The conceptual design of cable-supported bridges begins with determining the appropriate bridge type and its location. The site of the bridge plays a critical role in its design, especially when seismic factors are considered. In regions where earthquakes are expected, it is essential to factor in seismic performance. However, the process of selecting the most suitable bridge type typically focuses on factors other than seismic criteria, and seismic considerations are incorporated afterward. This approach is mainly due to the absence of exact techniques for comparing the seismic performance of various cable-supported bridge types. As a result, seismic design serves as a checking mechanism to determine whether the selected type is practical and suitable for the location.
For new bridges, the conceptual seismic design process can also apply to the seismic retrofit of existing bridges. However, retrofitting an older bridge is generally more complex than designing a new one because the options available for modification are more limited in the former. Conversely, a new bridge allows for broader flexibility in the design process, including seismic considerations from the start.
I. Conceptual Planning of Cable-Supported Bridge Design
The conceptual planning of cable-supported bridges involves several critical stages. First, the type of bridge and its location must be selected carefully. The site’s specific needs, including seismic risks, must be considered early in the planning phase. For example, when designing a bridge in a seismically active region, it is vital to account for how the bridge will respond to earthquake forces. This decision-making process influences the entire bridge design and sets the foundation for further structural and seismic considerations.
II. Cable-Supported Bridge Conceptual Design
In conceptual bridge design, various factors influence the choice of bridge type, including span length, environmental conditions, traffic loads, and aesthetic preferences. However, the seismic behavior of the bridge becomes a critical factor in determining the suitability of different designs. Cable-supported bridges, whether cable-stayed or suspension, are known for their ability to span long distances, which makes them ideal for various applications. The initial design considers the material, the shape, and the structural elements of the bridge, all while taking seismic risks into account.
III. Cable-Supported Bridge Conceptual Seismic Design
The seismic design of cable-supported bridges aims to ensure the structure is resistant to earthquake forces while maintaining its functional integrity. It involves three main considerations:
- Determining the seismic response features of the bridge
- Designing to avoid collapse during an earthquake
- Eliminating seismic vulnerabilities in bridge components
IV. Seismic Response Features of Cable-Supported Bridges
Cable-supported bridges are unique due to their long spans, which significantly influence their seismic response. A typical cable-stayed bridge, for example, has a fundamental vibration period ranging from 2 to 8 seconds, which is much longer than that of shorter-span bridges. This long vibration period results in a smaller seismic force but introduces the possibility of more significant deflections, especially under the influence of the P-delta effect. The P-delta effect refers to the amplified impact of vertical displacements during lateral movement, which becomes more pronounced in long-span structures.
Additionally, cable-supported bridges exhibit low damping ratios (1-2% for cable-stayed and 1.5-2% for suspension bridges), which means that they take a longer time to return to a stable position after an earthquake. The complexity of the bridge’s vibration modes also poses challenges: each component (cables, towers, deck, etc.) has its own vibration period, and these modes interact with one another, complicating the overall seismic behavior. Furthermore, cable-supported bridges are highly sensitive to ground motion at multiple supports, which can experience different seismic conditions due to variations in soil types and support distances.
Finally, large expansion joints are needed in cable-supported bridges to accommodate the movement caused by factors like temperature changes, seismic forces, and traffic loads. For example, the Rio-Antirrio Bridge in Greece is designed to accommodate movements of up to 2.5 meters under normal conditions and up to 5 meters in extreme situations.
V. Design to Avoid Collapse in Seismic Conditions
Several design strategies can be employed to enhance the earthquake resistance of cable-supported bridges. These strategies focus on increasing the flexibility and energy dissipation capacity of the structure to prevent catastrophic failure during seismic events.
Energy Dissipation
Energy dissipation devices help absorb the kinetic energy generated during an earthquake, reducing the seismic forces acting on the bridge. The most common types of dampers include fluid viscous dampers, friction dampers, and metallic yielding dampers. These devices are installed at various locations to reduce displacement and force demand. For example, large fluid viscous dampers can be installed between the bridge’s stiffening trusses and towers to reduce displacement, while smaller dampers can be used at critical points along the span to control vibration forces.
The use of energy dissipation devices was notably applied in the Rio-Antirrio Bridge, where viscous dampers were employed to reduce seismic impact.
Multiple Articulations
Incorporating multiple articulations in the bridge design allows for flexibility during an earthquake while maintaining rigidity against wind forces. The bridge can resist wind loads through rigid structural elements that connect the towers to the trusses. During an earthquake, these elements break, and damper devices are activated to absorb energy and reduce displacement. This strategy, used in the Rio-Antirrio Bridge, allows the bridge to withstand both wind and seismic forces without compromising safety.
Base Isolation
Base isolation systems, such as sliding bearings or gravel-supported foundations, can greatly enhance a bridge’s ability to resist seismic forces by decoupling the superstructure from the ground motion. For instance, the foundation of the Rio-Antirrio Bridge is supported by a layer of gravel, which allows horizontal movement during seismic events. This type of base isolation is also used in other prominent bridges, such as the Golden Gate Bridge.
Improve Ductility
Ductility refers to a structure’s ability to deform without losing its load-carrying capacity. Increasing the ductility of components such as towers, piers, and pylons is crucial to maintaining the stability of the bridge under seismic forces. Steel components can be made more ductile through lateral stiffening, while concrete components benefit from lateral confinement to enhance their seismic performance.
Redundancy Provisions
Given the unpredictable nature of earthquakes, redundancy is vital to ensuring the bridge can withstand unexpected loads. By designing alternate load paths through the addition of restrainers, shear keys, and catch blocks, engineers can provide backup mechanisms to prevent failure if certain components are damaged during an earthquake.
Strengthening of the Superstructure
Strengthening the connections of bridge components is another way to enhance seismic performance. These components, which bear the majority of the loads, must be designed as capacity protectors to ensure they can withstand seismic forces without failing.
VI. Seismic Vulnerability of Cable-Supported Bridge Components
Cable-supported bridges are composed of various structural components, and each one has the potential for seismic vulnerability. For example:
- Tower shafts may buckle during an earthquake.
- Connections and expansion joints may become damaged if the bridge components exceed their movement capacity.
- Foundations may suffer from soil liquefaction, especially in areas with loose, water-saturated soil.
At the conceptual design stage, engineers must carefully consider these vulnerabilities and design solutions to mitigate the risks. Using appropriate materials and employing specific design strategies can help ensure the bridge’s seismic resilience.
VII. Conclusion
Seismic design is a critical aspect of cable-supported bridge planning, particularly in earthquake-prone areas. The design must address the specific seismic response characteristics of the bridge, ensure it can avoid collapse during an earthquake, and mitigate vulnerabilities in key components. Through the use of advanced techniques like energy dissipation, multiple articulations, base isolation, and improved ductility, engineers can create bridges that are not only structurally sound but also resilient to seismic forces. In addition, redundancy and strengthening measures ensure that cable-supported bridges can withstand the unpredictable nature of earthquakes, making them safe and reliable for long-term use.