Doctor

Evaluating dynamic properties of structures through frequency records
Identification of structural damages. And Effectiveness of retrofit by evaluating dynamic properties.
In the modern world, highways, bridges, water/gas/petroleum pipelines, electrical transmission towers, dams, buildings, etc… serve a very important role in our society.
Structures like people might have some health problems.And that when becomes important to assess them in order to find the problems they might be facing. Those problems might be due to many reasons. Some of the reasons we can quote are: age, deterioration, lack of repair, misuse, and in some cases, because they were not designed for the current demand. Damage can also be due to natural hazards, such as earthquakes, typhoons, hurricanes, tornados,floods, etc…
Structural Damage detection is very important in order to prevent catastrophic events, and the assessment of those structures is required.
Structural damage is considered as a weakening of the structure that negatively affects its performance. Damage may also be defined as any deviation in the structure’s original geometric or material properties that may cause undesirable stresses,displacements, or vibrations on the structure. These weakening and deviations may be due to cracks, loose bolts, broken welds, corrosion, fatigue, etc.

The most spread method of damage detection has been and still is visual inspection, but unfortunately the efficiency of this method reduces as the structures become bigger and more complex. Visual inspection can be also costly and time consumingspecially in those areas where access is reduced. Another disadvantage of this method is that it cannot detect damages that are invisible to the human eyes.
There are other methods of none destructive techniques such as ultrasonic and eddy current scanning, acoustic emission, x-ray inspection, etc., provide options to detect the occurrence of damage. The methods are obviously ‘‘local’’ inspectionapproaches.
Structural damage identification through changes in dynamic characteristics, on the other hand, provide a ‘‘global’’ way to evaluate the structural state. Dynamics-based damage identification methods have drawn world-wide attention due to their infrastructural roles.”1
Structural damage causes a reduction of stiffness that leads to corresponding shifts in natural frequencies.Therefore, changes in structural parameters, such as natural frequency, mode shapes, or damping may be used as indicators of structural damage.

Huge improvements have taken place in the last few years in the fields of low-cost displacement and acceleration transducers signal conditioning and sampling hardware, electronic data acquisition systems. Those improvements have driven the interest of thescientific community toward the use of the dynamic response of structural systems as a tool to assess the damage and the safety. So the current tendency of the scientific research is towards using the dynamic response data to identify possible changes in the structural properties, with the aim to deduce information on the structural integrity of the relevant cross sections.

Due to the fact thatnatural frequencies are sensitive indicators of structural integrity, the relationships between frequencies changes and structural damage can be inferred. An analysis of periodical frequency measurements can be used to monitor structural condition. Since frequency measurements can be cheaply acquired, the approach could provide an inexpensive structural assessment technique.

The presence ofdamage or deterioration in a structure causes changes in the natural frequencies of the structure. The most useful damage location methods (based on dynamic testing) are probably those using changes in resonant frequencies because frequency measurements can be quickly conducted and are often reliable.

Reasons for full dynamic testing on structures:
Increase database of dynamic behavior of...