What holds up concrete pillars when building a bridge?

Answer 1

There are three types of foundations I have seen used for bridges: spread footings, piles, and caissons.

Spread footings are reinforced concrete pads built underground to "spread" out the pressure from the weight of the structure. These are almost never used for bridges (at least in the Chicago area... they may be more common in areas with different soil conditions.) because they do not have as much capacity as other types of foundations, and are more susceptible to settlement. In 10 years as a civil engineer, I have only seen 1 bridge that used spread footings.

Piles are long objects driven into the ground to hold up a structure. They can be made of steel beams (referred to as H-Piles), metal shells (steel tubes with a cap at the bottom, that are filled with concrete after being driven), precast concrete, or wood. Most pile-supported bridges I have seen use H-Piles or metal shells, with H-Piles being more common in my experience. While piles are being driven, engineers monitor the amount of energy required to force the pile into the ground. The pile is driven deeper and deeper until the required resistance is met. After the piles are driven, a reinforced concrete cap is constructed around the top of the piles, and the rest of the structure is built on top of that.

Caissons are shafts drilled in the ground that are then filled with reinforced concrete. Caissons can either tie in to a reinforced cap poured on top of them, or continue out of the ground and become the columns that you see supporting the bridge.

Piles and Caissons can either be friction bearing or end bearing. Friction bearing piles and caissons hold up the structure by using the friction between the sides of the piles or caissons and the soil around them. End bearing piles and caissons support the structure by transferring the pressure from the structure to a stronger layer of soil or rock at the tip of the pile or caisson. Piles driven into rock have “shoes” welded to the ends of them to prevent the end of the pile from damage during driving. End bearing caissons will often flare out to a larger diameter at the bottom to provide more area for the soil pressure to be spread over. This flared area is referred to as the bell.

The type of foundation is selected based on what is most economical considering existing soil conditions

Answer 2

It depends of the existing type of soil and its properties, if the bridge will cross a water body or not, and if the piles will be in contact with water or not.

If there's bedrock underneath, reaching it to have a firm support could be one possibility.

However, you may find areas where the bedrock is so deep that is not a cost effective alternative or sometimes where there's no bedrock at all. Then driving steel-shell-concrete piles is recommended. Steel piles are driven into the sandy ground without reaching any bedrock and then poured, this way the total weight of the bridge will be supported by the friction between the exterior surface of the piles and the sand.

Answer 3

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Answer 4

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Answer 5

Pryor to pouring a piller ,steel I-beams are driven into the ground to solid rock base.then the pillers are formed and reinforcing steel is placed inside forms to engineer's specifications.Concrete is poured and it bonds to pilings and rebar to form a free standing pillar....George

Answer 6

Concrete pillars are formed by pouring concrete into wooden forms. The wood is removed when the concrete hardens.

Answer 7

they have to be achored to the bedrock usually. sometimes they pound these very long steel poles deep into the ground to reach bedrock and sometimes they dig down and pour the cement right into the water just so long as the cement contacts bedrock.

Answer 8

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