Cable-supported bridges, which include suspension and cable-stayed bridges, are widely recognized for their ability to perform well during seismic events compared to other bridge types. However, despite their general resilience, certain structural elements remain vulnerable to earthquake forces. The performance of these bridges during earthquakes, and the seismic weak points they present, require careful consideration for the safety and integrity of these structures. This article aims to explore the seismic vulnerabilities in cable-supported bridges, examining suspension and cable-stayed bridges, as well as bridges under construction.
I. Introduction
Cable-supported bridges, due to their flexibility and design, typically exhibit good performance during earthquakes. Their suspension and cable-stayed systems allow them to absorb and dissipate seismic energy, reducing the likelihood of severe damage. However, certain structural components can still become weak points under extreme seismic forces. While many existing cable-supported bridges have not experienced significant earthquakes, it is crucial to investigate these weak points proactively, as effective strengthening techniques and designs are yet to be fully developed. The goal is to better understand and address these vulnerabilities to improve earthquake resistance in cable-supported bridges.
II. Types of Cable-Supported Bridges
Cable-supported bridges are typically categorized into three types:
- Suspension Bridges
- Cable-Stayed Bridges
- Bridges During Construction
Each of these bridge types exhibits its own set of strengths and vulnerabilities when subjected to seismic forces.
III. Performance of Suspension Bridges During Earthquakes
Suspension bridges are particularly susceptible to seismic forces due to their unique design and large span. Below are key components of suspension bridges that exhibit vulnerabilities during earthquakes:
A. Overview of Suspension Bridge Components
The essential components of a suspension bridge include towers, suspension systems (cables, saddles, suspenders), stiffening girders, foundations, and expansion joints. A diagram showing these components (Fig.2, Fig.3, Fig.4) illustrates the complexity of the system and highlights potential weak points.
B. Vulnerabilities in Suspension Bridges
- Towers The towers of suspension bridges bear the initial loads, including seismic forces. However, the tower plate cells, which form the structural base of the towers, are vulnerable to buckling. The buckling is typically caused by large tower displacement during an earthquake. The P-delta effect (a phenomenon where increased displacement leads to larger forces on the structure) significantly increases the risk of failure. Furthermore, foundation uplift can cause the tower to move side-to-side or forward-and-back, which shifts the load off its original position and exacerbates compression on the tower shaft. To address this, vertical stiffeners can be added to tower cells, and prestressed strands can be used to reinforce the concrete pedestals supporting the towers.
- Suspension System The suspension system, which includes cables, saddles, and suspenders, is generally one of the strongest components in terms of earthquake resistance. This strength is attributed to the large safety factor used in their design and the flexibility of the cables, which absorb much of the seismic shock. However, one notable weak point is the potential for slip between the cable saddles and the towers, particularly when large deflections occur, and the angle between the main span and the side spans becomes too great. This slip can reduce the effectiveness of the system in resisting seismic forces. Proper anchoring and reinforcement of the saddles are necessary to prevent this failure.
- Stiffening Girders Stiffening girders serve to support the live load and wind load of the bridge but are susceptible to significant damage during an earthquake that exceeds wind load expectations. Older suspension bridges, built with trusses, often experience damage to lateral braces and their connections. Strengthening these components, particularly the lateral braces, is crucial for seismic retrofitting.
- Foundations One of the most severe risks during an earthquake is the liquefaction of soil beneath the foundations. Liquefaction occurs when the ground loses its solid state and behaves like a liquid, which can cause catastrophic damage to the bridge’s foundations. Techniques such as stone columns, densification, and displacement piles can be employed to strengthen the soil and mitigate the risk of liquefaction.
- Wind Locks and Expansion Joints Wind locks and expansion joints are also weak elements of suspension bridges during earthquakes. When stiffening girders move laterally due to seismic forces, the suspenders, which are connected to these girders, experience large deflections. If the deflection exceeds the capacity of the wind locks and expansion joints, these elements will be damaged. The solution is to strengthen these connections and joints to accommodate larger movements.
IV. Performance of Cable-Stayed Bridges During Earthquakes
Cable-stayed bridges, like suspension bridges, are designed for long spans and flexible structures. While they share several vulnerabilities with suspension bridges, there are key differences in their performance during earthquakes.
A. Similarities with Suspension Bridges
Both cable-stayed and suspension bridges have similar structural elements, including long, flexible spans and towers. As such, cable-stayed bridges are vulnerable to similar seismic risks, such as tower buckling and soil liquefaction. However, the cable stays in cable-stayed bridges act as bracing elements, offering additional seismic resistance.
B. Key Differences
One of the primary differences between suspension and cable-stayed bridges is the seismic resistance of the towers. The towers in cable-stayed bridges benefit from the additional support provided by the cables, which act like bracings. As a result, the towers in cable-stayed bridges generally have better resistance to seismic forces compared to suspension bridges. However, if the towers are made of concrete, they may still be susceptible to damage during an earthquake.
C. Strong Components
The cable stays in cable-stayed bridges, much like the suspension system in suspension bridges, are the most resilient part of the structure. Their strength and stability under seismic loads are crucial to the overall performance of the bridge during an earthquake.
V. Earthquake Vulnerability of Bridges During Construction
Bridges under construction are more vulnerable to seismic forces than completed bridges, as the structural integrity is not yet fully realized. Partially completed bridges can suffer significant damage even from moderate seismic forces. Given that the likelihood of experiencing both wind and seismic forces simultaneously is rare, measures to protect these structures primarily focus on mitigating wind-induced vibrations during construction. However, precautions should also be taken to limit seismic damage during the construction phase.
VI. Conclusion
While cable-supported bridges generally perform well during earthquakes, they are not without vulnerabilities. Towers, suspension systems, stiffening girders, foundations, and expansion joints all present potential weak points under seismic forces. It is crucial to continue investigating and addressing these vulnerabilities through retrofitting and improved design techniques. Understanding these risks and proactively strengthening the bridge components will ensure the continued safety and longevity of cable-supported bridges in seismic regions.