Influence Of Model Assumptions Research Articles

The influence of model assumptions in a hybrid prediction tool for railway induced vibration

Within the SILVARSTAR project, a user-friendly frequency-based hybrid prediction tool is developed to assess the environmental impact of railway induced vibration. This will be implemented in the existing noise mapping software IMMI. The vibration level in a building in each frequency band is expressed as the product of source, propagation and receiver terms. A hybrid approach is used that combines experimental data with numerical predictions, providing increased flexibility and applicability. The train and track properties can be selected from a database or as numerical values. The user can select soil impedance and transfer functions from a database, pre-computed for a wide range of parameters with the MOTIV and TRAFFIC models. An experimental database of force densities, transfer functions, free field vibration and input parameters is also integrated into the new tool. The building response is estimated by means of empirical correction factors. Assumptions within the modelling approach can influence the prediction accuracy and the present paper aims to quantify these. We focus on the influence of train speed and soil properties on the compliance of the track-soil system, the dynamic axle loads, and the free field response, as well as on the load transfer between track and soil.

Influence of Model Assumptions on Radial Flow Investigations in Heavy-Ion Collisions

An effective way for the investigation of nuclear matter under the extreme conditions of high density and temperature is the study of nuclear fragmentation, realized in intermediate-energy heavy-ion collisions. The potential energy accumulated in the compression zone of the collision is converted into collective flow of matter in the later stages of the reaction. During the expansion of the system, fragments are formed in a clusterization process. In the final stage of the reaction, the fragments are moving along Coulomb trajectories and may deexcite by particle emission or secondary break-up.In order to verify the influence of the collective energy on the experimental energy spectra and extract quantitative information on the thermal and collective components, a model simulation was developed. In this procedure, the evolution of the disintegrating system in a multifragmentation process was described after the freeze-out stage with a Monte-Carlo approach. A reconstruction procedure for the fragment kinetic energies has shown the possibility to obtain the thermal and collective components from measured fragment spectra. In the present work, we scrutinize the model simulation for the evolution of the disintegrating system by testing the sensitivity of the collective expansion energy extraction on the model assumptions concerning the freeze-out characteristics, such as the size and shape of the freeze-out volume. Furthermore, thermal aspects of the early stage of the expansion are tested with an examination of the relationship between the assumption of the degree of thermalization of the emitting source and the extracted value of the collective energy.

Influence of Modelling Assumptions on the Seismic Risk of Industrial Precast Concrete Structures

This research evaluates the influence of different modelling assumptions on the global and local seismic risk assessment of code-conforming precast reinforced concrete buildings, specifically single-story industrial buildings. In particular the modelling of the system mass, the overhead crane, the beam-to-column and roof-to-beam connections and the cladding system are investigated. For this purpose, a case study resembling a new industrial building designed in accordance with the current Italian building code was selected. Typical dowel beam-to-column connections were considered and the influence of various modelling strategies investigated: perfect hinges, linear elastic connections and non-linear connections with a degrading hysteretic force-displacement model which was calibrated from available data on experimental tests. Three different types of roof-to-beam connections were investigated removing the assumption of rigid diaphragm, namely hot-rolled, cold-formed and socket welded connections. Initially, simplified planar models of single frames were considered to evaluate the influence of the different modelling strategies, then 3D models of the entire building were analyzed. Multiple-stripe non-linear dynamic time history analyses allowed to evaluate displacements, drifts, deformations and ultimate curvatures of the main elements and connections for various intensity measure levels. The seismic risk was assessed in terms of failure rate considering the collapse of both the columns and of the connections. The results show that the beam-to-column connections fail right after reaching yielding due to their low displacement ductility, leading to the loss of support of the beam and therefore increasing the collapse rate of the investigated structural typology.

The Influence of Model Assumptions on the Dynamic Impedance Functions of Shallow Foundations

The influence of model assumptions on the dynamic impedance functions of shallow foundations is investigated using finite elements in two studies. The first investigates the effects of model assumptions in different combinations including embedment of the foundation, variation of modulus with depth, and permanent load acting on the foundation. The second study is a parametric analysis investigating the effects of permanent load at varying soil depths and with different soil modulus coefficients. Shallow foundations in strata of frictional soil on top of bedrock are considered. Small-strain modulus and modulus reduction relationships are used in an iterative process to update the modulus due to the permanent load. The results show that model assumptions can have a large influence on impedance functions. The static stiffness coefficients differ, in some instances by more than 100%. The impedance functions, normalized with the static stiffness coefficients match each other well in the pre-resonance frequency range. However, in the frequency range above the fundamental frequency, the normalized impedance functions show a large variation. Further, the results show that the influence of the permanent load is largest in the case of shallow and stiff soil strata, both regarding normalized impedance functions as well as the static stiffness coefficient, which can be increased up to 67%. The change in fundamental frequency was however minimal.

Analysis of the influence of modelling assumptions on the prediction of the elastic properties of cardiac fibres

Although several numerical models of the human heart have been proposed in the literature, there are still several discrepancies among the results predicted by each model. These discrepancies can be attributed to the fact that each model has a number of assumptions and simplifications, which can limit the scope and precision of the numerical predictions obtained. Moreover, none of the works reported in the literature have assessed the influence of modelling assumptions on the predicted cardiac fiber elastic properties. In this paper a new passive mechanical model that combines the left ventricular (LV) pressure–volume in-vivo measurements with an indirect approach based on the finite element method (FEM), is proposed and used to analyze the influence of different modelling assumptions on the estimated elastic properties of the cardiac fiber. This analysis is carried out by varying modelling assumptions that are common to existing passive mechanical models. The results have shown that although the different modelling assumptions have a significant effect on the predicted value of the fiber elastic properties, they tend to lead to the same results. This suggests that simplified passive numerical models in combination with adjustment factors, are valid in comparison with more refined and complex LV passive models.

Influence of model assumptions about HIV disease progression after initiating or stopping treatment on estimates of infections and deaths averted by scaling up antiretroviral therapy.

BackgroundMany mathematical models have investigated the population-level impact of expanding antiretroviral therapy (ART), using different assumptions about HIV disease progression on ART and among ART dropouts. We evaluated the influence of these assumptions on model projections of the number of infections and deaths prevented by expanded ART.MethodsA new dynamic model of HIV transmission among men who have sex with men (MSM) was developed, which incorporated each of four alternative assumptions about disease progression used in previous models: (A) ART slows disease progression; (B) ART halts disease progression; (C) ART reverses disease progression by increasing CD4 count; (D) ART reverses disease progression, but disease progresses rapidly once treatment is stopped. The model was independently calibrated to HIV prevalence and ART coverage data from the United States under each progression assumption in turn. New HIV infections and HIV-related deaths averted over 10 years were compared for fixed ART coverage increases.ResultsLittle absolute difference (<7 percentage points (pp)) in HIV infections averted over 10 years was seen between progression assumptions for the same increases in ART coverage (varied between 33% and 90%) if ART dropouts reinitiated ART at the same rate as ART-naïve MSM. Larger differences in the predicted fraction of HIV-related deaths averted were observed (up to 15pp). However, if ART dropouts could only reinitiate ART at CD4<200 cells/μl, assumption C predicted substantially larger fractions of HIV infections and deaths averted than other assumptions (up to 20pp and 37pp larger, respectively).ConclusionDifferent disease progression assumptions on and post-ART interruption did not affect the fraction of HIV infections averted with expanded ART, unless ART dropouts only re-initiated ART at low CD4 counts. Different disease progression assumptions had a larger influence on the fraction of HIV-related deaths averted with expanded ART.

Influence of model assumptions on charring polymer decomposition in the cone calorimeter

This article addresses the one-dimensional modeling of a charring polymer decomposition in the cone calorimeter used to reproduce at bench scale the radiative heating from a fire. The rate-controlling phenomena are first discussed in a preliminary analysis of dimensionless numbers. Then, the role of three critical assumptions is highlighted by simulations: (1) transport of the gaseous products within the material or instantaneous release of gaseous products, (2) volume variation or constant volume, and (3) absorption of applied heat flux at the exposed face or through the thickness. Their influence in thermally thick regime is shown in particular on mass loss rate and time to extinction. Under the conditions tested, the influence of internal transport by convection on mass loss rate and time to extinction is minor. The assumption of constant volume appears to have a moderate influence on the mass loss rate and time to extinction. Variations of optical properties affect the numerical results by an increase of the maximum peak of mass loss rate and a decrease of time to extinction. Finally, the effects of applied heat flux and initial material thickness on the mass loss rate and time to extinction are important. With a higher heat flux or a smaller thickness, the decomposition is earlier, faster, and more intense.

Influence of modeling assumptions and aftershock hazard level in the seismic response of post-mainshock steel framed buildings

Seismic response under strong aftershocks has become an important topic in performance-based earthquake engineering recently. This paper examines the influence of the modeling approach in the response of a case-study four-story steel framed building designed with modern seismic provisions when subjected to two sets of aftershocks representing different aftershock-hazard levels. It was assumed that the building models experienced specific post-mainshock residual drifts prior of being subjected to the aftershocks. Results shown that including additional sources of stiffness and strength (i.e. inclusion of the interior gravity framing and participation of the slab in the beams) in the building analytical model lead to a different response under aftershocks in terms of maximum and residual interstory drifts than that of the analytical model commonly used in previous studies. Particularly, additional overstrength and post-yield stiffness ratio help to constrain residual drift demands under strong aftershocks, and decrease the probability of imminent demolition due to excessive post-mainshock residual drifts.

Influence of modelling assumptions in the expected loss evaluation of a precast industrial building

The economic losses arisen in the industrial sector after the Emilia earthquakes in 2012 highlighted the importance of conducting reliable seismic assessment analyses on the existing industrial building stock in order to ascertain both safety and potential losses associated to seismic events. To accomplish to such task, an accurate representation and quantification of the actual vulnerabilities in such buildings is required and reliable structural models need to be adopted.The paper investigates how various assumptions and levels of sophistication in finite element modelling affect the results in terms of economic losses, herein assumed as the reference decision variable. After the definition of the main seismic vulnerabilities of precast industrial buildings typical of the Italian territory, different types of finite element models are adopted and non-linear time history analyses conducted; in particular, the reinforced concrete fork at the top of the column is modelled in different ways, also considering the seismic retrofit.Appropriate fragility curves under selected engineering demand parameters are defined and provided within the Performance Based Earthquake Engineering methodology developed at the Pacific Earthquake Engineering Research Center for the assessment of the expected losses under a scenario-based earthquake. The influence of the modelling assumptions in the seismic risk estimate is evaluated. The results indicate that for the considered case study and in the absence of loss of support, simplified single-column models are suitable to estimate the expected losses.

A comparative study on the dynamics of tensegrity-membrane systems based on multiple models

This article investigates the modeling and dynamics of tensegrity-membrane systems. As extensions of tensegrity systems, tensegrity-membrane systems are composed of membranes, bars, and tendons. Significant system complexity and coupling between system components indicate that the behavior of the whole system needs to be investigated. For this purpose a complex nonlinear finite element model is developed to describe system dynamics. Numerical analysis results given by this nonlinear finite element model are compared with the results given by two other models based on different modeling assumptions and of lower complexity. Observations related to key dynamical properties and the influence of modeling assumptions are discussed in detail.

Influence of modeling parameters on the response of degrading systems to near-field ground motions

The influence of modeling assumptions on the seismic response of structures subjected to near-field ground motions is investigated. Emphasis is given on degrading systems, since real-world structures do not have infinite displacement capacity as many inelastic models assume, while such systems are able to explicitly take into consideration the effect of stiffness and strength degradation. Near-field ground motions are of particular interest, since, compared to far-field records, may cause increased demand caused by the velocity pulse in their fault-normal component. Single-degree-of-freedom (SDOF) oscillators with generic multilinear force–deformation backbones are first considered in order to derive general conclusions for a wide range of periods. The effect of every parameter that describes the backbone is studied separately in order to identify the sensitivity of the demand when the system is subjected to pulse-like ground motions. Moreover, a nine-story steel moment resisting frame is studied in order to show that the observations made on SDOF structures extend to multi-degree-of-freedom buildings.

USING CROSS-SECTIONAL BIOMONITORING STUDIES TO EXPLORE THE RELATIONSHIP BETWEEN HUMAN BODY BURDEN AND AGE

Background and Aims: Cross-sectional data sets are a compilation of different individuals whereas the longitudinal body burden trends are for only one individual over their entire lifetime. Numerous studies have reported cross-sectional biomonitoring data of polychlorinated biphenyls (PCBs) with human body burdens increasing continuously into old age. This relationship has been previously interpreted to indicate the role of age on bioaccumulation. We propose that what has been interpreted as an increase in body burden with age is actually a result of the temporal relationship between the sampling period and the peak in emissions. The purpose of this research was to investigate which factors control the concentration versus age relationship. Methods: Population cross-sections were generated from longitudinal calculations using the mechanistic model CoZMoMAN that links emissions to the environment with human body burdens. Body burden-age trends were generated from the longitudinal body burden calculations for human exposure to hypothetical chemicals with various partitioning and degradation properties. The influence of model assumptions and emissions peakedness are also examined. Results: The temporal relationship between the emissions scenario time trend and the biomonitoring sample collection period is the most influential factor controlling the shape of concentration-age trends for population cross-sections. For chemicals with degradation half-lives of 1 year or less, the relative concentration-age trend is always the same. Published biomonitoring studies for PCBs and PBDEs are interpreted in the context of the relationship between emissions and metabolic degradation. Conclusions: Bioaccumulation does not monotonically increase with age. The main predictors of cross-sectional body burden trends with age are the amount of time elapsed after emissions peaked and the chemical degradation rate.

Prediction of long-term tritium retention in the divertor of ITER: influence of modelling assumptions on retention rates

ERO modelling of long-term tritium (T) retention has been done for the divertor of ITER with graphite target plates assuming a certain beryllium influx into the divertor, eroded from the main chamber. The divertor beryllium (Be) influx relative to the deuterium ion flux has been fixed at 0.1% for the outer divertor and 1.0% for the inner divertor. In addition to the original B2-Eirene plasma background, the influence of variations of temperature and density in the divertor has been studied. Moreover, assumptions for enhanced erosion of redeposited carbon and effective sticking for hydrocarbons have been analysed. With graphite target plates, long-term tritium retention is dominated by T co-deposition in deposits. Within the studied parameter range, the modelling yields 200–500 possible ITER discharges without cleaning before reaching the safety limit of 700 g of in-vessel retained tritium. Surface temperature-dependent tritium amounts in carbon and beryllium deposits have been taken into account.

Influence of modeling assumptions on the seismic response of multi-span simply supported steel girder bridges in moderate seismic zones

Steel girder bridges are one of the most common bridge types in the central and south-eastern United States (CSUS). An understanding of their seismic response and assessment of their seismic risk has become a focus of the earthquake engineering community due to the increased awareness of the seismic hazard in the region. Analytical assessment of this seismic risk requires an evaluation of typical seismic responses and more of an understanding of the modeling parameters that significantly affect those responses. A seismic evaluation of a typical configuration for a multi-span simply supported steel girder bridge is performed for an approximate hazard level of 2% in 50 years using a nonlinear three-dimensional (3-D) analytical model. The results show significant vulnerabilities in the reinforced concrete columns and in the steel fixed and expansion bearings. Although the longitudinal loading of the bridge results in much larger demands compared with the transverse loading, some components of the bridge may still have appreciable damage under the transverse loading case. An analytical design-of-experiments screening study shows that modeling parameters such as loading direction and damping ratio are the most important in determining seismic response. Fixed bearing stiffness, among others, also significantly affects the response and should be considered carefully.

The influence of modeling assumptions on the modal analysis of vocal fold structures

Finite-element modal analysis studies of several vocal fold structural models were performed in vacuo under a variety of assumptions. The sensitivity of the vocal folds to changes in geometry and tissue properties was investigated. The Ritz method and the finite-element code adina were used to perform linear modal analysis of two-dimensional and three-dimensional models of the human vocal folds. The results allow for comparison and evaluation of various modeling approaches. In particular, the influence of geometrical constraints was shown to introduce errors of varying magnitude depending on the degree of anisotropy of the vocal fold tissue. [Research supported by the National Institute for Deafness and Other Communication Disorders.]