J Nondestruct Eval (2010) 29: 14–24 DOI 10.1007/s10921-009-0061-9 Ambient Vibration Tests of a Steel Footbridge Alemdar Bayraktar · Ahmet Can Altuni¸ sik · Barı¸ s Sevim · Temel Türker Published online: 1 January 2010 © Springer Science+Business Media, LLC 2009 Abstract The paper presents analytical and experimental modal analysis of a steel footbridge for modal parameter identification. 3D finite element model is established for the steel footbridge based on the design drawing and modal pa- rameters are obtained from deformed configurations. The field test is carried out by ambient vibration test under hu- man walking excitation. Both Peak Picking method in the frequency domain and the Stochastic Subspace Identifica- tion method in the time domain are used for the output- only modal identification. From the study, good agreement is found between mode shapes, but some differences in the natural frequencies and analytical frequencies are bigger. Keywords Ambient vibration test · Footbridge · Modal parameter · Peak picking · Stochastic subspace identification 1 Introduction In recent decades, there has been a trend towards improved mechanical characteristics of materials used in footbridge construction. It has enabled engineers to design lighter, slen- der and more aesthetic structures. As a result of these con- A. Bayraktar ( ) · A. Can Altuni¸ sik · B. Sevim · T. Türker Department of Civil Engineering, Karadeniz Technical University, 61080 Trabzon, Turkey e-mail: alemdar@ktu.edu.tr A. Can Altuni¸ sik e-mail: ahmetcan8284@hotmail.com B. Sevim e-mail: bsevim18@hotmail.com T. Türker e-mail: temelturker@hotmail.com struction trends, many footbridges have become more sus- ceptible to vibrations when subjected to dynamic loads. In addition to this, some inherit modeling uncertainties related to a lack of information on the as-built structure, such as boundary conditions, material properties, and the effects of non-structural elements make difficult to evaluate modal properties of footbridges analytically. For these purposes, modal testing of footbridges is used to rectify these prob- lems after construction. The modal testing procedures used in all of the articles are either frequency response function (FRF) based test- ing (using shaker or hammer excitation) or ambient vibra- tion survey. In the former both the input force and output response are measured. In the latter only the response is measured while the input force is environmental excitation (wind, human walking or traffic). The main advantages of ambient vibration testing are: (a) testing is cheap and fast, since the equipment for excitation is unnecessary, (b) test- ing does not interfere with the operation of the structure, (c) the measured response is representative for the real op- erating conditions of the structure. In this study, the ambient vibration tests are conducted on the steel footbridge because of the main advantages. Although the modal testing is one of the most pop- ular techniques for studying the behavior of engineering structures through a number of natural frequencies and mode shapes, there are only a few articles exist related to both experimental measurement and finite element model- ing specifically footbridge. The first extensive study related to vibration tests of footbridge was conducted by the UK Transport and Road Research Laboratory in the 1970s [1, 2]. Their work concentrated on presenting and discussing the experimentally identified modal properties. But, in the last two decades, a number of authors have started to pay more attention on modal testing of footbridges. Zivanovic et al.