How Do Bridge Foundations Work and Why Are They Important?
Bridges can simply not be built without foundations, the same way that cars cannot be built without engines and planes without wings. The foundations have to be perfect for a bridge to be functionable and safe.
The way in which bridges are formed depends on the type of bridge, which could be beam, arch, cable-stayed or suspension. Each bridge type is made uniquely to each other and differ in design and the type of foundation, to maintain stability and functionality.
Four types of bridges
1. Beam bridges
Beam bridges are the most simple and common form of bridge that requires strong foundations on either side of the crossing.
2. Arch bridges
Arch bridges use strong structures attached by an arch to withstand winds and remain stable.
3. Suspension bridges
Suspension bridges are made up of two towers either end of the bridge, that use a combination of tension and compression to transfer the weight to each tower.
4. Cable-stayed bridges
Cable-stayed bridges are reasonably modern, compared to the other types, with the first one being built by two German designers in Sweden in 1955. They have much greater stiffness, than compared to suspension bridges and use vertical compression to balance the weight between the towers.
What are bridge foundations?
Bridge foundations are a detrimental part of ensuring the stability of bridges. They require expert installation so that the foundation can transmit the load to the ground. The types of foundations that will be required depend on a handful of environmental factors, such as river flow, river depth, exposure conditions and bed conditions.
There are 2 main types of bridge foundations, strip footing foundations and piled foundations also referred to as shallow and deep foundations. The environmental factors, as mentioned previously, along with the weight of the load determine which foundation will work best. This is because you will then discover if you require shallow or deep foundations.
What is a strip footings foundation?
Strip footings (as the name suggests) is a continuous strip of concrete that spreads the weight of a load bearing structure and in general, are used for smaller structures. They are best suited to most different soil types as it is easier to install and holds a very suitable bearing capacity.
A strip footing transfers the load to a shallow depth and provides level support to smaller bridges. Shallow foundations are necessary for smaller and lighter bridges, where the loads imposed by a structure are low in relation to the bearing capacity of the local surface soils.
The thickness of the footings is about 0.8m-1.0m and has to withstand the bending moments and shearing forces created by the bridge’s abutment and piers.
There are many types of piers and abutments that you may find on a bridge, such as:
- Cylindrical piers
- Open piers
- Spill-through abutment
- Pile bent abutment
Bridge piers transfers the loads from superstructure to the foundation, and bridge abutments are substructures located at either end of the bridge to support its superstructure.
What is a pile foundation?
Building big bridges requires strong foundations that are able to hold a lot of weight. Pile foundations are made of steel and concrete and are located within long cylinder shaped columns to support the bridge structure. Deep foundations are narrow and long and transfer the load to the deep strata.
Screw piles can be easily installed with minimal noise and vibration and are used to create new pile foundations, or to repair existing bridges. Screw piles also greatly benefit the environment, compared to other types of deep foundation, as the avoidance of cement is favourable to the local ecosystem.
Pile foundation is the perfect choice for an environment that is affected with soil liquefaction because it can easily penetrate through the groundwater to the hard strata, where shallow foundations would struggle. Screw piles are the perfect choice if you’re looking at building bridges over rivers and other bodies of water, as the footings depth will be sufficient enough to withstand the scour caused by river flow.
ABC anchors provide 3 different screw piles:
To see the specifications of each version and their capacities, or to find out more click here.
Careful consideration of the bridge’s foundations is essential in building a sustainable bridge that is fit for purpose and suitable in the surrounding environment. If you are unsure as to what screw pile is necessary for your building goals, speak to one of the team here.