| 1. |
- Al-Ayish, Nadia, et al.
(författare)
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Environmental impact of concrete structures reinforced with GFRP bars : A simplified study on columns
- 2019
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Ingår i: Proceedings of the fib Symposium 2019. - : International Federation for Structural Concrete. - 9782940643004 ; , s. 1998-2005
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Konferensbidrag (refereegranskat)abstract
- Concrete has a significant influence on the global warming due to its high usage in the construction industry. There are a few different strategies to increase the sustainability potential of concrete structures. Most of these strategies involve reduction of the total clinker content. One strategy, which is often neglected due to its complexity, is to increase the durability of the concrete structure. By increasing the durability, the need for repair and maintenance is reduced and thus less resources are consumed during the service life. One of the main deterioration mechanisms in concrete structures is the corrosion of steel reinforcement. A strategy to increase the service life of concrete structures in harsh environment would therefore be to increase the durability of concrete or to use low- or non-corrosive reinforcement instead of traditional steel reinforcement. This paper focuses on the latter. Glass fibre reinforced polymer (GFRP) bars are non-corrosive and have emerged as an alternative to steel bars in reinforced concrete structures in harsh environment. They have other mechanical properties than steel and opens for alternative mix designs for concrete. However, the environmental impact of concrete structures reinforced with GFRP bars has not been fully investigated and most life-cycle assessment (LCA) studies have an exchange ratio of 1:1 between GFRP and steel bars despite differences in the mechanical properties. This paper studies the climate impact of concrete columns reinforced with GFRP bars through an LCA methodology, focusing on the functional unit.
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| 2. |
- Allahvirdizadeh, Reza, et al.
(författare)
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A framework recommendation for updating running safety design criteria of non-ballasted railway bridges using statistical investigations
- 2024
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Ingår i: Proceedings 12th European Conference on Structural Dynamics (EURODYN 2023). - : IOP Publishing. ; , s. 102008-
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Konferensbidrag (refereegranskat)abstract
- As far as the authors are aware, the threshold for vertical acceleration of the bridge deck was chosen based on the assumption that the induced dynamic loads would overcome gravity at higher accelerations, resulting in loss of contact between wheels and rail; however, the previous studies do not support this hypothesis. Considering these inconsistencies, a better understanding of the simplified design criteria is essential before conducting further studies suchas the calibration of partial safety factors. Therefore, this study considers a set of representative design scenarios to statistically compare wheel-rail contact loss with other criteria that can bederived from moving load models, such as vertical accelerations and bridge deck deflections. Based on the analyzes performed, deflection seems to be a better criterion than acceleration to control the running safety limit-state; although the results presented do not necessarily show avery strong correlation between these two criteria. Therefore, the k-means clustering approach isused together with 5% lower quantiles of the collected data to propose potential new thresholds. It should be noted that due to the limited number of analyzes, the approach presented in this study can be considered as a possible framework for further updates of the current design method rather than drawing general conclusions.
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| 3. |
- Allahvirdizadeh, Reza, et al.
(författare)
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Applicability of meta-model assisted reliability assessment for dynamic problems: a comparison between regression-based methods
- 2023
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Ingår i: Proceedings 14th International Conference on Applications of Statistics and Probability in Civil Engineering, ICASP14. - : Trinity College Dublin.
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Konferensbidrag (refereegranskat)abstract
- There is a growing intent among engineers, stakeholders, and decision makers to use probabilistic methods for infrastructure assessment or design objectives. However, the corresponding limit state for such problems usually requires the construction of complex computational models, usually using commercial software without parallelization capability. Such a requirement makes performing reliability analysis computationally prohibitive, which is even more challenging for dynamic problems, since a very short time step is required to obtain sufficiently accurate predictions. This concern has led to several methods being proposed to surrogate the limit state function with a generally black box called a meta-model. A variety of them, such as Kriging, Polynomial Chaos Expansion (PCE), Artificial Neural Networks (ANN), and response surfaces (e.g., polynomial, spline, or radial-base functions), have been adopted for this purpose. These meta-models are typically trained on a limited data set collected by computing the true responses of carefully selected input variables. Their applicability for assessing the probability of failure has been studied individually in the literature for both benchmark and practical problems. However, as far as the authors are aware, no comparison has been made between them for dynamic problems. This comparison needs to be made from the point of view of both accuracy and performance (number of calls to the limit state function). In this context, this paper takes a systematic approach to evaluate their performance under identical conditions, i.e., with similar training datasets. For this purpose, the dynamic response of railway bridges with different spans excited by the passage of trains with a wide range of speeds is used as a reference problem.
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| 4. |
- Allahvirdizadeh, Reza, et al.
(författare)
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Ensemble Meta-Models for Running Safety Assessment ofHigh-Speed Railway Bridges Considering Soil-StructureInteraction Effects
- 2022
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Konferensbidrag (refereegranskat)abstract
- Increasing operating speeds and axle loads of trains may induce higher verticaldeck accelerations on bridges, which may subsequently lead to the occurrence of ballast instabilities.This phenomenon not only increases maintenance costs but also leads to speed restrictionsunder operating conditions. In severe cases, it can also cause train derailment. Thishas been shown to be the governing criterion for the design of short to medium span lengthhigh-speed railway bridges, especially from the dynamic behaviour point of view. Despiteits substantial importance in the design of railway bridges, the conventional deterministic approachescannot achieve the desired level of safety (Allahvirdizadeh et al. 2020). Therefore,this article is devoted to the evaluation of the probability of violating running safety usingsimulation-based methods. In this context, different variables, including those for the bridge(span length, flexural rigidity, and geometric properties), for the train (axle load, dominantfrequency and damping ratio) and for the boundary conditions (soil and foundation properties)are considered. Due to the high computational cost and complexity of the consideredperformance function, a low-cost meta-model is trained using stack modelling concept as acombination of support vector machines (SVM), k-nearest neighbours (k-NN) and decisiontrees. Then, the range of maximum and minimum probabilities of exceeding the verticalacceleration threshold are evaluated as a function of train speed and bridge span length. Comparingthese boundaries with corresponding results of simply-supported bridges showed thatneglecting soil-structure interaction effects in shorter span bridges may result an underestimatedsafety of the system; however, for longer span bridges it may result in overestimatedsafeties.
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| 5. |
- Allahvirdizadeh, Reza, et al.
(författare)
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Estimating Running Safety Factor of Ballastless Railway Bridges Using Tail Modelling
- 2022
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Ingår i: Acta Polytechnica CTU Proceedings. - : Czech Technical University in Prague - Central Library. ; , s. 25-32
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Konferensbidrag (refereegranskat)abstract
- Excessive vertical acceleration of ballastless railway bridges subjected to vibrations induced by passing trains is one of the governing design criteria for bridges in high-speed lines. However, to the authors' knowledge, the corresponding design limit is not based on a solid theoretical or experimental background. Moreover, the traditionally applied safety factor also suffers from these concerns. Therefore, in the present study, a crude probabilistic approach is adopted to evaluate the consistency and reliability of this safety factor. For this purpose, deterministically designed bridges (using conventional methods) with short to medium spans are considered. Then, their reliability is evaluated using simulation-based techniques and extreme value theory, i.e., tail approximation. Then, the existing safety factor is calculated to evaluate the consistency of the current approaches, and possible new values are proposed based on the desired target reliabilities.
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| 6. |
- Allahvirdizadeh, Reza, et al.
(författare)
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Improved dynamic design method of ballasted high-speed railway bridges using surrogate-assisted reliability-based design optimization of dependent variables
- 2023
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Ingår i: Reliability Engineering & System Safety. - : Elsevier BV. - 0951-8320 .- 1879-0836. ; 238
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Tidskriftsartikel (refereegranskat)abstract
- Operating high-speed trains imposes excessive vibrations to bridges raising concerns about their safety. In this context, it was shown that some conventional design methods such as those related to the running safety suffer from a vague scientific background questioning their reliability or optimality. Therefore, the current article is devoted to updating the conventional design methodology, using Reliability-Based Design Optimization (RBDO) to propose the minimum allowable mass and stiffness which assures satisfying the target reliability. These proposed minimum design values can conceptually replace the conventional partial safety factor-based design method for running safety without the need for dynamic analysis. If the mass and stiffness resulting from the control of other limit states meet the proposed minimum values, the desired target reliability for running safety will be assured. This is achieved by adaptively training Kriging meta-models as a surrogate for the computational models decoupling the RBDO problem. In this regard, a new stopping criteria is proposed using mis-classification ratio of the cross-validated model; which reduces the generalization error of the trained meta-model and consequently the estimated failure probability. Moreover, due to the dependence of the design variables, the Copula concept is used to refine the augmented space and reformulate the RBDO problem.
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| 7. |
- Allahvirdizadeh, Reza
(författare)
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Improving the Dynamic Design Philosophy of High-Speed Railway Bridges Using Reliability-Based Methods
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Modern railway infrastructures, especially bridges, are exposed to significant vibrations with potential safety implications. In this context, previous studies have shown the inconsistency and inadequacy of some conventional design methods necessitaing them to be improved. The assessment of safety inherently deals with uncertainties. Therefore, the current study is dedicated to this objective using reliability-based methods. Of the various possible failure modes, the investigations presented here are limited to running safety and passenger comfort. The investigation of these limit-states requires constructing complex computational models with train-track-bridge interaction capabilities. However, the application of these computationally intensive models in the context of structural reliability does not appear to be feasible. Simplifying the system, the vertical acceleration and the deflection of the bridge serve as implicit limit-state measures. Initially, using First Order Reliability Method (FORM) revealed limitations in the application of the current safety factor, resulting in inconsistent reliability indices. Therefore, probabilistic design curves are proposed, defining minimum required bridge mass and stiffness based on cross-section types, span configurations and train speeds. These results are obtained by formulating a FORM-based optimization. Subsequently, the results are used to investigate the sensitivity of the estimated failure probabilities with respect to the contributing basic random variables. Acknowledging the limitations of FORM, surrogate-assisted simulation-based reliability assessments were used for further investigations. A comparison of the performance of widely used regression-based surrogate models under an identical active learning scheme showed the superior performance of the Kriging method over the others. Within areliability-based design optimization framework, this Kriging model facilitates the generation of new probabilistic design curves. This is achieved by reformulating the conventional method to account for the dependency between design variables using the copula concept. In addition, the surrogate model aided in calibrating the safety factor associated with the vertical acceleration threshold, leading to a proposal of 1.38 as a new safety factor. Subsequently, the influence of soil-structure interaction on the estimated reliability indices is evaluated using an ensemble of classification-based surrogate models. Results highlighted its beneficial contribution in terms of increased damping for shorter spans, countered by adverse effects due to frequency shortening in longer bridges. Finally, the epistemic uncertainties arising from the limited knowledge of the vertical acceleration threshold are investigated. It is found that neglecting these uncertainties can lead to an overestimation of allowable train speeds by about 13%.
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| 8. |
- Allahvirdizadeh, Reza, et al.
(författare)
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Minimum Design Requirements of High-Speed RailwayBridges Using Reliability-Based Design Optimization
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Constructing high-speed railway networks received a great attention during pastdecade raising new safety concerns on infrastructure (particularly bridges). It ismainly because of possible excessive vibrations due to higher operating speeds.In this regard, deficiency or vague scientific background for some of designmethodologies are shown by former studies; among which is the adopted safetyfactor for running safety. This limit state occurs if the load path from the trainto the bridge is disturbed by dislocating ballast particles because of excessivevertical accelerations of the bridge deck. Hence, the current article is devotedto update the conventional design methodology. For this objective, reliabilitybaseddesign optimization technique is employed to propose minimum allowablemass and stiffness (or maximum permissible fundamental frequency) which assuressatisfying target reliability level. This is achieved by adaptively trainingKriging meta-models to surrogate the computational models. It seems that theproposed method can result in lighter (economical) bridges in comparison tothe conventional design methodology; however, this beneficial aspect vanishesby increasing span length of the bridge.
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| 9. |
- Allahvirdizadeh, Reza, et al.
(författare)
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Partial safety factor calibration using surrogate models : An application for running safety of ballasted high-speed railway bridges
- 2024
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Ingår i: Probabilistic Engineering Mechanics. - : Elsevier BV. - 0266-8920 .- 1878-4275. ; 75
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Tidskriftsartikel (refereegranskat)abstract
- Traditionally, regulations employ semi-probabilistic methods with partial safety factors to control design limits. Calibrating these partial safety factors involves estimating the target reliability level and optimizing the partial safety factor values in order to minimize the deviation of the safety index between the considered design scenarios and the target value. This procedure necessitates performing a demanding amount of reliability analyses and is often carried out for simplified design situations. Therefore, high computational costs must be accepted for design problems formulated with complex computational models. This study implements a meta-modeling approach based on active learning in the partial safety calibration procedure, enabling its application to computationally intensive problems. Subsequently, the approach is applied to the running safety of ballasted high-speed railway bridges. This limit state implicitly accounts for the phenomenon of ballast destabilization, the occurrence of which disturbs the load path from the rail level to the bridge structure. The dramatic increase in train operating speeds in recent decades has increased the possibility of this design limit state being violated due to resonance. Despite the evident safety concerns, the adopted safety factors appear to be solely based on engineering judgments rather than calibration through higher-level reliability analysis. Therefore, the proposed calibration method is employed to determine the corresponding partial safety factors for various maximum allowable operating train speeds. The newly calibrated partial safety factors allow for a permissible maximum vertical acceleration of the bridge deck approximately 25% higher than the conventional design approaches. Therefore, incorporating these factors into the design procedure may lead to the construction of lighter bridges.
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| 10. |
- Allahvirdizadeh, Reza, et al.
(författare)
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Probabilistic dynamic design curves optimized for high-speed reinforced concrete railway bridges using first order reliability method
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Increasing the operating speed of trains in modern railway networks can induce greater actions on the infrastructure than was previously the case. This is due, in particular, to the occurrence of the resonance phenomenon in railway bridges, which is the focus of this article and was not traditionally considered as a concern. In this context, the vibrations experienced by bridges, both vertical accelerations and displacements, are limited by design regulations to ensure that the safety of train passages over bridges and the comfort of passengers are guaranteed. However, previous studies have shown that the conventional dynamic design methods do not always result in conservative designs, nor is the achieved safety always consistent. Therefore, a probabilistic approach is adopted in this study to optimize the cross-section properties of various railway bridges in a widedesign range including section types, span lengths, and number of spans. For this purpose, an iterative line search based optimization problem is formulated to minimize the depth of the cross-sections under consideration and consequently the linear mass of the bridges. Meanwhile, the associated failure probabilities of the above dynamic limit states are constrained to be less than the desired level of safety by incorporating them in to the optimization constraint. In this regard, First Order Reliability Method (FORM) is adopted to perform reliability analyses. Thus, the obtained results are presented in the form of design curves that may assist designers to select minimum cross-section dimensions satisfying the desired level of safety in terms of dynamic limit states. This objective can be achieved using the proposed design curves without the need to construct associated complex computational models and perform computationally expensive dynamic analyses.
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