Bridge column casing is implemented with the objective of enhancing the ductility, shear, and flexural capacity of the column. Additionally, in some cases, it can also aid in restricting radial dilating strain at plastic hinge regions. Various techniques are used for the implementation of bridge column casing, including concrete casing, steel casing, fiber reinforced plastic composite casing, and wire wrap casing. Each of these methods has its own unique benefits and can be employed based on the specific requirements of the project. Overall, the use of bridge column casing plays a critical role in ensuring the structural integrity and durability of bridge columns, thus enhancing their ability to withstand external loads and environmental factors.
Methods of Bridge Column Casing
In construction, there are various types of casings used for different purposes. One such type is the steel casing, which is a cylindrical tube made of steel used to provide support and protection to pipes and other infrastructure. Steel casings are commonly used in underground construction projects, such as tunnels and pipelines.
Another type of casing commonly used in construction is the bridge concrete column casing. These casings are typically made of reinforced concrete and are used to provide structural support and protection to bridge columns. They can also be used to enhance the aesthetic appeal of a bridge by adding decorative features.
Fiber reinforced plastic composite casings are another type of casing used in construction. They are made from a combination of fibers, such as fiberglass, and a polymer resin. These casings are known for their strength, durability, and resistance to corrosion. They are commonly used to protect pipes and other infrastructure from damage caused by the environment.
Wire wrap casings are a type of casing used to protect pipes and other infrastructure from external damage. They are made by wrapping a wire mesh around the surface of the infrastructure and covering it with a protective layer, such as concrete or a polymer resin. Wire wrap casings are commonly used in underground construction projects, such as oil and gas pipelines, to protect them from corrosion and other forms of damage.
Steel Casing of Bridge Columns
There are three classes of steel casings commonly used to improve the seismic performance of bridge columns. The first class is known as Class F, which involves completely grouting the steel casing and covering the entire height of the column. Class F casing is used to enhance both flexural and shear capacity of the column, making it suitable for columns with deficiencies in both these aspects. It effectively prevents slippage of column lap splices and reduces radial dilating strain to less than 0.001, making it highly effective in increasing confinement and improving shear strength. Detailed execution of Class F steel casing for bridge columns is shown in Figure-1 and Figure-2.
Fig.1: Detail of Class F Type of Column Casing
Fig.2: Bridge Column Steel Casing
The partial height column casing is the second category of column casing that covers only a portion of the column’s height. The main difference between class F and class P types is that class P includes a layer of polyethylene, which is not present in class F, as shown in Figure-3. It’s important to note that this type of steel casing is not suitable for improving the shear capacity of the column. Additionally, class P type allows for dilating strain greater than 0.001, which may result in a pin being created at the base of the column. In some cases, the partial casing can be extended to cover the full height of the column for aesthetic purposes, especially in locations where the column is highly visible.
Fig.3: Details of Class P Type of Bridge Column Steel Casing
Fig.4: Application of Class P Type of Column Casing
The third class of steel casing for column bridges is a combination of the two classes mentioned previously. This type of casing is used to reinforce columns that lack sufficient shear strength. Figure-5 provides further details on the specifications of the third class steel casing.
Bridge Concrete Column Casing
When a bridge column has an uncommon shape and needs to be aesthetically maintained after retrofit, a concrete casing can be a suitable option. To ensure that the new column design requirements are met, bridge column jacking is employed. This method not only increases the flexural capacity of the bridge column but also enhances its shear strength and improves its confinement.
The practical work involves placing stirrups or hoop reinforcement around the column, followed by drilling and bonding bars into the column, as shown in Figure-6. Finally, concrete is placed to maintain the aesthetic appearance of the column. It is important to note that this practical work can be messy and costly.
Fig.6: Placing and Bonding of Reinforcement Bars around Concrete Column
Fiber Reinforced Plastic Composite Casings of Columns
One potential method for enhancing the shear capacity and confinement of bridge columns is through the use of fiber reinforced plastic composite materials. This approach can offer a cost-effective solution to improving bridge column performance, particularly when it is not necessary to restrict the slippage of lap splices within anticipated plastic hinge areas.
One advantage of using fiber reinforced plastic composite materials is their flexibility, which allows them to be wrapped around bridge columns while maintaining the original shape of the column. This is especially important when the aesthetic appeal of the bridge element is a consideration.
Overall, using fiber reinforced plastic composites to enhance bridge column performance can be a practical and economic approach, particularly when the goal is to improve shear capacity and confinement without compromising the visual appearance of the structure.
Fig.7: Bride Column Casing using Fiber Reinforced Plastic Composite
Wire Wrap Casings of Bridge Columns
A method for prestressing columns involves manually wrapping strands around the column and inserting wedges between the strand and column interface. This technique can be used on different column shapes similar to those made of fiber reinforced plastic composite casing. However, it should be noted that the wire wrapping technique requires significant manual labor and is currently only used for circular columns.
Other construction methods for reinforced concrete columns include concreting, pedestals, and footings. Additionally, jacketing and collars can be used to strengthen columns and beams. It is important to consider the various failure modes of concrete columns, particularly compression members, when designing and constructing them.