Structural Health Monitoring of a Frame Using Random Vibration Analysis

In the past few years, the thin walled structure comprises an important and growing proportion of engineering construction industry. Their areas of applications increasingly ranging from aircraft, bridges, industrial buildings, warehouses, exhibition pavilions, concert halls, sport arenas, ships, oil rigs to storage vessel. They have a high load carrying capacity despite of their small thickness. The design calculation required to estimate the strength, stability and vibration of thin walled structure are among the most complex problems encountered in structural design. Dynamic testing of full scale steel frame structure is not possible because the damage may be so strong that it will be impossible to retrofit the frame building for it further using. Dynamic testing of a relatively small part of building (having the same structural scheme) is one of the possible ways for the solution of the above described problem. Before the dynamic testing is done, dynamic analysis of full scale and the small scale are carried out using ANSYS software for preliminary study about the vibration characteristics of the structure for its design consideration. This paper presents the structural health monitoring of a portal steel frame using transient vibration analysis which is followed by random analysis. The purpose of this paper is to present the result of an analytical study on an finite element method for damage detection of loosened bolt connections. For this, a laboratory specimen in the form of a portal frame is taken into consideration. The structure consist of a one bay frame, on which transient analysis followed by random vibration analysis is carried out. The accelerations data are measured for six different locations near the lap connections. Two states of structure are considered a healthy state and damaged one. The damage simulation is introduced by the means of loosening one out of two bolts alternatively at one of lap connections. The result focuses on the detection and localisation of structural damages such as loosened bolt, which usually causes a local reduction in stiffness. The method uses the comparison of natural frequencies and vibration energy from both healthy and damaged state obtained through Fast Fourier Transform (FFT) and analysis. It was observed that there is deviation in natural frequency and vibration energy of response obtained due to reduction in stiffness which indicates the presence of damage. There is a scope to extend the same work on composite plate structure.