Study of Modal Analysis Techniques to Evaluate the Damage Condition of RC Bridges with Heaving Moving Loads

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1 Study of Modal Analysis Techniques to Evaluate the Damage Condition of RC Bridges with Heaving Moving Loads S.A.C.N Wimalasuriya 1, Rajapaksha R.G.S.H 1, De Silva Sudhira 1 and De Silva Subashi 1 1 Department of Civil and Environmental Engineering Faculty of Engineering University of Ruhuna Hapugala, Galle SRI LANKA nisanchamith@yahoo.com,hesharahr@gmail.com Abstract: Reinforced concrete bridges that are constructed in highways and expressways have major influence on transportation sector of any country. However, due to long term deterioration, after facing extreme weather conditions and increasing of traffic loads many bridges are now operating at an overload beyond their design capacity and as a result these bridges are tend to damage. The damages are sometimes immediately visible but most of the time structural damages are hidden within the bridge. Still this has a risk of health and integrity of the bridge. The hidden damages are very difficult to identify through visual inspections and expensive to repair. This research presents the method of evaluating the current condition of four number of reinforced concrete bridges by assessing dynamic characteristics of the bridge. For the experimental measurements, tri-axial accelerometers were used where displacements were calculated from the acceleration waveforms to obtain the corresponding mode shapes. The natural frequencies were obtained by Fast Fourier Transformation of the analysed software. To simulate those modal parameters in the model of the structure, an applicable FEM was developed and validated with the present vibration data. By considering both result taken from real acceleration measurements and the model, form of the bridge was evaluated by doing modal analysis where the natural frequency and the mode shape were utilized as the two governing factors to assess the condition and the damage location of the bridge. Keywords: Damage Identification, Condition Evaluation, Modal Analysis, Structural Health Monitoring, Dynamic Characteristics. 1. INTRODUCTION Condition assessment or the evaluation of the current condition of a bridge to have a regular maintenance is really vital nowadays due to extreme weather condition and the increment of the traffic loads. In order to ensure their reliability, and particularly their stability and serviceability, it is important to analyse the bridge structure loaded by static and dynamic excitation. The condition evaluation of bridges initiate with the visual inspection of the bridge to determine the present condition (Petroff et al. 2012). The maintenance of bridge structures in a transport network is essential in order to make sure safety, in addition to providing cost-effective maintenance operation of the network to save replacement cost by maximum utilization of service period. However, the bridge management authorities in some countries have established a computer based system for the bridge management which is use to maintenance, rehabilitation and replacement of bridges. Currently, Structural Health Monitoring (SHM) in most of the developed countries is characterized by traditional visual inspection together with referencing of old assessment reports to maintain an accurate account of the bridges anagement have been developed in several developed countries of existing bridges to have a proper bridge management (Kawamura 2001). Major threats that a bridge can undergo primarily consist of the aging of the structural elements or deterioration, extreme loads such as those caused by seismic events or high winds and motor vehicle accident. Normally bridges are directly experienced severe environmental conditions and deterioration could occur with time. When a vehicle passes over a bridge, movement may generate in to tri-axial direction base on bridge stiffness. With the Deterioration, stiffness of bridge gets worse and eventually movement of the bridge can be increase. Uneven moment than design values can make bridge to collapse at the end. Tendency of recent and future research in this area is the development of new techniques of bridge assessment through the monitoring of dynamic nature of the bridge. 402

2 This research focus on evaluating method for current condition of reinforced concrete bridges by evaluating dynamic characteristics of the bridge or by conducting a modal analysis. To analyze the present condition of concrete bridges visual inspection data combine with non-destructive testing like Rebound Hammer, acoustic, ultrasonic, electromagnetic, and radiographic methods and vibration data in constant predicted time can be more effective (Neitzel et al. 2011). Dynamic testing and continuous monitoring based on modal analysis can play an imperative role in the management of any important civil infrastructures, since the experimental tools permit the periodic or constant assessment of bridges, dams, high-rise buildings, through the analysis of their response to ambient excitation, while they are in normal operation. Dynamic testing, or modal analysis, is a method that obtained natural frequencies and mode shapes from multi-degree of freedom systems where it is one of a significant method for determining modal characteristics of a bridge (Biggs et al. 2000). Identify the mode shapes and natural frequency of the particular bridge while contrast it from the initial natural frequency and the mode shape of the bridge to compare the results and ultimately to have proper bridge rating system is the key of this research. To improve the quality of bridge health monitoring data, more advanced measurement techniques have been implemented. Fixed hardware sensors such as strain gages, accelerometers, fibre optic sensors, and extensometers are becoming increasingly common in structural health monitoring (Yasar et al 2011) however, these devices often require power, implementation of data transmission methods, and only provide information for discrete points or along a line. This study focus on evaluating a method to assess the condition of bridges by its modal parameters where bridge evaluation is performed to determine the load-carrying capacity of all critical elements, and the bridge as a whole. The structural response of bridges to dynamic loads contains common characteristics regardless of the load type and structural system. Dissimilar to the structural response to static loads, the dynamic response of a structure depends on several parameters such as material properties, damping, mass of the structure, accelerations, velocity of moving loads and modes of vibration. When a vehicle passes over a bridge, movement may generate in to tri-axis direction base on bridge stiffness (Sooriyaarachchi et al. 2015). With the Deterioration, stiffness of bridge gets lower value and ultimately movement can be increase. Uneven movement than designer allowed can happen to collapse of bridge. Natural Frequency and the mode shape of the bridge is the governing factor of this study which can be identify after receiving the raw data from bridge vibrations. 2. METHODOLOGY Vibration data, basically frequency, acceleration and displacement were collected by fixing three number of accelerometers at the location of two supports and the mid of the span of a single span on the bridge deck as shown in Figure 1. Data were captured according to dynamic load excitations by moving loads. The instrumental setup consists of these accelerometers were connected properly to the bridge deck in order to reduce any relative movements between sensor and bridge where all three inputs were connected to a universal data re The recorded given as acceleration and displacement data form and the graph of Dominant Frequency vs. Displacement Amplitude and finally the analysis was done to obtain the experimental natural frequency and the mode shape of the bridge. Experimental mode shapes were obtained by the displacement data sets given by the three measuring points of the bridge after a proper analyse Development of Validated FEM Formulation of a Finite Element Model (FEM) is a very important part of understanding bridge response to evaluate the existing bridge. One reason for creating an FEM was to document the current state of the bridge through an analytical model, thus establishing a quantitative baseline for future comparison. In that case ANSYS workbench was used to develop the FEM of the particular bridges as shown in Figure 2 by establishing initial condition of the bridge to the status of the newly constructed bridge with the help of structural drawing of the particular bridge requested from the Road 403

3 Development Authority (RDA). Displacements, mode shapes and corresponding frequencies between the FEM and test measurements are validated and better idea of the bridge condition with respect to the primary bridge is taken to ultimately rate the condition of the bridge. The recorded frequencies were used to validate the FE models in order to evaluate the current condition of the bridges. Figure 1 Experimental Setup Figure 2 FEMs of Bridges 2.2. Validating the FEM Validating the FE models with the experimental test results was very tough work at the beginning after creating the models with ANSYS. In validating the FEMs the Natural frequency in vertical direction given by the experimental test results were utilized. Initially the natural frequency in vertical direction obtained from data sets were much lower than the natural frequency given by the FEM which act as the newly constructed bridge. Because it is obvious that with the time natural frequency is getting reduce with the bridge deterioration. As an example the natural frequency given by the FEM of the Bridge 1, Bridge 2, Bridge 3 and Bridge 4 initially were 9.7Hz, 6.9Hz, 6.2Hz and 5.2Hz respectively in corresponding mode shape (1st mode shape) where in vibration data sets it was 9Hz, 6Hz, 5.7Hz and 4.2Hz respectively which represent the current bridge as shown in Figure 3. So this change between the natural frequencies were validated by making the FEM of the bridges damaged. The particular bridges were validated by following methods with the help of the visual inspection test results obtained from RDA. Changing the material properties Changing the boundary conditions Changing the cross section details Compressive strength (Concrete/Steel) Tensile Strength Poisson Ratio After these changes to the models of four bridges the natural frequencies were taken up to 5-10% of the frequencies given by the vibration data where all the four bridge models were validated with a comprehensive effort. Also the change of the boundary condition of the models made the natural frequency change in a percentage of 30% -40% which can be more reliable at the condition assessment stage and it can be identify as the failure of supports of the bridge. As the final stage, the condition assessment of these bridges have been carried out by identifying the percentage of the damage done to validate the model from the initial condition to the existing condition of the bridge. Damaged point identification was done by dividing the bridge model into ten parts and material properties were assigned each separately. Then material properties were changed in each part separately and corresponding natural frequency response was recorded. 404

4 3. RESULTS AND DISCUSSION Figure 3 Validation of Natural Frequency Three accelerometers at the two supports and the mid of a bridge span were connected as previously mentioned and these tri-axial accelerations are recorded in the data sampling rate of 1024/s. Graph of Displacement Amplitude vs. Dominant Frequency and graph of Displacement vs. Time were obtained as shown in Figure 4 and 5 respectively. Vertical Dominant Frequency 9Hz, Displacement Amplitude=0.407m Longitudinal Dominant Frequency 9Hz, Displacement Amplitude=0.312m Transverse Dominant Frequency 9Hz, Displacement Amplitude=0.251m Figure 4 Displacement Amplitude vs. Frequency Figure 5 Displacement vs. Time at Mid Span Mode Shape 1 (9.7Hz) Mode Shape 2 (16.2Hz) Mode Shape 3 (21.8Hz) Mode Shape 4 (24.2.2Hz) Figure 6 Mode Shapes of FEM 405

5 3.1. Condition Assessment After the model validation it was the final stage where the condition assessment of the particular four bridges were evaluated. At the validation stage, the bridges were damaged by the methods discussed earlier with reference to the initial condition of the bridge. At that stage the damages did to the bridges were calculated as a percentage by given a weighted average value for different structural elements of the bridge with respect to the significance like bridge deck (Weighted Average of 1.5), girders (Weighted Average of 2), brazing elements (Weighted Average of 0.5) and etc. After the calculation of the damaged did to the bridges by that method, identified the brides were damaged by these percentages shown in Figure 7. Figure 7 Damage Percentage of Bridges Mode shapes as shown in Figure 6 for bridge case 1 were compared and analysed with respect to the undamaged (initial) condition of the bridge. It was obvious that the all the four bridges were oscillated in the first mode shape in corresponding natural frequencies where it can be clearly seen from the experimental value and FEM. After that bridges were more damaged from the existing condition (from the validated model) and identify the reduction of natural frequency as a percentage and the increment of the area given by the mode shape graph as a percentage shown in Figure 8. These two values (natural frequency reduction percentage & area increment percentage of the mode shape graph) ultimately will be used to identify the damaged condition of the bridge as shown in Table 1 where it is necessary to evaluate the bridge after some years by deploying a vibration measurement of same bridge. Figure 8 Variation of Mode Shape 1 with the Damage Percentage in Bridge 1 406

6 Table 1 Damage Condition of Bridges Bridge Bridge 1 (Continuous Bridge 7m Span) Bridge 2 (Continuous Bridge 20m Span) Bridge 3 (Continuous Bridge 22m Span) Bridge 4 (Continuous Bridge 35m Span) Reduction of Natural Frequency % Increment of the Area of 1 st Mode Shape Graph % Damaged Condition % Damage Location Identification Sometime the bridge models were divided into ten parts and its material properties were changed starting from existing condition until it suppress from the bridge. The natural frequency change during this process was recorded. Same procedure was followed for other nine elements and corresponding natural frequency change was recorded. The natural frequency changing percentage vs distance was plotted and it shows in Figure 9. By using the graph of natural frequency changing percentage, damage location can be approximately identified. For an example, if the natural frequency changing percentage is 4, then the damage location is either 1.7m or 12.7m, or both along the bridge. Figure 9 The Natural Frequency Changing Percentage Vs. Distance 407

7 4. CONCLUSION Management and maintenance of existing concrete bridges is a difficult engineering concern with public safety and economic implications. Successful treatment limits the extensive effects of structural deficiency, deterioration, and destruction of bridges. Instrumentation and monitoring is the only tool that can assist the trustworthy assessment required before any involvement. Bridge structures are evaluated by ordinary techniques cannot forecast actual structural score by itself. Either Static load test or dynamic response test is the way to enhance usual expectation. Structural evaluation of existing bridges is a difficult process comprising of a series of incorporated system components and measures. Modern bridge management requires the development of integrated administrative and engineering solutions, which are not only technically and commercially feasible, but also hands-on and quick. Deficiency of a structure can be considered as an undesirable weakening of the structure that has a negative effect on its performance, and affects the safety of the structural system. It also can be undesirable stresses, displacements or vibrations in the structure. Most of current approaches on natural frequency and mode shape based structure evaluation methods are more reliable than other non-destructive methods and visual inspection data. Based on the final results the following conclusions were identified, The proposed mode shape based approach and natural frequency based approach is feasible for condition assessment in the RC bridges. Mode shape based approach can be used to decide whether the structure is damaged or not and also the damaged percentage according to the shape of the mode. Natural frequency changing percentage can be used to detect the presence of damage. From this approach the damage location can be also identified. 5. ACKNOWLEDGEMENT The authors wish to express their fullest gratitude to the Faculty Research Grant 2016 of Faculty of Engineering, University of Ruhuna, and Eng. Anil Prasad from Southern Province Road Development Authority. 6. REFERENCES pp Indian Journal of Engineering & Materials Sciences, Vol. 19, Biggs, R.M., Barton, F.W., Gomez, J.P., Mass modeling and analysis of reinforced- -R4). Damir, Z., Mirsad, T. and Radomir F., 2015, 'Identification of Modal Parameters of Bridges Using Ambient Vibration Measurements', Shock and Vibration, Volume (2015), Article ID , pp Design of Civil Infrastructure Systems, pp , Virginia, ASCE. Life-Cycle Cost Analysis and Laksiri, P., Sudhira, D. and Chandana, N., 2010, 'Structural Assessment of Reinforced Concrete Bridge Structures Exposed to Chloride Environment', International Conference on Sustainable Built Environment (ICSBE-2010), pp

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