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Pier Protection and Warning Systems for Bridges Subject to Ship Collisions

Appendix A
Alternative Surveillance and Warning Systems for The Sunshine Skyway Bridge Across Tampa Bay

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Introduction

The occurrence of a catastrophe such as the one that took place when the ship Summit Venture collided with the Sunshine Skyway Bridge on May 9, 1980, is very difficult, if not impossible, to predict. In most cases the possibility of such an occurrence is so remote that it is not considered in the design of structures such as the Sunshine Skyway Bridge. However, when such an accident does occur, attention is sharply focused on the prevention of accidents of this nature.

The purpose of this report is to document the results of a study conducted to identify alternative systems which could be installed on the Sunshine Skyway Bridge to provide detection of catastrophic failures of the bridge and warning motorists of such events. Systems which warn motorists of inclement weather conditions on the bridge such a fog or high winds were also included in this study.

Surveillance and Warning Systems - System Elements

Three basic categories for design were selected prior to the development of alternative surveillance and warning systems. System elements were identified for each category. Listed below are the three design categories and the system elements contained in each. Short descriptive paragraphs and advantages and disadvantages follow each system element.

I. Design Category - Prevention

  1. System Element - Pier Markings
    Amber lights are installed on each pier which supports the structural steel elements of the bridge. The amber lights outline the path of the channel beneath the bridge and identify the most critical piers closest to the channel. See Figure 1.

    Advantages
    • Provide positive guidance
    Disadvantages
    • Energy consumption
    • Maintenance
    • Limited visibility conditions
    • Limited visibility conditions hamper effectiveness

  2. System Element - Subsurface Attenuation Devices
    Subsurface attenuation devices such as dredge material, junked vehicles, etc., are placed parallel to the edge of the channel at a depth so as not to interfere with non-channel restricted waterway traffic. Of course ships are slowed or stopped when they impact the subsurface attenuation device. See Figure 2.

    Advantages
    • Low probability of impact
    • Reduces damage when impacted
    • Not affected by environmental condition
    Disadvantages
    • Cost
    • Environmental constraints
    • Maintenance
    Figure 1. Pier Markings for Sunshine Skyway Bridge
    Figure 1. Pier Markings For Sunshine Skyway Bridge


    Figure 2. Substance Attenuation Devices For Sunshine Skyway Bridge
    Figure 2. Substance Attenuation Devices For Sunshine Skyway Bridge

  3. System Element - Fender System
    An extensive fender system is constructed along each side of the channel. The fender system outlines the path of the channel beneath the bridge and slows or stops off course ships. See Figure 3.

    Advantages
    • Positive guidance
    • Reduces damage when impacted
    • Not affected by environmental conditions
    Disadvantages
    • Maintenance
    • Environmental constraints
    • Cost

II. Design Category - Detection

Probabilistic Determination

  1. System Element - Ship Channel Surveillance (Laser)
    A laser beam is projected along the edge of the ship channel at a height so as to be affected by larger vessels only. When the laser beam is disrupted, an off course ship and possible collision with the bridge is indicated. See Figure 4.

    Advantages
    • Early warning of erratic ship maneuvers
    • Operator not required
    Disadvantages
    • Cost
    • Maintenance
    • Environmental constraints
    • Verification
    • False detections

  2. System Element - Radar System
    A ground level radar system similar to those used in airports is installed near the bridge. An operator is able to track ships and verify their position relative to the channel and bridge. See Figure 5.

    Advantages
    • Complete tracking of ships
    • Self-verification
    • Reliability
    • Not affected by environmental conditions
    • Early warning of erratic ship maneuvers
    Disadvantages
    • Cost
    • Maintenance
    • Operator required

  3. System Element - Closed Circuit Television
    Closed circuit television cameras are located on the bridge and provide visual surveillance of the ship channel. The operator and television monitors are located at one of the toll facilities for the bridge. See Figure 6.

    Advantages
    • Early implementation
    • Self-verification
    • Early warning of erratic ship maneuvers
    • Roadway incident detection
    Disadvantages
    • Part-time operator required
    • Limited visibility conditions hamper effectiveness

    Figure 3. Fender System For Sunshine Skyway Bridge
    Figure 3. Fender System For Sunshine Skyway Bridge


    Figure 4. Laser Ship Channel Surveillance for Sunshine Skyway Bridge
    Figure 4. Laser Ship Channel Surveillance for Sunshine Skyway Bridge



    Figure 5. Radar System for Sunshine Skyway Bridge
    Figure 5. Radar System for Sunshine Skyway Bridge


  4. System Element - Radio Beam Guidance System
    Directional radio beam transmitters are installed on the bridge and aimed toward the center of the channel. A portable radio beam receiver is carried on board by the pilot and the system provides the pilot with information on the ship's position relative to the center of the channel and distance from the bridge. See Figure 7.

    Advantages
    • Early warning of erratic ship maneuvers
    Disadvantages
    • Cost
    • Maintenance
    • Environmental constraints
    • False detections

    Impact Occurrence

  5. System Element - Bridge Continuity
    A conduit containing a pair of conductors is attached to the bridge structure. The pair of conductors are connected to a sensor which monitors electrical continuity of the conductors. A catastrophic failure of the bridge structure would break the continuity of the conductors and the sensor would detect this occurrence. See Figure 8.

    Advantages
    • Low Cost
    • Reliability
    • Not affected by environmental constraints
    • Low maintenance
    • No operator required
    • Early implementation
    Disadvantages
    • Does not provide time to warn 100% of motorists
    • Malfunction of system creates severe impact on motorist behavior

  6. System Element - Pier Vibration
    A vibration sensor is placed on each pier which supports the structural steel elements of the bridge. Pier vibration above a threshold value which would be created by the impact of a ship is detected by the sensor. See Figure 9.


    Figure 6. Closed Circuit Television System for Sunshine Skyway Bridge
    Figure 6. Closed Circuit Television System for Sunshine Skyway Bridge



    Figure 7. Radio Beam Guidance System for Sunshine Skyway Bridge
    Figure 7. Radio Beam Guidance System for Sunshine Skyway Bridge



    Figure 8. Bridge Continuity System For Sunshine Skyway Bridge
    Figure 8. Bridge Continuity System For Sunshine Skyway Bridge



    Figure 9. Pier Vibration System for Sunshine Skyway Bridge
    Figure 9. Pier Vibration System for Sunshine Skyway Bridge

    Advantages
    • Low Cost
    • Early implementation
    • No operator required
    • Not affected by environmental conditions
    • Detection of non-catastrophic damage
    Disadvantages
    • Does not provide time to warn 100% of motorists
    • Maintenance
    • False detections

  7. System Element - Roadway Delineation (Reflectors)
    Reflective button markers are attached to the hand rails at a close, uniform spacing. The presence of these markers at night would indicate a continuous structure, their absence would indicate a section of the bridge is missing.

    Advantages
    • Low Cost
    • Early implementation
    • No operator required
    • Low maintenance
    Disadvantages
    • Low effectiveness
    • Limited visibility conditions hamper effectiveness
    • Verification
    • Notification
    • Does not provide time to warn 100% of motorists

  8. System Element - Roadway Delineation (Rail Lighting)
    Rail Lights, which are continuous longitudinal sections of lights for roadway illumination, are attached to the bridge hand rails. The presence of these lights at night would indicate a continuous structure, their absence would indicate a section of the bridge is missing.

    Advantages
    • Early implementation
    • No operator required
    • Improves nighttime traffic operations
    Disadvantages
    • Does not provide time to warn 100% of motorists
    • Limited visibility conditions hamper effectiveness
    • Limited effectiveness
    • Maintenance
    • Driver education

III. DESIGN CATEGORY - WARNING

  1. System Element - Passive Signing with Flashers
    Passive signs with static messages are installed on the bridge at regular intervals. Both environmental and operational messages are displayed on the signs. These messages are in effect when the flasher is activated. See Figure 10.

    Advantages
    • No operator required
    • Low cost
    • Low maintenance
    • Early implementation
    Disadvantages
    • Malfunction of system creates severe impact on motorist behavior
    • No positive control
    • Limited visibility conditions hamper effectiveness
    • Psychological effects on motorists
    • Does not provide time to warn 100% of motorists
    • Message not observed by all motorists

    Figure 10. Passive signing with Flasher for Sunshine Skyway Bridge
    Figure 10. Passive signing with Flasher for Sunshine Skyway Bridge

  2. System Element - Dynamic Signing
    Variable message signs are located over the roadway at regular intervals across the bridge. Both environmental and operational messages are displayed on the signs in response to operator commands or detection systems. See Figure 11.

    Advantages
    • Can be used for other applications
    • Good Target value
    • Early implementation
    Disadvantages
    • Not positive control
    • Maintenance
    • Message not observed by all motorists
    • Does not provide time to warn 100% of motorists

  3. System Element - Signals and Gates
    Drawbridge type signals and gates are located on the bridge at several locations prior to the main span structure. Signals and gates are activated by detection systems. See Figure 12.

    Advantages
    • Positive control
    • No operator required
    • Early implementation
    Disadvantages
    • Psychological effects on motorists
    • Maintenance

  4. System Element - Lane Control Signs
    Lane control signs are located over each roadway lane at one quarter mile spacings across the high rise section of the bridge. Roadway lanes are closed by changing the lane control sign message from a green arrow to a red "X" on command from a system operator. See Figure 13.

    Advantages
    • Can be used for other applications
    Disadvantages
    • Not positive control
    • Maintenance
    • Maintenance
    • Message not observed by all motorists
    • Does not provide time to warn 100% of motorists


    Figure 11. Dynamic Signing for Sunshine Skyway Bridge
    Figure 11. Dynamic Signing for Sunshine Skyway Bridge



    Figure 12. Signals and Gates for Sunshine Skyway Bridge
    Figure 12. Signals and Gates for Sunshine Skyway Bridge



    Figure 13. Lane Control Signs on Sunshine Skyway Bridge
    Figure 13. Lane Control Signs on Sunshine Skyway Bridge

  5. System Element - Audible Alarm and Signs
    Audible alarms are installed integral to the operation of the passive signs with flashers (Figure 10) or dynamic signs (Figure 11). See Figure 14.

    Advantages
    • Low cost
    • Early implementation
    • No operator required
    Disadvantages
    • Malfunction of system creates severe impact on motorist behavior
    • Message not observed by all motorists

  6. System Element - Citizen Band Radio
    A citizen base radio is located in the toll facility. An operator reports condition on the bridge over CB channel 9. See Figure 15.

    Advantages
    • Low cost
    • Greater coverage area
    Disadvantages
    • False calls
    • Message not observed by all motorists
    • Not reliable
    • Does not provide time to warn 100% of motorists

  7. System Element - Traffic Signals
    Pedestal mounted traffic signals (red and amber sections only) are located on both sides of the roadway at one quarter mile spacings across the high rise section of the bridge. Warnings (flashing amber) or bridge closure (steady red) are indicated on the traffic signal by operator command or input from detection systems. See Figure 16.

    Advantages
    • Recognized roadway control
    • Early implementation
    Disadvantages
    • Energy
    • Not positive control
    • Maintenance

Alternative Surveillance and Warning Systems - Development and Selection

By combining various prevention, detection and warning devices presented in preceding sections of this report, ten alternative surveillance systems were developed. Each system element in the alternatives is listed in Table 1. The overall system cost and complexity increases as the alternative number increases. To achieve adequate system effectiveness; however, the surveillance system must be moderately complex, and the system cost is therefore relatively high.

Listed in Table 2 are eight surveillance system goals and numerical goal weights developed to quantitatively evaluate the alternative surveillance systems. The technique used involved the appraisal of intangible system benefits by numerically weighting goal achievements. For example, alternative system nine employs the use of a fender system which is considered to be an excellent positive guidance system for ships; therefore, a value of twenty-four out of a possible twenty-five was assigned to the first system goal for system nine. Performance values assigned to each goal were then totalled. Alternative systems six, seven, eight and nine had total goal achievement values of 64 or higher. The prevention, detection and warning techniques used in these four surveillance systems are considered to be the best choice for implementation on the Sunshine Skyway Bridge.


Figure 14. Audible Alarm and Passive Signs with Flashers for Sunshine Skyway Bridge
Figure 14. Audible Alarm and Passive Signs with Flashers for Sunshine Skyway Bridge



Figure 15. Citizen Band Radio for Sunshine Skyway Bridge
Figure 15. Citizen Band Radio for Sunshine Skyway Bridge



Figure 16. Traffic Signals on Sunshine Skyway Bridge
Figure 16. Traffic Signals on Sunshine Skyway Bridge


Table 1 Alternative Systems Development Surveillance and Warning System for Sunshine Skyway Bridge
 Alternative 1Alternative 2Alternative 3Alternative 4Alternative 5Alternative 6Alternative 7Alternative 8Alternative 9Alternative 10
PREVENTION          
Pier MarkingsXXXXXXXX  
Attenuators         X
Fender System        X 
DETECTION (PROBABILISTIC          
Laser System    XXXXX 
Radar System          
CCTV       XX 
Radio Beam     XXXX 
Weather InstrumentXXXXXXXXXX
DETECTION (IMPACT OCCURRENCE)          
Bridge ContinuityX X       
Pier Vibration XXXXXXX  
Delineation (Reflectors)XXX X     
Delineation (Offset-Spot Rail Lighting)   X XXX  
WARNING           
Passive Signing & FlashersXX        
Dynamic Signing  XXXXXXXX
Gates     XXXX 
Lane Control Signs     X    
Audible Alarm & Signs           
C.B. Radio          
Traffic Signals     X XXX

Table 2 Alternative Systems Goal Achievement Chart
System GoalsGoal WeightAlternative System 1Alternative System 2Alternative System 3Alternative System 4Alternative System 5Alternative System 6Alternative System 7Alternative System 8Alternative System 9Alternative System 10
1. Positive Guidance to Ships25555510222222240
2. Positive Bridge Protection20000000001018
3. System Reliability15125561010141414
4. Warning Conveyed to 100% of Motorists150000121414141414
5. Positive Vehicle Traffic Control102244699994
6. Positive Guidance to Motorists52224244400
7. Minimal maintenance & Operation Costs54443211004
8. Improvements in Roadway Operations51134354433
Total10015162325416564677457

Conclusions and Recommendations

Based on the results of this study, it is recommended that Alternative Systems 6, 7, 8 and 9 be evaluated in detail to determine which system would be the most cost effective for installation on the Sunshine Skyway Bridge. System 6 is estimated to be the least costly system and System 8 is the most expensive.

Functionally, Systems 6, 7 and 8 are basically the same, with the differences being in the maintenance, operation and initial costs of these systems.

Installation of a surveillance and warning system could take place during reconstruction of the Skyway Bridge. However, with the present two-way traffic conditions on the remaining bridge, consideration should be given to early implementation of the system on this existing section.

Figure 8. Bridge Continuity System for Sunshine Skyway Bridge
Figure 8. Bridge Continuity System for Sunshine Skyway Bridge

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Updated: 04/07/2011
 

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