Introduction
Steel cantilever bridges are remarkable engineering structures that provide efficient and elegant solutions for spanning wide rivers and other large bodies of water. These bridges are characterized by their distinctive cantilever design, which allows for the construction of long, uninterrupted spans without the need for intermediate supports.
A cantilever is a structure that is anchored at one of its ends only and projects horizontally the other end into space without any support. In the principle of a cantilever bridge, this end helps to support the bridge central span which is called as the cantilevered span.
Design Provisions
The basic principle of the load transfer is the bottom part of each cantilever is anchored into the ground, while the upper end of the cantilever supports the bridge itself.
A common way to construct steel truss spans is to counterbalance each cantilever arm with another cantilever arm projecting the opposite direction. The resulting arrangement results in a balanced cantilever i.e. a span supported out of cantilever action. The action which is facilitated by the a counterbalance on each of its ends. The counterbalancing arms are called anchor arms.
Designing a steel cantilever bridge involves careful consideration of various factors such as the length of the span, anticipated loads, and environmental conditions.. These specifications outline design provisions for material strength, load factors, fatigue resistance, and deflection limits as per appropriate design code.
Components of the bridge
The entire bridge is divided into 3 different parts.
· The main span
· Anchor spans
The main span is divided into Suspended arm and two cantilever arms. The anchor span is the anchor arm, which is attached to the counterweight and provides statbility to the cantilevers on both sides of the bridge. Furthermore, Truss based cantilever bridges are complex structures with many different components together making the gist of the entire edifice. Some of the components of the bridge are as follows:
· Vertical posts
· Floor beams
· End floor beams
· Lateral beams
· Slab deck
· Bracings
· Diagonals
· Lateral bracings
· Stringers
· Portal bracings
Geometry of Sections
The geometry of the sections used in the construction of a steel cantilever bridge depends on the bridge's span length and the anticipated loadings. Typically, these bridges employ truss structures consisting of diagonals, verticals, and horizontals to efficiently distribute loads. The depth and arrangement of the truss members are optimized to ensure structural stability while minimizing the weight of the bridge.
Typically, al connections are bolted and welded for extra anchorage strength. Bolted and welded connections play a crucial role in the fabrication of steel cantilever bridges. These connections provide flexibility during construction, ease of maintenance, and the ability to replace individual members if necessary. High-strength bolts are used to join the various components of the bridge, including the truss members, main girders, and bracing elements. The bolted connections are designed to transmit the applied loads between the connected elements effectively. The connection design includes considerations for shear, tension, and bending moments to ensure structural integrity.
Foundation
The foundation of a steel cantilever bridge is critical to its overall stability. The type of foundation required depends on several factors, including soil conditions, water depth, and the height of the bridge piers. Common foundation types for these bridges include deep pile foundations, such as driven piles or drilled shafts. The foundation design takes into account the vertical and horizontal loads exerted by the bridge and ensures that the soil can safely support these loads without excessive settlement or lateral movement
Truss Mechanism of Load Transfer to the Footing
Truss members efficiently distribute the applied loads, such as vehicular traffic and wind forces, from the deck to the bridge piers or abutments. The diagonals of the truss carry tension or compression forces, while the verticals and horizontals primarily handle axial and bending loads as shown in figure. These forces are transferred to the foundation through the bridge piers or abutments, which are designed to withstand the resulting vertical and horizontal reactions.
Steel truss cantilevers support loads by tension of the upper members and compression of the lower members. The structure distributes the tension loads via the anchor arms at the sides to the outermost supports, while the compression is carried to the foundations beneath the central towers.
Fabrication of the Bridge
The fabrication of a steel cantilever bridge involves several stages. After the detailed design is completed, the steel sections are fabricated in a workshop or factory.
The individual members are then transported to the construction site and assembled using the bolted connections. Welding is also employed to enhance the strength and rigidity of certain connections. Once the sections are joined, the bridge is erected progressively, typically starting from the abutments or piers and moving toward the center of the span.
Advantages
High Load Capacity
Cantilever bridges have the ability to carry heavy loads due to their structural design. The dense and rigid nature of cantilever decks allows them to support significant weight, making them suitable for accommodating heavy dead loads of the edifice.
Long Span Capabilities
Steel cantilever bridges are capable of spanning long distances without intermediate supports. This feature is particularly advantageous when crossing wide rivers or valleys where constructing piers or supports may be impractical or costly.
Durability and Strength
Steel, as a construction material, offers excellent durability and strength properties. It has a high resistance to corrosion, weathering, and fatigue, ensuring the longevity and structural integrity of cantilever bridges even in harsh environmental conditions.
Minimal Disruption during Construction
Unlike some other bridge types, cantilever bridges require minimal disruption during the construction process. With their ability to avoid extensive use of falsework, which can impede traffic or pose safety risks, cantilever bridges offer a more convenient and efficient construction process, minimizing interruptions to transportation routes.
Aesthetic Appeal
Cantilever bridges often possess an architectural elegance that adds to the aesthetic appeal of a landscape. The long, uninterrupted spans and graceful truss designs of steel cantilever bridges can become iconic landmarks and enhance the visual appeal of an area.
Reduced Maintenance Costs
Steel structures are known for their low maintenance requirements. Steel cantilever bridges typically require less maintenance and repair work compared to other bridge types, resulting in cost savings over the life cycle of the structure.
Flexibility in Design
Steel allows for greater flexibility in designing cantilever bridges, enabling engineers to optimize the structure's performance based on specific project requirements. The versatility of steel fabrication techniques and the availability of a wide range of steel sections contribute to the adaptability and versatility of cantilever bridge design.
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