Chapter 17 Vibration Serviceability Assessment of an In-Service Pedestrian Bridge Under Human-Induced Excitations Amir Gheitasi, Salman Usmani, Mohamad Alipour, Osman E. Ozbulut, and Devin K. Harris Abstract Pedestrian bridges may experience significant vibrations under pedestrian traffic and wind loads. Design codes address the vibration limit state levels either by ensuring the frequency ranges associated with typical pedestrian passages are outside the lower fundamental frequencies of the structure or by restricting the maximum accelerations below the limits for pedestrian comfort. This paper discusses vibration serviceability assessment of a highly trafficked local pedestrian bridge based on the field dynamic tests. The selected bridge is a 60-m-long three-span steel structure with a continuous reinforced concrete slab supported on two longitudinal steel girders. First, a finite element model of the pedestrian bridge is developed to obtain the natural frequencies and mode shapes. Then, ambient vibration tests are conducted to validate the modal characteristics of the pedestrian bridge. Next, the dynamic response of the bridge in terms of peak accelerations is determined both experimentally and analytically under various pedestrian excitations. Finally, the implications of the results for the serviceability limit state assessment of the pedestrian bridge are discussed. Keywords Modal analysis • Dynamic testing • Vibrations • Footbridges • Serviceability 17.1 Introduction As opposed to highway or rail road bridges, pedestrian footbridges are not typically subjected to heavy traffic loads. In their design and construction process, several light and slender members are being used to not only take advantage of the low strength demand on the system, but also to fulfill the architectural demands which more often than not seek artistic designs with longer spans. These geometrical characteristics of the pedestrian footbridges make them susceptible to human induced vibrations, as they tend to have natural frequencies similar to those detected for the pedestrian walking. Although current design methodologies guarantee the safety and stability of the lightweight footbridge structures, it is their serviceability, in terms of level of comfortless and safety that pedestrians feel, that has been the major concern affiliated with this specific types of structures. Some well-known examples of serviceability issues associated with in-service pedestrian bridges include the London Millennium footbridge [1], the Pont du Solferino in Paris [2], and the T-Bridge in Japan [3]. In order to address these challenges, several code and design provisions have provided guidelines to control the vibration of footbridges to satisfy the corresponding serviceability limit states. Eurocode5 [4], British National Annex [5], French Guideline SETRA [6], and the European guideline HIVOSS [7] are the examples of existing guidelines which have characterized the pedestrian loading under crowd conditions and provided methodologies to predict the response of the bridge system using the SDOF approach or finite element analysis [8]. In the design codes, the serviceability of a given footbridge system is being evaluated through limiting the natural frequencies of the system to avoid pedestrian-vibration induced resonance. The corresponding limits for the structural modal frequency are called critical frequencies. If the given footbridge fails to satisfy the limits of the code, the serviceability of the structure shall then be evaluated by limiting the levels of vibration (in terms of maximum acceleration) under human induced excitations. The values of modal frequencies for any given footbridge can be obtained through experimental testing (vibration testing) or computational simulations (finite element analysis). However, the accuracy of the numerical results highly depends on the modeling approach, details of the structural components included in the model, and the corresponding analysis approach. The experimental data can also be used to calibrate the numerical models, so that further details on the response of the structure can be collected from the structural analysis. As opposed to the natural frequencies and mode shapes that are intrinsic characteristics of any structural system, the acceleration response of a structure under human excitations is a function of A. Gheitasi • S. Usmani • M. Alipour • O.E. Ozbulut ( ) • D.K. Harris Department of Civil and Environmental Engineering, University of Virginia, Charlottesville, VA, USA e-mail: ozbulute@yahoo.com © The Society for Experimental Mechanics, Inc. 2016 M. Allen et al. (eds.), Dynamics of Coupled Structures, Volume 4, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-29763-7_17 163
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