Structural damage risk

The evaluation of structural damage risk is a very complex subject. There are several techniques to evaluate the risk of damage on bridges including visual inspection, non-destructive testing such as ultrasonic testing, X-ray, and infrared thermography, dynamic testing and numerical risk assessment.

Traditional visual inspection tools of bridges can only detect obvious damages like disruption, cracks or rust on the surface of bridges and must be conducted at least biennially. These methods are likely to seem the easiest way to maintain the structural integrity of bridges and viaducts, however it involves deploying technical teams on-site for an extended period of time and the access to very complex parts of the bridge as cables and hangers of suspension bridges and the girders of viaducts and arch structures.

In a recent past, engineers have started to deploy in more critical infrastructures dynamic testing using Structural Health Monitoring (SHM) procedures as a complementary process to the visual inspection. SHM involves the use of various sensors and measurement techniques to gather data about the structure, which is then analyzed to detect any signs of problem with the bridge. Within the most common sensors placed in bridges are accelerometers, strain gauges, displacement transducers, acoustic and optical sensors, and GPS systems.

Vibrations for SHM

Vibration-based techniques to SHM is becoming increasingly more important for the maintenance, inspection and safeguard of structural integrity of bridges. Vibrations are normally gather with accelerometers installed on the structures as they are relatively easy and economical to obtain. Thus, vibration-based techniques are a promising tool for structural analysis and evaluation. In the following, we will explain how MATEREO analyzes the structural damage risk.

Accelerometers

Accelerometers are physical devices that measure acceleration, which is the rate of change of velocity over time. They work by detecting the motion of a mass inside the device when it is subjected to acceleration. The motion of the mass is then converted into an electrical signal, which can be used to determine the magnitude and direction of the acceleration. There are different types of accelerometers, including piezoelectric, capacitive, optical fiber and MEMS (Micro-Electro-Mechanical Systems) accelerometers.

MATEREO uses wireless accelerometers from Omnidots with MEMS technology, that communicates data instantaneously via 4G/LTE (Figure 1). Installing these accelerometers is incredibly fast and easy. We can play with several accelerometers in the same bridge, however their portability allows to deploy the same unit in different bridges and take temporary samples of data that can be used to verify the risk of damage in different stages of bridge's life-cycle.

Vibration intensity

The level of structural vibration does not need to be necessarily considered intolerable by the bridge users to have a the risk of structural damage. The vibration values and their limits need to be analyzed individually for each infrastructure.

Structural vibration limits for particular damage risks can be classified according to the level of vibration intensity. According to Beards (1996) the?limit lines of vibration intensity are given in terms of vibration amplitude and frequency for various levels of damage can be defined according to image in Figure 2.

At MATEREO dashboard, we provide to the end-user the risk of damage as a variable of the current vibration intensity. As long as the measurements of vibrations are accessed in real-time, we create real-time plots of vibration intensity to verify if the values are in which region of Beards (1996) diagram.

Vibration frequency and amplitude are analyzed for the last 24h to get an updated state of the bridge, resulting in a plot as the one represented in Figure 3. Purple dots represent the various frequencies analyzed. Most of them are in the I-No damage region of the graphic, which indicate a bridge with no apparent risk of damage.

References

Beards, C. F. (1996) Structural Vibration: Analysis and Damping. Arnold,?London. ISBN 0 340 64580 6