Structural Vulnerability, Risk Assessment and Land Use Issues for Transportation Infrastructure

May 18, 2005

Shay K. Burrows, P.E. Senior Structural Engineer

OutlineIdentify threats to bridges and tunnelsDescribe the effective defense strategyHow land use plays a major part in supporting the defense

April 25, 2023

Are Bridges and Tunnels Really Targets?Bridges and tunnels are attractive terrorist targets due to:Economic importance to traffic and commerceSymbolism (I.e. Golden Gate Bridge, Lincoln Tunnel, etc.)Cost and time for replacementPublic impact from an attackRelatively high vulnerability, both susceptibly and structurally

Source: Transportation Security Administration

Are Bridges and Tunnels Really Targets?Encyclopedia of Afghan Resistance

“When you destroy large bridges by explosives, loading the middle part will destroy the netted area (the roadway), the explosives should be

combined with others placed at the two pressure points. This will destroy the bridge.”

Pressure point #2

Pressure point #1

Pressure point #3

Terrorist TacticsPossible Course of Action – Decision Criteria

High probability of successImpact – maximize damage and casualtiesRealistic Logistics – easily obtained materialsSpeed – minimize placement and priming timeSecrecy and surpriseSimplicity and easy coordinationFlexibility

Terrorists Threats to BridgesMechanical cutting devicesChemicals (acids, corrosives, etc.)ThermiteTorchesArea denial

Chemical/ biologicalSmall bombs, or just the threat of one

Terrorist Threats to Bridges

Explosion!Bombs constitute a high percentage of terrorist attacks worldwide

Explosive EffectivenessDepends on:

TypeAmount (the more the better!)Location

Internally placedExternal contactStandoff

Decreasing effectiveness

Decreasing time on target

Explosive Location

Source: FM5-250 and ConWep

5'

2.5'

Center of mass:x lb

Contact, elevated, un-tamped: 34x lb

6'

Standoff of 6':267x lb

Required C-4 Charge Size for

Breaching of Concrete

Vehicle BombsHistorically, the terrorist weapon of choice

Can use more explosivesDoesn’t require any time on target

Mitigation – In GeneralThe goal is to implement measures which are appropriate and effective for a particular risk, yet economical and do not interfere with a bridge’s operation

Construct an Effective DefenseResponse and Recovery

Prepare to deal with multiple, cascading eventsDeterrence

Make them know you are watchingDenial

Physically limit accessDetection

Use security, sensors, surveillanceDefense

Apply permanent standoff, structural hardening

Defense PriorityFirst Priority

Develop an accelerated response and recovery planSecond Priority

Deter, deny, detectThird Priority

Defend with standoffFourth Priority

Defend with structural toughening

How effective land use can help:

- Denying access

- Providing standoff

Deny Access – Fencing and Area ControlFencing or bollards to deny vehicle access or parking

Provide Standoff on LandPermanent barriers may provide sufficient standoff, including bollardsTemporary barriers can be used to close roads/lanes or provide standoff around critical elements during a heightened alert or specific threatAny barrier is better than none, since standoff is more important

What We’re Trying to Prevent

Contact Information

Shay K. Burrows, P.E. FHWA- Resource Center

10. S. Howard StreetSuite 4000Baltimore, MD 21201

(410) 962-6791 shay.burrows@fhwa.dot.gov