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Technical V IV Unacceptable risk III Apply engineering judgement II Acceptable risk I A B C D Severity Figure 2: Risk matrix to variations in design assumptions should be to achieve acceptable levels. Optional additional considered. reduction measures may also be suggested to further reduce the risks. The findings of the Level of risk and acceptability risk assessment will feed back into the decision For a qualitative assessment, a risk matrix (Figure making process of the design and operation of the 2) is a convenient method of ranking the risks. Each building. All the sources of data, assumptions and hazard is plotted on the risk matrix according the uncertainties in the assessment should be included appropriate severity and likelihood category. The in the report. hazards should be evaluated in order starting with the highest risk hazard. Structural provisions For a quantitative assessment the level of risk For Class 3 buildings where the risks are not greater can be calculated by multiplying the likelihood with than those for Class 2B buildings the structural the severity. provisions required for robustness will be the same The acceptability criteria should be provided by as those for Class 2B buildings. Where there are the authorities or the client. Indicative information increased risks from hazards, additional structural on what levels of risk may be acceptable can be provisions are likely to be required to reduce the obtained from various sources. For example, consequences of the hazards. The types of structural the nuclear6 and the offshore5 industries have provisions that may be appropriate include: guidance regarding acceptable levels of risk but designing members to resist specific accidental these are generally expressed in terms of risk of actions or generic accidental loading and enhancing death per individual per year. Other indicative redundancy within the structure so that alternative information, from Reference 9, is the acceptable load paths can be utilised in the event of a hazard annual frequency of collapse of ‘critical’ bridges of occurring and thereby avoiding disproportionate 0.01 in 100 years giving an annual frequency of 10-4 collapse. and the acceptable annual frequency of collapse of ‘regular’ bridges of 0.1 in 100 years giving an annual References frequency of 10-3. 1. The Building Regulations 2000 (amended 2004). The Stationery Office Risk Reduction Measures 2. Building Regulations 2000 – Approved Document The hierarchy of risk reduction measures (starting A (2004 Edition). The Stationery Office with the most preferred) is given below. 3. Guidance on meeting the robustness a) Inherent safety – eliminate the possibility of requirements in Approved Document A. hazards occurring The Steel Construction Institute, 2005 b) Prevention – reduce the likelihood of hazards 4. New approach to disproportionate collapse c) Detection – measures for early warning of Stuart Alexander. The Structural Engineer, 7 hazards December 2004 d) Control – limiting the size of the hazards 5. Guidance on risk assessment for offshore e) Mitigation – protection from the effects of installations. HSE, 2006 hazards 6. The tolerability of risk from nuclear power f) Emergency response – planning for evacuation stations. HSE, 1992 and access for emergency services 7. Gas explosions in buildings in the UK. Ellis & Currie, The Structural Engineer, 6 Oct 1998 Reporting 8. BS EN 1991-1-7: 2006 Eurocode 1 Part 1-7: The final stage of the systematic risk assessment General actions – Accidental actions is to report the findings. The report should include 9. Guide specification and commentary for vessel all the hazards and their associated level of risk collision design of highway bridges. AASHTO, with explanations of why the risks are acceptable 1991 and what reduction measures have been necessary 32 NSC March 2007


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