Development of EDP Program for Damage Analysis of Timber Bridges Based on Vibration Measurements

This Bachelor thesis deals with the topic "Computer Program for Timber Bridge Damage Analysis Based on Vibration Measurements." It mainly refers to the structural testing of timber bridges.

Timber structures have the big disadvantage that the damage to the material cannot be determined early by a visual check of the material surface like it is the case of the steel or concrete structures. The reason for this is that the natural decay process of timber, which occurs at a permanent moisture of more than 20% inside the timber, damaging the timber from the inside. A test method for timber structures aims at determining the state inside the timber. However, the test methods used for this usually cause damage and are therefore rather unsuitable for a recurring test. This is why a test method for the measurement of mode shapes and their natural frequencies has been developed. This is possible because the natural frequency of a structure mainly depends on the bending stiffness.

Author	Sebastian Krug
University	Biberach University of Applied Sciences, Germany

The objective of this thesis is the development of a program for a damage analysis of timber bridges. The prerequisite for using the program is that the natural frequencies are determined on a presumably damaged timber bridge.

The developed program provides the option to enter all relevant data that is necessary for the automated generation of a 3D bridge model. The program Microsoft-Excel is utilized for this. Afterwards, the 3D model is automatically generated in the RSTAB program. The required communication between the different programs is possible via the COM interface.

After the virtual 3D model has been completely created, the natural frequencies were calculated in the DYNAM Pro add-on module. Then, it was necessary to compare the actually recorded measured values and the measured values determined analytically in the program. Since it is assumed that the virtual model represents a real bridge structure in the optimal state, there should be no difference between the real measured and the virtually calculated natural frequencies. However, if it is determined that the real structure has lower natural frequencies than the virtual model, it can be deduced that the real structure has a lower stiffness than the virtual model.

In order to calculate this stiffness difference, the program allows for determining the difference of the modulus of elasticity of the material by using the Newton method. Using the iteration according to Newton, it is possible to iterate up to three moduli of elasticity at the same time, and thus to determine the timber component degree of damage.

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