During recent detailed inspections of timber bridges, concerns have been raised in relation to excessive movement of timber piles under traffic loads. Bridge inspectors are regularly reporting that piles on timber bridges are “pumping” that is displacing vertically under each axle group as a heavy vehicle passes over the bridge.
Current assessment of pile movement is extremely subjective since it is based on an inspector touching the pile and sensing the traffic-induced vibrations. This observation has proven to be unreliable in determining whether the pile identified is actually displacing vertically, or whether the pile is simply vibrating.
The objective of this project is to develop a cost effective process to monitor the performance of timber bridge piles where the bridge inspection has identified excess movement in those piles. We wish to learn how deterioration of a timber pile affects its vibration response and determine:
- Whether the vibration response is adequately sensitive to timber deterioration
- Whether it can be used as an objective inspection practice
The initial concept of this project is to develop a methodology that incorporates the use of Tri-axial accelerometers, data-logging and simple analysis to monitor the vibration or displacement of suspect piles. The intention is that this system is simple and robust enough to deploy quickly by the bridge inspection crew with monitoring expected to be short term with a duration of one to three days.
The project would also include an assessment, based on the results, on the suitability, or otherwise, of the process. A successful result would lead to this becoming a standard tool in our inspection and assessment of timber bridges.
Approach and Scope of Works:
The project will place instrumentation on bridges in various conditions. The data will to be analysed by a student with a civil engineering/data science background to determine how sensitive the response of the bridge is to a deteriorated pile. It is envisaged that this will involve some structural modelling and analysis work as well.
The investigation is to measure the vibrations induced by traffic loads to understand how stiffness reductions, resulting from deterioration of the timber as well as connections within the pile length, affects the natural frequencies of the bridge. The measured accelerometer data would also enable the identification of whether the vibration is axial or flexural. The emphasis is on identifying differences in the relative vibration behaviour of sound timber piles, deteriorated timber piles and potentially, repaired timber piles as well.
Benefits to Main Roads:
Through the work it is aimed to explore the ability of using X,Y,Z displacement data as a objective indicator of distress in timber piles. Where concerns are raised in relation to excessive pile movements, the problem is usually deep under water which requires an inspection by a diver or by underwater remote operated vehicle. Therefore there is a high benefit if an objective assessment is made available to guide whether further inspections are warranted.