the development of uneven foundation settlement, the pagoda has deteriorated severely. During the years of 1981 to 1986, Chinese civil engineering community carried out an important intervention project to restore this ancient pagoda which was then on the verge of collapse. At conclusion of the project, the foundation settlement and the structure inclination have been well stabilized (Yuan Jianli, et al. 2004). As present, the pagoda maintains a tip deflection of 2.325 meter with an angle of inclination of 2°48'.

Figure 4. Physical and mathematical models.

4.2 Primary parameters and preliminary model

Figure 3 shows the picture and plane of the Huqiu pagoda and Table 3 lists the dimensions at each storey. Based on the actual survey data and documented records (Suzhou Huqiu pagoda archives, 1988), material properties and their variation range were determined (Table 4). Because various materials were used during the different repair projects, the compressive strength of the brick masonry with larger variation at every storey, and the mortar of brick masonry is mainly a mixture of clay-lime-sand, so the elastic modulus of the pagoda is a main uncertain factor on dynamic behavior model.

First, a 3D physical model of the Huqiu pagoda was developed using AutoCAD software. The model is divided by floors and areas of known damage or retrofit of a large scale were incorporated into the model. Next, the physical model was converted into a 3D finite element model with the ANSYS finite element program. A preliminary model of the Huqiu pagoda, as shown in Figure 4, was established by further defining element properties, element meshing, and boundary conditions. Structural flaws of a relatively smaller scale are not included in this preliminary model. Finer meshes were used for connection and transition areas such as door holes and walls, floor slabs, joints of each floor. The number of elements in the model added up to total of 73,241 (Yao Ling, 2003), and the Power Dynamics technique is adopted for the analysis of modal parameters.

Figure 5. Test point locations.

Figure 6. Auto-power spectrum of test point 3.

Figure 5. Test point locations.

4.3 Field testing of dynamic characteristics

The dynamic characteristics of the Huqiu Pagoda, prominently its natural frequencies and mode shapes, were tested by the ambient vibration method. The main testing instrument used was an INV-306 intelligent signal collection, process and analysis system manufactured by Orient Vibration and Noise Technology Research Institute of China. It uses large scale signal collection and analysis software DASP (Data Auto Sample and Process System), and the maximum sampling frequency of the system is 100 KHz. The effective frequency scope of horizontal-velocity sensors (Type 891-II) is 0.01-100 Hz.

For the appropriate test parameters selection, the dynamic characteristics of Huqiu pagoda were estimated before the field test. According to the Eq. (2), the natural period of Huqiu pagoda is about 0.84 second, namely the first frequency is 1.19 HZ. Besides, the truncated frequency of primitive signal was taken at 20 HZ and sampling frequency at 60 HZ during the ambient vibration testing.

Seven testing points were instrumented on the pagoda with their relative elevations shown in Figure 5. Eight sensors were used for the testing, one is placed on the reference point and the other seven are placed on the floors of the first to seventh storey. The reference

Figure 7. Transfer function of test point 3.

point was fixed on the second floor (i.e., the eighth point), which was also the location of the sixth point. The signals of eight sensors were collected at the same time. All points were sampled in the X direction first, and were sampled in the Y direction again. To satisfy the request of higher orders frequencies to test time, each sampling time is more than 20 minutes.

The auto-power spectrum as well as the transfer function at every test point (shown in Figs 6 and 7 for testing-point 3, respectively) was analyzed. The natural frequencies and mode shapes were obtained by the software DASP using the improved EFDD technique (Ying Huaiqiao and Liu Jinming, 2002) to fit testing data (shown in Figure 8). Tables 5 and 6 list the natural frequencies and mode shapes of the first four orders respectively.

4.4 Modification of modal parameters

By comparing and analyzing on the sensitivity of main structural parameters such as the tower eaves, leaning

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