Movable bridges are a remarkable feat of engineering, designed to change their position or shape to allow the passage of vessels and boats through waterways. Unlike traditional bridges, which are static, movable bridges open or adjust to accommodate water traffic, making them a versatile solution in areas where navigation is crucial. These bridges offer cost-effective solutions since they eliminate the need for long approaches and high piers. When the waterway is opened to vessels, traffic over the bridge is temporarily halted, and once the passage is clear, the bridge returns to its normal position for road traffic.
Types of Movable Bridges
There are several types of movable bridges, each designed to meet specific needs. While many of these types are no longer in widespread use, some remain practical and desirable in various applications. Among the various types of movable bridges, three stand out for their practicality and common use. These include the bascule bridge, the vertical lifting bridge, and the swing bridge. Additionally, there are several special types of movable bridges, which, while rare, offer unique solutions to specific engineering challenges.
Bascule Bridge
The bascule bridge, also known as a drawbridge, is one of the most commonly used movable bridges. It consists of a bridge span that pivots on an axis, typically perpendicular to the bridge’s longitudinal centerline. The horizontal pivot point is strategically placed around the center of gravity of the bridge to ensure balance, making it easier to open and close the span.
One of the key features of the bascule bridge is the weight distribution. Depending on the bridge’s use, the weight can be adjusted to facilitate either the opening or closing of the span. When the waterway needs to be opened for vessels, the weight distribution is termed “counterweight heavy,” as it helps in lifting the span. Conversely, when closing the bridge, the weight distribution is referred to as “heavy span.”
There are two primary types of bascule bridges: single leaf and double leaf. The “leaf” refers to the part of the bridge that moves during the opening process. Less commonly, triple and quadrupole bascule bridges are also constructed. The bascule bridge is ideal for many situations due to its reliability, structural soundness, and cost-effectiveness in both construction and operation.
Vertical Lifting Bridge
The vertical lifting bridge is another widely used movable bridge, particularly in areas where long spans are required. This bridge features a span, often of a truss type, that is supported by towers at each end. A counterweight system is used to balance the weight of the span. The span is lifted vertically to open the waterway, using ropes and rotating sheaves to transfer motion from the towers to the span.
There are several variations of vertical lifting bridges, including double, triple, and quadruple vertical lifts. The latter two types are ideal for crowded areas, such as those near terminals or ports, where space is limited. If the machinery responsible for lifting the span is fixed on the span itself, the bridge is referred to as a span-drive vertical lift bridge. Alternatively, when the machinery is fixed on the towers, it is termed a tower-drive vertical lift bridge.
Vertical lifting bridges are especially advantageous when stability is critical, and they are often preferred when a long span is necessary, as they offer substantial strength and reliability.
Swing Bridge
A swing bridge is designed to rotate horizontally around a vertical axis, enabling the passage of vessels through the bridge. The bridge is fixed on a horizontal plane supported by a pivotal pier, which acts as the central point for rotation. Unlike bascule or vertical lifting bridges, swing bridges do not move vertically but instead rotate horizontally to clear the waterway.
When a swing bridge is closed, the end of the span must be supported by piers or abutments, especially when the span is long. To open and close the bridge, complex machinery is used, such as retractable rollers, wedges, jacks, or shoes to lift the bridge span. This design allows the swing bridge to operate smoothly, but it also makes it more complex compared to other movable bridges.
One of the advantages of the swing bridge is its relative resistance to wind forces. Because the wind force tends to have a less significant effect on the bridge’s stability compared to bascule and vertical lifting bridges, swing bridges can remain more stable during adverse weather conditions. However, the wind’s impact should not be entirely dismissed, as it can increase the overturning moment, especially if the bridge is fixed at its center.
Swing bridges typically feature truss or plate girder spans, with the latter becoming more popular in modern designs due to its cost-effectiveness.
Special Types of Movable Bridges
While not commonly constructed today, there are several special types of movable bridges that have been developed over the years to address unique engineering challenges. The decline in the construction of these bridges is attributed to advancements in materials, load-bearing capacity, and safety considerations. Special movable bridges include:
- Retractile Bridges: These bridges are designed to retract or slide to one side to open a waterway.
- Pontoon Retractile Bridges: Similar to retractile bridges, but supported by floating pontoons.
- Pontoon Swing Bridges: A combination of swing and pontoon technology, where the bridge is mounted on floating pontoons that swing to clear the waterway.
- Shear Pole Swing Bridges: This bridge uses a shear pole system to facilitate rotation.
- Folding and Curling Bridges: These bridges fold or curl up to allow passage for boats or ships.
- Removable Spans: Bridges with spans that can be entirely removed to open the waterway.
- Submersible Bridges: These bridges are designed to submerge into the water to allow ships to pass over.
- Tilt Bridges: Bridges that tilt to provide a passage.
- Transporter Bridges: These bridges carry vehicles across a waterway using a moving platform.
- Jet Bridges: Used in specific applications, these bridges involve the use of jet propulsion systems.
The development of these special types of movable bridges was largely driven by the unique needs of specific locations and technological constraints at the time. However, modern materials and safety standards have made many of these designs obsolete, as newer technologies have taken their place.
Conclusion
Movable bridges are an essential part of infrastructure in regions where water traffic is crucial for transportation. While the traditional bascule, vertical lifting, and swing bridges remain the most practical and commonly used types, special types of movable bridges still serve as innovative solutions for specific challenges. The evolution of movable bridges demonstrates the adaptability of engineering solutions to meet the needs of both land and water traffic, and as technology advances, these bridges will continue to evolve to support the future of transportation.