Damage Assessment of Civil Engineering Structures and Bridges Using Nonlinear Dynamic Characteristics

Bridges are an integral part of every culture and an indispensable element of a country's infrastructure. The high technical demands, social benefits of bridges and economic reasons require a faithful maintenance, regular inspections and permanent repair work in order to ensure safety and serviceability as well as to expand durability and lifetime. Normally, the detection of damage in bridges is mainly done by visual examination. However, defects as for instance partial rupture of a prestressing cable cannot be visually observed. Since a few decades the number of dynamic tests, carried out for a better assessment or monitoring of the structural behaviour, increases. These tests base on the fact that damage changes dynamic structural properties like eigenfrequencies, modeshapes and damping. However, the fact, that the sensitivity of changes in modal properties to structural defects is sometimes low and other damage detection methods are often unfeasible in case of small damage is the motivation for this thesis. The focus of the present work is to investigate the sensitivity of the non-linear dynamic behaviour to structural defects and how far it could be useful for damage detection and assessment in practice. The basic idea is that non-linear dynamic effects, i.e. the amplitude dependency of the modal parameters and the occurrence of higher harmonics, vary with changes in structural condition, due to cracks or failure of tendons for example. The overall aim is to enlarge the field of non-destructive vibration based damage detection methods and to assess the feasibility using non-linear dynamic characteristics.Therefore, a number of laboratory tests with reinforced concrete beams, prestressed concrete structures and dynamic tests on two bridges with regard to non-linear dynamic behaviour are presented. The results are compared to the results of conventional vibration based damage assessment methods and static load tests.The use of changes in non-linear dynamic characteristics turns out to be another useful tool, which should be considered in the framework of structural health monitoring using dynamic methods.