Miroslav Sykora

Target reliability for existing structures considering economic and societal aspects

Abstract Specification of target reliability levels is one of the key issues of the assessment of existing structures. ISO 13822:2010 and ISO 2394:2015 indicate procedures for specification of target reliability levels by optimisation of the total cost related to an assumed remaining working life of a structure. These approaches are critically compared with human safety criteria, with target levels based on a marginal life-saving costs principle, and with recommendations of present standards. Optimal target reliability levels are then derived in the representative case study for an existing structural member. It appears that the requirement to reach the same target reliability levels for existing and new structures is uneconomical. Decisions made in the assessment can result in the acceptance of the actual state or in the upgrade of an existing structure. Two reliability levels are thus needed – the minimum level below which the structure is unreliable and should be upgraded, and the target level indicating an optimum upgrade strategy. It is recommended that these levels be established using economic optimisation and the marginal life-saving costs principle, as both the approaches take into account the costs of safety measures and the failure consequences.

Failures of Roofs under Snow Load: Causes and Reliability Analysis

Collapses of a number of roofs in Europe during the winter 2005/2006 initiated discussions concerning reliability of the roofs exposed to snow loads. Presented overview of extensive investigations of structural failures is focused on causes and consequences of failures. Main observed causes may be subdivided into human errors and insufficient code provisions. Collapses developed from local failures particularly in cases of insufficiently robust structures. Probabilistic reliability analysis reveals that the model for snow loads in the Eurocodes should be modified. Obtained experiences provide valuable background information for future revision of current standards and for forensic assessments of failures of structures exposed to snow loads.

Robustness of Structures: Lessons from failures

Abstract Robustness is considered as an attribute of a structural system that relates to its ability to fulfil its function in the face of adverse events. It is difficult to quantify robustness. The focus of COST action TU0601 has been on developing a framework to quantify robustness and on identifying methods and strategies to improve the robustness of structures. The objectives of this paper are to present an analysis of different failures from the point of view of robustness and to identify measures that directly or indirectly contribute to robustness. It is concluded that structural form plays a major role but it is essential to ensure good management processes for design and construction.

Uncertainty in Resistance Models for Steel Members

Abstract Resistance of steel structures is primarily dependent on material properties, geometry and uncertainties related to an applied model. While materials and geometry can be relatively well described, the uncertainties in resistance models are not yet well understood. In many cases significant efforts are spent to improve resistance models and reduce uncertainty associated with outcomes of the model. However, these achievements are then inadequately reflected in the values of partial factors. That is why the present paper clarifies a model uncertainty and its quantification. Initially a general concept of the model uncertainty is proposed. Influences affecting results obtained by tests and models and influences of actual structural conditions are overviewed. Statistical characteristics of the uncertainties in resistance of steel members are then provided. Simple engineering formulas, mostly based on the EN 1993-1-1 models, are taken into account. To facilitate practical applications, the partial factors for the model uncertainties are derived using a semiprobabilistic approach.

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.

Determination of target safety for structures

Codes of practice aim at guaranteeing structures having the risks acceptable to the public and the minimum total costs over a design working life. However, current criteria for structural design provide a broad range of target reliability indices, specified often for different reference periods even though their recalculation for different reference periods is indeterminate due to mutual dependence of failure events. General approaches for selecting target reliability levels are discussed in view of costbenefit optimisation and human safety aspects. Design strategies seem to be driven by economic arguments rather than by human safety criteria.

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.

Methodology for risk assessment of road bridges endangered by blast attack

Malfunction of bridges due to natural or man-made hazards causes severe consequences in transportation networks. Decision making about protective measures should take basis in optimising expected costs. Uncertainties in input data should be treated by probabilistic methods. In general, total costs cover initial and operational costs, and direct and indirect consequences of structural failure. Bayesian (causal) networks provide an appropriate tool for such considerations. Case studies indicate that particularly the analysis of bridge response to a blast attack is a complex issue.

Optimising Monitoring: Standards, Reliability Basis and Application to Assessment of Roof Snow Load Risks

Abstract This paper addresses the optimisation of monitoring and decision-making criteria regarding the future use of structures. The current state of practice in standards and reliability framework are initially summarised. A general methodology for obtaining cost-optimal decisions is then presented on the basis of limit state design, probabilistic reliability analysis and cost estimates. The implementation of the risk-based approach is illustrated in a case study dealing with the roof of a stadium located in northern Italy. As the roof fails to comply with the requirements of the Eurocodes, the installation of a permanent monitoring system is recommended to allow for a real-time reliability assessment. The results demonstrate the potential of monitoring systems and probabilistic reliability analysis for supporting decisions regarding safety measures such as snow removal or temporary closure of the stadium, as well as reflecting the need to implement the discussed procedures in future standards.

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.