional seismic waves, the damping ratio increased from 0.045 to 0.096, which exceeded 0.005 that is damping ratio of RC structure defined in Chinese specification (Load code for the design of building structures GB 50009-2012). Due to the damage of the model structure, the stiffness gradually decreased, and it decreased by 25.23% until the test was finished. Under the vertical earthquake, the vibration of the model structure is axial, it causes the vertical seismic force in model. The concrete cylinder is subjected to compression or even tensile stress, which results in horizontal circumferential cracks appear at the height of 3m and 7m, indicating that the two sections are dangerous part of the model structure.
3.3 Acceleration response
By analyzing the acceleration response at each measuring point of the model structure, the maximum acceleration at different heights under different PGA was obtained (Table 8). The acceleration amplification factors of the model structure under each working condition were further calculated by Equation (3), which is the ratio of the measured peak acceleration to the same direction peak acceleration at the base. It is an important indicator of the dynamic response of the model structure.(Fig. 16).
(3)
Where: βi is the acceleration amplification factor at different height, ai, max is the maximum acceleration at measuring point, a0, max is the maximum acceleration of the shaking table.
Table 8 lists the maximum acceleration of model structure under different PGA. With the increase of PGA, the maximum acceleration also increase. Under the action of different PGA (0.113g, 0.158g, 0.225g, 0.900g), the average maximum acceleration are 0.401g, 0.519g, 0.713g and 1.979g, respectively.The receiver tower model consists of concrete cylinder and copper truss, but the acceleration responses of the two substructures are quite different. Under the same earthquake, the acceleration response of the copper structure is smaller than that of the concrete cylinder (Fig. 16), this is may attribute to the vertical whiplash effect that could reduce the acceleration response of copper structure.
The acceleration response of the concrete cylinder: Acceleration reaches the maximum value at the height of 2-8m under all seismic waves. Meanwhile the acceleration amplification factors under the artificial waves are the largest among the three seismic waves, indicating that different types of seismic waves have different effects on the structural response. As the PGA of the input seismic waves gradually increase, there is no obvious change of the envelope diagrams of acceleration amplification factor, which has rarely been found in previous studies. As can be seen, β is the ratio of the PGA at measuring points to the PGA at the base, it is possible to present the same value under vertical earthquake. Under the PGA of 0.113g, 0.158g, 0.225g, all of the acceleration amplification factors are less than 1.0 at the height of 11m, while it is larger than 1.0 with the PGA of 0.900g. It shows that the acceleration response at the top of concrete cylinder is enhanced under the vertical earthquakes with high PGA. The acceleration response of copper structure: Under low seismic intensity, with the PGA increases, the acceleration amplification factors decreases, and it reduces to the minimum values when the PGA is 0.225g. But under high seismic intensity, for example, with the PGA of 0.900g, the acceleration amplification factor of the copper structure is close to that of the concrete cylinder, the reason may be that the artificial mass on the copper structure increases the seismic response when subjected to vertical earthquakes with high value of PGA.
In term of the change rule of acceleration response, the two substructures are the same. That is, the acceleration response increases first and then decreases gradually along the height, and the turning point of acceleration is located at the 2/3 height of the respective substructures, in which the acceleration response of concrete cylinder starts to decrease at the height of 8m, and the copper structure begins to decrease at the height of 12.5m. It indicates the range that from the height of 1/3 to 2/3 of each structure is the concentrated area of seismic response, which should be strengthened in seismic design.
3.4 Displacement response
Based on the second integration of the acceleration time history response measured by the acceleration sensor, the maximum displacement of the model structure at different height relative to the base under different levels of earthquakes can be obtained (Table 9). To analyse the displacement response better, the vertical displacement envelope diagrams are shown in Fig. 17.
Table. 9 and Fig. 17 show that with the increases of the PGA, the maximum displacement gradually increases, especially when the PGA is 0.900g, the displacement is almost twice compared with that of the PGA of 0.225g. The displacement response of the model structure reaches its maximum at the height of 4m, and the maximum vertical displacement is 4.96mm under the artificial wave. In view of the overall model, displacement response of concrete cylinder is larger than that of copper structure, and the displacement response is generally larger in the range from 4m to 7m, which shows that the displacement response of the model structure under vertical earthquake is mainly concentrated in the concrete cylinder. That may account for the severe damage of the model at the height of 3m and 7m, which is consistent with the test phenomenon. But generally speaking, the vertical displacement response of the model structure is always at a low level, indicating that the axial deformation of the model is not obvious under the vertical earthquakes. The displacement response of the connection part between copper structure and concrete cylinder is almost the same, which shows that the connection is reliable and the performance is good.
The displacement response characteristics of the concrete cylinder are similar to that of the acceleration responses, that is to say, the displacement response increases first along the height and then decreases gradually. The displacement turning point is located at the 2/3 height of the concrete cylinder, and the displacement begins to decrease at 7m, illustrating the range of concentration area of the displacement response, which should be paid more attention. The vertical displacement of the copper structure is relatively small, and it is so tiny that can be neglected compared with the deformation ability of copper material.
3.5 Vertical seismic force
According to the acceleration response and structural mass distribution of the model structure, the vertical seismic force of the model structure under different levels of earthquakes is calculated. The specific results are shown in Table 10, and the envelope diagram of the vertical seismic force is depicted in Fig. 18.
Fig. 18 shows the vertical seismic force increases with the increase of PGA. Under different PGA action, the maximum vertical seismic force occurs at the height of 2m. The main reason is that both of the acceleration and mass distribution are the largest at this position. It gradually decreases along the height and reduces to the minimum at the top of tower. Under the action of seismic waves with the PGA of 0.900g, the maximum vertical seismic force of the concrete cylinder at the height of 2m is 145.6kN, which is about 18 times compared with the maximum vertical seismic force of the copper structure whose maximum value is 7.8kN at the height of 12.5m. As shown in Fig. 18, the vertical seismic force of the model structure is mainly concentrated within the height of 2m-8m. This is because the vertical acceleration and mass distribution change along the model structure, causing the vertical seismic force to show the above-mentioned change trend. The damage of the concrete cylinder is also concentrated in this area, and the vertical seismic force is considered as the reason of structural failure.