In this post I'll be covering some general aspects of why my project (ERS10) is required. In particular I will discuss the growing necessity of damage detection in bridges and some historical development down the years. I hope you'll enjoy!
The identification of structural damage in bridges is a research topic that has generated significant attention in recent years. The primary reason for its surge in popularity is an aging road and rail infrastructure, which is subjected to traffic loading conditions that far surpass their original design criteria. This unprecedented increase in loading is accelerating structural fatigue, which in turn reduces service-life. Some fatigue assessments carried out on the most common reinforced concrete bridge types constructed in Brazil since 1950 found that shorter span bridges, in the range of 7 to 10 meters, may have their fatigue performance in danger if a 100 year design-life is required . As a consequence, it was deemed that a straightforward, non-destructive assessment method of bridge deterioration is urgently required.
Currently, non-destructive assessment methods entail visual inspections, hammer tests and localised damage assessment methods. These methods, although useful and inexpensive, have numerous limitations; they are infrequent, taking up to nine years between inspections , dependent on the competence of inspectors and are confined to localised damage and external deterioration, while the true global bridge condition remains relatively unknown. Additionally, as bridge infrastructure continues to age and deteriorate, the frequency of inspection must increase to counteract the reduction in safety of these structures. This task is made more difficult due to its sheer enormity. Recent figures show that Europe's bridge count is circa one million, and of Europe's half a million rail bridges, 35% are over 100 years old . This leads to a need to reduce uncertainty regarding bridge condition through other, more efficient means apart from traditional inspection techniques.
The concept of using measured vibrations to discern damage in structures has been employed for some time. For instance, some early research by German engineers in the 1950's used vibration intensities, attained from measured accelerations, as an empirical indicator of damage in buildings . The use of monitoring a bridge's natural frequency over time to detect damage in structures was originally proposed by Adams et al.  in the late 1970s. It was a promising development as frequency is a product of a structure's mass and stiffness, and it was thought that monitoring natural frequencies over time would show how a structure's stiffness declined. However, there are many limitations to this methodology, for instance; changes in frequency would not locate damage accurately, as cracking in different locations can cause frequency changes of equal magnitude.
Apart from natural frequencies, other modal properties such as mode shapes, damping ratios and modal curvatures have been traditionally used to detect damage. For instance, cracking in a cross-section will increase internal friction and thus raise the value of the section's damping ratio, however, damping ratios are heavily influenced by vibration amplitude and measuring them from vibration data produced large standard deviations, which impair their accuracy and effectiveness as a reliable damage indicator.
The core problem is that bridges are monitored over long periods of time and are subjected to large temperature fluctuations, harsh storms and numerous traffic scenarios. These varying conditions affect changes to a bridge's stiffness and mass in a non-linear manner, which in turn alters the bridge's modal properties. This is evident in Peeters & De Roeck's  assessment of the Z-24 Bridge in Switzerland, where significant variation in the bridge's natural frequency was observed when the ambient temperature dropped below freezing point (see Figure 1). The cause of this bi-linear behaviour was attributable to the newly solidified ice in the bridge deck and supports contributing to its stiffness.
Figure 1. Z-24 Bridge - Natural Frequency v Temperature - after 
So, that's all I will cover for now. I hope the above few paragraphs give you an idea of the need of an efficient condition assessment methodology of bridges across Europe, and that it also portrays some of the difficulties imposed by using vibration data, in particular, modal properties.
See you again soon!
 Rodrigues, F., Casas, J.R. & Almeida, P. (2013). "Fatigue-Safety assessment of RC bridges. Application to the Brazilian highway network", Structure and Infrastructure Engineering, Vol. 9, N. 6, 2013, pp.601-616.
 Federal Highway Administration. (2008) "Bridge Evaluation Quality Assurance in Europe", Technical Report Document, FHWA-PL-08-016, March.
 MAINLINE. Maintenance, renewal and improvement of rail transport infrastructure to reduce economic and environmental impacts. (2013) Deliverable D1.1: "Benchmark of new technologies to extend the life of elderly rail infrastructure" European Project. 7th Framework programme. European Commission.
 Koch, H.W. (1953). Determining the effects of vibration in buildings, V.D.I.Z., Vol. 25, N. 21, pp. 744-747
 Adams R.D., Cawley P., Pye C.J., Stone B.J. (1978) "A vibration technique for non-destructively assessing the integrity of structures." Journal of Mechanical Engineering Science. 20: 93–100.
 Peeters, B & Roeck, G.D. (2001) " One-year monitoring of the Z24-Bridge: environmental effects versus damage events ". Earthquake Engineering and Structural Dynamics, 30, 149-171.