engleski

Assessing bridge serviceability by evaluation of dynamic parameters

Deterioration of concrete bridges presents a major concern in countries worldwide. In Croatia structural degradation is enhanced by aggressive maritime environment with frequent high winds. Chloride attack followed by cracking, delamination, splitting and peeling off of concrete is identified as major deterioration mechanism. Nevertheless, structural damage cannot be attributed only to severe exposure conditions as design errors and construction faults significantly contributed to rapid degradation of concrete bridges. The paper shall discuss implementation of monitoring dynamic parameters of a bridge for assessing bridge serviceability. The advantage of such approach is that bridge condition is evaluated in non-destructive manner. For this purpose either permanent or temporary instrumentation may be utilized. In most cases, accelerometers are used for measurements. Dynamic loading is provided by heavy test vehicles crossing the bridge at different speeds and by wind effects. In-situ measurements of dynamic behavior are used to obtain modal response data. Key dynamic parameters such as natural frequencies, mode shapes and structural damping are determined from the measured data. Analytical finite element model is developed according to execution drawings, if available, and subsequently calibrated according to the test results. The calibrated analytical model enables evaluation of stiffness and load carrying capacity of damaged structure, and assessment of bridge serviceability. These kind of dynamic tests are supposed to be conducted in condition assessment procedures, which are to be set for all Croatian highways. The first dynamic testing is to be conducting during static and dynamic load testing before bridges opening to the traffic. The results of initial tests represent pivot data for further examinations of the bridge that are set in national bridge management system. Measuring the vertical displacements of the bridges experimentally is not a trivial matter. Displacement transducers measure relative displacements and thus need a stationary reference base. Establishing such a base requires access under the bridge, which is not always possible. Even where it is possible to access under the bridge, the height of some bridges makes the installation very difficult and impractical. During tests experimental data on three dynamic parameters were obtained, and results are shown in following diagrams with respect on age of the overpass. The described procedure was successfully applied for serviceability assessment of many bridges in Croatia and can be deemed as an efficient and reliable technique. If applied in regular time intervals, according to the predefined maintenance plan, it provides identification of anomalies in structural performance, thus indicating structural changes or damage.

bridge; assessment; dynamic testing; natural frequency; damping

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