1. Risk or Consequence? The Concept of Consequence Based Explosion Resistant Design Sirous Yasseri IMECHE Seminar on Engineering Structures Survival: Blast and Impact Protection University of Nottingham, 7th October 2010

2. Goals of a Design Risk management Hazard driven measures
Technology driven measures
Regulatory driven measures
Risk management
Economic limitations
Demographic limitation
Environmental limitation

3. Elements of Design Process
Exposure (Explosion load with certain probability of being exceeded)
Setting the context
Vulnerability (Probability of a Damage State being reached or exceeded for a given Explosion
In this framework three levels are distinguished, namely the exposure, the vulnerability and the robustness. The exposure can be considered an indicator of the hazard potential for a given object or system of consideration. Considering earthquakes the exposure EX is an inherently uncertain phenomenon with probabilistic characteristics usually provided in terms of earthquake intensities and corresponding return periods. The vulnerability P(D EX) can be considered an indicator of the immediate consequences (or damages to the system) associated with a given exposure event. Considering an explosion event the vulnerability is associated with significant uncertainty and is appropriately described by a probability distribution of different damage states of structures and lifelines conditional on the exposure event, e.g. the earthquake intensity. The robustness is an indicator of the indirect consequences due to the damages of the considered system. Considering again the event of an earthquake the robustness is associated with the conditional probability of losses of various degrees conditional on the exposure and a given damage state.
Consequence (probability of financial loss, injury and fatality)
Remediation to keep loss below tolerable level (Risk Reduction measure and their cost effectiveness)

4. Expression for Expected Consequences
Decision variable
risk of losses DV
Damage measure
casualties
capital loss
downtime DM
Engineering demand parameter
displacement
drift
etc
EDP
Intensity measure
hazard curve
level of explosion IM

5. Events leading to gas explosion
Events leading to gas explosion. BLEVE – Boiling Liquid Expanding Vapor Explosions
The process of a gas cloud being ignited with the result of a rapid increase in pressure is defined as a gas explosion. Before an explosion is possible, there are several events that must occur. These events are illustrated in Figure.
As the diagram shows, it is of course not possible to have an explosion without release of gas. Secondly, the gas must be ignited and an ignition can either lead to fire or an explosion. In this paper, it is only the explosion which is treated and not fires, even though they are more common as Figure suggest.
(Gex-Con, 2006)

6. Example of exceedance curve
There are several ways of dealing with risk analysis of gas explosions on oil platforms. One of the most accepted methods is is known as probabilistic explosion risk assessment. The method uses CFD calculations to both simulate gas dispersion and pressure from the explosion. These simulations are treated using statistics and changes the parameters which influence the explosion. Some parameters are wind speed, wind direction, size of leakage and direction. The result is a so-called exceedance curve and an example is given in Figure.
Figure indicates that the frequency, and thus the probability, of a large pressure due to an explosion decrease as the presure grows. This means that if an explosion would occur it is most likely that the pressure would be relatively small. The exceedance curve is used to estimate if the probability of a certain pressure can be accepted and if the pressure is not acceptable a redesign must be considered. For example, the structural members are designed to a pressure of 2[Barg] which means that this pressure or a higher pressure occurs once every 10,000 years because the frequency is If this frequency can be accepted, then the design pressure is correct. It is of course possible to experience a higher pressure e.g. 5[Barg] but the frequency of such a pressure is about 10-6, cf. Figure.

7. Possible Threats to Civil Engineered Systems

8. Frequency matrix Rank Frequency Category Annual Frequency Range 6
Very Often
>10
Day - Month 5
Often
1 to 10
Month -Year 4
Probable
0.1 to 1
1 to 10 years 3
Moderate
0.01 to 0.1
10 to 100 years 2
Rare
0.001 to 0.01
100 to 1000 years 1
Very rare to
1000 to 10,000 years
Extremely rare
<
>10,000 years
One example of a frequency matrix is shown in Figure 3. The matrix expresses how
often any given undesired event occurs. The frequency is partly given by a qualitative
scale going from “very frequently” to “extremely rare”, and partly by a quantitative
scale, which roughly describes the frequency and repetition periods for an event as a
numeric value. It is important that both the qualitative scale and the quantitative scale
are defined in a way that attains a broad agreement around the concepts. It should
be noted that the quantitative scale is logarithmic.

9. Communicate and Consult
High-level framework
Communicate and Consult
Objectives
Stakeholders
Criteria
Key issues
Establish the Context
What risks?
How might they occur?
Identify risks
Controls
Probability
Consequence
Rank
Evaluate risks
Identify options
Assess options
Set priorities
Evaluate treatment options
Develop and implement future improvement plan
Treat risks
Monitor and Review
Source: Developed from the International Infrastructure Management Manual, 2006 and the AS/NZS 4360: 2004

10. Defence in Depth
Robustness works as Russian Matryoshka dolls. Outside each layer of protection, there is another layer containing it, and monitoring and maintaining it… but is there a larger doll outside that doll, protecting it?
If yes what is that layer

11. Philosophy of Seveso II Directive
I N S P E C T I O N S
Land-Use
Planning
Safe
Management
Technology
Information to the Public
Emergency
Planning
Demonstrate safety
in the Safety Report

12. The Bow Tie Analysis

13. What do we mean by performance?
Building
What is the cost? P
= RISK
Fatality/Injury (safety)
£ Money (damage)
Downtime (loss of use)

14. Peak reflected overpressures Pr (in MPa) with different W-R combinations

15. Quantitative range of performance
Force parameter
Displacement
parameter
The damage evaluation procedures are performance-based; that is, they measure acceptability (and changes in acceptability) on the basis of the degree to which a structure achieves one or more performance levels for the hazard posed by one or more hypothetical future earthquakes. A performance level typically is defined by a particular damage state for the components of a building. Commonly-used performance levels, in order of decreasing amounts of damage, are Collapse Prevention, Life Safety, and Immediate Occupancy. Hazards associated with future hypothetical earthquakes commonly are defined in terms of ground shaking intensity with a certain likelihood of being exceeded over a defined time period, or in terms of a characteristic earthquake likely to occur on a given fault. The combination of a performance level and a hazard defines a Performance Objective. For example, a common Performance Objective for a building is that it maintain Life Safety for ground motion with a ten percent chance of exceedance in fifty years.
$, % replacement
25% % %
25% % %
25% % %
$, % replacement
$, % replacement
0.0
0.0
Casualty rate
Casualty rate
Downtime, days
Downtime, days

16. The Connected World System of interdependent critical infrastructures
All elements are linked by arrows indicating characteristic relations between them.

17. Initial & Lifecycle Costs
The cost of a facility should be seen in terms of both the initial cost as well as the life-cycle cost. In some projects, initial cost of setting up the structure may seem low, but in the long-run, cost of maintaining and upgrading it may be high. To capture such financial burdens during the planning stage itself, life-cycle cost is relied upon to get a more meaningful reflection of the viability and sustainability of the facility.
Determining the initial cost of constructing and protecting a facility involves a number of factors. But, one factor that seems to directly affect protection cost is the stand-off distance of the facility from the un-protected access point nearest to it. All protection strategies that are affected by the level of threat, say by the amount of charge used in the explosive (in terms of equivalent tons of TNT), are seen as variable costs, implying that they depend on stand-off distance. On the other hand, protection strategies or measures that do not depend on the level of threat, e.g., the hardware required at the security surveillance control room or the space required to house the
facility, are seen as fixed costs. Since designers do not have control on the level of threat, they often attempt to reduce the initial cost of the structure by increasing the stand-off distance, because the peak blast pressure is inversely proportional to the square of the stand-off distance. But, increasing the stand-off distance has the downside of increased perimeter from the facility to be protected and increased land cost. Thus, one can identify an optimal stand-off distance from the partial initial cost, which is the sum of cost of protection (i.e., hardening cost) and cost of stand-off (i.e., land cost and perimeter protection cost). The fixed costs need to be added to this partial initial cost to obtain the initial cost (Figure 1.2).
Contributors to initial cost of fatality (Source: FEMA 427)

18. Vulnerability DM

19. Vulnerability Functions - Definition
Collapse Prevention
Damage Control
Serviceability
Probability of Reaching a Limit Response
Blast Overpressure Severity

20. Vulnerability Functions – Wider Applications
Effect on other infrastructure
Economic Consequence
Societal Reaction
Population of Buildings
Horizontal Axis – Blast Overpressure
Vertical Axis – Effect on a System or a Region

21. Performance based design
Demand for specific hazard level
Building Damage States
Force
parameter
Immediate
occupancy
Life
safety
Collapse
prevention
The damage evaluation procedures are performance-based; that is, they measure acceptability (and changes in acceptability) on the basis of the degree to which a structure achieves one or more performance levels for the hazard posed by one or more hypothetical future earthquakes. A performance level typically is defined by a particular damage state for the components of a building. Commonly-used performance levels, in order of decreasing amounts of damage, are Collapse Prevention, Life Safety, and Immediate Occupancy. Hazards associated with future hypothetical earthquakes commonly are defined in terms of ground shaking intensity with a certain likelihood of being exceeded over a defined time period, or in terms of a characteristic earthquake likely to occur on a given fault. The combination of a performance level and a hazard defines a Performance Objective. For example, a common Performance Objective for a building is that it maintain Life Safety for ground motion with a ten percent chance of exceedance in fifty years.
Displacement parameter
Performance Levels

22. Qualitative range of performance
Performance level
Damage
Downtime
Collapse
Prevention •
Severe structural damage
Incipient Collapse
Probable falling hazards
Possible restricted egress
Probable
total loss
Demand for specific hazard level
Life Safety •
Probable structural damage
No Collapse
No falling hazards
Adequate emergency egress
Possible
total loss
Damage
Control •
Slight structural damage
Life safety attainable
Essential systems repairable
Moderate overall damage
2 to 3
weeks
Immediate
Occupancy •
Negligible structural damage
Life safety maintained
Essential systems operational
Minor overall damage 24
hours

23. Expected losses for an event
Inelastic analysis results
slight
moderate
extensive
complete
slight
moderate
extensive
complete
Fragility relationships
Probability
Displacement
Earthquake force
Earthquake force
on building
on building
Expected losses for event
Casualties
Repair/replacement costs
Downtime
Pushover curve
Pushover curve
Global displacement
Global displacement
of building
of building

24. Traditional code-based design
Building
Hazard Assessment .
structural model
Are consequences acceptable?
Yes: Design goals are satisfied
No: Propose suitable remediation

25. Consequence based Blast Resistant DesignDV
Compute the loses.
Consequence based Blast Resistant Design DM
Characterize the damage.
EDP
Analyze the structure. IM
Determine the hazard.

26. Vulnerability – Measures of Input and Response
Random Variables
Strength Parameters
Geometric Parameters
Correlation & Distribution
Response Limit States
Serviceability
Damage Control
Collapse Prevention
Input Motion
Captures Response Conservatively
Includes a Measure of Variability
Requires Scaling

27. Structural analysis Nonlinear model Multiple response history analyses
EDP
Nonlinear model
Multiple response history analyses
Sets of demands for 10 individual records
Statistically generated sets of correlated demands for 200 realizations

28. Consequence function for repair costsDM

29. Vulnerability Minimization
Existing Vulnerability
Measures of Intervention

30. Consequence-Based Engineering
Start
4. Decision Making
Done
confident that consequences will be acceptable
yes
Are Consequences Acceptable?
Damage Analysis
Rapid Assessment
1. System Definition
define system of interest
define hazard
define characteristics
Should Acceptable Consequences be Redefined? no
5. Refine Hazard Estimate
further refine hazard for more precise loss assessment
yes no
Should Parameters be Refined?
6. Refine Inventory Estimate
further refine inventory of built environment for more precise loss assessment
2. Rapid Estimate of
Consequences
quick assessment of
likely consequences
yes
Are System Interventions Possible? no
7. Refine Fragility Relations
further refine vulnerability of built environment with refined hazard and more precise response analyses
3. Define Acceptable
Consequences
define stakeholder needs no
Allows mitigation action plans to be prescribed in an optimal way that minimizes risk across an entire system of interest.
Can continually be improved with research using advanced technologies in risk assessment and visualization.
Steps defined in four categories:
rapid assessment
decision making
damage synthesis
consequence minimization
yes
Consequence Minimization
10. Prescribe System Interventions to Minimize Consequences
rehabilitate or demolish vulnerable structures, construct new structures, re-route network flows, re-manage land use, etc.
8. Re-Assess Social Impact
assess social and economic consequences of event in terms of refined hazard and inventory estimates and fragility relations
Inappropriate Solution
consequences will NOT be acceptable
9. Re-Visualize Consequences
examine effects of system alterations on reducing consequences

31. Take Home Lesson
Not always possible to calculate risk and hence loads associated with a given risk.
It is better to design for consequences that we know we can tolerate.
A consequence based Design methodology was outlined for Situations where risk cannot be well defined
Never Forget the connected world when determining consequences; or take credit when no Layer of Protection is provided
For After Sale Care write to