On the Dynamic Assessment of a Cable-Stayed Footbridge: The Iron BridgeColmenares, Daniel; Ruhani, Shaho; Karoumi, Raid
doi: 10.1088/1742-6596/2647/12/122016pmid: N/A
Human-induced loads may produce resonance when the forcing frequency coincides with the natural frequency of the system. In this work, the dynamic assessment of a pedestrian cable-stayed bridge in Uppsala, Sweden is presented. The dynamic properties of the system have been identified and a loading scenario is evaluated. A comparison between the theoretical and measured acceleration of the bridge is made using a detailed finite element model. Different modelling aspects are considered and evaluated by studying their influence on the natural frequencies of the system such as the tie rods system, railings, and boundary conditions. Moreover, a parametric analysis of the elastic modulus of the cast iron material with respect to the boundary conditions of the system is presented to quantify the uncertainties of the system. Special focus is given to the resonant response of the first natural frequency of the bridge potentially compromising the serviceability limit state of the structure. Furthermore, the scenario of a single pedestrian jogging on the spot is evaluated and a hypothetical standing crowd is considered to illustrate the benefits of taking into account the human-structure interaction effect. A considerable reduction of the dynamic response of the system is found, highlighting the importance of the human-structure-interaction effect.
Modal analysis of a footbridge under pedestrian traffic and additional shaker loadingPurpura, Laura; Güner, Hüseyin; Hoffait, Sébastien; Dënoel, Vincent
doi: 10.1088/1742-6596/2647/12/122015pmid: N/A
This paper investigates the possibility of identifying the modal properties of footbridges while they are subjected to pedestrian traffic using the CSI algorithm. The analysis is based on vibration data collected from a footbridge under different pedestrian densities. The results show that the CSI algorithm provides consistent modal properties, but the identified damping ratio seems to be significantly influenced by pedestrians. This is likely due to the human-structure interaction. Identified apparent damping ratios appear plausible, but of course not representative of the bare structure. Overall, while the CSI algorithm helps alleviate the limitations of the authors’ previous approache to modal identification with a custom shaker, the practical usefulness of identifying modal properties of footbridges under pedestrian traffic is therefore limited.
A Multiaxial Test Framework for the Evaluation of Human Gait-Induced Loads on Lateral Harmonic SurfacesCastillo, Bryan; Thomson, Peter; Marulanda, Johannio
doi: 10.1088/1742-6596/2647/12/122004pmid: N/A
The introduction of increasingly resistant and light-weight materials in the construction industry, coupled with the hypothesis of a global regeneration of urban structures with higher technical and aesthetic requirements, has resulted in civil structures such as bleachers, stairs, slabs, and foot-bridge being vulnerable to excessive vibrations due to dynamic loads, especially human-induced loads. These loads present adaptive phenomena due to structural vibrations generated by the coupling effects of Human-Structure Interaction (HSI). Two main aspects are considered in the effects of HSI: the change in dynamic properties of the structure due to the additional presence of non-stationary mass, and the degree of coupling between people in transit, as well as between them and the structure. Therefore, this paper focuses on the study of the last aspect considered through the development of a Dynamic Platform, the Human-Structure Interaction Multiaxial Test Framework (HSI-MTF), to acquire three-dimensional loads induced by human gait under the effects of lateral harmonic motions. An experimental campaign was conducted with a test subject to evaluate gait loads under lateral sinusoidal movements and on rigid surface. The lateral loads, and frequency content induced by the human gait during the HSI-MTF lateral surface displacement protocols were analyzed.
A general approach to model human-structure interaction in the case of slender structuresVanali, M.; Manzoni, S.; Berardengo, M.; Lucà, F.
doi: 10.1088/1742-6596/2647/12/122003pmid: N/A
The paper deals with a general approach for modeling human-structure interaction in slender structures. Vertical vibrations are addressed in the paper. The proposed approach relies on the model of the complete system and on a solution procedure capable to account for the most complex dynamic conditions. The effects of changes in pedestrians positions along the structure and of changes in pedestrians postures during motion are combined, therefore taking into account different and time varying mechanical impedances.Moreover, the developed model describes the coupled system (i.e., structure plus people on the structure) through the definition of two different actions of pedestrians: passive and active forces. This approach was used to evaluate the dynamic interactions between a light slender structure and pedestrians. The proposed approach was validated by carrying out experimental tests on a staircase and results show a good match between the predicted vibrations and the experimentally found ones.
Experimental study of active human-structure interaction during running on footbridgesLottefier, Jasper; Broeck, Peter Van den; Vanwanseele, Benedicte; Nimmen, Katrien Van
doi: 10.1088/1742-6596/2647/12/122008pmid: N/A
Slender footbridges are predominantly sensitive to human-induced vibrations. While the previous two decades have focussed on pedestrian-induced vibrations, topical research questions are investigating the impact of running actions. The currently available load model for vibration serviceability assessments only applies to a single runner on a rigid surface. The present work investigates whether low-frequency vertical vibrations of the supporting surface affect the running motion, known as active human-structure interaction. To facilitate characterisation, an extensive measurement campaign was organised. First, a treadmill was placed on a rigid surface as reference and secondly on a vibrating footbridge, excited by a mechanical shaker. The measurement setup enables the simultaneous registration of the contact forces, body motion, running motion metrics on a step-by-step basis and the structural response. The results indicate that human-structure interaction phenomena do occur while running on a vertical vibrating surface. The main effects include changes of the peak contact force, determined by the relative phase between the body motion and the footbridge’s motion. Moreover, when running (near) resonance, two stable running regimes can occur. The first resembles a non-resonant runner (constantly alternating between in- and out-of-phase) while the second resembles resonant running by keeping a fairly constant phase with the structure.
Determination of cable tension force in pedestrian suspension bridge short hangers based on finite element model updatingEreiz, Suzana; Duvnjak, Ivan; Pajan, Jurica; Bartolac, Marko; Damjanović, Domagoj
doi: 10.1088/1742-6596/2647/12/122013pmid: N/A
The hangers are the important element on suspension bridges that transfer the forces from the deck to the main cable. To verify their capacity and identify possible risks, the forces transferred by the hangers and the corresponding tension must be determined. However, this is often done using standard equations for determining cable forces. Due to the different lengths of the hangers and the way in which their tensioning is achieved, the effects of boundary conditions occur that need to be considered and that require an update of the finite element model at the local level (when the whole bridge structure is observed). This paper presents a method for determining the cable tension of hangers that combines the experimentally determined dynamic properties (natural frequencies and mode shapes) and the numerical model updating. In addition to the usual approach based on the determination of the natural frequency of hangers, this paper presents an approach based on the mode shapes of the hangers. Special attention is paid to the boundary conditions coefficient. The method is applied on suspension bridge case study.
Lightweight footbridges subjected to streams of pedestrians: tests and discussion of a crowd-structure interaction modelGallegos-Calderón, Christian; Naranjo-Pérez, Javier; Jiménez-Alonso, Javier F.; Díaz, Iván M.
doi: 10.1088/1742-6596/2647/12/122012pmid: N/A
Human-induced vibrations in slender lightweight footbridges have been extensively investigated in the last 30 years. In existing guidelines, the dynamic response of a footbridge can be assessed considering the pedestrians as external non-interacting loads acting on the structure. For a more realistic load model, individuals defined as single Degree of Freedom (DOF) dynamic systems moving along the structure have been employed by researchers and practitioners. Hence, Human-Structure Interaction (HSI) is considered in the dynamic analysis. Adopting this approach to calculate the vibration levels on a footbridge may lead to a costly computational problem when dealing with a crowd due to the large number of DOFs associated to the pedestrians. Recently, a frequency-domain procedure based on a coupled crowd-structure system has been proposed by the authors to overcome the aforementioned issue. To further validate the proposal, the analytical results should be contrasted with experimental measurements. In this sense, this paper presents the dynamic response of two footbridges under the action of streams of walking pedestrians. A laboratory fibre reinforced polymer structure is firstly analysed, and an in-service cable-stayed steel footbridge is secondly studied. The vibration serviceability of both pedestrian structures is assessed considering 0.20 and 0.50 pedestrians/m2. For each bridge, the experimental results are compared with predictions computed through the application of load models from existing guidelines. In addition, the frequency-domain proposal is employed to calculate the steady-state structural response while accounting for HSI. Since dynamic parameters of the human body must be defined for the second approach, different values available in literature are employed. Results demonstrate the overestimation of the response when provisions stated in current design documents are used. Also, the benefits of the novel procedure to consider Crowd-Structure Interaction on lightweight footbridges are highlighted.
Vibration serviceability of footbridges in crowded conditions: crowd dynamics simulations vs guidelines’ predictionsTubino, F; Venuti, F
doi: 10.1088/1742-6596/2647/12/122002pmid: N/A
Vibration serviceability assessment in crowded conditions requires a reliable human-induced loading model taking into account pedestrian interaction. Current guidelines provide simplified procedures to determine the maximum dynamic response based on very simplified loading models. The main objective of this paper is to assess the reliability of current guidelines for vibration serviceability assessment of footbridges. With this aim, an extensive campaign of numerical simulations of bidirectional pedestrian traffic is carried out through an agent-based model, considering variable pedestrian densities and deck widths. The results of numerical simulations are first compared against the experiments available in the literature in terms of fundamental diagram of the mean walking speed as a function of pedestrian density. Then, starting from numerical simulations, the dynamic response of a class of footbridges is estimated numerically and compared with guidelines’ predictions in order to assess their reliability.
Field-testing and serviceability assessment of a lively footbridgeMulas, Maria Gabriella; Fortis, Cristina; Lastrico, Giulia
doi: 10.1088/1742-6596/2647/12/122005pmid: N/A
Following a previous work, the paper describes the serviceability assessment of a footbridge over the Lambro River near Milano (Italy), based on the comparative analysis of on-field tests and of the outcome of the Hyvoss guidelines. The 3-span footbridge, for a bicycle-pedestrian mixed use, is 107 m long, 4.4 m wide and roughly symmetric about both mid-span and the longitudinal axis. A reinforced concrete (RC) deck is supported by a steel structure: two longitudinal welded I-profiles, braced in their lower part, support the transverse beams that, in turn, support the RC deck. Ambient vibration tests identified the footbridge modal properties, detecting the fundamental bending mode, with the maximum amplitude recorded at mid-span, at 1.75 Hz, well within the critical range of excitation from walking pedestrians. Hence, a series of forced vibration tests was subsequently performed, involving single pedestrians or groups of up to 12 people. Sensors, located as in AVTs, recorded footbridge acceleration, both horizontal and vertical. In all the tests, pedestrians walked in resonance conditions with the first mode frequency along straight trajectories. To excite further the first mode, their spatial configurations were symmetric about the longitudinal axis of the bridge. Difference among tests concerns not only the number of test subjects (TSs), but also their spatial configuration and/or the TSs involved in each test. The comparison among tests sharing similar spatial configurations, with TSs either in a row or in a column, allows a serviceability assessment directly from the experimental footbridge response. The numerical analyses are based on a FE model previously developed, showing an excellent reproduction of the first bending mode, but a poor simulation of the second torsional mode. The outcome of the analyses performed according to HiVoSS guideline is in good agreement with the experimental results, in spite of the very low crowd density of the on-field tests and the unsatisfactory reproduction of the torsional mode within the FE model.
A first step towards the development of load models for the vibration serviceability assessment of footbridges under running actionsStrobbe, Senne; Lottefier, Jasper; Broeck, Peter Van den; Nimmen, Katrien Van
doi: 10.1088/1742-6596/2647/12/122007pmid: N/A
The state of the art involving dynamic running actions is limited to single-person load models valid for running on a rigid laboratory floor. Little to no expertise is available on running excitation as a load scenario for civil engineering structures. The development of realistic load scenarios requires input on (1) intra-person variabilities, (2) inter-person variabilities and (3) human-structure interaction phenomena, and their impact on the resulting structural response. This contribution uses vibration measurements on the lower back of running persons to identify and characterize intra- and inter-person variabilities of the dynamic running load. Based on these results, the impact of these variabilities on the resulting structural response is investigated numerically. The results show that as the impact of intra-person variability is negligible, it can be disregarded in design calculations. However, the impact of inter-person variability is shown to be considerable, reducing the resulting structural response with 64% and more. Realistic ranges of inter-person variability should therefore be considered in design calculations.