Target Level Of Safety Research Articles

交通流の分散による航空機の近接通過頻度の軽減

RVSM, which is the vertical separation minimum reduced from 2,000 ft to 1,000 ft at flight levels (FL) between 290 and 410 inclusive, was implemented on 30 September 2005 within the Japanese domestic airspace for efficient aircraft operation. Prior to the implementation, safety assessment for the airspace in assumed RVSM environments was carried out using the collision risk model. As a result of the assessment carried out in 2004, an estimate of collision risk averaged for whole target airspace met a maximum allowable level of collision risk, i.e. 2.5×10^<-9> [accidents / flight hour], called the technical target level of safety (TLS). However, a collision risk value for G581 route didn't meet the TLS. The G581 route system was restructured for reducing aircraft passing frequencies by traffic flow division. It's important to investigate the change in passing frequency values of the restructured airspace. Using flight progress data, the passing frequencies for the G581 route system were estimated every month. The results obtained are as follows. (1) An estimate of the collision risk after the restructuring for the G581 meets TLS. (2) The average passing frequency of post-restructuring is one twentieth of that of pre-restructuring.

Inverse reliability measures and reliability-based design optimisation

Several inverse reliability measures (e.g. Probabilistic Performance Measure (PPM) and Probabilistic Sufficiency Factor (PSF)) that are essentially equivalent have been introduced in recent years as measures of safety. The different names for essentially the same measure reflect the fact that different researchers focused on different advantages of inverse measures. These advantages include improved computational efficiency of Reliability-Based Design Optimisation (RBDO), accuracy in Response Surface Approximations (RSAs) and easy estimates of resources needed for achieving target safety levels. This paper surveys these inverse measures and describes their advantages compared with the direct measures of safety such as probability of failure and reliability index. Methods to compute the inverse measures are also described. RBDO with inverse measure is demonstrated with a beam design example.

防波堤を対象とした円弧すべりに関する信頼性設計法の適用

It is necessary to determine the target safety level in order to introduce level-1 reliability-based design method. We discussed the target safety level against the circular arc slip failure of breakwaters under ordinal condition, for which the effect of the wave forces are not taken into account. Based on the comparison of average safety level under the current design method with that of the minimum expected total cost including the economic loss, it is shown that safety level by the current design method is very high. The partial factors for level-1 reliability-based design method are proposed.

Reliability-based design optimization using probabilistic sufficiency factor

A probabilistic sufficiency factor approach is proposed that combines safety factor and probability of failure. The probabilistic sufficiency factor approach represents a factor of safety relative to a target probability of failure. It provides a measure of safety that can be used more readily than the probability of failure or the safety index by designers to estimate the required weight increase to reach a target safety level. The probabilistic sufficiency factor can be calculated from the results of Monte Carlo simulation with little extra computation. The paper presents the use of probabilistic sufficiency factor with a design response surface approximation, which fits it as a function of design variables. It is shown that the design response surface approximation for the probabilistic sufficiency factor is more accurate than that for the probability of failure or for the safety index. Unlike the probability of failure or the safety index, the probabilistic sufficiency factor does not suffer from accuracy problems in regions of low probability of failure when calculated by Monte Carlo simulation. The use of the probabilistic sufficiency factor accelerates the convergence of reliability-based design optimization.

Why the Eurocontrol Safety Regulation Commission Policy on Safety Nets and Risk Assessment is Wrong

Current Eurocontrol Safety Regulation Commission (SRC) policy says that the Air Traffic Management (ATM) system (including safety minima) must be demonstrated through risk assessments to meet the Target Level of Safety (TLS) without needing to take safety nets (such as Short Term Conflict Alert) into account. This policy is wrong. The policy is invalid because it does not build rationally and consistently from ATM's firm foundations of TLS and hazard analysis. The policy is bad because it would tend to retard safety improvements. Safety net policy must rest on a clear and rational treatment of integrated ATM system safety defences. A new safety net policy, appropriate to safe ATM system improvements, is needed, which recognizes that safety nets are an integrated part of ATM system defences. The effects of safety nets in reducing deaths from mid-air collisions should be fully included in hazard analysis and safety audits in the context of the TLS for total system design.

Radar Inaccuracies and Mid-Air Collision Risk: Part 2 En Route Radar Separation Minima

A review of safety targets for en route ATC radar separation suggests that the existing target level of safety (TLS) is over-cautious. If risk budgeting principles are followed consistently, a ‘radar TLS’ of 1·0×10−9 fatal aircraft accidents per flying hour is appropriate. This rate is consistent with Joint Aviation Authorities (JAA) guidance on system failure conditions leading to catastrophic accidents. Dynamic and static calculations using published data are compared. The new methodology shows where there are problems with the traditional static calculations, and how to improve the estimation. A further improvement introduces a simple robust model of the controller's decision processes. The focus is not on describing what controllers would generally do, but on setting criteria based on what they could not reasonably be expected to do. This additional ingredient into the calculation adds realism and ensures that attention is focused on hazardous correlated errors. Focused data collection would be an essential component of new risk estimates. The key information required would be on radar performance and the nature and frequency of use of radar separation, including the relative velocities for proximate events at closest point of approach and the frequency of correlated gross errors (through a conditional probability factor). If this factor is not properly taken into account, then the data collection and analysis could be inefficient.

Analysis on Target Safety Level for Egress from a Fire Room and a Fire Floor in Typical Office Buildings

The Japanese Ministry of Construction's Notification 1441 (2000) on Method for Egress Safety from a Fire Floor is composed of a set of prescribed characteristic input values for fire load density, occupant density, etc., coupled with a set of simplified formulae to calculate fire and evacuation behavior. The nominal egress time margin is calculated by combining these values and formulae with building design variables, such as room area and ceiling height. To clarify the degree of safety implemented in the verification method, the stochastic variability in input values and uncertainties in calculation formulae have been examined. Based on existing research and survey results, variability and uncertainty are expressed in terms of probability density functions. The target levels of egress from a fire room and a fire floor were also discussed in this study. The calculation method was analyzed using the methods of Reliability Engineering. The Limit State Function was selected as the difference between the nominal time margin of egress safety and the actual, stochastic time margin of egress safety. By examining the safety level of Limit State Function, the Target Safety Index and the Acceptable Probabity of Failure implied in the Verification Method is clarified for typical office building in Japan. This work has clarified that: (1) Two variables which contain inevitable stochastic variation (Fire Load Density and Occupant Density) are recognized as Type A variables, and six variables which contain uncertainties due to the incompleteness of the prediction method (Walking Velocity, Specific Flow Rate, Time to Start the Evacuation, Fire Growth Rate over combustible contents, Fire Growth Rate over lining materials, and Smoke Filling Time) are recognized as Type B variables. (2) The coefficients of variation are large in both Type A and Type B variables, ranging from 13 to 235 %. There is a need to reduce the uncertainties associated with Type B variables. (3) The Partial Safety Factors vary in the range of - 0.45 to 4.97. (4) The implied safety index of the calculation method is in the range of 1 to 3 depending on the method of smoke control and floor area on the condition of egress from a fire room, and in the range of 2 to 3 it is fairly independent on the method of smoke control and floor area on the condition of egress from a fire floor. (5) In comparison with Type A variability, the effect of Type B uncertainties (prediction error) is not negligible. (6) Possible combination of partial safety factors are proposed by 0.2, 2-6, 0.9 for fire load density, escape start time and smoke filling time on the condition of egress from a fire room for typical office buildings with single side corridor in Japan, and by 0.7, 4.8, 2.6 and 0.9 for fire load density, escape start time, fire growth rate and smoke filling time on the condition of egress from a fire floor.

Calibration methods for reliability-based design codes

The calibration methods are applied to define the optimal code format according to some target safety levels. The calibration procedure can be seen as a specific optimization process where the control variables are the partial factors of the code. Different methods are available in the literature, but the choice of an appropriate method is not usually an easy task. Each method can be shown to have a better performance for a particular kind of problems. The scope of this paper is to underline the advantages and disadvantages of the classical methods, to define their domain of validity and to propose new efficient methods. In our approach, the accuracy is balanced with time efficiency by the mean of iterative scheme using approximate methods. The presented ideas are clarified by four numerical examples of parabolic performance function, column buckling, stability of submarine shells and structural elements.

Structural safety assessment of a pontoon-type VLFS considering damage to the breakwater

A structural safety assessment of a pontoon-type very large floating structure (VLFS) surrounded by a gravity-type breakwater was carried out for extreme wave conditions by considering the damage to the breakwater. Bending and shear collapses are considered to be a failure mode of the floating structure, while overturning damages the breakwater. The probability of the breakwater overturning, and the transmitted wave height before and after damage to the breakwater, are evaluated using design formulae for port and harbor facilities in Japan. The ultimate bending and shear strengths of the floating structure are calculated by the idealized structural unit method (ISUM) and FEM, respectively. The calculated failure probability for the floating structure is compared with the specified target safety level. It was found that the floating structure under consideration is most likely to fail by bending in transverse waves, and that the corresponding failure probability satisfies the target level.

典型的なオフィスビルにおける居室避難安全水準 : 建物火災に対する目標避難安全水準の分析 その2

In this study, the target safety level of egress from fire room was discussed. The calculation method defined in Ministry of Construction's Notification 1441 (2000) was analyzed by using the method of reliability engineering. The limit state function was selected by the difference between nominal margin of egress safety and actual, stochastic margin of egress safety. By examining the safety level of limit state function, the target safety index implied in the verification method is clarified for typical office building layout. The results show that (1) the implied safety index is in the range of 1 to 3 depending on the method of smoke control and floor area. (2) In comparison with type A variability (fuel and occupant load), the effect of type B uncertainties (prediction error) is not neeligible. (3) Possible combination of partial safety factors are proposed by 0.2, 2-6, 0.9 for fuel load density, escape start time and smoke filling time for typical office buildings with single side corridor.

典型的なオフィスビルにおける階避難安全水準 : 建物火災に対する目標避難安全水準の分析 その3

In this study, the target safety level of egress from fire floor was discussed. The calculation method defined in Ministry of Construction's Notification 1441 (2000) was analyzed by using the method of reliability engineering. The limit state function was selected by the difference between nominal margin of egress safety and actual, stochastic margin of egress safety. By examining the safety level of limit state function, the target safety index implied in the verification method is clarified for typical office building layout with single-side corridor. The results show that (1) the implied safety index is in the range of 2 to 3 fairly independent on the method of smoke control and floor area. (2) Possible combination of partial safety factors are proposed by 0.7, 4.8, 2.6 and 0.9 for fuel load density, escape start time, fire growth rate and smoke filling time.

Design Philosophy Issues of Fiber Reinfored Polymer Reinforced Concrete Structures

The conventional design philosophy for reinforced concrete (RC) relies heavily on the ductile properties of steel. These ductile properties are used as a fuse and conceal the large uncertainty in the determination of modes of failure caused directly by concrete. Current design guidelines for fiber reinforced polymer (FRP) RC structures have inappropriately adopted the same design philosophy used for steel RC, leading either to the adoption of conservative safety factors or reduced structural reliability. A reliability-based analysis of FRP RC beams shows that the current, very conservative partial safety factors for FRP reinforcement on their own do not influence the structural safety of overreinforced concrete elements. Proposals are made for the modification of the material partial safety factors to achieve target safety levels.

Probability of Collision of Aircraft with Dissimilar Position Errors

The natural safety metric in air traffic management is the probability of collision between aircraft : due to its extremely low probability of occurrence, it is virtually impossible to measure directly in simulations. It is shown that the rms position error is an indirect safety metric because it specifies an upper bound for the probability of collision. The latter can be calculated from the rms position error of two aircraft, whether it is the same σ (similar aircraft) or different σ 1 and σ 2 (dissimilar aircraft) ; the latter more general case is considered here. The rms position error is easy to implement as a safety metric : it just requires monitoring of the deviation between the actual trajectory and the intended trajectory. The actual trajectory could be specified by real flight data or a real-time or fast-time simulation; it could include en route flying, terminal maneuvers, and ground movements, as long as a minimum separation distance is defined for each phase. It is shown that International Civil Aviation Organization (ICAO) target level of safety (TLS) of low probability of collision is attained if the rms position error σ does not exceed about 1/11 th σ ≤ 0.09L of the minimum separation distance L. This assumes both aircraft have the same rms position error; in this sense they are similar aircraft. In the case of dissimilar aircraft, with distinct rms position errors, σ 1 > σ 2 , the probability of collision will increase by up to a factor of five if σ 1 and σ 2 do not differ by more than a factor of nine. This is a relatively small increase, which remains within ICAO target level of safety (TLS) of5 x 10 -9 per flight hour, if L/σ ≥ 12 is raised, that is, the rms error is about 1/12th of the separation distance.

Development of Truck Weight Regulations Using Bridge Reliability Model

Historically, truck regulations have maintained controls on axle and gross weights with legal load formulas based on limiting allowable stresses in certain types of bridges. These stress limitations do not usually lead to consistent or defensible safety levels and also ignore the cost impact of the weight regulation on the national bridge network. This paper illustrates how new truck weight regulations can be developed to provide acceptable safety levels. Target safety levels are derived from existing AASHTO bridge evaluation and rating procedures applied to structures showing adequate performance levels. Reliability indices are used to relate the statistics of bridge load effects, based on either existing or proposed truck weight regulations, to the dynamic behavior and resistance variables of existing bridges. The sensitivity of the results to various assumptions and errors in the database is also analyzed. An accompanying paper reviews the consequences of adapting such a formula on the safety of existing bridges. The deterministic analysis as well as a reliability assessment are performed in the accompanying paper to review the consequences of adapting such regulations on the U.S. bridge network using the National Bridge Inventory files.

Bending Moment Capacity of Pipes

In this paper, the bending moment capacity for metallic pipes has been investigated to provide criteria for optimizing the cost effectiveness in pipeline seabed intervention design. An analytical solution for the ultimate load-carrying capacity of pipes subjected to combined pressure, longitudinal force, and bending has been derived and thoroughly compared against results obtained by the finite element method. The derived equations can be used for high-strength materials with isotropic as well as anisotropic stress/strain characteristics, and may be applied for pipelines, risers, and piping if safety factors are calibrated in accordance with appropriate target safety levels. [S0892-7219(00)00504-5]

Safety of long railway tunnels

Planning and designing railway tunnels with an explicit reference to safety issues is becoming of utmost importance since the combination of high speed, mixed goods–passenger traffic and extreme length of the new tunnels under design or concept evaluation, have sensitively modified the inherent safety of the railway tunnel. Although the probability of occurrence of accidental events may still be considered rather low, the possible consequences of such events in long tunnels can be catastrophic, therefore raising the overall risk to levels that may be no more acceptable. The scope of this paper is to illustrate the state-of-practice related to risk analysis of long railway tunnels. First, ambitious tunnel projects are briefly reviewed. The applicable risk-analysis procedures are then described and discussed. The problem of risk appraisal is addressed and quantitative target safety levels are proposed. Safety systems for risk reduction are outlined.

Safety instrumented functions and safety integrity levels (SIL)

This paper discusses two performance-based standards, ANSI/ISA S84.01 and IEC d61508, and the requirements they place upon user companies of electrical, electronic and programmable electronic safety related systems (E/E/PE SRS) or Safety Instrumented Systems (SIS). To comply to the requirements of the standards, a user company would have to: (a) identify the safety target level of the process; (b) evaluate the hazardous events that pose a risk higher than the safety target level; (c) determine the safety function(s) that must be implemented in an SIS to achieve the safety target level; (d) implement the safety functions in an SIS and evaluate its safety integrity level (SIL); (e) install, test and commission the SIS; and (f) verify that the installed SIS does in fact reduce the process risk to below the safety target level. Several risk analysis techniques that can be used to comply with the aforementioned requirements are discussed and a simple example is used to illustrate the use, advantages and disadvantages of the techniques. The evaluation of SIL of the SIS (probability to fail to respond to a process demand) is outside the scope of this paper.

A Reliability-Based Calibration Study for Upheaval Buckling of Pipelines

Abstract The present paper describes a reliability-based design procedure against upheaval buckling of rock or soil-covered pipelines. The failure mode considered is “snap-through” buckling. The study is performed using state-of-the-art design methodologies, including an assessment of all known uncertainties related to the load and capacity, measurements, surveys, and confidence in the applied models. A response surface technique is applied within the level III reliability analysis. Target safety levels are discussed for both SLS and ULS conditions, and a case-specific reliability-based calibration study is performed in order to derive a consistent design format.

Optimum Reliability-Based Design Loads Due to Natural Hazard

The reliability concept is now widely accepted in structural engineering as a tool to find the most appropriate structural system for a particular site in terms of safety and economy. The probability of failure Pf, or the reliability index β, are explicitly cited in some codes to determine design loads. However, target reliability tends to be a standard value specified by code writers, mainly based on calibration with established design regulations.Instead of calibration, the degree of safety against natural hazards can be represented in terms of optimum reliability, as determined by the minimum total cost principle, with a reliability index obtained as a closed-form solution of a simplified model. Such simplified models can include parameters for the cost-up ratio, the load effect coefficient of vibration cov, the separation factor and the normalized failure cost.Through the examination of the cost-up ratio and estimation of the normalized failure cost, design loads can be determined using a simple first order, second moment reliability index formula. This procedure for determining target safety or design load levels for snow, wind and earthquake loads is discussed. The effects of availability on statistical information of these loads are also presented.

衝突危険度による洋上複合間隔ルートの安全性評価-III : NOPACルートの横方向の衝突危険度

The passing frequency is one of the important parameters of the collision risk model used for assessing the safety of an airspace. In this paper, lateral passing frequency values and relative speeds for North Pacific (NOPAC) routes were calculated and lateral collision risk was estimated using these values under several assumptions. The results obtained are as follows. (1)Passing frequency values are 0.124(opposite) and 0.0074(same). (2)Relative along-track speed, |ΔV|, is 28.9kt and relative crosstrack speed, |y|, is 11.6kt. (3)The estimated collision risk value under 100NM lateral separation is 1.7x10^<-9>. This value satisfies the TLS(target level of safety) value which is proposed by the ICAO.