Hi all,
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 [1]. 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 [2], 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 [3]. 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 [4]. The use of monitoring a bridge's natural frequency over time to detect damage in structures was originally proposed by Adams et al. [5] 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 [6] 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 [6]
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!
Bibliography
[1] 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.
[2] Federal Highway Administration. (2008) "Bridge Evaluation Quality Assurance in Europe", Technical Report Document, FHWA-PL-08-016, March.
[3] 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.
[4] Koch, H.W. (1953). Determining the effects of vibration in buildings, V.D.I.Z., Vol. 25, N. 21, pp. 744-747
[7] 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.
[6] 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.
