1,721,117 research outputs found
Modelling train-induced vibration of structures using a mixed-frame-of-reference approach
No full textA novel computational modelling approach for prediction of environmental vibration is introduced. The model is formulated in both moving and fixed frames of reference, with a mixed frame of reference formulation introduced to couple the two frames of reference. The resulting system is able to model a vehicle travelling on an infinite railway track, formulated in a moving frame of reference, interacting via the soil with a structure (i.e. building), formulated in a fixed frame of reference. The method utilizes a semi-analytical soil model with the structures modelled using three-dimensional finite elements. Two solution procedures of the full system are proposed: partial coupling, where some secondary effects from reflected waves propagating through soil are disregarded, and full coupling, where the vehicle–track–soil–structure is modelled as a fully coupled system. Both proposed solution procedures offer a one-step approach for solving the whole system in the frequency–spatial domain. The usage of the model is demonstrated in two example cases: one analysing a simple building structure near a railway track, using the partial coupling solution procedure, and another one analysing the behaviour of a vehicle model traversing over a rigid block embedded inside the soil, using the full coupling solution procedure. The introduced modelling approach offers a computationally efficient solution procedure, at the same time being applicable to a wide array of application cases
Nonlinear analysis of earthquake-induced vibrations
No full textThe report is a support for the study of structural behaviorunder certain loads.First, simply suported steel beams and simply supported reinforcedconcrete beams, under uniformly distributed loads, areanalyzed, by both analytical and numerical methods. Second,embedded columns and multy-storey structures, made out rstof steel and then reinforced concrete, are studied.The analytical equations are derived according to the theoryinvolved and then encoded in Matlab so they can be usedin then future for other similar problems. For the numericalanalysis, two Finite Element programs are used, i.e. Abaqusand a 2D code created in Matlab.The report is a useful tool for the ones who want to studyin detail reinforced concrete sections, plastic behavior, andalso for the ones who want to learn how to model structuressubjected to seismic loads in Abaqus, by means of accelerograms.The report is supported by a DVD. Its content can also be requestedby contacting the author at [email protected] [email protected]
Dynamic Modeling of a Bridge Subjected to Seismic Waves
No full textI denne kandidatopgave undersøges udbredelsen af seismiske bølger op gennem et givetantal jordlag og videre igennem en brokonstruktion med en total længde på 2 km. Deter kun horisontale forskydningsbølger der undersøges, da horisontale accelerationerer mest kritisk for de fleste konstruktioner i forhold til vertikale accelerationer. Forseismisk design har lokale dæmpnings- og isærforstærknings effekter stor betydningmht. udbredelsen af seismiske bølger.Bølgeudbredelsen gennem bløde jordlag er modeleret ved brug af en endimensionaldynamisk lineær viskoelastisk semi-analytisk Domain Transformation Method (DTM)model, samt ved brug af en dynamisk viskoelastisk endimensional Finite ElementMethod (FEM) model. Resultaterne for de to modeller er sammenlignet for at evalueremodellerne, og de stemmer godt overens. Modellerne er løst i frekvensdomænet forat gøre det muligt at anvende hysteretisk dæmpning, som er anset for at være enudmærket dæmpningsmodel for jord. En delvis lineær model er introduceret i FEMmodellen for at tage hensyn til reduktion af tangentforskydningsmodulet samt forøgelseaf dæmpningsfaktoren ved stigende forskydningstøjninger.Jordskælvsdata fra jordskælvet i Aqaba 1995 er anvendt. Responsen fra de anvendtejordlag er bestemt og derefter brugt som input til de bropiller, som står på de blødejordlag. Der sker betydelig forstærkning, når jordskælvet udbredes op gennem de blødejordlag. Bølgeudbredelsen for broen er bestemt ved anvendelse af en tredimensionaldynamisk viskoelatisk FEM model. Både tryk, vridnings samt forskydningsbølger kanmodelleres med modellen.Et lille parameterstudie er udført, hvor den horisontale retning for de seismiske bølgervarieres. En anden parameter som varieres er den tilsyneladende hastighed som deseismiske bølger udbreder sig med gennem klippen under de bløde jordlag. Variationaf denne parameter medfører en forsinkelse af signalet fra bropille til bropille. Resultatetaf parameter studiet viser, at en udbredelses retning langs med broen er mest kritisk.Nogle af bropillerne er placeret i vand, hvorfor hydrodynamisk masse er anvendt imodellen.In this thesis the propagation of seismic waves through a number of soft soil layers andup into a bridge with a total span of 2 km is investigated. Only horizontal shear waves(SH-waves) are modeled, as most buildings are often most vulnerable to excitations inthe horizontal direction. In seismic design the local site effects of soft layers is oftenof big significance, as the amplification of seismic waves when they propagate throughsoft soil layers is often quite significant.The wave propagation through the soft soil layers are modeled using both a dynamiclinear viscoelastic semi-analytic Domain Transformation Method (DTM) and a dynamiclinear viscoelastic Finite Element Method (FEM) model. The models are compared inorder to validate the results, and the results fit quite nicely. The models are solved in thefrequency domain in order to use hysteretic damping, which is recognized as a gooddamping model for soils. A partly linear model is introduced in the FEM soil model inorder to take into account decrease of the small strain shear modulus and increase ofthe damping ratio as the shear strain amplitude increases.Strong motion data for the 1995 Aqaba Earthquake, which had a moment magnitudeof 7.3, is used. The soil response is determined and used as an input to the bridgemodel. It is found that the soft soil layers amplifies the seismic waves with severalmagnitudes. The wave propagation through the bridge is modeled using a dynamiclinear viscoelastic three dimensional FEM beam model. Both compressional, torsionaland shear wave propagation through the bridge can be modeled. The angle at whichthe the SH-waves hits the bridge can also be varied, which is used in a small parameterstudy.Another parameter, which is investigated is the delay at which the seismic waves hitsthe different columns. The results of the parameter study show that the critical directionof the earthquake is parallel to the bridge due to the fact that high normal stresses andthus compressional waves occur in the bridge deck. The bridge is located in water andhydrodynamic mass is therefore applied in the bridge model. Soil structure interactionis also taken into account by using a lumped parameter model to model the rotational aswell as the torsional stiffness of the foundations which are located on the soft soils. Forthe columns founded directly on bedrock, however, the rotational and torsional stiffnessis assumed infinitely stiff
Comparison between 2D and 3D considerations of a railway embankment
No full textDette projekt omhandler numeriske betragtninger af banedæmninger i 2D og 3D. Hovedformålet er at opstille tendenser, som gør det muligt at simplificere tidskrævende 3D analyser til ækvivalente 2D analyser. Der er udført et state of the art for at redegøre for eksisterende studier vedrørende betydningen af simplificeringen af tre- til todimensionelle betragtninger af banedæmninger. Dette omhandler blandt andet studier vedrørende de betydningen af de geometriske variationer i tredje dimension. Desuden er et todimensionelt lineær-elastisk FE-program opbygget i MATLAB, som kan anvendes til bestemmelse af influenslængden. Dette Fe-program beskriver lastfordelingen i den tredje dimension vinkelret på dæmningstværsnittet og tager desuden højde for den trykspredning, som forekommer i dæmningen. Fe-programmet anvendes sammen med de kommercielle FE-programmer PLAXIS 2D og 3D til at opstille en tendens for influenslængden i anvendelsesgrænsetilstanden ved at sammenligne de maksimale flytninger i 2D og 3D. I brudgrænsetilstanden viste det sig at være sværere at beskrive en tendens for influenslængden, da bestemmelsen af den præcise brudlast i PLAXIS var problematisk. Dette skyldes måden hvorpå brudlasten beregnes i PLAXIS, hvilket besværliggjorde en direkte sammenligning mellem brudlasterne i 2D og 3D.This master of science project considers 2D and 3D representations of railway embankments with a focus on constituting relationships that make it possible to simplify the computationally expensive 3D cases into equivalent 2D cases. As a result the problem formulation is stated as:- Which studies have so far been carried out concerning the simplification from 3D to 2D representations of railway embankments?-How can 3D representations of a loaded railway embankment be simplified to equivalent 2D representations based on numerically determined lengths of influence in the service limit state and ultimate limit state?A state of the art survey has been carried out to investigate existing studies within the field. From the survey it was seen that the safety factor in 3D is generally larger than the safety factor of a corresponding 2D case. The influence of geometric variations in the third dimension have also been investigated for turns and corners and these effects generally amplify the difference between the safety factors. Other studies also consider the influence of the height and the width of the embankment.In order to simplify the 3D states to equivalent 2D states a 2D linear elastic finite element program has been developed in MATLAB. A model was set up in the MATLAB program to determine the load distribution in the direction perpendicular to the cross-section plane of the embankment. Based on the MATLAB-model and models created in the commercial FE programs PLAXIS 2D and 3D it was possible to establish correlations between the length of influence for two load cases in the service limit state. In the first load case a single load of 250 kN was applied on each rail while 4 loads of the same size were applied on each rail in the second load case. Furthermore, it was verified that superposition principles were applicable to determine the corresponding deformations arising from other load cases than the two specific load cases considered.In the same way it was sought to establish a correlation in the ultimate limit state. However, it was not possible to obtain satisfactory results of the ultimate load in PLAXIS 3D due to the way that these loads are determined in PLAXIS. Consequently, it is necessary to determine the ultimate load in a different manner to obtain reliable results of the ultimate loads but this lies within a future study
Ground Vibrations: Effects of masses placed on or in the ground
No full textDenne rapport omhandler et reelt og aktuelt problem for bygninger påvirket af vibrationer genereret af forskellige årsager. Projektet fokuserer på formering af vibrationer gennem jorden og undersøger løsninger for at mindske størrelsen af disse vibrationer. En relativ ny teori er analyseret for at forhindre vibrationerne ved at placere en masse mellem vibrationens kilde og modtageren, som enten kan være placeret på jorden eller indlejret i jorden.Problemet er analyseret gennem to metoder; laboratorie tests foretaget på en skaleret model og numerisk analyse. Forskningsarbejdet er startet med at skalere vibrationernes karakteristika ved brug af tilgængelige materialer til anvendelige laboratorie dimensioner. Derudover er materialernes og set uppets adfærd trin-for-trin undersøgt ved at bruge specialiseret udstyr (accelerometer, impulshammer og Pulse software til at bearbejde data) i format af frekvensresponsfunktioner (FRF) analyse. Undersøgelsen leder op til to endelige forsøgsopstillinger, og hver af dem med mulighed for at blive tilpasset med en række masse konfigurationer. Den første model simulerer et enkelt jordlag hvor massen er placeret på overfladen mellem exciter og en modtager. Den anden model forestiller et tykkere jordlag med massen indlejret i selve jorden. Efterfølgende er en række numeriske modeller simuleret med software programmet Abaqus CAE med de samme randbetingelser og materiale data, som blev brugt i forsøgene. Resultaterne fra laboratorie forsøgene og de numeriske modeller er derefter sammenlignet, hvorefter de numeriske modeller er valideret. Det er observeret, at den numeriske model matcher de relative simple modeller bedre, hvor sandsynligheden for usikkerheder er mindre. Effekten af at tilføje masse kunne observeres ved alle test set ups både eksperimentelt og numerisk. Den respons registreres dæmpning af de accelerationer på nogle frekvensområder, men på samme tid øget respons kunne observeres ved andre frekvenser. Den væsentligste positive indflydelse observeres ved at tilføje flere masser mellem exciter og modtager der simulerer en periodisk konfiguration af masserne.This report concerns a real and actual problem of buildings affected by vibrations generated by various urban activities. The project focuses on the vibrations propagation through ground and investigates solutions to reduce the magnitude of these vibrations. In order to mitigate vibrations a relative new idea is studied by using masses placed in between the vibration source and the receiver which can be either placed on the ground surface or embedded in the ground.The problem is studied from two different approaches, a small scale laboratory experiment and by numerical models. The research work starts with the task of scaling the vibrations characteristics to convenient laboratory dimensions while using accessible materials. Furthermore the behavior of the materials and of the test setup is investigated step by step using specialized equipment (accelerometers , impact hammer and a data processing software PULSE) in the format of a frequency response function (FRF) analysis. The investigation lead to a number of two final experimental test setups, each of them having the possibility to be adapted to a series of added masses configurations. The first model simulates a single soil layer with masses positioned on the surface between an exciter and a receiver. The second model resembles a thicker layer of soil with the masses embedded in the ground volume. Subsequently, using the commercial software Abaqus CAE a series of numerical models are modeled with the same boundary conditions and the same materials used in the experimental analysis. The results from the experimental and numerical analysis are then compared and the ability of the numerical models of simulating this phenomenon is tested. It was discovered that the numerical model matches better relative simple models where less uncertainties are likely to be introduced. The effect of added masses could be observed in all test setups configurations for both experimental and numerical analyses. The response recorded attenuation of the accelerations at some frequency ranges but in the same time increased response could be observed at other frequencies. The most significant positive influence was observed when multiple masses were placed in between the exciter and receiver simulating a periodic configuration of the masses
M.Sc. in Civil and Structural Engineering:3rd Semester and Master’s Thesis Ideas 2014
Full text linkThe following pages contain a list of project ideas proposed by the scientific staff at the Department of Civil Engineering, Aalborg University, and a number of companies. Most of the project ideas in this catalogue may form the basis for long and short master projects as well as regular 3rd semester projects at the M.Sc. programme in Civil and Structural Engineering
Prediction of wave propagation in soils using semi-analytic method
No full textGround-borne noise induced by trains andconstruction works has become day by daya constant issue to the tranquility of urbanareas. Population increase and city developments mean that more areas will be subjectedto uncomfortable noises and the need for tranquility is ever more necessary. So far theseinduced vibrations have been hard and expensive to predict, thus less time consuming andmore accurate computational models are required. This is also the focus of this master’sthesis where a semi-analytical model developed by several researchers will be evaluated.The semi-analytic model is used to predict thepropagation of waves in a media due to vibrations from external source and the impactthey have on the surrounding area. A thorough investigation of the model is performedby taking into account the influence of the soilproperties in the dynamic amplification.This master’s thesis also includes an experimental part, where geophones are used tomeasure the vibrations from trains at a location specified by COWI. The data obtainedfrom the measurements is used to find theproperties of the soil and further validate thesemi-analytic method.To conclude, a comparison between the measured vibrations and the ones simulated fromthe model are analysed and compared in orderto obtain a closer fit as possible
Modelling of Random Variations in Soil Properties
No full textGeotechnical data is constrained with uncertainties. Among them is the natural variabilityof the soil which is a type of uncertainty that cannot be reduced. When designing astructure, this unpredictability of the soil has to be taken into account. The traditionalway of doing this is based on a deterministic design, where partial safety factors are appliedto the characteristic value of the loads and strength parameters, leading to a designbased on increased loads and decreased strengths.The purpose of the thesis is to model the random variations in soil properties using aprobabilistic approach, based on the random field theory, in which soil properties arecharacterized in terms of their means and standard deviations, and through a realiabilityanalysis to calibrate the partial safety factor for the undrained shear strength. The partialsafety factor in Denmark is set to gcu = 1:8, which is quite high compared to the generalfactor proposed in Eurocode 7, gcu = 1:4.Two-dimensional cross-correlated random fields for the undrained shear strength andYoung’s modulus of the soil are generated based on the matrix decomposition method.The probabilistic finite element model is created by using different packages; Abaqus,Python scripts, Fortran subroutines and Matlab scripts. 1000 Monte Carlo simulationsare performed, resulting in 1000 bearing capacities of the structure. The data is thenfitted and combined with the probability density function of the load a number of realizationsare performed with the crude Monte Carlo simulation technique. Through thisthe partial safety factor of the undrained shear strength is calibrated to a correspondingtarget reliability index. This whole procedure is performed for two examples, one for astrip footing and one for an embankment.The reliability-based calibration shows that the partial safety factor can be reduced significantly,from gcu = 1:8 to gcu = 1:5, in both cases, by which the costs are reducedtoo.In addition, the effects of the correlation length on the failure mechanism is investigated,and it is found that the failure mechanism is highly dependent on the correlation length
Dynamic analysis of Soil-Foundation Interaction
No full textDette projekt består af en dynamisk analyse af Jord-Fundament Interaktion, dvs. Jord-Struktur Interaktion (JSI) og Struktur-Jord-Struktur Interaktion (SJSI). Formålet med projektet er at studere forskellige bølge udbredelses mønstre. Dette er blevet undersøgt i tids domæne. De anvendte frekvenser repræsenterer menneskeskabte, miljømæssige vibrationer.Først, er det grundlæggende teori forklaret og baseret på denne teori, er en simpel model for overflade bølger beskrevet.Dernæst, er finite element metoden beskrevet, hvorefter disse modeller er sammenlignet med hinanden.Efter der er redegjort for grundlæggende egenskaber og antagelser, er de faktiske mønstre på jordoverfladen undersøgt i tids domæne. Forskellige parametre er analyseret og deres respektive indflydelse på resultatet er forklaret.Den sidste del af rapporten fokuserer på JSI, hvor fundamentet er påvirket med forskellige konfigurationer af puls og harmonisk belastning og det vertikale respons af dette er analyseret i en afstand, ved et andet fundament. Dette resulterer i et forslag om at simplificere modellen.This project consists of a dynamic analysis of Soil-Foundation Interaction, i.e. Soil-Structure Interaction (SSI) and Structure-Soil-Structure Interaction (SSSI). The aim of the project is to study different wave propagation patterns. This has been studied in time domain. The used frequencies will be representative for man-made environmental vibrations.Firstly, the basic theory is explained and based on this theory, a simple model is described for surface waves.Secondly, the finite element model is described and based on both models, a comparison between the methods is made.After all basic properties and assumptions, the actual patterns in the soil is studied in time domain. Different parameters are analysed and their respective influence on the results is explained.The last part of this project contains the SSI, wherein different foundation configurations are excited with pulse and harmonic loads and the vertical responses at another foundation, some distance away, are analysed. This results in a suggestion for simplification of the model
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