Risk-Based Approach


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Risk-Based Approach
  By Armedito Ramadhan 1 Design-to-Code and Design-to-First Principle: There are a number of approaches that can be used to create a safety design of a structure. These approaches are generally categorized as either design-to-code   or design-to-first principle  . Design-to-code  is a methodology that relies on safety factor and empirical engineering practices. On the other hand, design-to-first principle  does not rely on safety factor but explicitly define the safety objective and performance standard of the structure. This method allows the development of innovative solutions and optimization of the design. However, each of the approach has their own advantage and disadvantage thus should be used in accordance to the given safety or business case. Below is a diagram showing the different approaches to create a safety design of a structure: The prescriptive (traditional) approach  consists of applying local regulations, industry codes and standards and good engineering practices. In effect, the safety objectives are implicitly defined and the design process focuses primarily on the means to reach them. The approach can be very efficient and well controlled for conventional design cases. The traditional design process entails sizing the structure so that the stress in the component does not reach a critical value. There are two types of critical values: the limited loads that are determined by the engineer and the requirements dictated by the design specifications. Usually a safety factor   is applied to the limited load, known as design loads, and the component is sized to meet the larger of the two loads. However, the adoption of the safety factor does not have a foundation based upon mathematical equations and reliability theory. Instead, it is a historical convention that has been accepted because ‘there have not been too many failures    when it is used’. Clearly, the traditional meth od is neglecting these errors which cause costly design failures. These errors are caused by variations or uncertainties, which on individual components are negligible, but if it is applied to the entire system can cause unforeseen failure modes. The performance-based approach  relies on the explicit definition of the safety objectives and performance standard expected from the structure. This requirement is translated in the design of structure as displacement and rotational limit. The design shall be developed to fulfill these objectives in a more flexible Realistic hazard scenario: Codes, standards, good engineering practices Structure Design   Safety Objectives (mitigation strategy) Performance levels   Probability scenarios Desin accidental load Traditional / Prescriptive Design Deterministic scenarios   Performance-Based Design or   Performance Criteria: disp. & rotational limit   Safety factor     Risk-Based Design Safety Studies: Probability and consequence analysis   Risk quantification     Safet critical element Desin accidental load   2 manner compared to the prescriptive approach. As a design-to-principle method, the performance based approach allows the development of innovative solutions and optimization of the design. The performance-based approach requires the definition of realistic hazard scenario (e.g. fire, earthquake, collision), which could be deterministic or probabilistic. The risk-based approach  combines together a frequency analysis and a consequence analysis in order to evaluate the risk from the potential accidental events. This approach allows managing the risk through the design of facility and providing opportunities for design optimization. Compared to the performance based design, the risk based design sets more focus on the risk that gives the biggest impact to safety, environmental, and business loss (ones with high probability of failure and high consequence). Indeed if only the worst case is considered, the facility would not be designable. The drawback of the risk-based approach is it requires more resources because dedicated analysis studies need to be performed (i.e. frequency analysis, consequence analysis, etc.) Table below provides a summary of strength and weakness of each approach. Prescriptive / Traditional (focus on means) Performance Based (focus on objectives) Risk Based (focus on risk & consequence) Strengths    Very efficient for conventional cases.    Well-known and well controlled.    Straightforward application.    Compliance is easy to demonstrate for the designer, to endorse (owner) and to accept (authority, classification society).    More flexible to cope with project specificities.    Explicit definition of objectives and associated performance criteria.    Optimization of mitigation measures (cost reduction, reduced MTO, less time on construction site for implementation).    Cover special cases    Particular risk can be managed and assured through the design phase.    Providing opportunities for design optimization.    Safety critical components are determined. Weakness    Implicit objectives.    Special cases not covered.    Long process for acceptance of any deviation to the codes & regulations.    Does not have a foundation based upon mathematical equations and reliability theory.    Acceptance criteria may be more difficult to define (by owner or authorities) or acceptance may be more difficult to grant.    More resources /skills needed for each step of the detailed design process.    Time consuming during engineering phase (demonstration that the system satisfies the performance criteria)    Safety Management System required during the entire lifecycle of the facility to account for potential design modifications which can change scenarios.    Acceptance criteria may be more difficult to define or acceptance may be more difficult to grant.    More resources /skills needed for dedicated frequency and consequence analysis study.    Time consuming during engineering phase. Since the above approaches serve different purpose, it cannot be concluded which approach is better than the others. They should be used in consideration of the given safety or business case. In some cases, a combination between them may be needed to meet the safety and functional objective.   3 Case example: Design of an Offshore Platform in the Caspian Sea (Citation: Arup Engineering seismic course material; R. Gibson et al) Arup was the detailed designer of a self-installing gravity based structure (GBS) in a highly seismic zone in the Caspian Sea. Arup became involved after an elastic analysis based prescriptive design approach predicted the platform to have inadequate capacity to withstand significant seismic events. The Arup team adopted a performance based approach, in combination with advanced non-linear analyses, to demonstrate the suitability of the design for use in this location and satisfied the performance requirements of ISO 19901-21. Graph below shows the non-linearity behaviour considered in the non-linear time history analysis results in lower response spectrum compared to the forced-based prescriptive method. Following ISO requirement for non-linear time history method, the performance level for structure is defined as shown in table below. The performance-based approach:    Demonstrated structural feasibility    Avoided major redesign    Proved adequacy of individual members    Saved material and cost. Although, the above example did not perform any risk-based analysis, a risk-based analysis can be done if there is particular risk to be concerned. For example, after doing safety studies and risk quantification we consider that the crane will be frequently used and the consequence of collapsing gives the biggest impact to safety, environmental, business loss (high probability of failure and high consequence). A risk-based design will use this information to design the member under the crane to be bigger and have redundancy of fatigue.   4 Case example: Seismic Safety Design Approaches: Past and most present design approaches for seismic safety are prescriptive, based on following rules rather than explicitly quantifying performance/risk (ASCE standard 7). For earthquake loads/demands, past approach is typically designed against deterministic earthquake scenarios or probabilistic seismic hazard maps. Although seismic safety was implied by these past design approaches, the seismic risk for the resulting structures, e.g. annual probability of seismically-induced failure) was not explicitly quantified in the design process, or in the development of the design process by regulators. To be able to explicitly quantifying performance and risk, a risk quantification analysis is needed. The quantified risk is the integral of fragility and hazard. The fragility curve depends on details of individual elements of the building. Not just for building collapse, the target annual probability could be of earthquake-induced: -   Casualties -   Repair costs -   Loss of use The following codes cover the risk-based design approach related to earthquake and structure:    Risk-Targeted Maximum Considered Earthquake (MCE R ) Ground Motions for designing new buildings and other structures, 2012 International Building Code.    A Performance-Based Approach to Define the Site-Specific Earthquake Ground Motion, U.S. Nuclear Regulatory Commission (NRC) Regulatory Guide (RG) 1.208 (2007).    Development of Next-Generation Performance-Based Seismic Design Procedures for New and Existing Buildings, ongoing Applied Technology Council (ATC) Project #58, funded by U.S. Federal Emergency Management Agency (FEMA) Reference: Gruenwald, J., Risk-based structural design: Designing for future aircraft Luco, N., Performance and Risk Based Design Approaches for Seismic Safety, USGS. Hocquet, J., Challenges in Using Risk and Performance Based Design Method for FLNG
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