Árpád Rózsás | TNO - Academia.edu (original) (raw)

Papers by Árpád Rózsás

Data-Centric Engineering

The decreasing cost and improved sensor and monitoring system technology (e.g., fiber optics and ... more The decreasing cost and improved sensor and monitoring system technology (e.g., fiber optics and strain gauges) have led to more measurements in close proximity to each other. When using such spatially dense measurement data in Bayesian system identification strategies, the correlation in the model prediction error can become significant. The widely adopted assumption of uncorrelated Gaussian error may lead to inaccurate parameter estimation and overconfident predictions, which may lead to suboptimal decisions. This article addresses the challenges of performing Bayesian system identification for structures when large datasets are used, considering both spatial and temporal dependencies in the model uncertainty. We present an approach to efficiently evaluate the log-likelihood function, and we utilize nested sampling to compute the evidence for Bayesian model selection. The approach is first demonstrated on a synthetic case and then applied to a (measured) real-world steel bridge. T...

Engineering Fracture Mechanics, Apr 1, 2022

Structural Safety, May 1, 2017

In structural reliability the dependence structure between random variables is almost exclusively... more In structural reliability the dependence structure between random variables is almost exclusively modeled by Gauss (normal or Gaussian) copula; however, this implicit assumption is typically not corroborated. This paper is focusing on time-variant reliability problems with continuous stochastic processes, which are collection of dependent random variables and to our knowledge are not modeled by other than Gauss copula in structural reliability. Therefore, the aim of this contribution is to qualitatively and quantitatively analyze the impact of this copula assumption on failure probability. Three illustrative examples are studied considering bivariate Gauss, t, rotated Clayton, Gumbel, and rotated Gumbel copulas. Timevariant actions are modeled as stationary, ergodic, continuous stochastic processes, and the PHI2 method is adopted for the analyses. The calculations show that the copula function has significant effect on failure probability. In the studied examples, application of Gauss copula can four times underestimate or even 10 times overestimate failure probabilities obtained by other copulas. For normal structures agreement on copula type is recommended, while for safety critical ones inference of copula type from observations is advocated. If data are scare, multiple copula functions and model averaging could be used to explore this uncertainty.

This paper examines the reliability level of the Eurocode’s partial factor design (PFD) method fo... more This paper examines the reliability level of the Eurocode’s partial factor design (PFD) method for steel frames subjected to snow load. This is motivated by the frequent damage and collapse of structures under heavy snowfall [1, 2]. We present a general approach which can help to comprehensively investigate the compliance of current Eurocode partial factor provisions and to assess whether the collapses most frequently observed at steel structures can be attributed to the deficiency of standardized provisions. To alleviate the limitations of previous studies, the general framework is intended to efficiently perform parametric reliability analysis using state-of-the-art numerical methods, has the potential to identify the crucial parameters, the sufficient modelling level and to justify the assumptions made in previous studies. Additionally the effect of epistemic uncertainties through Bayesian statistical analysis is also taken into account. The presented approach is illustrated through a simple steel frame, which is analysed considering various load-ratios. First, actual snow measurements are evaluated using classical, maximum likelihood based and Bayesian methods. Lognormal, Generalized Extreme Value (GEV) and Gumbel distributions are considered. Using these results partial factor based optimization of the frames is performed. The firefly metaheuristic algorithm is chosen to carry out the optimization [3]. This optimization is used to get fully utilized structures per PFD which is indispensable for reliability assessment of the method. Cross-sectional level limit state function is considered taking into account the global stability of the frame approximately. The objective function is the overall weight of the structure; the constraints are enforced through penalty functions. After the PFD, the safety level of the structures is analyzed. The reliability analysis is performed using FORM, SORM and importance sampling Monte Carlo (ISMC) methods. Based on the completed analyses, the main conclusions are the following: (i) The safety level of steel frames subjected to snow load is fairly sensitive to the distribution type used to describe the dominant variable load. This assertion is valid irrespectively of the load-ratio. (ii) Frames designed per partial factor method in accordance with Eurocode, i.e., using Gumbel distribution assumption and frequentist statistics (MLE) to derive characteristic loads, may not have adequate safety level in the high load-ratio region with the same assumptions made in the reliability analysis. If GEV distribution is used in the reliability analysis the safety level significantly decreases, by ~14% compared to Gumbel model, which means a 7 times increase in the probability of failure. (iii) In case of GEV distribution the Bayesian posterior model gives about 80% higher probability of failure than the GEV-MLE. (iv) The epistemic uncertainty has a significant effect on the reliability of steel frames, in case of Gumbel distribution the incorporation of epistemic uncertainty into the reliability analysis yields to ~35% increase in the probability of failure. For the GEV distribution the increase is about 400-500%. (v) The reliability level substantially varies with the load-ratio, this cannot be captured by one single partial factor of the snow load. It should be emphasized that the statistical analysis is based on a 50-year long observation and the numerical analysis is limited to few examples. The investigation is being extended to further design cases and to the sensitivity analysis of measurements data, to able to draw general conclusions. REFERENCES [1] Geis, J, Strobel, K, Liel, A, “Snow-Induced Building Failures”, Journal of Performance of Constructed Facilities, Vol. 26, pp. 377-388, 2011. [2] Holicky, M, Sykora, M, “Failures of Roofs under Snow Load: Causes and Reliability Analysis” in Forensic Engineering 2009, American Society of Civil Engineers, pp. 444-453, 2009. [3] Yang, X-S, Engineering Optimization: An Introduction with Metaheuristic Applications, John Wiley & Sons, Hoboken, New Jersey, 2010.

Target reliability levels, design and remaining working lifes and reference periods recommended i... more Target reliability levels, design and remaining working lifes and reference periods recommended in various national and international codes and guidelines are sometimes inconsistent. Design working lifes for new structures are typically 50 to 100 years for different types of structures. For existing structures the remaining working life is often smaller than 50 years. Recommended target reliability indexes are usually given for one or two reference periods (one year and 50 years), without an explicit link to the design working life. In this paper a general framework is given in order to make the reliability index independent of the design working life and to provide guidance for specification of the target reliability level for different situations. The study is based on probabilistic economic optimization. Using this methodology, annual targets - in terms of failure rates - can be set for new structures, and (deteriorating) existing structures, given the marginal cost for reliability, failure consequences and the uncertainty in loading and resistance. © 2018 Delft University of Technology. All rights reserved.

Taylor & Francis Group eBooks, 2019

The objective of this paper is to explore the added value of full-probabilistic nonlinear finite

I am grateful to my supervisors Nauzika Kovács and Miroslav Sýkora for their guidance and continu... more I am grateful to my supervisors Nauzika Kovács and Miroslav Sýkora for their guidance and continuous support during the completion of this work. I owe my gratitude to László Dunai for allowing me to pursue the whim of my interest over the course of my studies. This work would have never been completed without his support and his trust in me. I am indebted to László Gergely Vigh for the valuable discussions and for jointly completed research. I highly appreciate his personal and professional help, which he is always eager to provide even amid his numerous activities. I am thankful to all members of the Department of Structural Engineering for the friendly and supportive environment, for sharing their knowledge, and for nurturing my engineering insight. Special thanks go to Kitti Gidófalvy, Piroska Mikulásné Fegyó, and Mansour Kachichian for the personal conversations. I am grateful to the members of Klokner Institute, particularly to those of the Department of Structural Reliability, where I spent a wonderful 10-month research stay. I am especially grateful to Miroslav Sýkora for making my stay in Prague an unforgettable experience, for the invigorating professional conversations, and for his friendship. Special thanks to commander-in-chief Mogyi for her professional help and for her companionship in our journey c. I am obliged to the countless people of past and present who shaped my world view and who's work and ideas are indispensable and inseparable part of my thinking. I wish I were able to add a grain of sand to the mountain of their amassed knowledge. This work was partly supported by the Ph.D. Scholarship of the Hungarian State and by the International Visegrad Fund Intra-Visegrad Scholarship (contract no. 51401089), both are highly appreciated. The numerical analyses are mainly completed using Matlab (Matlab, 2015), FERUM (Der Kiureghian et al., 2006), and R (R Core Team, 2015). The work and commitment of the developers of these applications are highly appreciated. In the spirit of reproducible and open research all of the related scripts, processed data, and results are available from the author and partially from the repositories of the following GitHub account: https://github.com/rozsasarpi.

Structural Concrete

This paper presents a reliability‐based calibration method for design code formulas. The method i... more This paper presents a reliability‐based calibration method for design code formulas. The method is demonstrated on the shear design formulas in Eurocode 2 and fib Model Code 2010 (MC2010). We found that the partial factor in the current Eurocode 2 is about 20% lower than the optimal value and, thus, provides an insufficient safety margin. The obtained optimal partial factor in the (modified) Eurocode 2 and MC2010 formulas is 1.53 and 1.36, respectively. The difference stems from higher accuracy and, hence, lower uncertainty of the MC2010 model in predicting experimental results. Hence, on average, the MC2010 formula leads to about 13% larger design resistances compared to Eurocode 2 given that the target reliability for both design formulas is the same. To stimulate and facilitate future structural code development and derivation of partial factors, we make the used computer code freely available.

Structure and Infrastructure Engineering, Oct 12, 2022

Structure and Infrastructure Engineering, 2019

Europe has an extensive transport infrastructure network where bridges play a vital role. Most of... more Europe has an extensive transport infrastructure network where bridges play a vital role. Most of them were built as part of the post-World War II reconstruction effort, meaning that we, as society, are already facing the beginning of the end of their design life. This shows the necessity of efficient approaches, complementing visual inspections, for early detection of damage that might jeopardise structural integrity and ultimately might lead to loss of life. This work introduces a novel, pro-active structural health monitoring (SHM) approach to better identify and quantify representative damage types, on prestressed concrete bridges. Based on a numerical simulation of a comprehensive case study available in the literature: the Lez ıria Bridge, the results show that damage can be identified with good accuracy for early stages of damaged bridges. Pier settlements and prestress losses are the damage types where the severity is quite accurately quantified even for lower damage severity levels. Once the damage type is identified, it is found that a pair of two vertical displacements reveals to be enough to quantify the damage extent. The results also show potential in the utilisation of the approach for a rational and efficient design of monitoring systems towards damage identification.

me.bme.hu

Contrary to the previous design practice-which is overwhelmingly based on elasticity-the new Euro... more Contrary to the previous design practice-which is overwhelmingly based on elasticity-the new European standard allows the mobilization of plastic reserves of composite bridges. Hitherto, no comprehensive study was made to assess the achievable ...

International Journal of Architectural Heritage

Structural Concrete, Apr 3, 2022

Engineering Fracture Mechanics

Computers and Geotechnics, 2022

Statistical uncertainties, arising from the uncertainty of parameter estimation and model selecti... more Statistical uncertainties, arising from the uncertainty of parameter estimation and model selection, are often neglected in probabilistic assessment of engineering structures. However, few previous studies indicate that this might cause severe underestimation of extreme loads and lead to insufficient structural reliability. This contribution aims to qualitatively and quantitatively investigate the effect of this simplification on extreme values of wind speed that are commonly associated with design values. The probabilistic modelling of basic wind speeds is thoroughly investigated. Moderately high temporal resolution data – daily 10 min averaged maxima from three distinct one hour long measurement sessions – are obtained from the Carpatclim database, covering a 50-year observation period. Data for Budapest are taken into account as a representative example. Block maxima and peak over threshold approaches are applied to extract maxima and to fit associated distributions. Frequentist ...

Szerkezetek megbizhatosagi analizise soran az elettartamra vonatkoztatott tonkremeneteli valoszin... more Szerkezetek megbizhatosagi analizise soran az elettartamra vonatkoztatott tonkremeneteli valoszinűseget gyakran a serulekenysegi-veszelyeztetettsegi gorbe modszerevel hatarozzuk meg. A cikkben peldakon keresztul illusztraljuk a modszer alkalmazasat, a serulekenysegi es veszelyeztetettsegi gorbek előallitasanak modjat extrem hatasoknak (tűz, foldrenges, ho) kitett szerkezetek teherbirasi vizsgalatara. Szemleltetjuk a kulonboző hatasok es a szerkezeti tonkremenetel valoszinűsegi leirasanak nehezsegeit es a gyakorlatban alkalmazott kozelitesek korlatait.

12th International Conference on Structural Analysis of Historical Constructions, 2021

Deciding whether crack patterns are similar is one of the main components in damage assessment of... more Deciding whether crack patterns are similar is one of the main components in damage assessment of masonry structures, since the degree of similarity between patterns often correlates with the likelihood of having similar causes. Hence, quantifying crack pattern similarity is instrumental in understanding damage. Similarity checks usually take place in experts' mind, comparing an observed pattern with hundreds of patterns they have seen in the past and recorded in their memory. The observed pattern is then related to damage causes they find the most plausible based on their experience, understanding of physics, and local conditions. Currently, it is not understood how the crack pattern comparisons are made by the human brain and experts have difficulties in verbalizing and formalizing their decision making process. Therefore, deciding on the similarity of masonry crack patterns is solely performed by experts. This approach is expensive, limited by the availability of experts, and yields only qualitative answers.

Observations are inevitably contaminated by measurement uncertainty, which is a predominant sourc... more Observations are inevitably contaminated by measurement uncertainty, which is a predominant source of uncertainty in some cases. In reliability analysis, probabilistic models are typically fitted to measurements without considering this uncertainty. Hence, this paper intends to explore the effect of this simplification on structural reliability and to provide recommendations on its treatment. Statistical and interval-based approaches are used to quantify and propagate measurement uncertainty. They are critically compared by analyzing ground snow measurements that are often affected by large measurement uncertainty. It is propagated through the mechanical model of a generic structure to investigate its effect on reliability. Parametric studies facilitate to analyze the effect of key parameters, such as measurement uncertainty, coefficient of variation of ground snow load, and distribution type. The interval analysis is performed as a hybrid interval-probabilistic analysis. Measuremen...

Data-Centric Engineering

Engineering Fracture Mechanics, Apr 1, 2022

Structural Safety, May 1, 2017

Taylor & Francis Group eBooks, 2019

The objective of this paper is to explore the added value of full-probabilistic nonlinear finite

Structural Concrete

Structure and Infrastructure Engineering, Oct 12, 2022

Structure and Infrastructure Engineering, 2019

me.bme.hu

International Journal of Architectural Heritage

Structural Concrete, Apr 3, 2022

Engineering Fracture Mechanics

Computers and Geotechnics, 2022

12th International Conference on Structural Analysis of Historical Constructions, 2021

This study is motivated by the sharp contrast between physical and probabilistic models of civil ... more This study is motivated by the sharp contrast between physical and probabilistic models of civil engineering. The current practice focuses on physical models while probabilistic ones are relatively underdeveloped. This unbalance can even hinder advances in physical models. Moreover, this rarely acknowledged asymmetry creates the illusion that our deterministic models accurately capture all or at least the main aspects of reality. Thus, this thesis aspires to subtly adjust this imbalance by focusing on probabilistic models.

The main contribution is that it explores often neglected or oversimplified aspects of probabilistic analysis in civil engineering. These distinct but related issues are: (i ) selection of an appropriate distribution type; (ii ) effect of statistical uncertainty; (iii ) measurement uncertainty; (iv) long-term trends; and (v) dependence structure. These are demonstrated through analyzing extreme ground snow loads, although they are inevitably present for most random variables. Snow action, which has recently led to numerous structural failures in Central Europe, is treated as a vehicle of illustration to give a sharp focus to the study.

Methods developed in mathematical statistics, probability theory, information theory, and structural reliability are applied to tackle these issues. Fully statistical analysis of snow extremes is undertaken in conjunction with structural reliability analysis. The popular civil engineering approaches are compared with more advanced techniques that are able to capture the neglected effects in the former approaches. Snow water equivalent data of the Carpathian Region from more than 600 meteorological stations are analyzed, thus the results are representative for lowlands, for highlands, and for mountains as well. Furthermore, extensive parametric analyses are performed to extend the investigations to random variables other than ground snow intensity.

Based on the analysis of the above-listed issues the following main conclusions are drawn:

(i) It is demonstrated that mountains and highlands are better represented by Weibull, while lowlands by Fréchet distribution. In comparison, the currently standardized Gumbel model recommended in Eurocode often appreciably underestimates the snow maxima of lowlands and thus leads to overestimation of structural reliability.

(ii) It is shown that current practice, which neglects statistical uncertainties (parameter estimation and model selection uncertainties), can yield to even multiple orders of magnitude underestimation of failure probability. Bayesian posterior predictive distribution and Bayesian model averaging is proposed to account for parameter estimation and model selection uncertainties, respectively.

(iii) Statistical and interval based approaches are used to explore the effect of prevalently neglected measurement uncertainty on structural reliability. It is demonstrated that such simplification can lead to an order of magnitude underestimation of failure probability. If sufficient data are available to infer the probabilistic model of measurement uncertainty, then the statistical approach is recommended. Otherwise, the interval approach is advocated.

(iv) Using non-stationary extreme value analysis, statistically significant decreasing trend is found in annual ground snow extremes for most of the Carpathian Region. For some locations the effect of the trend on structural reliability is practically significant. This change is favorable from safety point of view as it increases reliability. Hence, revision of current regulations due to long-term trends is not needed from safety reasons, though it might be desirable from economic considerations. However, record lengths are insufficient to draw strong conclusions and to include trends in predicting extreme values with return periods of hundreds of years.

(v) Study of the widely used Gauss (normal, or Gaussian) copula assumption of time-continuous stochastic processes is performed. It reveals that Gauss copula can four times underestimate or even ten times overestimate time-variant failure probabilities obtained by other adopted copulas (t, Gumbel, rotated Gumbel, and rotated Clayton). Model averaging is proposed as a viable approach to rigorously account for copula function uncertainty.

Based on these findings, practical recommendations are made for normal and safety critical structures. The effects and proposed approaches are illustrated through real life examples. Reliability of the more than 130 years old wrought iron structure of Eiffel-hall and a steel hall of Paks Nuclear Power Plant is analyzed. The tackled challenges are general and relevant for most random variables such as wind, traffic, and earthquake actions. Therefore, the findings can be applied for these as well and they can help to draft more consistent standards and to build safer structures.

The primary purpose of this thesis is to investigate the plastic reserve of composite plate girde... more The primary purpose of this thesis is to investigate the plastic reserve of composite plate girder bridges. These structures are suitable for this due to the synergetic combination of the concrete and steel. The former provides the “cheap” stiffness and strength in compression while the steel in tension ensures the ductility. However, the theoretical and experimental aspects of plastic design are well established only in the US provisions are available for the designers.

The aim was to inquiry the plastic design in the framework of the Eurocode through an existing elastically designed bridge. In the first part of the study the necessary theoretical background is overviewed, the related literature is examined. The main emphasis is placed on the ultimate load bearing capacity, which is determined using various limit states, such as first hinge, incremental collapse and plastic collapse. The safety levels of these limit states were also investigated. To ensure the ductility of the pier-sections innovative structural solutions gathered and evaluated. The selected bridge is a composite, plate girder, continuous structure formed by three spans (30,0-40,0-30,0m). This was redesigned following plastic principles, the relevant provisions and the findings of the researchers.

The calculations showed that − for the original structure − the traffic load could be increased by ~30 and ~60% over the first yield in case of using first hinge and shakedown limit states, respectively. It was found that the safety levels of these limit states at least reach or exceed that of the first yield or first hinge. It should be noted that these results reflect only one example; nevertheless, they are in good agreement with the American results. The redesign yielded to a structure with cleaner lines with considerably less section transition and about 25% structural steel saving. Based on the calculations and international data the plastic design of girder bridges appears to be a promising way, at the same time more research required.

Régi- és új típusú vasúti öszvérhidak, Jun 2010

The aim of this study is to show the enormous potential of composite bridges which is not yet ful... more The aim of this study is to show the enormous potential of composite bridges which is not yet fully exploited after more than one century since its first appearance. It concentrates on the field of railway bridges. After a brief overview of the historical evolution of composite bridges from Melan patent to widespread usage, it gives a summation about the Hungarian railway bridges compared with the European average regarding span, age and type. The second part of this paper gives an extensive description about the innovative steel-concrete composite railway bridges. These are mainly beam bridges which solve the problems of hogging area of continuous structures. Bridge types which are introduced by this study:
– Concrete filled (CF) hollow section girder
– CF steel pipe
– CF narrow-width steel box
– CF I girder
– Rolled
– Welded
– Double composite section
– External longitudinal post-tensioning
– Preflex girder
At the end it gives an overview about the composite arch bridges regarding the construction methods.