Case Study: Structural Health Monitoring For Bridge Structures
Structural Health Monitoring
Structural Health Monitoring (SHM) is a method to determine integrity state of a structure and to estimate its remaining useful life on a continuous real-time basis. In the other word SHM is used for damage diagnosis, and prognosis of structures. Unlike traditional Non-Destructive Evaluation (NDE) methods, Structural Health Monitoring techniques use measurement difference at a location at two different times to identify the condition of the structure.
SHM consist of sensor networks that monitor the behavior of structures under load. Non-Destructive Evaluation inspects the characterization and location of damage after the structure is taken out of service temporarily while SHM do the same during the operation time of the structure.
The main objective of infrastructure monitoring is replacing schedule-driven maintenance with condition-based maintenance.
Developed countries encountered with growing problem of deterioration of infrastructures. In Germany, more than 80% of the federal highway bridges have signs of deteriorations, which need repair and maintenance. Their costs were estimated by more than 6.8 billion Euro. In the United States more than 150,000 bridges, which are about 25% of the U.S. bridges, are considered structurally deficient.
Traditional approaches, such as visual inspection, are time consuming and leave the possibility for hidden undetected damage. These drawbacks have promoted the development of structural health monitoring over the past decades. The only reliable method to manage the current situation of infrastructures and prevent catastrophic events now is using SHM.
Currently, visual inspections and nondestructive evaluations (NDE) are used to inspect of the state of structural integrity of the majority of civil structures. It is done by governmental officials or private engineers.
Global health monitoring methods determine whether or not damage is present in the entire structure. Visual inspection, and tap tests are two common traditional methods of global health inspection. Local health monitoring methods are used to locate and measure the damage. An example for local health monitoring methods is Non-destructive evaluation (NDE). NDE is a group of inspection methods. Some of most well-known NDE methods are ultrasonic and acoustic emissions, X-ray and Eddy currents (Farrar et al. 2001). Ultrasonic guided waves are used to measure the state of stress, and eddy current techniques are used to locate cracks. NDE is time consuming, expensive, and needs access to potentially damaged parts of the structure which is not always possible. For instance, although X-ray method is a well-known and popular method of NDE, it needs access to both side of the under examination location which makes it difficult to use.
The main limitation of visual inspection is that it requires access to the potentially damaged regions of the structure, which may be not possible or dangerous to access. Traditional approaches leave the possibility for hidden undetected damaged components which are in critical condition. An example of the difficulty to detect structural components in a critical condition is the recent collapse of the I-35W Bridge in Minneapolis. I-35W Bridge ages 40 years and were designed to stand 100 years. It collapsed on 1 august 2007 in the rush hours and while workers were resurfacing it.
Causes of the I-35W Bridge collapse were as follow. Corrosion on the gusset plates had been underestimated by bridge inspectors, limited access that inspectors had to the gusset plates, a design error combined with overload of the bridge. The design error was not discovered by inspectors since it was not a standard practice for the inspectors to look for such a design errors. The thickness of gusset plates was 1/2” while they should have been 1”. There were not any SHM sensors installed on the bridge and it had to be inspected by using traditional methods. All causes of this collapse refer to use of traditional inspection methods. If the bridge were equipped by SHM instruments corrosion of gusset plated could have been detected, inspectors would not have needed to access inaccessible places of the bridge, and the overload of gusset plates would have been recorded by sensors.
USA Federal Highway Administration mandates evaluation of condition of bridges and rating them for their safety and reliability every two years. Mainly, tap tests and visual inspection are used in these evaluations. Tap test is tapping the surface of structure and listening to audible variations to determine if voids or de-bonding exist. Voids near the surface of concrete, de-bonding of wraps, and significant cracks can be found by tap test. The US has a bridge inventory of over 500,000 highway bridges with a span length (distance between two intermediate solid beams) over 8 m. This large number of bridges makes required bridge inspections not always achievable.
Instruments and sensors Used in SHM
A large variety of sensors are used for SHM. The more common types of them are as follow. Micro-electromechanical System (MEMS), accelerometers, nuclear magnetic resonance (NMR) capsules to detect chloride ions, Shearography to detect out-of-plane displacements. Shearography is an optics-based technique, which measures surface displacement of a loaded structure (Francis, Tatam, & Groves, 2010). LIDAR to capture 3D position of objects, infrared thermography. Radar techniques are used to obtain three dimensional views of reinforcing steel in concrete slabs.
Fiber optic sensors are used for measurements of cracks in concrete, and vehicle counting. Instrumentation in bridges usually consists of displacement transducers, strain gauges and accelerometers, global positioning system (GPS), anemometers, corrosion sensors, cable tension force sensors. Sensors need to withstand the damaging effects of the alkali or salty environment. Therefore, they are covered by protective layers like boxes or plastic pipes.
Stiffness or flexibility identification methods are one of SHM methods that are capable of finding damages in a structure. For that, a model from the structure is prepared and data from the installed sensors on the structure is compared with the output of the model. When the sensed signals are significantly different from the predicted output from the model, it is likely that damage has occurred.
In continue, as examples of structures which instrumented by SHM sensors two cases of One Rincon Hill Tower, Golden Gate Bridge, and new I-35W Bridge are discussed briefly.
One Rincon Hill Tower instrumentation
The One Rincon Hill Tower is a 62-story residential building located in downtown San Francisco. It equipped with 72 accelerometers, and Sensors array in the central core wall at different stories.
These data are very useful for Post-earthquake damage assessment in order to ensure the safety of its residents (Nagarajaiah et al. 2016).
Golden Gate Bridge instrumentation
The Golden Gate Bridge is located at the entrance of the San Francisco Bay, and was opened to traffic in 1937. The total length of the bridge is 2790 m, with a main span of 1280 m. The structure consists of suspended steel trusses supported by braced steel towers.
The instrumentation of the bridge consists of 69 accelerometers and 4 relative displacement sensors, a free-field station on the south side of the bridge, and a wireless sensor network consisting of 320 sensors (Pakzad et al. 2008).
New I-35W Bridge
The new I-35W Bridge was built in 2008. More than 300 sensors embedded or mounted to the bridge provide information to engineers about the structural behavior of the bridge. In particular Roctest vibrating wire sensors and Smartec SOFO sensors measure how the bridge is reacting to load and the temperature of concrete. Additional Senscore corrosion sensors measure the chloride penetration in concrete.
One in four bridges in the United States needs significant repair or cannot handle automobile traffic. Deterioration of the United States civil infrastructures has promoted SHM programs. SHM determines performance of structures and estimate their remaining useful life. SHM is crucial to detect hidden structural damage that cannot be detected by visual inspection. SHM is used as part of a decision-making process for civil infrastructures during their operation and after extreme events.
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