As part of the Brite-EuRam project BE96-3157 SIMCES (System Identification to Monitor Civil Engineering Structures) the three span box bridge Z24 in Switzerland was monitored during almost one year before it was artificially damaged. In the preceding monitoring period the influence of environmental conditions, such as humidity, wind and especially temperature, on the bridge eigenfrequencies was studied. The goal of the subsequent damage tests, corresponding to realistic and relevant cases, was to prove that damage could be detected, localised and quantified by considering changes in eigenfrequencies and modeshapes. Some of the main conclusions are that ambient vibrations treated by proper system identifications algorithms can provide accurate results for eigenfrequencies and modeshapes, that it is mandatory to filter beforehand the influence of environmental conditions and that small, stiffness degradation producing damage can be detected if the corresponding eigenfrequency diminutions surpass 1%.

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