A bridge pier is a type of structure that extend to the ground below or into the water. It is used to support bridge superstructure and transfer the loads to the foundation. The bridge piers 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, dry lands on which highway systems are built as overpasses
It can vary in size and shape (Fig. 1 and Fig. 2) Based on aesthetics, site, space, and economic constraints such as beam and V shapes. The designer must specify appropriate pier shape for specific application. Sometimes, bridge piers are designed to guarantee proper load bearing of roadway, and in other cases, it can be constructed in such a manner that it would not prevent proper river water flow. Additionally, the design of bridge pier can be modified based on the other forces that act on the bridge such as high winds that may enforced certain design over another one.
Types of Bridge Piers
Based on the Structure of Piers
Piers are categorized into two major types based on its structure which include solid piers and open piers. These types are further classified into several types:
1. Solid Piers
Solid piers possess solid and impermeable structure, and usually constructed from bricks, stone Masonry, mass concrete or reinforced concrete. Solid piers are categorized into solid masonry piers and solid reinforced concrete piers
1.1 Solid Masonry Piers
It is constructed from brick masonry, stone masonry, and concrete. For economic reasons, the outer part of solid masonry pier is built from stone masonry and the inner part is filled with the help of mass concrete.
1.2 Solid Reinforced Concrete Piers
Solid reinforced concrete piers are mostly constructed from reinforced concrete and commonly rectangular in cross-section. It is used in the case where the height of the piers is more and the solid masonry piers would not be strong enough to bear the load and can be uneconomical.
2. Open Piers
Open piers permit the passage of water through the structure and classified into the following types:
2.1 Cylindrical Piers
Cylindrical pier is constructed from cast iron or mild steel cylinder 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.
2.2 Column Piers or Column Bent
This type of pers 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.
2.3 Multicolumn or Pile Bent
Multicolumn or pile bent or frame bent piers are composed of two or more column that supports a cap. Isolating 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.
2.4 Pile Pier or Pile Bents
Pile pier is the modification of multicolumn bent and used for the type of bent on low height and short span structure. So, pile pier or pile bents are specified when the ground is unstable and the low piers are required.
2.5 Trestle Pier or Trestle Bent
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.
Based on Construction Materials
3. Masonry Piers
This may include stone masonry and brick masonry. Masonry piers are generally massive that 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.
4. Mass Concrete Piers
Similar to masonry piers, Mass concrete piers massive which in turn 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.
5. Reinforced and Prestressed Concrete Piers
Reinforced concrete or prestressed concrete piers have small cross- sectional area compare with masonry and mass concrete piers. That is why such pier require much less foundation area in addition to offering less obstruction to waterway.
Reinforced concrete or prestressed 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 per cent of the gross cross-sectional area. Where brick and stone materials arc costly, it is generally found economical to use reinforced concrete or prestressed concrete piers.
Based on Load Transfer Mechanism
6. Fixed Piers
Fixed piers support a fixed bearing and subjected both to transverse and longitudinal forces
7. Free Piers
Free piers support free bearings and transfer only axial forces from the bearing to the foundations.
8. Hammerhead or Cantilevered Piers
Hammer head pier, which is also termed as solid shaft pier, has a single solid concrete cross section upon which a cap is placed. This type of pier is used to support steel girder or precast prestressed concrete superstructures. It is mostly constructed in urban areas where space limitation is a concern. Not only does it aesthetically pleasing but also occupy small spaces, thereby providing more room for the traffic underneath.
Generally, spread footing are recommended as a foundation for hammerhead piers. The major axis of solid shaft pier shall be in the direction of the stream flow otherwise circular or small rectangular cross section need to be selected. Standards for the use of hammerhead piers are often maintained by individual transportation departments.