Bridge Piers: Types and Functions
A bridge pier is a type of structure that extends to the ground below or into the water. It is used to support bridge superstructure and transfer the loads to the foundation. The bridge pier can be constructed to be substantially attractive and strong in order to withstand both vertical and horizontal loads. It also does not hinder water flow or tide if the bridge spans the water.
Bridge piers may be built using concrete, stone, or metal. Concrete is commonly specified as construction materials provided that the pier is submerged in water since metal is prone to rust in water. It is constructed in many locations like waterways, or dry lands on which highway systems are built as overpasses.
Requirements of Bridge Piers
- It should effectively transfer loads from superstructure to foundation without failure.
- It should withstand all force actions.
- The material for the piers should be easily available.
- It should have pleasant appearance.
- Its design should be simple.
- The piers should be durable against weathering, impacts and corrosion.
- It should have minimum repair and maintenance cost.
- It should have stability against lateral and longitudinal force actions, such as seismic, wind, ice, currents, and impacts.
Types of Bridge Piers
Classification based on structure of piers
The type of piers to be used in the bridge depends on the type of bridge, sub-soil conditions as well as the procedure adopted for the construction of bridge. The bridge can be broadly divided into two parts:
These piers have solid and impermeable structure. The generally used materials used for the construction of this type of piers are bricks, stone masonry, mass concrete, RCC, etc. They may be further classified as:
- Solid Masonry Piers
- Solid RCC Piers
The piers which are constructed with brick masonry, stone masonry, concrete etc. are known as solid masonry piers. Sometimes it is seen that in solid masonry piles the outer portion is constructed with the stone masonry and the inner part is filled with the help of mass concrete. In this way, it can save the cost of construction.
The solid piers which are constructed with the reinforcement are called RCC piers. These piers are generally rectangular in cross-section and are generally used where the height of the piers are more and the solid masonry piers will not be strong enough to bear the load and can be uneconomical.
Open piers permit the passage of water through the structure and classified into the following types
- Cylindrical Piers
- Column Piers or Column Bent
- Pile Pier
- Trestle Pier or Trestle Bent
Cylindrical pier is constructed from cast irons or mild steel cylinders which are filled with concrete. This type of pier is suitable for bridges with moderate height. In certain cases, horizontal and diagonal steel bracing may be used to improve stability.
This type of piers is suitable for bridge with significant height. It consists of a cap beam and supporting columns forming a frame. Column bent piers can either be used to support a steel girder superstructure or be used as an integral pier where the cast-in-place construction technique is used. The columns can be either circular or rectangular in cross section. They are by far the most popular forms of piers in the modern highway system.
Multicolumn or frame bent piers are composed of two or more columns that support a cap. Isolated footing is used for this type of piers if the spacing between columns are large otherwise combined footing would be more suitable. There is a problem of debris collection when the water is allowed to flow between the columns.
Pile pier is the modification of multicolumn bent and is used for the type of bent on low height and short span structure. So, pile pier is specified when the ground is unstable and the low piers are required.
Trestle pier is composed of column with bent cap at the top. It is suitable for bridges in locations where river bed is firm and water current is slow. It is also employed for flyovers and elevated roads.
Classification Based on Materials
This may include stone masonry and brick masonry. Masonry piers are generally massive and may lead to obstruction of linear waterway and increase the loads on foundations. Masonry solid shaft piers are built on open raft foundation where the possibility of scour is nil. Pile foundations are also possible for such type of piers.
Mass Concrete Piers
Similar to masonry piers, mass concrete piers obstruct linear waterway and increase loads on foundation. Pile foundations can be used for mass concrete piers. Additionally, open raft foundation is used for mass concrete solid shaft piers provided that scouring is not anticipated. In mass concrete piers, no reinforcement is required from structural considerations but nominal reinforcement is provided for temperature and shrinkage effect.
Reinforced and Pre-stressed Concrete Piers
Reinforced concrete or pre-stressed concrete piers have small cross-sectional area compared with masonry and mass concrete piers. That is why such piers require much less foundation area in addition to offering less obstruction to waterway.
Reinforced concrete or pre-stressed concrete cellular piers arc suitable for major bridges where both the span and the depths are considerable and the self-weight of the piers should be as minimum and the section modulus as maximum as possible.
For reinforced concrete piers, the percentage of longitudinal reinforcement should neither be less than 0.8 nor more than 8% of the gross cross-sectional area. Where brick and stone materials arc costly, it is generally found economical to use reinforced concrete or pre-stressed concrete piers.
Classification of Bridge Piers Based on Force Transfer Mechanisms
Based on the type of forces the pier is subjected to, bridge piers are classified as fixed and free piers. In a most simple explanation, the piers supporting a fixed bearing are called fixed piers and those supporting free bearings are called free piers. Fixed piers are subjected to transverse and longitudinal forces whereas free piers transfer only axial forces from the bearing to the foundations. For multi-span bridges, the number of fixed and free piers depend on the length of bridge, size of pier and magnitude of long and trans-forces superstructure can be subjected to. For regular multi-span continuous bridges under 200-300m one fixed pier is provided.