A plate girder is a type of built-up I-beam section commonly used in structural engineering to carry heavy loads that cannot be supported economically by rolled I-sections. It is constructed by riveting or welding steel plates into the shape of an I-beam. Plate girders are crucial in applications where large spans and high load-carrying capacities are required. In this article, we will discuss the primary components of a plate girder and their respective functions in ensuring the strength and stability of the girder.
Components of a Plate Girder
A typical plate girder consists of three key components: the web, the flanges, and the stiffeners. Each of these components plays an essential role in distributing the loads applied to the girder and ensuring its structural integrity. Let’s explore each component in more detail.
1. Web
The web is the deep, central vertical plate in a plate girder. It plays a critical role in separating the two flange plates by a required distance. Its primary function is to resist the shear forces that develop in the plate girder when it is subjected to load.
In terms of load distribution, the web transfers the shear force from the applied load across the girder to the supporting structure. Without a properly designed web, the plate girder would be unable to effectively resist the shear forces, which could lead to failure. Thus, the web is a vital component for maintaining the girder’s overall strength.
2. Flanges
Flanges are the horizontal components of the plate girder that are placed at the top and bottom, separated by the web. Their primary function is to resist the bending moments acting on the girder due to applied loads.
The top flange of the plate girder resists compression caused by the bending moment, while the bottom flange resists tensile forces. The dimensions of the flanges, including their width and thickness, must be carefully calculated to ensure they can provide sufficient resistance to the bending moments. This ensures that the plate girder remains stable and doesn’t undergo excessive deformation under loading. Properly sized flanges help in effectively distributing bending stresses and preventing failure.
3. Stiffeners
Stiffeners are additional components integrated into the plate girder to enhance its load-carrying capacity and prevent structural failure due to buckling. There are two primary types of stiffeners: vertical stiffeners and horizontal stiffeners. Both types of stiffeners play distinct roles in ensuring the girder’s performance.
A. Vertical Stiffeners
Vertical stiffeners are placed at right angles to the flanges and are often referred to as transverse stiffeners. These stiffeners are essential in preventing the web from buckling under concentrated loads and ensuring the overall stability of the girder.
Vertical stiffeners are classified into two types:
- End Stiffeners (Bearing Stiffeners): These stiffeners are located at both ends of the plate girder. Their primary role is to transfer the load from the girder to the support. In addition, end stiffeners help to prevent the web from crushing due to the compressive forces acting on the girder’s ends. This function is particularly important, as a crushed web could compromise the girder’s structural integrity.
- Intermediate Stiffeners: These stiffeners are positioned between the end stiffeners and are required when concentrated loads are applied to the girder. When the web’s thickness is inadequate (i.e., less than 1/85th of the girder’s depth), the web is prone to buckling due to shear forces. In such cases, intermediate stiffeners are used to reinforce the web and prevent buckling, thereby improving the girder’s overall strength and stability.
B. Horizontal Stiffeners
Horizontal stiffeners, also known as longitudinal stiffeners, are placed parallel to the flange plates. Their primary function is to enhance the buckling strength of the web by resisting lateral forces. These stiffeners ensure that the web does not buckle under the applied shear forces and helps maintain the overall stability of the girder.
There are two types of horizontal stiffeners:
- Continuous Horizontal Stiffeners: These stiffeners run across the entire girder, connecting all the transverse stiffeners. They help distribute the load from the flanges and the web, and they enhance the girder’s overall load-bearing capacity. Continuous stiffeners are crucial in girders with longer spans or higher load requirements.
- Discontinuous Horizontal Stiffeners: These stiffeners are placed between the transverse stiffeners without touching them. Unlike continuous stiffeners, discontinuous stiffeners do not participate in load transfer from the flange or the web. Instead, they provide additional resistance to buckling, helping to maintain the structural integrity of the girder’s web under shear stress.
Conclusion
Plate girders are vital structural elements in engineering, designed to support heavy loads over large spans. The three main components of a plate girder—the web, flanges, and stiffeners—each play an essential role in ensuring the girder’s ability to resist shear, bending, and buckling under applied loads.
- The web resists shear forces and maintains the distance between the flanges.
- The flanges resist bending moments, with the top flange handling compression and the bottom flange handling tension.
- The stiffeners, both vertical and horizontal, provide additional strength by preventing web buckling and ensuring load distribution.
By understanding the functions of these components, engineers can design plate girders that meet the specific demands of a project, ensuring safety, stability, and longevity in the structures they support.