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What are concrete I Section girders?


Concrete I-section bridges are common in bridge construction due to their structural efficiency and versatility. Concrete I-section girders are a common choice for bridge design due to their structural efficiency and versatility.


These bridges consist of a concrete deck supported by a series of concrete beams, known as I-beams, arranged in a rectangular configuration. The I-beams provide the main load-bearing capacity of the bridge, while the concrete deck serves as a surface for vehicles to travel on.

A concrete I-section girder bridge

These girders consist of a horizontal top flange, a vertical web, and a horizontal bottom flange, forming an "I" shape in cross-section. The web decreases in thickness at the centre while thickening at the ends where it rests on the bearing. The top flange almost remains same throughout the length of the beam, while the bottom flange may thicken at the center or as per design requirement.

I section Girders at different locations along length

The top and bottom flanges provide lateral stability and resistance to bending moments, while the web serves as the primary load-carrying member, resisting shear forces and torsion. The I section beams are connected by a diaphragm at the ends and at the centre. The diaphragm increases stiffness and flexural capacity of the entire edifice.


Basic Arrangement of the I girder on a pier

The design of the bridge plays a crucial role in its behavior. Factors such as the width and depth of the I-beams, the spacing between them, and the thickness of the concrete deck all influence the strength and stability of the bridge. In the design of concrete I-section girders for bridges, several factors must be considered. The primary load-carrying capacity of the girders is determined by the size and strength of the concrete, as well as the dimensions of the top and bottom flanges and the web. The spacing of the girders is also an important consideration, as it determines the width and depth of the deck slab and the overall stiffness of the bridge. In addition to the structural considerations, the design of concrete I-section girders for bridges also takes into account the construction process and the long-term durability of the bridge.

The girders are designed to withstand the forces associated with the construction process, such as the weight of the forms and the loads associated with placing and curing the concrete. The analytical model must consider the construction stages as load inducing and subtracting stages that may create positive or negative stresses on the girder. The girders must also be designed to withstand the effects of long-term loading (Creep and shrinkage) and the harsh environmental conditions to which they will be subjected.

This often involves the use of specialized concrete mixes and reinforcement, such as steel rebar or fiber-reinforced polymer bars, to enhance the girders' strength and durability.


One important aspect of the design process is ensuring that the girders are adequately reinforced to resist the tensile forces that develop in the web and flanges under load. This is typically accomplished with steel rebars, which are placed in a as per the shape and post tensioning cable profile along its length.

RF for I girder

The longitudinal bars are flexure reinforcement and higher diameter bars are provided at the bottom flange. A cover of 50 mm is maintained for the reinforcement throughout. Since the post tensioning cables run throughout the girder, at some parts they clash with this arrangement of rebars. At these crucial locations, the rebars must have necessary shape modifications and be appropriately represented and highlighted in the drawing. The rebar table indicates the shape of the bar ans the length of the bar respectively. This table must also include the angles as per girder shape uniqueness. Nonetheless, bars are dowelled from the top forming rings throughout the length of the girder, this is practiced to enable crane lifting and placement operations on site.

RF dowel bar

Once the reinforcement is placed, the steel shuttering can be installed and closed. It is open from the top from where the concrete is poured. While concreting, the concrete mix has to be very precise since higher wc ratio will result in the honey combing at bottom flange. Additionally, due to rebar and cable caging, it is difficult to reach the bottom of the flange with vibrators. Hence, surface vibrators are used.


I section girders are simply supported beams and their behavior is exactly similar to simply supported flexure members. Sometimes, I section girders span more than one span, the details of such multi span I section girders will be discussed in a separate article.

The post

The behavior of concrete I-section bridges is primarily determined by the properties of the materials used and the design of the bridge. Concrete is a strong and durable material, but it is also prone to cracking under certain circumstances. To prevent cracking, the concrete used in I-section bridges is typically reinforced with steel bars, which help to distribute the load more evenly and increase the overall strength of the bridge.


Once the girder has been casted and the concrete has gained sufficient strength, the post tensioning process is the next step.

What is post tensioning? Read here

"post tensioning for I section girder' Read here

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