The International Information Center for Structural Engineers

Dynamic properties of an eleven-span motorway bridge at different levels of excitation

G-W Chen,S. Beskhyroun,P. Omenzetter.
Book Title or Journal
New Zealand Society for Earthquake Engineering (NZSEE) Annual Technical Conference
Number of pages
Bridge dynamic properties measured under a given vibration intensity condition would give a true picture of the behaviour for that particular condition. However, the use of the model derived from such data may not be reliable when applied for the prediction of response under a different vibration intensity condition. Therefore, it is necessary to investigate the structural dynamic behaviour at different levels of excitation in detail. This paper focuses on the experimental investigation of modal property variability at different levels of excitation. Both weak ambient vibration tests (induced by nearby traffic, wind and possibly microtremors) and forced vibration tests with different applied input force induced by eccentric mass shakers were performed on the Nelson St off-ramp bridge (an 11-span post-tensioned concrete, box girder structure forming a part of the motorway network in Auckland’s CBD). Three separate system identification methods, namely peak-picking (PP), the frequency domain decomposition (FDD) and the data-driven stochastic subspace identification (SSI) method, were applied for accurate structural modal parameter identification. It was found that the three output only identification techniques are able to extract natural frequencies of the structure reliably, while the time domain SSI method yields the best mode shape estimates and PP may not be able to give accurate mode shape estimates for some modes. The variability of the dynamic properties for the 1st vertical and lateral bending modes was examined. A general trend of decreasing natural frequencies and increasing damping ratios was observed with increased level of vibration intensity.
Bridges,dynamic properties