Substructure & Foundation Options for Short Span Applications
The substructure of the bridge consists of the portion of the bridge that supports the entire structure on the given surrounding soil.
Substructure:
The substructure consists of the portion of the bridge that supports the entire structure on the given surrounding soil. There are varying designs due to the different soil conditions for each bridge site and the different weights of the structures for each project.
In some cases, the nature of the superstructure will also influence the choice of foundation type, especially when integral substructures are used. Designers are advised to involve qualified geotechnical engineers early in the bridge design process to help select the appropriate foundation type.
Foundation:
The foundation of a bridge structure creates a smooth transition allowing the internal forces to flow between the bridge and the ground. Foundations are the ‘bridges’ or links between the bridge and the ground.
There are really only two types of bridge foundations – although each has many variations – ‘spread’ foundations and ‘piled’ foundations. Both types must transfer forces into the soil beneath the bridge.
Spread Foundation:
The loads, the forces, from the bridge are spread over an area large enough for the ground to bear them.
Spread foundations are rather like the snowshoes you might wear to prevent you sinking into deep snow – the stereotype is like a tennis racquet – in fact the French term is raquette à neige.
The principle of a spread foundation applies if you were ever unfortunate enough to find yourself caught in quicksand. The best advice is not to struggle but to lie flat and still on the surface to try to spread your weight. If no one is around to help pull you out then try to swim – quicksand is liquefied sand and so is much denser than water. Swimming in it will be very hard work but it could save your life.
Piles Foundation:
Piles are long slender columns in the ground that transmit the loads to lower depths where they can be carried.
A common type of pile is the “driven pile” – these are typically steel piles hat are knocked into the ground like massive nails driven into timber by hammering. But. the final choice of pile depends on the ground conditions and soil strata.
If there is bed rock some way down then the piles can be driven or cast so that the loads are taken directly to the rock – they are known as end-bearing piles. Bored piles are usually end bearing with sometimes quite large diameters.
However if there is no bedrock near enough to the surface then the piles may be driven or cast into stiffer ground. The piles are then designed to transmit the load to the ground through the friction between the surfaces of the pile and the ground. Driven piles may be end bearing or friction piles or a combination of both.
The resources in this section will describe some applications, provide illustrations and evaluate the application to short span modular steel bridges.
For a list of bridge substructure and foundation suppliers, please see Find-A-Supplier on the SSSBA website.
