Potential Cause and Impact of the Biliran Bridge Shaking Incident

The absence of seismic activity and the observation of strong winds during the event strongly suggest that aeroelastic effects played a significant role. Here's how:

1. Wind-Induced Oscillations

  • Aeroelastic Flutter: High winds can interact with the bridge structure, causing oscillations. Flutter occurs when aerodynamic forces match the bridge's natural frequency, amplifying vibrations in both the truss and deck. This phenomenon was famously responsible for the Tacoma Narrows Bridge collapse in 1940.
  • Vortex Shedding: Steady winds flowing around the truss elements may have created alternating low-pressure zones, leading to periodic forces that caused vertical and lateral vibrations.
  • Galloping or Buffeting: Strong gusts of wind could have exerted uneven forces on the truss and deck, initiating wave-like motions.

2. Dynamic Interaction with Vehicles

  • The presence of vehicles, even stationary ones, could have contributed to the shaking by altering the dynamic load distribution. If the vehicles stopped during the shaking, their static weight could have amplified oscillations under wind forces.

3. Structural Deterioration

  • The bridge's advanced age (over 45 years) and reported weakening likely exacerbated the shaking. Corroded truss members, deteriorated joints, or loosened connections could reduce stiffness, making the structure more prone to dynamic amplification.

4. Inadequate Damping

  • Older bridge designs like the Biliran Bridge may lack modern damping systems, making them vulnerable to vibrations caused by dynamic forces such as wind.

Impact of Vertical Shaking

The observed vertical wavelike motion and shaking of the truss and deck are serious concerns with multiple implications:

1. Structural Fatigue

  • Repeated vibrations stress structural components, accelerating fatigue in steel trusses and connections. This could lead to microcracks, further weakening the bridge over time.

2. Loss of Joint Integrity

  • Oscillations can strain connections between truss members and between the truss and deck, potentially loosening bolts, rivets, or welds.

3. Deck and Truss Damage

  • The vertical shaking of the floor deck could cause localized cracking or delamination in reinforced concrete components, while excessive movement in the truss may lead to deformation or misalignment.

4. Foundation and Abutment Stress

  • Prolonged shaking transmits dynamic forces to the bridge’s supports, potentially destabilizing foundations or causing settlement.

5. Safety Risks

  • Such visible shaking undermines public confidence and poses immediate risks to users. While the governor declared it "safe," strong winds and visible vibrations highlight the need for thorough inspection.

Recommendations to Address the Issue

To ensure the bridge's safety and longevity, the following actions are recommended:

1. Immediate Inspections

  • Conduct a detailed structural integrity assessment focusing on:
    • Steel truss members for fatigue cracks or corrosion.
    • Connections (bolts, rivets, welds) for looseness or damage.
    • Concrete components for cracks, spalling, or delamination.
  • Use non-destructive testing (NDT) methods such as ultrasonic testing or radiography to detect hidden damage.

2. Dynamic Analysis

  • Perform a dynamic load and wind-resistance analysis to determine the bridge's natural frequencies and susceptibility to wind-induced oscillations.

3. Retrofit with Modern Damping Systems

  • Install tuned mass dampers (TMDs) or viscous dampers to reduce vibrations and enhance stability under dynamic loads like wind or vehicles.

4. Strengthen Structural Components

  • Reinforce corroded or weakened steel members and replace deteriorated connections. Apply protective coatings to prevent further corrosion from the marine environment.

5. Wind Deflectors or Barriers

  • Consider adding aerodynamic modifications such as wind deflectors or barriers to mitigate the impact of strong winds on the truss and deck.

6. Traffic Management

  • Restrict access to vehicles exceeding load limits and impose temporary closures during strong winds until retrofitting is complete.

7. Long-Term Monitoring

  • Install sensors to monitor vibrations, stress, and environmental conditions (e.g., wind speeds) in real time to detect potential issues early.

8. Future Replacement Planning

  • Given the bridge’s age and vulnerability, develop a replacement or major rehabilitation plan to ensure long-term safety and functionality.

Summary

The vertical shaking of the Biliran Bridge was likely caused by strong winds inducing aeroelastic effects compounded by its age and structural limitations. While the immediate declaration of safety may calm public fears, visible oscillations warrant urgent action. A comprehensive assessment, retrofitting, and monitoring are essential to prevent further deterioration and ensure the safety of this historic and vital infrastructure. Proactive measures now will mitigate risks and extend the bridge’s service life while safeguarding the community.