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Experimental and analytical model analysis of Babolsar's steel arch bridge

Beygi, M. H. A ; Sharif University of Technology | 2006

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  1. Type of Document: Article
  2. DOI: 10.1201/b18175-84
  3. Publisher: Taylor and Francis/ Balkema , 2006
  4. Abstract:
  5. The paper presents the experimental and analytical model analysis of a steel-girder arch bridge. The field test is carried out by ambient vibration testing under traffic excitations. Both the peak picking method in the frequency domain and the stochastic subspace identification method in the time domain are used for the output-only model identification. A good agreement in identified frequencies has been found between the two methods. It is further demonstrated that the stochastic subspace method provides better mode shapes. The three-dimensional finite element models are constructed and an analytical model analysis is then performed to generate natural frequencies and mode shapes in the three-orthogonal directions. The finite element models are validated to match the field natural frequencies and mode shapes. This paper concentrates on both experimental and analytical model analysis of a steel arch bridge over the Babolrood River located in Babolsar in Iran. Field model testing was carried out using ambient vibration testing under "natural" excitations induced by traffic & crossing truck (weight = 32 ton). The main parts of Babolrood River Bridge are steel arches. The bridge is a steel arch with a length of 90 m. This bridge was designed about 70 years ago. The superstructure of the bridge consists of the vertical and lateral load carrying systems, and the deck system & the horizontal bracing system. The arch span consists of 15 diagonally braced members. The main attached on both sides of the arch and the floor system is suspended through these. The floor system consists of about 200 mm thickness concrete slab supported by five longitudinal stringers. The stringers are placed on the transverse built-up floor beams and the bearing consists of pin and roller combinations to allow rotation and translation. The top shoe of this bearing is connected to the bottom flange of the steel girder, which is then connected to the pin. The slots in the bottom flange of the steel girder allow translation are forced vibration tests and ambient vibration tests. Forced vibration tests are directly related to the application of standard techniques of experimental model analysis in which the structure is excited by artificial means such as shakers or drop weights. Relatively long records of response measurements are required and the amplitude of measurements is relatively small in the ambient vibration test. The field model testing of the arch steel Babolrood River Bridge was carried out using the method of ambient vibration. The equipment used to measure the response consisted of triaxial seismometers linked to their own data acquisition system. The system contained digital recording strong motion velocity graph. A lock was positioned at each station with the seismometers oriented in the vertical, transverse and longitudinal directions. Seismometers were connected to the data acquisition system by shielded cables & connected to GPS. All measurements were taken by placing the instruments on the pavement due to the limited access to the actual floor beams and the testing time constraints involved. Measurement stations were chosen at each joint (panel point) of suspenders connected to the deck. As a result, a total of 16 locations (8 points per side) were measured. Three test setups were conceived to cover the planned testing area of the arch of the bridge. A reference location, here in after referred to as the base station, was selected based on the mode shapes from the preliminary finite element model. Each setup consisted of four base triaxial seismometers stations and four moveable triaxial seismometers stations. Data from three test setups for each of the right-hand lane and left-hand lane were measured. Once the data were collected in one setup, the moveable stations were moved to the next locations while the base stations remained stationary. Each setup yields a total of twelve sets of data from moveable stations and twelve sets of base station data. This sequence was repeated three times to get measurements on all stations. The sampling frequency on site was chosen to be as high as 1,00 Hz to capture the short-time (higher-frequency) transient signals of the ambient vibration in detail. The ambient vibration measurement was simultaneously recorded for 360 s at all seismometers, which resulted in total 18000 data points per data set (channel). During one of the tests, normal traffic was allowed to flow over the bridge at normal speeds. The sampling rate of 1,00 Hz is too high for the frequency range of interest. However, with the development of data acquisition systems of high computer speed and capacity, there is no difficulty in working with a high sampling rate. Re-sampling and application of a low-pass filter can be easily done by the digital computer and software in the office, but the field test is not easily repeatable. A higher sampling rate in the field provides the possibility to study the effect of sampling rate on the extraction of the model properties. Ambient excitation does not lend itself to FRFs or IRFs calculations because the input force is not measured in an ambient vibration test. Therefore, a model identification procedure will need to base itself on output-only data. Two complementary model analysis methods are implemented here. They are the rather simple peak picking (PP) method in the frequency domain and the more advanced stochastic subspace identification (SSI) method in the time domain. Although the input forces are not measured when performing ambient vibration measurements, this problem can be circumvented by adopting an adapted model identification technique. In this technique, the base station (reference) signal is used as "input" and the corresponding FRFs and coherence functions are computed for each response measurement point with respect to this station. The measured data are first de-trended which enables the removal of the dc components that can badly influence the identification results. A 1,00 Hz sampling frequency on site results in a frequency range from 0 to 50 Hz. For most bridges, however, the frequency range of interest lies between 0.5 and 15 Hz and contains at least the first ten natural frequencies. For the peak picking method in the frequency domain, the ANPSDs for all measurement data without decimating are shown in Fig. 8. The peak points are clearly shown and then the frequencies can be picked up. Though the PP method is fast and provides a reliable identified natural frequency in most cases, it sometimes cannot yield enough good mode shapes. Three-dimensional linear elastic finite element models of the arch span of the Babolrood Bridge have been constructed using LUSAS 13.5 finite element analysis software. The model is developed for both the analytical model analysis and earthquake response analysis, and represents the structure in its current as-built configuration. The arch members, girders, stringers, floor beams, and bracing members are modeled by two-node beam elements that have three translational degrees of freedom (DOFs) and three rotational DOFs at each node. It can be seen that the displacements in experimental method higher than analytical model. The FE analytical model analysis was validated by experimental model analysis in terms of natural frequencies and mode shapes. Theoretically, a perfect model would match all experimentally determined mode shapes and frequencies exactly. In practice, it is not expected to be a perfect match between all analytical and measured model properties. Therefore, only the most structurally significant modes and frequencies are used in the comparison process. In addition, the higher modes identified through ambient vibration measurements are not reliable since the higher modes are not excited sufficiently. To facilitate the seismic evaluation/retrofit of the Babolsar River Bridge on, the dynamic properties have been studied by analytical model analysis with the 3D finite element method and by experimental model analysis with ambient vibration testing. A relatively simple model testing procedure on a real and large bridge under actual working conditions is presented. The potential usefulness is being able to verify analytical models and to monitor the health performance for large bridges. © 2006 Taylor & Francis Group
  6. Keywords:
  7. Analytical models ; Application programs ; Arch bridges ; Arches ; Base stations ; Concrete slabs ; Costs ; Data acquisition ; Degrees of freedom (mechanics) ; Digital computers ; Electric measuring bridges ; Finite element method ; Flanges ; Floors ; Frequency domain analysis ; Instrument testing ; Life cycle ; Location ; Low pass filters ; Maintenance ; Natural frequencies ; Rivers ; Seismographs ; Seismology ; Software testing ; Steel beams and girders ; Steel bridges ; Steel testing ; Stochastic models ; Stochastic systems ; Stringers ; Vibration analysis ; Vibration measurement ; Earthquake response analysis ; Finite element analysis software ; Natural frequencies and modes ; Stochastic subspace identification ; Stochastic subspace identification methods ; Stochastic subspace methods ; Three dimensional finite element model ; Three translational degrees of freedoms ; Time domain analysis
  8. Source: 3rd International Conference on Bridge Maintenance, Safety and Management - Bridge Maintenance, Safety, Management, Life-Cycle Performance and Cost, Porto, 16 July 2006 through 19 July 2006 ; 2006 , Pages 235-237 ; 0415403154 (ISBN); 9780415403153 (ISBN)
  9. URL: https://www.taylorfrancis.com/chapters/edit/10.1201/b18175-86/experimental-analytical-model-analysis-babolsar-steel-arch-bridge-beygi-kazemi-lark-tabrizian