Bridges & Structures

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 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 4. Laser Ship Channel Surveillance 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 7. Radio Beam Guidance System for Sunshine Skyway Bridge
   
   
   
   
   Figure 8. Bridge Continuity 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
   
   
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 12. Signals and Gates for 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 15. Citizen Band Radio for Sunshine Skyway Bridge

Figure 16. Traffic Signals on Sunshine Skyway Bridge

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

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