Why Bridges Don't Sink
Bridges rely on deep foundations like piles made of timber, concrete, or steel to support concentrated forces. Piles resist vertical, lateral, and uplift loads, with alternatives like drilled shafts for quieter installation in soft soils or underwater.
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Bridges are unique structures that face specific challenges due to the concentration of forces on their piers and abutments. To support these loads, engineers use deep foundations like piles, which can be made of timber, concrete, or steel. Piles are driven or drilled into the ground to provide structural support. They resist vertical loads through end-bearing on strong soil or rock layers and skin friction along the pile walls. Piles can also withstand lateral loads and uplift forces, making them versatile foundation elements for bridges and buildings. However, driven piles can be noisy and disruptive during installation, leading to the development of alternative solutions like drilled shafts. Drilled shafts involve drilling a hole, placing reinforcing steel, and filling it with concrete, making them suitable for soft soils or underwater installations. The process includes using casings and tremies to maintain hole integrity and prevent water intrusion during concrete pouring. Overall, deep foundations play a crucial role in ensuring the stability and longevity of bridge structures under various loading conditions.
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18 comments> The tagline of the Pile Driving Contractors Association is “A Driven Pile is a Tested Pile” because, just by installing them, you’ve verified that they can withstand a certain amount of force. After all, you had to overcome that force to get them in the ground. And if you’re not seeing enough resistance, in most cases, you can just keep driving downward until you do!
This is because the shapes are different. I beams are typically more slender through the web because the goal is to concentrate mass at the flange for moment capacity because they’re beams and geared towards bending. H piles are thicker in the web with the web thickness usually similar to the flange because the use case requires axial capacity and various constructability considerations. I beams turned into W (wide flange) and S sections in the standard shapes and H beams are called HP sections.
You’ll often see them cross-specified for foundation work but it’s rare that you’d choose an HP section over a more efficient section like a W or S for something “out of the ground.”
I believe this is the same fundamental engineering method used in a swamp by Herbert's father in Monty Python and the Holy Grail. [0]
There is always bedrock, but in some places your pile would have to be really long to reach it:
> The gravel deposits of 100 m (330 ft) are the deepest in the south of Munich and decrease towards the north.
(from https://en.wikipedia.org/wiki/Munich_gravel_plain - not saying this is anything really extraordinary, but it's the area I'm most familiar with)
Videos: https://www.youtube.com/channel/UCMOqf8ab-42UUQIdVoKwjlQ
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