Target Reliability Levels Research Articles

Influence of Reinforcing Steel Corrosion on Life Cycle Reliability Assessment of Existing R.C. Buildings

Time-dependent reliability assessment is a crucial aspect of the decision process for rehabilitation of existing reinforced concrete structures. Since the assessment strongly depends on degradation of materials with time, the paper focuses on the influence of corrosion in reinforcing steel on time-reliability curves of relevant reinforced concrete (r.c.) structures, built in Italy in the 1960s, belonging to different building categories. To realistically represent the probability distribution functions (pdfs) of the relevant properties of reinforcing steel and concrete commonly adopted in the 1960s, stochastic models for steel yielding and concrete compressive strength have been derived, by means of a suitable cluster analysis, from secondary databases of test results gathered at that time in Italy on concrete and steel rebar specimens. This cluster analysis, based on Gaussian mixture models, provides a powerful tool to “objectively” extract material classes and associated probability density functions from databases of experimental test results. In the study, different degradation conditions and several reinforcing steel and concrete classes are considered, also aiming to scrutinize their influence on the time-dependent reliability curves. Finally, to stress the significance of the study, the time-dependent reliability curves so obtained are critically examined and discussed also in comparison with the target reliability levels currently adopted in the Eurocodes.

Аbout target reliability levels establishing in national codes for structural design

В соответствии с действующим ТКП EN1990 [5] конструкции следует проектировать таким образом, чтобы в течение проектного срока службы, с соответствующим уровнем надежности и оптимальными экологическими затратами, они: (1) воспринимали все воздействия и влияния, появления которых следует ожидать в процессе эксплуатации; (2) удовлетворяли требованиям пригодности к нормальной эксплуатации и (3) долговечности. Перечисленные требования являются взаимосвязанными и совместно формулируют целевые значения мер надежности, вносимые в нормы. Принятая в настоящее время классификация конструкций (классы СС), включенная в актуальный EN1990 [8], не нашла широкого применения при реальном проектировании, основана на неясной качественной дифференциации и описании последствий отказа конструкции или конструктивной системы. Результаты анализа различных подходов к назначению требуемых мер надежности (анализ рисков с “F-N” кривыми, монетарная оптимизация, принцип предельных затрат на спасение жизни, LQI), приведенные в статье, позволили сделать вывод о том, что целевые уровни надежности следует устанавливать, рассматривая экономическую (стоимостную) оптимизацию совместно с принципом предельной стоимости спасения жизни (LQI-принцип), оба эти подхода учитывают последствия отказа и стоимость мер обеспечения безопасности.

Effects of the move towards renewables on the power system reliability and flexibility in South Korea

The plan to shift towards renewable energy has recently become the central part of the energy policy on the power system in South Korea. The sudden shift towards renewable energy has raised questions regarding the reliability and flexibility of the power system. This paper proposes a research framework to evaluate the new policy in South Korea from various aspects using three simulation models in a series. The first optimal generation model finds the optimal electricity generation mix and provides the total generation cost and environmental impact of the given long-term capacity expansion plan. The other two simulation models assess the reliability and flexibility of power systems, respectively. Within the research framework, we introduce a new probabilistic index to quantitatively measure the flexibility. The results of applying the framework into the new policy show that the policy fails to guarantee the target reliability level and increases costs and emissions. Achieving target system reliability will require additional capacity, and system flexibility is very sensitive to the type of capacity added. If the Korean government decides to add more capacity, natural gas turbine power plants turn out to be a good option from the point of economic, environmental, and flexibility considerations.

Reliability-Based Design Optimization of a Cemented Prosthesis in a Femur Undergoing Bone Remodeling.

This study proposes a numerical methodology to minimize the bone mass loss in a femur with a total hip arthroplasty procedure, considering uncertainties in the material parameters and using a reliability-based design optimization (RBDO) procedure. A genetic algorithm (GA) is applied for optimization, and a three-dimensional finite element (FE) model associated with the bone remodeling procedure is proposed and described to account for the internal and external femoral bone behavior. An example of a femoral prosthesis design is presented as a basis for discussion of the proposed methodologies, and the corresponding reliability level is evaluated. Constraints on the strength of all materials and target reliability levels are inputs to the optimization model. The main prosthesis dimensions and Young modulus are the design variables. The proposed methodology is compared with a well-known deterministic optimization (DO) procedure and the results show that it is important to consider the uncertainties in this kind of problem since in this case, the a posteriori reliability may be low.

System-based calibration of offshore Malaysia environmental action factors for the ISO 19902 PETRONAS offshore fixed platforms

Previously, the action factors, which are generally used in the region of Malaysia is based on the data from the Gulf of Mexico (GoM) or the North Sea, where the environmental condition is much harsher than the offshore Malaysia. Direct application of the high load factors in these regions can result in a waste of economic resources. Based on a reliability-based Load and Resistance Factor Design approach, the current study is carried out for the provision of system-based approach for the calibration of Offshore Malaysia Annex Extreme Environmental Action Factors based on the data from Malaysian waters. With the accessible SEAFINE and HYCOM hindcast database, long-term statistics of extreme environmental load have been derived for six platform locations in Malaysian waters, following the response-based Load Statistics Module (LSM) process. The Generic Load Model (GLM) including the wave-in-deck forces, which has been employed for transferring the wave motion statistics to structural response statistics, is developed in the paper. The structural resistance uncertainty is characterised by the minimum reserve strength ratios (RSRs) derived from a theoretical model that relates with environmental load factor. A structural system reliability analysis based on the FORM method has been performed for a range of minimum values of theoretical RSR for jacket structures fully designed to ISO Code 19902. Partial action factors associated with extreme environmental loads are calibrated to achieve a target reliability level in terms of reliability index.

Reliability-based design of FRP flexural strengthened reinforced concrete beams: Guidelines assessment and calibration

In the past few decades, many guidelines and standards for FRP strengthened reinforced concrete (RC) structures have been presented based on the theory of reliability-based limit state with the load and resistance factor design (LRFD) approach. The LRFD method consists of two formats: the resistance reduction factor format and the material partial safety factor format. In this paper, several well-recognized design guidelines with these two formats for FRP flexural strengthened RC beams have been assessed based on model error and reliability index. First, the model error of each design guideline was evaluated based on a collected test database. Then, the reliability index was obtained by the importance sampling (IS) method, in which the optimum density function is determined by the first-order reliability method (FORM). Finally, the calibration of resistance reduction factor or FRP partial safety factor was performed with different target reliability indexes. The results show that: (1) The model error of each failure mode is different for each design guideline. For flexural failure, China’s GB 50608 2010 has the lowest model error. For intermediate crack-induced (IC) debonding, UK’s TR 55 2012 shows the highest model error. For end debonding, Italy’s CNR-DT 200 R1/2012 is less conservative. (2) It is difficult to meet the pre-given reliability level for some failure modes in several design guidelines using the originally suggested design parameters. For example, GB 50608 2010, fib T5.1 2019 and JSCE 2001. (3) The calibrated resistance reduction factors or the FRP partial safety factors are provided to satisfy the target reliability level. However, it is meaningless and uneconomical to adopt partial safety factors that are too large or reduction factors that are too small to fit into a design. Other options can be chosen such as applying additional anchorages to avoid end debonding, increasing the FRP bonding area, and reducing the FRP thickness to avoid the probability of IC debonding failure.

Partial factor methods for existing structures according to fib Bulletin 80: Assessment of an existing prestressed concrete bridge

AbstractThe assessment of existing reinforced concrete structures is one of the major aspects for engineers and practitioners. In particular, existing infrastructures, as bridges and viaducts, are extensively exposed to environmental actions, materials aging, degradation, and variation of magnitude of traffic loads during their service life. Hence, the assessment of existing structural systems assuming the same criteria conceived for the design (i.e., partial factor method—EN 1990) can be too conservative and, sometimes, may lead to unnecessary and expensive structural interventions. In this context, fib Bulletin 80 defines the partial factor methods suitable for the assessment of existing reinforced concrete structures accounting for their residual service life, information from in situ and laboratory tests, measurements of variable actions and reduced target reliability levels according to both economical and human safety criteria. The methodologies proposed in fib Bulletin 80 have been applied to assess the safety of an existing prestressed reinforced concrete bridge built in 90s and located in Italy. The results are compared to the outcomes from the assessment performed according to EN1990 and, finally, limits and advantages of the methodologies proposed by fib Bulletin 80 are discussed.

Reliability-based calibration of partial factors for the design of temporary scaffold structures

In this work, partial factors for the design of temporary scaffold structures are calibrated based on reliability calculations. Scaffolds are commonly designed in accordance with the rules in standards (e.g. EN12811, EN12810) and information found in codes of good practice. However, there is no clear proof that these design procedures result in an appropriate and consistent safety level for scaffold structures. The present study proposes appropriate target reliability levels for façade scaffolds, based on which partial factors are determined. The results are based on simulations of different scaffold design situations, considering samples generated by Latin Hypercube Sampling (LHS). The results of these simulations are processed by applying FORM analyses for buckling and yielding of the scaffold elements. As such, the inherent reliability corresponding to current design practice is determined. Based on the inherent and target reliabilities, partial factors are calibrated using two methods: the Adjusted Partial Factor Method and an optimisation procedure based on least-square averaging. Concluding, annual reliability indexes in the range 2.5–3.5 are found. These lead to partial factors lower than or equal to those used in current design practice. Hence, there is an optimisation possible in the design of temporary structures compared to current practice, leading to more economical scaffold designs.

Assessment of an existing prestressed concrete bridge according to the partial factor method for existing structures (fib Bulletin 80)

The assessment of existing reinforced concrete structures is a critical aspect for engineers and practitioners. In particular, existing infrastructures, as bridges and viaducts, are extensively exposed to environmental actions, materials aging, degradation and variation of magnitude of traffic loads during their service life. Hence, to perform the assessment of existing structural systems assuming the same criteria conceived for the design (i.e., partial factor method – EN 1990) can result to be too conservative and, sometimes, leads to unnecessary and costly structural interventions. In this context, fib Bulletin 80 defines a new partial factor method suitable for the assessment of existing reinforced concrete structures and infrastructures accounting for their residual service life, information from in situ and laboratory tests, measurements of variable actions and reduced target reliability levels according to economical and human safety criteria. The methodologies proposed in fib Bulletin 80 have been applied to the assessment of an existing precast box section pre-stressed reinforced concrete bridge built in 90s and located in north of Italy. The results are compared to the outcomes from the assessment performed according to EN1990 and, finally, limits and advantages of fib Bulletin 80 methodologies are discussed.

On the Capacity of 5G NR Grant-Free Scheduling with Shared Radio Resources to Support Ultra-Reliable and Low-Latency Communications.

5G and beyond networks are being designed to support the future digital society, where numerous sensors, machinery, vehicles and humans will be connected in the so-called Internet of Things (IoT). The support of time-critical verticals such as Industry 4.0 will be especially challenging, due to the demanding communication requirements of manufacturing applications such as motion control, control-to-control applications and factory automation, which will require the exchange of critical sensing and control information among the factory nodes. To this aim, important changes have been introduced in 5G for Ultra-Reliable and Low-Latency Communications (URLLC). One of these changes is the introduction of grant-free scheduling for uplink transmissions. The objective is to reduce latency by eliminating the need for User Equipments (UEs—sensors, devices or machinery) to request resources and wait until the network grants them. Grant-free scheduling can reserve radio resources for dedicated UEs or for groups of UEs. The latter option is particularly relevant to support applications with aperiodic or sporadic traffic and deterministic low latency requirements. In this case, when a UE has information to transmit, it must contend for the usage of radio resources. This can lead to potential packet collisions between UEs. 5G introduces the possibility of transmitting K replicas of the same packet to combat such collisions. Previous studies have shown that grant-free scheduling with K replicas and shared resources increases the packet delivery. However, relying upon the transmission of K replicas to achieve a target reliability level can result in additional delays, and it is yet unknown whether grant-free scheduling with K replicas and shared resources can guarantee very high reliability levels with very low latency. This is the objective of this study, that identifies the reliability and latency levels that can be achieved by 5G grant-free scheduling with K replicas and shared resources in the presence of aperiodic traffic, and as a function of the number of UEs, reserved radio resources and replicas K. The study demonstrates that current Fifth Generation New Radio (5G NR) grant-free scheduling has limitations to sustain stringent reliability and latency levels for aperiodic traffic.

Reliability-Based Serviceability Limit State Design of a Jacket Substructure for an Offshore Wind Turbine

The development of a structurally optimized foundation design has become one of the main research objectives for offshore wind turbines (OWTs). The design process should be carried out in a probabilistic way due to the uncertainties involved, such as using parametric uncertainties regarding material and geometric properties, and model uncertainties in resistance prediction models and regarding environmental loads. Traditional simple deterministic checking procedures do not guarantee an optimized design because the associated uncertainties are not fully considered. In this paper, a reliability analysis framework is proposed to support the optimized design of jacket foundations for OWTs. The reliability analysis mainly considers the serviceability limit state of the structure according to the requirements of the code. The framework consists of two parts: (i) an important parameter identification procedure based on statistical correlation analysis and (ii) a finite element-simulation-based reliability estimation procedure. The procedure is demonstrated through a jacket structure design of a 3 MW OWT. The analysis results show that the statistical correlation analysis can help to identify the parameters necessary for the overall structural performance. The Latin hypercube sampling and the Monte Carlo simulation using FE models effectively and efficiently evaluate the reliability of the structure while not relying on a surrogate limit state function. A comparison between the proposed framework and the deterministic design shows that the framework can help to achieve a better result closer to the target reliability level.

Uncertainty quantification of the bond stress – displacement relationship of shoring anchors in different geologic units

ABSTRACTThe design of anchored wall systems is governed by the bond strength between the grout and the surrounding soil/rock in the grouted zone. Published work on the reliability of anchored retaining systems focuses on ultimate limit state considerations to ensure a target level of reliability against pullout of the anchor in the bonded zone. These studies do not shed light on the uncertainty in the bond stress – displacement relationship of the grout/soil interface. This uncertainty exists as a result of spatial variability in the soil/rock properties even within the same geologic unit. The primary objective of this paper is to quantify the uncertainty in the bond stress – displacement relationship to aid the serviceability limit state design of anchored wall systems. To achieve this objective, a database of shoring anchor tests from real projects that were executed in several sites around Beirut is assembled and analysed. The tests are categorised based on three different geologic units (Limestone, Marls, and Clays) that are common to many geologic settings in the world. The bond stress – displacement relationship is modelled with a hyperbolic model and the statistics of the model are derived from the assembled database and classified based on the geologic units analysed.

Risk-Informed Requirements for Design and Assessment of Structures Under Temporary Use.

The relatively high failure rates, with important consequences in many cases, suggest that the implicitly acceptable risk levels corresponding to temporary civil engineering structures and activities might exceed the bounds of normally acceptable levels associated with different societal activities. Among other reasons, this may be attributed to the lack of a rational approach for the assessment of risks associated with the different technologies supporting these activities in general, and for structures in particular. There is a need for establishing appropriate target reliability levels for structures under temporary use taking into account specific circumstances such as reduced risk exposure times. This issue is being addressed in this article. Acceptance criteria for building-structure-related risks to persons obtained in prior studies are adapted to the special circumstances of nonpermanent risk exposure. Thereby, the general principle followed is to maintain the same risk levels per time unit as for permanently occupied buildings. The adaptation is based on the statistical annual fatality rate, a life safety risk metric that allows for a consistent comparison of risks across different societal activities and technologies. It is shown that the target reliability indices taking account of the temporary use of buildings might be significantly higher than the values suggested for permanently used structures.

Development of partial factor design method on bending strength of piles for Japanese Specifications for Highway Bridges

ABSTRACTThis paper describes the development of a partial factor design method on the bending strength of piles for the Japanese Specifications for Highway Bridges. First, uncertainties in mobilised bending moments and yield bending moments were evaluated by Monte Carlo simulations. Second, the reliability of piles designed by the previous specifications were evaluated on the basis of reliability analysis considering uncertainties in the mobilised bending moments, yield bending moments, and other factors. Finally, a partial factor design method utilising a survey subsurface investigation method and ground type was developed to reach target reliability levels determined by the Standards.

A risk-based approach for calibration of design codes

This paper contributes to the ongoing discussion on the tentative target reliability levels for the design of structures and calibration of codes. The proposed extension of the existing approaches for code optimisation allows a consistent calibration of codes through all levels of design. The basis is taken from the normative decision theory as the rational strategy for taking decisions under risk. The method proposed supports code-making by optimizing reliability elements accounting for i) the safety format adopted in the code, ii) the design level that is regulated, and iii) the differences among the design cases regulated by the same code. Some of the decision theoretical aspects that are object of discussions in the research community are addressed. The work aims at providing a more robust and transparent background to target reliabilities in design standards.

Fatigue reliability analysis for medial tibial stress syndrome

Medial Tibial Stress Syndrome MTSS is frequently diagnosed as a severe clinical issue which occurs due to the cycle loadings (fatigue failure). Since the mechanical properties of the bones are random variables, therefore, there is a need for a probabilistic approach to rationally assess the reliability level of the fatigue failure of the tibia. The main contribution of this paper is to introduce a novel limit state function to determine the fatigue damage state and the reliability index of MTSS based on the different loading and aging conditions. In this study, the tibias of the several people at the different ages are subjected to the fatigue loadings. The load carrying capacities of the considered specimen are determined based on the reliability analysis. Indeed, several 3D finite element analyses are performed to find out the damage states of the tibias. Accordingly, the reliability-based stress analysis is accomplished to localize the most vulnerable zone of the bone. As it was expected, the higher loading cycles associated with the elder bones is related to the lowest fatigue reliability level. Finally, the target reliability level of MTSS is proposed to deliberate the safe level of the loading condition on the tibia in terms of the walking distance.

Probabilistic Analysis of Tunnel Liner Performance Using Random Field Theory

Tunnel liner is conventionally designed according to the deterministic design methods. However, the loads transmitted from the surrounding ground to the liner are very uncertain. In other words, the tunnel liner is subjected to nonuniform loading conditions which may affect its performance. This is mainly due to the inherent randomness and spatial variability of ground properties. In addition, since borehole drillings are usually carried out before tunnel excavation, the measured borehole data should be considered and reflected in the tunnel liner design practice, especially when it comes to the reliability‐based design using the conditional random field theory. This paper presents an investigation of the tunnel liner performance from the perspective of probabilistic analysis and reliability‐based design, and a probabilistic procedure for evaluating the tunnel liner performance is fully described. The random behavior of ground properties is accounted for by the conditional random field theory, and the performance of tunnel liner is examined via finite‐difference modeling in FLAC 3D. The evaluation procedure is performed within the Monte Carlo simulation framework. An illustrative hypothesized tunnel is introduced to demonstrate the application of the probabilistic evaluation procedure, and the effects of site characterization parameters and conditioning on the tunnel liner performance are also inspected in a series of parametric studies. Before reaching conclusions, the reliability‐based design of a real tunnel is performed with respect to the liner strengths and thickness, given target reliability levels, which provides some insights into a more reasonable and economical design of tunnel liner.

Full probabilistic geotechnical design under various design scenarios using direct Monte Carlo simulation and sample reweighting

It is common to encounter a diversity of design scenarios with different values and statistics of loads and/or geotechnical parameters in design practice of engineering geology and geotechnical engineering. Maintaining a uniform level of reliability close to the target reliability under various design scenarios is a key goal of geotechnical reliability-based design (RBD), which is a fundamental challenge for semi-probabilistic RBD methods because a wide range of design scenarios can be involved in geotechnical design practice. This paper develops an efficient RBD updating approach under a full probabilistic design framework, which combines direct Monte Carlo simulation (MCS) with a sample reweighting technique to efficiently obtain final designs under different design scenarios by a single simulation run, avoiding repeatedly performing direct MCS for different design scenarios. This leads to considerable computational saving, particularly as the number of design scenarios concerned is large. The proposed approach is illustrated through a pad foundation design example and a rock slope design example. Results show that the proposed approach properly calculates failure probabilities of different possible designs under different design scenarios by a single run of direct MCS, provided that sufficient failure samples are generated in the direct MCS run to cover failure regions of different design scenarios, which are used to determine the updated final designs. Reliability of the final designs obtained from the proposed approach for various design scenarios is generally uniform and is close to the target reliability level prescribed for the design purpose. Moreover, the proposed approach is able to link site investigation efforts to design saving in an efficient and straightforward manner. It also provides insights into the sensitivity of the final design to uncertain parameters involved in design.

Copula‐Based Chance‐Constrained Hydro‐Economic Optimization Model for Optimal Design of Reservoir‐Irrigation District Systems under Multiple Interdependent Sources of Uncertainty

AbstractThis paper presents a new explicit stochastic hydro‐economic optimization model for reservoir‐irrigation district systems design affected by multiple interdependent sources of uncertainties. The model solution determines both optimal design and long‐term operation policies of the systems while accounting for uncertainties of reservoir inflow, rainfall, crop yield, crop price, and production costs. The problem is formulated as a chance‐constrained program in which the dependence structure of discrete and continuous random coefficients is accounted for using copula. In addition to design variables such as the sizing of reservoir capacity and the irrigable area, optimal reservoir operation policies and irrigation management strategies are determined. The model performance and significance is tested in the case study of Chamshir hydrosystem in Iran, consisting of Chamshir Dam and downstream irrigation districts. Various scenarios assessing baseline conditions, risk attitude, and the impacts of change in climatic inputs and upstream conditions are also evaluated. The model results are used to quantify the interrelationships and trade‐offs among optimal values of design parameters, optimal irrigation policy, decision‐makers' risk‐attitude, uncertainty levels of agro‐economic factors, and target reliability levels of meeting crop yield and water requirement. The results demonstrate that smaller sizes of the reservoir and the irrigation district and stress‐avoidance irrigation policies, rather than deficit‐irrigation policies, are preferred when the decision maker is risk‐averse and uncertainties in agro‐economic factors and reliability levels are large. Additionally, we show how the optimal cropping pattern and irrigation strategies are affected by climate change‐induced rainfall variations and the alteration of conditions upstream of the project.

Reliability analysis of welded and bolted connections in cold-formed steel sections

Abstract This article shows a study of the level of reliability of welded and bolted connections in cold formed steel members, for some limiting states, adopted by AISI and Brazilian codes. The aim of this study is the assessment of the reliability index β for a variation of the nominal live-to-dead load ratios as well as comparison of the value found considering different load combinations. The first order reliability method is used to calculate the reliability index β. In this study, reliability indices smaller than 3.5 were obtained, especially for bolted connections. Consideration of the model errors and FORM method, lead to significant reductions in reliability indices, which are found to be less than the recommended target reliability levels.