Bridges are fundamental structures connecting communities and facilitating transportation across various terrains and obstacles. Their enduring presence allows for the movement of people and goods. Beneath their roadways and pathways lies a complex interplay of structural elements, each contributing to the bridge’s stability and ability to withstand immense forces. Among these components, the girder stands as a primary support.
Understanding the Bridge Girder
A bridge girder serves as a primary horizontal structural component designed to support the bridge deck and transfer loads to the underlying piers or abutments. It acts as a backbone for the bridge, bearing the weight of the structure itself, along with traffic and environmental forces. While often used interchangeably with the term “beam,” a girder refers to a larger, more robust element engineered to span greater distances and carry more substantial loads.
This foundational support is designed to resist bending and shear forces that arise from the weight it carries. By efficiently managing these forces, the girder ensures the bridge deck remains stable and safe for passage. It gathers distributed loads from the bridge deck and channels them downward to the bridge’s vertical supports, such as piers and abutments.
Varieties of Bridge Girders
Bridge girders come in various shapes and are constructed from different materials, each suited for particular bridge designs and load requirements. The choice of shape and material impacts how the girder handles stress and distributes weight across a span.
One common shape is the I-beam girder, named for its cross-sectional appearance resembling the letter “I.” These can be “rolled steel girders,” factory-produced in standardized lengths, typically up to 100 to 120 feet. Alternatively, “plate girders” are fabricated by welding separate steel plates together to form the web and flanges, offering greater flexibility in height and shape, and allowing for spans exceeding 300 feet.
Another prevalent type is the box girder, which features an enclosed, hollow, box-like cross-section. These girders provide strong resistance to twisting forces, making them suitable for curved bridges or those subjected to significant torsional stresses. T-girders, with a “T” shaped cross-section, are also used in certain applications.
In terms of materials, steel and concrete are predominantly used for girders. Steel girders are valued for their high strength-to-weight ratio and durability. Concrete girders are also widely employed, often incorporating steel reinforcement bars to enhance their tensile strength.
More advanced concrete girders include pre-stressed and post-tensioned types. Pre-stressed concrete girders have steel tendons tensioned before the concrete is poured and cured, while post-tensioned girders involve tensioning tendons after the concrete has hardened, within ducts cast into the concrete. These pre-stressing techniques introduce internal compression, which helps the concrete resist tensile forces, allowing for longer, more efficient spans.
How Girders Support Bridges
Girders support bridges by efficiently managing and transferring the forces exerted upon them. When a load, such as traffic or the bridge’s own weight, is applied to the deck, the girders beneath experience two primary forces: compression and tension. Compression is a pushing or squeezing force that attempts to shorten a material, while tension is a pulling or stretching force that tries to lengthen it.
In a typical I-beam girder, the upper part, known as the top flange, is primarily under compression from the downward load. Simultaneously, the lower part, or bottom flange, is subjected to tension as it attempts to stretch. The vertical section connecting these flanges, called the web, experiences shear forces and helps maintain the distance between the flanges, contributing to the girder’s overall stability.
The design of a girder, particularly its height, significantly influences its ability to carry loads. A deeper girder provides greater resistance to bending, allowing it to span longer distances or support heavier weights. These forces are then systematically transferred through the girder to the bridge’s supports. This process ensures the bridge remains structurally sound.