Dynamic Tests in Bridge Health Monitoring

Bridge Health Monitoring (BHM) usually refers to the process of implementing a damage detection and characterization strategy for bridge structures management.^{[1,2,3,4,5,6,7]} Here, damage is defined as changes to the material and/or geometric properties of a structural system, including changes to the boundary conditions and system connectivity, which adversely affect the system’s performance.

Process of BHM can be generally defined as an act of acquiring, processing, communicating and archiving information about the structure actions and the actions’ effects on a structure over a given period of time.^[8,9,10]

Procedures of Bridge Health Monitoring can be divided into two main categories (Figure 1): –

load-independent procedures, comprising of regular as well as irregular (special) inspections based on visual examination and results of the non-destructive testing (NDT) and/or semi-destructive testing (SDT),

Figure 1

Load-dependent dynamic tests play a very important role in BHM, because bridge structures are exposed to various dynamic loads, for example, moving live loads, time varying wind loads and so on. The dynamic effects are taken into account while designing bridges and play an important role as one of the basic structure performance indicators during the whole lifetime. The results of the bridge dynamic monitoring and analyses carried out for many years offer valuable information for structure health assessment and bridge management.

Predefined dynamic tests under controlled loads (proof load tests) are typically applied before opening a new bridge structure to traffic or after completing major rehabilitation works. Short- and long-term monitoring is executed under ambient traffic loads and environmental impacts during operation of a bridge. All dynamic tests are based on transducers for sensing physical quantities, programmable electronic equipment for acquiring, processing and communicating data and algorithms that define how data acquisition, processing and communication is performed. Proof load tests and short-term monitoring (24 hours to few days) are usually executed by means of temporally installed sensors, while the long-term monitoring requires permanently mounted system.

The load-dependent monitoring procedures, based on dynamic tests, involve the observation of a bridge structure over time using periodically sampled response measurements from an array of sensors, the extraction of damage-sensitive features from these measurements, and the analysis of these features to determine the current state of structure health. For long-term monitoring, the output of this process is periodically updated information regarding the ability of the structure to perform its intended function in light of the inevitable aging and degradation resulting from operational environments.

Systematics of bridge dynamic tests considering various methods of structure vibration excitation, the measured and analyzed parameters as well as possibilities of their applications in Bridge Health Monitoring are presented in Figure 2. Taking into account the types of structure vibration excitation, three main groups of bridge dynamic tests can be distinguished:^{[11,12,13,14,15,25]}–

ambient vibration tests under dynamic loads caused by special vehicles or regular traffic as well as vibrations initiated by environmental loads (e.g., wind, earthquakes);

Figure 2

For the observation of bridge structure reactions to traffic loads and other influences as well as for bridge performance and health monitoring, the following physical quantities are usually measured:^{[1,2,3,4,5,6,7,8,9,10,13,15,25]}–

linear and angular displacements,

On the basis of the measured values, the following structure dynamic parameters are typically calculated and analyzed (Figure 2): –

Dynamic Amplification Factors (DAF),

The most popular measuring techniques and types of sensors used in bridge dynamic tests for identification of the physical quantities are presented in Table 1.

In Bridge Health Monitoring process, the results of dynamic tests are generally applied for (Figure 2): –

identification of loads − assessment of the magnitude as well as the spatial and temporal distribution of forces acting on a structure,^{[3,5,7,16,22,25]}

Generally, the results of bridge dynamic tests create background for assessment of bridge structural condition and for the prediction of the remaining service life of the structure. This information often becomes fundamental for efficient management of bridge operation and maintenance.^[3,4,5,6,7]

Each dynamic test of bridge has to be designed individually, taking into account specific problems of the observed structure and particular limitations associated with a type of vibration excitation as well as goals of planned tests. Nevertheless, some general steps in design and execution of bridge dynamic tests can be distinguished, as presented in Figure 3.

Figure 3

In the first step, all goals of the dynamic tests should be defined as a basis for design of architecture of a measuring system. In the same time, a theoretical model of the bridge structure is usually created and applied to the calculation of the range of the measured physical quantities and its spatial distribution in a structure. In the next phase, all components of the measuring system should be designed, including type and parameters of sensors, sampling rate, data transmission system, data processing procedures and so on. After detailed design of all components and installation of the whole system on the bridge, the validation procedure should be performed. Experimental values should be compared to the results calculated by means of theoretical model of the structure. Validation process should also confirm that the required precision of measurements is achieved.

In the case of positive results of validation procedure, the testing system can be accepted for application. Some modifications of measuring system as well as theoretical model of the structure are normally required in the situation of problems with system approval.

Execution of the planned dynamic tests should be preceded by detailed inspection of structure condition and the inspection should be repeated after the dynamic tests. All changes of the structure condition before and after the tests (defects) should be included in the theoretical model of the bridge and in the architecture of the measuring system, if needed.

The final step of the bridge dynamic test consists of processing of the output data and their application in the Bridge Health Monitoring practice.

In Poland, the dynamic proof load tests of bridges^[17] are standardized in separate but similar codes for steel structures^[23] and for concrete structures^[24]. The tests are performed under predefined loads in the form of vehicles (trucks or locomotives) as well as special controlled exciters.

Requirements and conditions specified for road and railway bridge dynamic tests are presented in Table 2. The main criterion of bridge selection to preliminary dynamic load tests is theoretical length of the span. Dynamic tests of railway bridges should be obligatorily executed on all concrete structures with spans longer than 15 m and steel structures with spans longer than 21 m. For road bridges, the dynamic tests are required for steel structures with spans longer than 21 m and for concrete structures – for individually selected bridges.

For all types of bridges, the positive results of static loads are required to execute the dynamic tests. During the standard dynamic tests, the loading vehicles are passing through the bridge with a constant speed, starting with the speed 10 km/h. For each consecutive test, the speed is increased by 20 km/h up to the maximum allowable speed v_max.

During the tests of concrete road bridges, the artificial bumps of height up to 10 cm can also be used to simulate potential irregularities of the road surface (e.g., potholes).

Short- and long-term dynamic monitoring (see Figure 1) is executed under ambient traffic loads and environmental impacts during the operation of a bridge. During such tests, some maintenance activities are often necessary. It includes mostly: –

modifications or replacement of the measuring system components,

Taking into account the bridge structure excitation method (see Figure 2), two types of groups of dynamic characteristics can be distinguished: –

vibration parameters of the structure subjected to a specific load, mainly special vehicles or normal traffic (simultaneous vibration of the bridge and vehicles), identified during the so called operational tests with ambient excitation,^{[5,8,9,10,13,17,20]}

Operational tests usually use typical traffic (vehicles, trains, pedestrians) as the source of excitation to identify the response signal of the structure on the specific dynamic load. Identified parameters of the signal (excited frequencies and operational deflection shapes) give information about the whole structure-vehicle system behavior. In case of the use of normal traffic as excitation, the results of operational tests are biased by additional mass of the live load moving along the structure. Results can be more accurate when the following assumptions are fulfilled: –

excitation has to be applied randomly in terms of space and spread on the whole structure,

Modal testing of structure, based on the analysis of the response of the structure on a defined and measured excitation (exciters, special techniques), gives an answer on its natural frequencies, scaled mode-shapes and modal damping coefficients. In this method, applying of excitation do not affect the tested structure, neither by added mass nor by modification of stiffness, meeting the assumption that the obtained results should reflect characteristics and performance of the real bridge structure.

Both kinds of tests offer very useful knowledge, which supports the assessment of serviceability and condition of the structure in the process of bridge health monitoring as a part of Bridge Management Systems.

Dynamic tests play more and more important role in health monitoring and management of bridge infrastructure (e.g.,^{[1,2,3,4,5,6,7,8,9,17,20]}). The tests may be carried out both in a permanent way by means of built-in gauges mounted on a structure as well as within systematic inspections and arranged tests of a limited duration with application of external measuring devices.

The main goals of dynamic tests in Bridge Health Monitoring process can be formulated as follows: –

on-line observation of bridge technical condition and serviceability based on predefined performance indicators,