Steel bridges are an essential part of modern infrastructure, offering durability, strength, and versatility. The classification of steel bridges is based on several factors, primarily the type of traffic they carry, their main structural system, and the position of the carriageway relative to the main load-carrying members. Understanding these classifications helps engineers determine the most suitable design for a given situation, whether it’s for road traffic, railway systems, or a combination of both. In this article, we will explore the classification of steel bridges in detail.
I. Classification Based on the Type of Traffic Carried
Steel bridges can be classified according to the type of traffic they are designed to carry. There are three primary types:
A. Highway or Road Bridges
Highway or road bridges are designed specifically to carry vehicular traffic, such as cars, trucks, and buses. These bridges are typically built to accommodate significant weight loads and withstand constant traffic movement. Road bridges are commonly seen on highways, overpasses, and in urban areas to cross rivers, valleys, or other obstacles.
B. Railway or Rail Bridges
Railway bridges are designed to carry trains and railway vehicles. They are built to endure high dynamic loads generated by moving trains, including the effects of vibrations, braking forces, and the weight of the trains themselves. Rail bridges are typically found along railway tracks crossing rivers, roads, or other infrastructure elements.
C. Road-Cum-Rail Bridges
These bridges are a combination of both road and rail traffic, designed to support both vehicle and train traffic simultaneously. Road-cum-rail bridges are often used in locations where both transportation types are essential, such as in urban areas with mixed traffic or where space constraints make it necessary to accommodate multiple traffic modes in one structure.
II. Classification Based on the Main Structural System
The structural system of a steel bridge refers to the design used to support and distribute loads. Different structural systems are employed based on the span of the bridge, the width of the carriageway, and the type of traffic. Here are the main types:
A. Girder Bridges
Girder bridges are one of the most common types of steel bridges, where the main load-carrying members are girders that support the bridge deck. The primary structural action in girder bridges is flexure, or bending, between the vertical supports.
- Types of Girder Bridges:
- Solid Web Girders: These are simple and cost-effective for shorter spans.
- Truss Girders: Used for medium to long spans, with a truss configuration to improve strength.
- Box Girders: Used for continuous spans and are more efficient in carrying loads.
Girder bridges are suitable for spans up to 50 meters for plate girder bridges and up to 250 meters for box girders. Truss bridges are ideal for spans between 30 meters and 375 meters, while cantilever bridges are designed for spans of 300 meters to 550 meters. Girder bridges can be further categorized into simple spans, continuous spans, and suspended and cantilevered spans.
B. Rigid Frame Bridges
Rigid frame bridges feature a structural system where the longitudinal girders are made continuous with the supporting vertical or inclined members. The main forces at play are flexure and axial force, which are carried by moment-carrying joints. Rigid frame bridges are most effective in spans ranging from 25 meters to 200 meters. This system is suitable for locations with limited height clearance and where a continuous structure is required.
C. Arch Bridges
Arch bridges are designed with arches as the primary load-bearing element. The loads are transferred to the foundations by the arches, which act under axial compression, with some bending forces involved. Arch bridges are highly efficient for longer spans, typically ranging from 200 meters to 500 meters. These bridges offer aesthetic appeal and are often used in scenic or iconic locations.
D. Cable Stayed Bridges
Cable stayed bridges feature cables that are arranged in vertical or near-vertical planes to support the main longitudinal girders. These cables are anchored at the bottom to the girders and are suspended from one or more tall towers. This design is especially economical for spans ranging from 150 meters to 700 meters. Cable stayed bridges are commonly used for river crossings, highways, and railways where long spans are required.
E. Suspension Bridges
Suspension bridges are typically used for the longest spans, where the bridge deck is suspended from cables that stretch across the gap. The cables pass over tall towers at either end of the bridge and are anchored to the ground. Suspension bridges are ideal for spans over 500 meters and are often used for crossing wide bodies of water or other large gaps where other bridge types would be impractical.
III. Classification Based on the Position of Carriageway
The position of the carriageway relative to the main structural system also influences the classification of steel bridges. The three primary types are:
A. Deck Type Bridge
In a deck type bridge, the carriageway rests on top of the main load-carrying members. This is the most common arrangement in girder and truss bridges. In a deck type plate girder bridge, the roadway is placed on the top flanges of the girders, while in a deck type truss girder bridge, the roadway is placed at the top chord level. This configuration is ideal for shorter spans and provides easy access to the underside of the bridge for maintenance.
B. Through Type Bridge
In a through type bridge, the carriageway rests at the bottom of the main load-carrying members. The roadway or railway is placed at the level of the bottom flanges or bottom chord in truss girders. This arrangement requires additional bracing of the top flange or lateral support for the top chord to prevent compression. Through type bridges are often used in cases where a greater height clearance beneath the bridge is necessary.
C. Semi-Through Type Bridge
A semi-through type bridge features a carriageway positioned between the top and bottom of the main load-carrying members. The lateral restraint in this type is typically provided by the U-frame action, which includes the verticals and cross beams working together to stabilize the structure. This type of bridge is commonly used when space constraints prevent the use of other types.
IV. Conclusion
The classification of steel bridges is a critical aspect of civil engineering, as it directly influences the design and construction process. By understanding the different types based on traffic, structural system, and carriageway position, engineers can make informed decisions that optimize both the functionality and safety of the bridge. Whether it’s a road bridge, a rail bridge, or a combination of both, the right classification ensures that each bridge is suited to its specific purpose, span requirements, and environmental conditions.