Jana Marková

Target Reliability Levels in Present Standards

Abstract The target reliability levels recommended in national and international documents vary within a broad range, while the reference to relevant costs and failure consequences is mentioned only very vaguely. In some documents the target reliability index is indicated for one or two reference periods (1 year, 50 years or life-time) without providing appropriate links to the design working life. This contribution attempts to clarify the relationship between the target reliability levels, costs of safety measures, failure consequences, reference periods and the design working life

Bayesian network application for the risk assessment of existing energy production units

A Bayesian network is applied in this contribution in order to assess the risks of a selected production unit in a fossil power station. A general framework for the risk assessment of production units of a power station is presented first by implementing statistical methods and Bayesian networks. Special emphasis is given to the input data consisting of failure rates which are obtained on the basis of recorded data and expert judgements. The consequences of failure are divided into economical and human (societal): economic consequences include outages of key technological devices, societal consequences cover potential injuries and fatalities. Probabilistic risk assessment methods are applied to the selected production unit of a power station. The influence of the uncertainties in the considered technical parameters on the availability of the unit is assessed and the acceptance of the calculated availability represented through the mean value and the standard deviation is discussed. Societal risks given in terms of weighted injuries and fatalities are obtained and respective risk acceptance criteria are presented. Uncertainties affecting the risks are discussed. It appears that the proposed framework provides a valuable assessment of the influence individual devices and their components on availability and societal risk. For that purpose the used methodology, intentionally simplified for operational applications, includes important factors affecting risks of production units. It is concluded that Bayesian networks are a transparent method for the probabilistic risk assessment of complex technological systems. The results of the performed analyses can be easily updated when additional information becomes available as illustrated in characteristic examples.

Bayesian Network Application for the Risk Assessment of Existing Energy Production Units

A Bayesian network is applied in this contribution in order to assess the risks of a selected production unit in a fossil power station. A general framework for the risk assessment of production units of a power station is presented first by implementing statistical methods and Bayesian networks. Special emphasis is given to the input data consisting of failure rates which are obtained on the basis of recorded data and expert judgements. The consequences of failure are divided into economical and human (societal): economic consequences include outages of key technological devices, societal consequences cover potential injuries and fatalities. Probabilistic risk assessment methods are applied to the selected production unit of a power station. The influence of the uncertainties in the considered technical parameters on the availability of the unit is assessed and the acceptance of the calculated availability represented through the mean value and the standard deviation is discussed. Societal risks given in terms of weighted injuries and fatalities are obtained and respective risk acceptance criteria are presented. Uncertainties affecting the risks are discussed. It appears that the proposed framework provides a valuable assessment of the influence individual devices and their components on availability and societal risk. For that purpose the used methodology, intentionally simplified for operational applications, includes important factors affecting risks of production units. It is concluded that Bayesian networks are a transparent method for the probabilistic risk assessment of complex technological systems. The results of the performed analyses can be easily updated when additional information becomes available as illustrated in characteristic examples.

Predicting Service Life of Chimneys and Cooling Towers Based on Monitoring

Actual conditions and prediction of residual service life of existing reinforced concrete chimneys and cooling towers should be assessed considering technical data obtained from monitoring and the principles of Eurocodes, ISO standards and fib Model Code 2010. In the contribution the criteria for assessing carbonation progress are discussed in detail, utilising experience and long-term measurements of the major power producer in Central Europe.

Estimating design resistance of wrought balcony girders

Abstract The contribution is focused on reliability of balcony girders of a Czech national heritage monument. As preliminary reliability assessment suggests insufficient resistance, a series of nondestructive tests supplemented by a single tensile test are performed and evaluated by the statistical methods. Values of material properties, recommended in standards for historic materials, seem to be overly conservative and it is advised to specify properties of historic metallic materials by tests.

Long-Term Monitoring of a Composite Steel-Concrete Bridge on the Prague Ring Road

The composite steel-concrete bridge across the “Slavičí údolí” on the Prague ring exposed to extreme traffic is selected for present traffic flow monitoring. The response of the bridge to the traffic loads is monitored using strain gauges. The record of voltage is converted to the amount of relative deformation, respectively stress in the bottom flange of steel I beam of the bridge. In parallel with the measurements, the computer slab - wall model based on finite element method is developed. The computer model is used for comparison of calculated and measured stresses and deflections in the flanges and to determine the natural frequencies of the examined bridge structure. Calibrated measurements are used for three-year monitoring of the effects of road traffic on bridge facilitating national decision in the National Annex to EN 1991-2.

Reliability Assessment of Existing Concrete Bridges

Reliability Assessment of Existing Concrete Bridges The remaining working life of an existing reinforced concrete bridge is analysed considering the serviceability limit states of crack width. Two new models of crack width provided Eurocode EN 1992 and in the Model Code 2010 are applied. The probabilistic methods of structural reliability are used for assessing the crack width limits and remaining working life of existing concrete bridges. It appears that the initial reliability with respect to the serviceability requirements resulting from the original design of the bridge seems to satisfy the required target reliability level recommended in Eurocodes. However, the reliability index significantly decreases with the reduction of the reinforcement area due to corrosion. Ocena Niezawodności Istniejących Mostów Betonowych Pozostały okres żywotności istniejącego żelbetowego mostu został poddany analizie, w której szerokość pęknięcia przyjęto za graniczne kryterium użyteczności. Wykorzystano dwa nowe modele pęknięcia, których dostarczyły Eurocode EN 1992 i Model Code 2010. Probabilistyczne metody niezawodności strukturalnej zastosowane do oceny granicznych szerokości pęknięcia i pozostałego okresu żywotności istniejących mostów ze zbrojonego betonu. Należy sądzić, że początkowa niezawodność w odniesieniu do wymagań użyteczności wynikających z oryginalnego projektu mostu wydaje się zadowalać wymagany docelowy poziom niezawodności zalecany przez Eurokody. Jednakże w wyniku korozji współczynnik niezawodności znacznie maleje wraz z redukcją strefy zbrojenia

Probabilistic Analysis of Crack Width

Probabilistic analysis of crack width of a reinforced concrete element is based on the formulas accepted in Eurocode 2 and European Model Code 90. Obtained values of reliability index b seem to be satisfactory for the reinforced concrete slab that fulfils requirements for the crack width specified in Eurocode 2. However, the reliability of the slab seems to be insufficient when the European Model Code 90 is considered; reliability index is less than recommended value 1.5 for serviceability limit states indicated in Eurocode 1. Analysis of sensitivity factors of basic variables enables to find out variables significantly affecting the total crack width.

Uncertainties of Crack Width Models

Crack widths for verification of serviceability limit states can be calculated by simplified formulas provided by codes of practice or by numerical analysis based on nonlinear finite element methods. Authors performed a pilot study with the aim to develop a methodology for assessment of model uncertainty involved in the crack width analysis. Experimental data of four beams were used as a reference of real physical evidence for model validation. Two crack models were investigated, namely the model proposed by the fib Model Code 2010 and the numerical model based on fracture mechanics and finite element method. The results indicate that both models tend to underestimate the maximal crack width. The mean crack widths are well simulated by the numerical model.

Reliability Assessment of Lateral Halls of Svijanska Arena

Decision on repairs or strengthening of existing structures with special focus on heritage structures or structures with areas where people may congregate has to be based on detail technical survey according to ISO 13822 and ISO 2394. The extent of structural intervention has to be always well planned taking into account economic, societal and environmental aspects. It is necessary to consider whether instead of significant structural intervention which could lead to unneeded economical loss of existing structures or also to the loss of heritage value of historic monuments, it is not better to make optimisation of structural intervention and accept the procedure for effective risk management.