7 results on '"Ingólfsson, Einar Thór"'
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2. Pedestrian-induced lateral vibrations of footbridges:Experimental studies and probabilistic modelling
- Author
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Ingólfsson, Einar Thór
- Abstract
Denne afhandling omhandler vandrette fodgængerinducerede svingninger svarende til dem der bl.a. blev observeret på Londons præstigefulde gangbro, Millennium Bridge i året 2000 og Solferino-broen i Paris i 1999. I særdeleshed har den midlertidige lukning af førnævnte broer gjort ingeniører og arkitekter bevidste om de potentielle svingningsproblemer som fodgængere kan forårsage i gangbroer. Som en følge heraf er der opstået en ny forskningsretning som søger at forstå effekten af fodgængerinducerede laster på gangbroer og kvantificere interaktionen mellem fodgængere og et vandret bevægende underlag. I det sidste årti er antallet af videnskabelige publikationer indenfor dette område steget stødt og stadig flere tilfælde af broer der har oplevet lignende voldsomme svingninger er blevet offentliggjorte. Til trods for dette findes der på verdensplan kun et fåtal normer og retningslinjer med vejledninger til undgåelse af disse problemer i nye gangbroer. Disse vejledninger bygger typisk på en antagelse om at fodgængerinducerede vandrette laster er proportionale med broens svingningshastighed, hvorfor deres effekt kan modelleres som negative dæmpere hidrørende fra en synkroniseret bevægelse af fodgængerne. Denne ekscitationsmekanisme er ofte karakteriseret som Synkroniseret Vandret Ekscitation (e. Synchronised Lateral Excitation – SLE). Observationer fra et begrænset antal fuldskala gruppeforsøg har eftervist forekomsten af en form for ustabilitet, karakteriseret ved en uforholdsmæssig stigning i den vandrette respons ved en forøgelse af antallet (udover et kritisk antal) af fodgængere på broen. Fænomenet skyldes en form for interaktion mellem fodgængerne og den vandrette bevægelse af broen, men de styrende mekanismer i forbindelse med lastfrembringelsen debatteres stadig. I denne afhandling præsenteres et omfattende litteraturstudie der omhandler fodgængerinducerede vandrette laster, deres effekt på gangbroer og teoretiske modeller af interaktionen derimellem (e. Human-Structure Interaction). Det vises her at der eksisterer forskellige hypoteser vedrørende interaktionens karakter. Mange af disse beror udelukkende på teoretiske modeller og mangler fyldestgørende empirisk dokumentation til at understøtte og retfærdiggøre deres anvendelse. I særdeleshed betvivles vigtigheden af personers synkronisering til broens svingninger som forudsætning for udvikling af voldsomme sideværts svingninger. I dette studium er der udført eksperimentelle undersøgelser, hvis formål er at bestemme de laster som hidrører fra personers gang på et vandret bevægeligt underlag, nærmere betegnet et løbebånd. Lasterne er bestemt under to forskellige forhold. I den første del fastholdes løbebåndet mod vandret bevægelse hvorefter den anden del er karakteriseret ved en påtvungen sinusformet bevægelse ved forskellige kombinationer af frekvens (0.33- 1.07 Hz) og amplitude (4.5-48mm). Dette studie omfattede 71 personer der tilsammen xi tilbagelagde 55 km fordelt på næsten 5000 individuelle forsøg. En detaljeret analyse af forsøgspersonernes bevægelsesmønstre afslørede at synkronisering mellem personen og det bevægelige underlag ikke er nødvendig for at forårsage store vandrette svingninger i gangbroer. Derimod fremkommer de store svingninger fra hastighedskorrelerede kræfter i form af negativ dæmpning, der kan opstå uafhængigt af forholdet mellem gangfrekvensen og frekvensen af den vandrette bevægelse. Disse kræfter fremkaldes af accelerationer af kroppens tyngdepunkt forårsaget af underlagets bevægelse, hvorfor de beskrives som selvinducerede (e. self-excited). Grundet den store spredning i forsøgsresultaterne, samt det faktum af menneskeinducerede laster generelt er underlagt en stor usikkerhed, er der udviklet en stokastisk model der beskriver frekvens- og amplitudeafhængigheden af de vandrette laster hidrørende fra fodgængere. Parametrene i modellen er baseret direkte på forsøgsresultaterne og udgør dermed et robust og statistisk pålideligt værktøj til modellering af fodgængerinducerede vandrette vibrationer. Det demonstreres her, at responsen af gangborer udsat for last fra menneskegrupper er følsom over for valg af diverse inputparametre. Disse tæller blandt andet fordelingen af gangfrekvenser i gruppen, størrelsen af lasten fra de øvrige omgivelser eksempelvis vind, samt lastvarigheden. Valget af disse parametre har en afgørende betydning for antallet af fodgænger, der kan forårsage voldsomme svingninger i et givet scenarie. Afslutningsvis præsenteres et forenklet stabilitetskriterium der kan anvendes til at bestemme det kritiske antal personer som kan ophæve konstruktionens dæmpning og dermed forårsage voldsomme svingninger. The dissertation investigates the phenomenon of excessive pedestrian-induced lateral vibrations as observed on several high-profile footbridges. In particular, the temporary closures of both Paris’ Solferino Bridge (1999) and the London Millennium Bridge (2000) have led to an understanding on the part of engineers and architects of the need to evaluate the potential for footbridge vibrations that can be attributed to pedestrians. Within the scientific community, the closures have also led to the initiation of a new tract of research, focused on the understanding of pedestrian loading, bridge response and their interaction. In the last decade, a significant amount of research has been carried out in this field. As a consequence, numerous other bridges of different length and type have been found prone to similar excessive lateral vibrations when exposed to large pedestrian crowds. However, only few national and international codes of practice and official design guidelines currently exist to help the designer address this issue. Most of these are based on the main hypothesis, that pedestrian-induced lateral loads can be modelled as velocity proportional or as negative dampers, resulting from the synchronised lateral movement of pedestrians. This excitation mechanism is often characterised as Synchronous Lateral Excitation (SLE). Reports from a limited number of controlled pedestrian crowd tests have verified the existence of a transition point at which a rapid increase in the lateral bridge response is triggered. This disproportionate increase in the lateral vibration response is caused by a dynamic interaction between the pedestrian and the laterally moving structure, although the governing mechanism which generates the load is still disputed. In this thesis, a comprehensive literature review is presented, solely focused on pedestrianinduced lateral forces, their effect on footbridges and existing theoretical models of humanstructure interaction. It is shown that different hypotheses exist about the nature of this interaction, many of which are only supported by theoretical modelling and lack sufficient experimental evidence to support their applicability. Especially, the importance of human-structure synchronisation for the development of large footbridge vibrations is questionable. Therefore, an extensive experimental campaign has been carried out to determine the lateral forces generated by pedestrians during walking on a laterally moving treadmill. Two different conditions are investigated; initially the treadmill is fixed and then it is laterally driven in a sinusoidal motion at varying combinations of frequencies (0.33 – 1.07 Hz) and amplitudes (4.5 – 48mm). The experimental campaign involved 71 test subjects who covered approximately 55 km of walking distributed on almost 5000 individual tests. An in-depth analysis of the movement of the pedestrians that participated in the experimental campaign reveal that synchronisation is not a pre-condition for the ix development of large amplitude lateral vibrations on footbridges, as walking frequencies remain largely unaffected by the lateral motion. Instead, large amplitude vibrations are the result of correlated pedestrian forces in the form of negative damping that can be generated irrespective of the relationship between the walking frequency and the frequency of the lateral movement. These forces are self-excited in the sense that they are generated by the movement of the body’s centre of mass, which in turn is caused by the lateral acceleration of the underlying pavement. Due to the random nature of the human-induced loadings and a large scatter in the experimental data, a novel stochastic load model for the frequency and amplitude dependent lateral forces is presented. The parameters in the model are based directly on the measured lateral forces from the experimental campaign. Thereby, the model is currently the most statistically reliable analytical tool for modelling of pedestrian-induced lateral vibrations. It is shown that the modal response of a footbridge subject to a pedestrian crowd is sensitive to the selection of the pacing rate distribution within the group, the magnitude of ambient loads and the total duration of the load event. The selection of these parameters ultimately affects the critical number of pedestrians needed to trigger excessive vibrations in a particular simulation. Finally, a simplified frequency dependent stability criterion is presented, for which the critical number of pedestrians needed to cancel the inherent modal damping of a footbridge can be obtained.
- Published
- 2011
3. Lateral Pedestrian-Induced Vibrations of Footbridges: Characteristics of Walking Forces
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Ricciardelli, Francesco, primary, Mafrici, Michele, additional, and Ingólfsson, Einar Thór, additional
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- 2014
- Full Text
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4. Pedestrian-Induced Lateral Forces on Footbridges
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Ingólfsson, Einar Thór, primary, Georgakis, Christos T., additional, Jönsson, Jeppe, additional, and Ricciardelli, Francesco, additional
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- 2011
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5. Modeling Spatially Unrestricted Pedestrian Traffic on Footbridges
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Živanović, Stana, primary, Pavić, Aleksandar, additional, and Ingólfsson, Einar Thór, additional
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- 2010
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6. Modeling Spatially Unrestricted Pedestrian Traffic on Footbridges.
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Zˇivanovic, Stana, Pavic, Aleksandar, and Ingólfsson, Einar Thór
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RANDOM walks ,FOOTBRIDGES ,MATHEMATICAL models ,PEDESTRIANS ,MONTE Carlo method ,SIMULATION methods & models - Abstract
The research into modeling walking-induced dynamic loading and its effects on footbridge structures and people using them has been intensified in the past decade after some high profile vibration serviceability failures. In particular, the crowd induced loading, characterized by spatially restricted movement of pedestrians, has kept attracting attention of researchers. However, it is the normal spatially unrestricted pedestrian traffic, and its vertical dynamic loading component, that is the most relevant for vibration serviceability checks for most footbridges. Despite the existence of numerous design procedures concerned with this loading, the current confidence in its modeling is low due to lack of verification of the models on as-built structures. This is the motivation behind reviewing the existing design procedures for modeling normal pedestrian traffic in this paper and evaluating their performance against the experimental data acquired on two as-built footbridges. Additionally, the use of Monte Carlo simulations is also investigated. Possible factors that cause discrepancies between measured and calculated vibration responses, including possibility of existence of pedestrian-structure dynamic interaction, are discussed. [ABSTRACT FROM AUTHOR]
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- 2010
- Full Text
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7. PEDESTRIAN-INDUCED LATERAL FORCES ON FOOTBRIDGES.
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Ingólfsson, Einar Thór, Georgakis, Christos T., Jönsson, Jeppe, and Ricciardelli, Francesco
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FOOTBRIDGES , *PEDESTRIAN areas , *PEDESTRIAN areas design , *VIBRATION (Mechanics) , *PAVEMENTS , *FREQUENCY response - Abstract
This paper investigates the phenomenon of excessive pedestrian-induced lateral vibrations as observed on several high-profile footbridges. The vibrations are a consequence of human-structure interaction, in which the forces generated by the pedestrians depend strongly on the vibration of the underlying pavement. An extensive experimental analysis has been carried out to determine the lateral forces generated by pedestrians when walking on a laterally moving treadmill. Two different conditions are investigated; initially the treadmill is fixed and then it is laterally driven in a sinusoidal motion at varying combinations of frequencies (0.33-1.07 Hz) and amplitudes (4.5-48 mm). The component of the pedestrian-induced force which is caused by the laterally moving surface is herewith quantified through equivalent velocity and acceleration proportional coefficients. It is shown that large amplitude lateral vibrations are the results of correlated pedestrian forces in the form of negative damping, with amplitudes that depend on the relationship between the step frequency and the frequency of the lateral movement.. [ABSTRACT FROM AUTHOR]
- Published
- 2012
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