Understanding structural behavior is accomplished through analysis using the methods now common to most civil engineers. Finite element models are useful tools that, when created and used appropriately, can provide significant insight into the behavior of the structure. Finite element programs can perform static and dynamic analyses that can help guide the selection of sensors and their locations. In turn, the models can help interpret the results. However, the structure as it exists may behave different from the model and so calibrating the model is important. A common way of calibrating models, and more importantly also determining the safety of a bridge, is by performing load tests.
3.3.1 Load Determination & Event Detection
The ultimate goal of bridge inspections and structural health monitoring is to provide a method of determin- ing whether the bridge is still safe for operation. Determining where damage has occurred and its severity is typically just an intermediate step in determining the damage’s effect on the ability of the bridge to safely carry loads and determining appropriate repair and retrofit strategies. A bridge manager will feel a greater immediacy if told his bridge can no longer safely carry a semi-truck than if told that a beam has seen a certain amount of strain increase or a shift in natural frequencies. Getting from the metric to the rating can be a challenge.
Strain gages can only measure the changes in strain that occur after the gage itself has been installed. However, to determine the load capacity of the bridge, the structural response to the dead loads that have already been applied must be known. The response of the bridge to a known live load can be observed and the structural response calculated. This process is generally referred to as a load test of which there are two types [37]. The first type of load test is a diagnostic test. In this test, a vehicle or vehicles of known weight are driven on a defined course so the load and application points are known. The load level is designed to be
below the elastic load limit. Strain and deflection sensors record data at strategic locations. The data is used to determine the load distribution and stiffness characteristics of the bridge usually in conjunction with a FE model. The other type of load test is a more direct method in determining the load capacity of the bridge and is called a proof test. In this test, the loads in the truck are increased until either the target load that the manager wants the bridge to be rated at is reached, or the sensors start to observe nonlinear behavior indicating the elastic limit has been reached.
Diagnostic load tests have become a common tool in evaluating structures and validating finite element models [38–40]. However, they can be costly and obtrusive as they involve closing the road to traffic so that only the known loads are acting on the structure.
In a structural health monitoring context, which can be considered as continuous diagnostic testing, the loads that cause sensor response are not typically known. Therefore in the diagnostic algorithms of the monitoring system, when the newly acquired response is compared to historical responses it may not be discernible if the response represents a change in load or a change in the structure. Various methods have been proposed to determine the loads that are causing the measured responses in a strain monitoring system. In a system called Restricted Input Network Activation Scheme (RINAS), Su [41] proposed using a camera placed on the bridge and image processing techniques to determine what type of vehicles were crossing the bridge. Environmental data can also be collected so that the ideal events – the passage of an isolated semi-tractor at night – are the only ones used as input to diagnostic algorithms. Su further proposed, but did not implement, using RFID tags and data collected at state DOT weigh stations to further supplement the knowledge as to the loads of the vehicle.
Another, more direct way of determining the loads is by installing a weigh-in-motion (WIM) system at the bridge site. [42] WIM systems can take advantage of a number of sensor technologies and could therefore be easily integrated into an already existing structural health monitoring system. The principle behind the scales and embeddable mats that make up a WIM is the same as for a diagnostic test on the bridge. The WIM is calibrated with a known load and then it can use its response to future events to determine the load. The WIM needs regular repair and maintenance and has to be replaced every few years.
An alternative to trying to determine loads for a structural health monitoring application was reported by Whelan et al. [43] Their proposal used a multimetric wireless sensor system that was capable of measuring both strain and acceleration. After installation, a diagnostic load test was performed with a loaded truck to establish an initial structural assessment. Recognizing that the loads would not be known between the initial and any future load testing, the sensors would operate as a vibration monitoring system that would perform
anomaly detection continuously. If an anomaly were detected the sensors would alert the bridge manager to schedule an inspection and load rating using the extant wireless strain sensors. In this way, the load ratings which are easily incorporated into the current National Bridge Inventory databases are performed as the bridge inspection community learns to accept vibration based analyses.
3.3.2 Steel Bridges
When developing an SHM system for a bridge, the material and structural system used are important con- siderations. As shown in Table 2.2, thirty percent of the nation’s bridges are considered to have steel as their primary construction material. These steel bridges come in a number of shapes and sizes. Steel truss bridges may be the most easily recognized for the typical motorist but steel box girder bridges and many suspension or cable stayed bridges can be classified as steel bridges.
Each of the different types of steel bridges will present a different set of concerns for the bridge manager. The concerns for each bridge will require a different approach to structural health monitoring and dictate which sensors should be used and where they should be placed. Monitoring the corrosion of individual wire strands in the suspension cables of a bridge requires different sensors and methods than would be used for corrosion monitoring of a steel box girder. The SHM approach needed is determined by the structural system and the likely failure mechanisms of the bridge. Therefore, understanding the design of each bridge so as to identify its potential weaknesses is essential to creating a useful SHM system. An understanding of the engineering practices and knowledge (or lack thereof) of the era in which the bridge was designed is also important in identifying the best SHM system for a specific bridge. A brief history of steel bridge construc- tion will be given in Section 4.1.1 with emphasis on the engineering material and knowledge advancements that contributed to the design of the Rock Island Bridge.