Recommendations for Bridge and Tunnel Security
Appendix C: Case Study in Bridge and Tunnel Risk Assessment
Risk Assessment Approach
This appendix describes the risk assessment method used to help determine how to allocate resources for mitigating the adverse effects of terrorist acts on critical transportation facilities and the occupants of those facilities. Decisions on how best to spend mitigation funds require a rational and systematic risk-based approach that considers, for each facility, the combination of hazard occurrence likelihood, consequences given the occurrence, and socioeconomic importance of the facility. Risk assessment methods for mitigation decisions related to natural hazards are fairly well-established. The application of these methods to non-natural hazards (i.e., acts of terrorism) is relatively new. There is no well-established comprehensive procedure that can be used to determine how terrorism mitigation funds should be spent given a finite list of facilities, mitigation alternatives, and agency-defined constraints.
We used a rational and systematic risk assessment method for prioritizing alternatives for mitigating the effects of acts of terrorism. The method makes use of several key sources of information, including the following:
- Prior work in seismic risk assessment for retrofit prioritization (Maroney, 1990; Sheng and Gilbert, 1991; Kim, 1993; Babaei and Hawkins, 1993; Hart Consultant Group et al., 1994; King and Kiremidjian, 1994; Basoz and Kiremidjian, 1995; Audigier et al., 2000)
- DOD procedures for addressing physical threats in facility planning (U.S. Department of Defense, 1994)
- AASHTO Guidelines for highway vulnerability assessment (AASHTO, 2002)
- U.S. Department of Justice state preparedness support program (U.S. Department of Justice, 2000)
- Analytic Hierarchy Process for consensus decision making given multiple attributes (Saaty, 1980)
The risk to a facility due to a man-made hazard is represented as the combination of the following three factors as shown in Figure C-1:
- Importance Factor (IF). A measure of the socioeconomic impact of the facility's operation, computed as a weighted combination of the following attributes of the facility:
- Historical and symbolic importance
- Replacement value
- Importance as an emergency evacuation route
- Importance to the regional economy
- Importance to the regional transportation network
- Annual revenue value
- Criticality of the utilities attached to the facility
- Military importance
- Exposed population on or in the facility
- Occurrence Factor (OFi). A measure of the relative probability or likelihood of threat i occurring, computed as a weighted combination of the following:
- Level of access
- Level of security
- Visibility or attractiveness of the facility
- Level of publicity
- Number of times the facility has been threatened in the past
- Vulnerability Factor (VFi). A measure of the consequences to the facility and the occupants given the occurrence of threat i, computed as a weighted combination of the following:
- Expected damage to the asset
- Expected down-time or closure of the facility
- Expected number of casualties
Expressed in equation format, the risk score (RS) for a given facility, is written as follows:
RS = IF X Σ [OFi X VFi ] (1)
where OFi, VFi, and IF are defined as above, and Σ denotes the summation over all considered threats to the facility.
Each of the factors in Equation (1) is a number between 0 and 1, computed using a multi-variate utility method. In this method, each factor is computed as the summation of the weighted values (between 0 and 1) of the attributes that define the factor as follows:
IF = Σ [wj X vj(xj)] (2a)
OF = Σ [wj X vj(xj)] (2b)
VF = Σ [wj X vj(xj)] (2c)
where xj is the value of attribute j (e.g., very high), vj(xj) is the function or table that maps xj to a utility value (between 0 and 1; e.g., very high corresponds to 1), wj is the weighting factor on attribute j, and Σ denotes the summation over all considered attributes for the factor. See Figure C-1 for a graphical depiction of the above discussion.
The weighting factors used for combining the attributes that make up each of the factors listed above are developed using the pair-wise comparison procedure in the Analytic Hierarchy Process, whereby each member of the decision making group assigns a numerical value to the relative influence of one attribute over another. The scores are averaged and used to compute the weighting factors, which are then reviewed by the group as a whole and revised until all members of the group are satisfied with the results. Figures C-2 through C-4 show the relative weights for the attributes used to compute the Importance Factor, Occurrence Factor, and Vulnerability Factor, respectively.
After the weighting factors have been developed, the risk assessment method proceeds as follows for each facility:
- Compute the Importance Factor (Equation (2a)) by assigning values to the attributes that contribute to the factor
- Identify vulnerable components of the facility
- Identify credible threats to each component
- Compute the Occurrence Factor (Equation (2b)) and Vulnerability Factor (Equation (2c)) for each threat by assigning values to the attributes that contribute to the two factors
- Compute the baseline Risk Score (Equation (1)) for the facility as a combination of the Importance, Occurrence, and Vulnerability Factors as shown in Figure C-1
- Identify the mitigation projects for the facility and the threats that will be mitigated
- For each mitigation project, re-compute the Occurrence and Vulnerability Factors (Equations (2b) and (2c)) given the presence of the mitigation project, and then re-compute the Risk Score (Equation (1))
- Rank the mitigation projects in terms of reduction in Risk Score compared to the baseline Risk Score for the facility computed in Step 5
Risk Assessment Results
The result of this risk assessment effort is a ranked list that identifies the benefit of enacting each mitigation project. The costs (in terms of capital expenditure, operation and maintenance, and disruption) were developed in a parallel effort and used with these results in an explicit cost-benefit analysis to identify the final list of mitigation projects to pursue.
Prior to developing the final ranked list of projects based on the cost-benefit comparison, several intermediate results were examined to ensure that the final results would be both rational and practical. For example, Figure C-5 shows the ranking of the eight facilities by Importance Factor. Figures C-6 through C-11 show the breakdown of each facility into the vulnerable components or threat targets.
The final list of mitigation projects, ranked by the ratio of benefit (in terms of reduction in facility Risk Score) to project cost, is given in Table C-1.
Figure C-12 shows the resulting ranked list in a chart format to help illustrate the comparison of mitigation project benefits and costs.
Risk Assessment References
AASHTO, 2002, A Guide to Highway Vulnerability Assessment for Critical Asset Identification and Protection, Prepared by SAIC, Washington, DC.
Audigier, M.A., Kiremidjian, A.S., Chiu, S.S., and King, S.A., 2000, "Risk Analysis of Port Facilities," Proceedings of the 12th World Conference on Earthquake Engineering, paper no. 2311.
Babaei, K., and Hawkins, N., 1993, Bridge Seismic Retrofit Planning Program, Report WA-RD 217.1, Washington State Department of Transportation, Olympia, WA.
Basoz, N., and Kiremidjian, A.S., 1995, Prioritization of Bridges for Seismic Retrofitting, Report No. 114, John Blume Earthquake Engineering Center, Department of Civil Engineering, Stanford University, Stanford, CA.
Hart Consultant Group et al., 1994, Seismic Risk Decision Analysis for Kaiser Permanente Pasadena, Final Project Report, Santa Monica, CA.
Kim, S.H., 1993, A GIS-Based Risk Analysis Approach for Bridges Against Natural Hazards, Ph.D. Dissertation, Department of Civil Engineering, State University of New York, Buffalo, NY.
King, S.A., and Kiremidjian, A.S., 1994, Regional Seismic Hazard and Risk Analysis Through Geographic Information Systems, Report No. 111, John Blume Earthquake Engineering Center, Department of Civil Engineering, Stanford University, Stanford, CA.
Maroney, B., 1990, CALTRANS Seismic Risk Algorithm for Bridge Structures, Division of Structures, California Department of Transportation, Sacramento, CA.
Saaty, T.L., 1980, The Analytic Hierarchy Process, McGraw-Hill, New York, NY.
Sheng, L.H., and Gilbert, A., 1991, "California Department of Transportation Seismic Retrofit Program: The Prioritization and Screening Process," Proceedings of the Third U.S. National Conference on Lifeline Earthquake Engineering, pp. 1110-1119.
U.S. Department of Defense, 1994, TM 5-853/AFMAN 32-1071, Volume 1, Chapter 3, Planning Phase, Washington, DC.
U.S. Department of Justice, 2000, Fiscal Year 1999 State Domestic Preparedness Support Program, Washington, DC.
Figure C-1. Components in Risk Assessment for an Individual Facility
Figure C-2. Relative Weights for Attributes Used to Compute Importance Factor
Figure C-3. Relative Weights for Attributes Used to Compute Occurrence Factor
Figure C-4. Relative Weights for Attributes Used to Compute Vulnerability Factor
Figure C-5. Ranking of Facilities by Importance Factor
Figure C-6. Breakdown of Bridge 1 into Vulnerable Components and Mode of Access
Figure C-7. Breakdown of Tunnel 1 into Vulnerable Components and Mode of Access
Figure C-8. Breakdown of Tunnel 2 into Vulnerable Components and Mode of Access
Figure C-9. Breakdown of Bridge 2 into Vulnerable Components and Mode of Access
Figure C-10. Breakdown of Bridge 3 into Vulnerable Components and Mode of Access
Figure C-11. Breakdown of Bridge 4 into Vulnerable Components and Mode of Access
Table C-1. Final Ranking of Mitigation Projects by Benefit/Cost Ratio
|Rank||Facility||Location||Reduction in Risk Score|| Project Cost
|1||Bridge 1||Main Pier Base B||0.16||753|
|2||Other Facility 1||Element A||0.30||1,872|
|3||Tunnel 1||Vent Buildings - Buildings||0.48||7,857|
|4||Other Facility 1||Element B||0.34||8,243|
|5||Bridge 1||Anchor Element B||0.11||2,840|
|6||Bridge 1||Anchor Element A||0.10||2,840|
|7||Other Facility 1||Element C||0.23||6,982|
|8||Tunnel 1||Approach Viaduct||0.12||3,891|
|9||Bridge 1||Main Pier Base A||0.32||13,937|
|10||Bridge 4||Tension Hangers||0.05||2,944|
|11||Tunnel 1||Vent Bldgs. - Tunnel Ceilings||0.16||12,619|
|12||Tunnel 1||Approach Plaza||0.03||2,787|
|13||Tunnel 2||Vent Buildings - Buildings||0.10||9,142|
|14||Tunnel 2||Vent Bldgs. - Tunnel Ceilings||0.13||12,523|
|15||Bridge 1||Deck Level||0.30||30,869|
|16||Bridge 2||Main Piers||0.10||12,048|
|17||Other Facility 1||Element D||0.05||7,432|
|18||Tunnel 1||Admin. Building||0.01||434|
|19||Bridge 1||Tension Hangers||0.07||12,363|
|20||Bridge 1||Approach Highway||0.15||32,686|
|21||Other Facility 2||Element A||0.01||1,950|
|22||Bridge 4||Main Span Abutment||0.02||5,891|
|23||Bridge 3||Main Piers||0.09||24,649|
|24||Other Facility 1||Element E||0.10||31,754|
|25||Other Facility 2||Element B||0.02||6,896|
|26||Tunnel 1||Tunnel Structure||0.51||222,723|
|27||Tunnel 2||Tunnel Structure||0.35||186,735|
|28||Other Facility 1||Element F||0.03||20,516|
|29||Bridge 4||Compression Members||0.01||8,687|
|30||Bridge 2||Main Span||0.08||64,996|
|31||Bridge 3||Main Span||0.07||108,718|
Figure C-12. Chart Illustrating Comparison of Benefits and Costs for All Mitigation Projects