Determining individual member stiffness of bridge structures using a simple dynamic procedure Jianchun Li ", Bijan Samali a and Keith Crews a "Centrefor Buill Infrastructure Research. Faculty of Engineering. University of Technology, Svdney. NSW, Australia A reliable determination of the structural condition of timber bridges presently requires costly load testing. A new dynamic based testing method was developed by authors to reduce the cost and shorten the testing time. The method has been successfully used to undertake field-testing of more than 40 timber bridges across NSW. The dynamic testing procedure involves the attachment of accelerometers undemeath the bridge girders. The bridge girders are then excited by a modal hammer. The method requires tests with and without extra mass. so that the overall flexural stiffness of the bridge can be obtained. However. in order to accurately estimate the load carrying capacity of the bridge. it is necessary to obtain the stiffness values of individual members from test results without complicating the current testing procedure. In this is paper. the authors review the dynamic testing procedure and propose a method to determine individual member stiffnesses for a bridge structure based on the field dynamic testing data. The outcomes of this work not only enable more accurate prediction of the load carrying capacity of the bridge but will also identify defective members of the bridge structure. 1. Introduction Local Government in Australia is responsible for the operational management and maintenance of over 20.000 bridges. More than 70'}o of these bridges comprise aging timber bridges. the load capacity and structural adequacy of many of which have been impaired over time. A major challenge facing Local Government nationally is to develop effective strategies for the maintenance and rehabilitation of the extensive timber bridge stocks which fOI111 a key component of the road network under its control. Raising the efficiency and reliability of bridge maintenance practices of local government has the potential not only to minimise costly unscheduled emergency repairs. but also to reduce the overall maintenance costs. whilst improving the operational effectiveness of its road network. The field testing of over 40 timber bridges in NSW has been undertaken and fOl111s part of the second phase of an earlier project sponsored by the Institution of Public Works Engineering Australia (IPWEA) in 1999. As part of that project. a new testing regime. based on dynamic measurements. was developed and a thorough pilot study on the single span Cattai bridge in Baulkham Hills Shire was undertaken to demonstrate the potential of the proposed procedure [1.3]. The second phase had as its principal goal the further development and implementation of the procedure and enabling equipment for the cost-effective determination of the load deformation characteristics and load carrying capacity of a wide variety of short-span bridges[2]. Coupled with specially developed analysis software. this provides a measure of the structural adequacy of the bridge and a reliable basis for devising appropriate maintenance or remedial measures. In this paper. this new dynamic testing approach will be reviewed and a method based on modal analysis will be proposed to determine the stiffness of individual bridge members. which will enhance the dynamic testing approach. 2. Review of the new approach to the management of bridge assets Procedure The new dynamic bridge assessment procedure involves the attachment of accelerometers undemeath the bridge girders and the measurement of the vibration response of the bridge superstructure unloaded and with one or more loads (such as a truck. water tanker. grader. concrete blocks. etc. of knO\\11mass) applied at midspan. The excitation is usually generated by a modal impact hammer. The resulting dynamic responses are measured with low frequency and high sensitivity accelerometers. which are robust and simple to install. The data is logged and the bridge deck properties evaluated. using dynamic signal analyses on a standard computer with special software. Two sets of frequencies are measured for the bridge. 'as is'. and when loaded by the extra mass. From the resulting frequency shift due to added mass. flexural stiffness of the bridge can be calculated. Figure 1 summarizes. schematically. the testing-analysis-assessment procedures which comprise the new dynamic method of bridge assessment. Effective APVC Proceedings. 2003