The foundation and types of foundations

The foundation is a group of structural elements and its mission is to transmit the loads of the construction or elements supported to the ground by distributing them in a way that does not exceed its admissible pressure nor produce zonal loads. Because the strength of the floor is generally less than that of the abutments or walls it will support, the area of ​​contact between the floor and the foundation will be proportionally larger than the supported elements.

Purposes:
• Be strong enough not to break by shear.
• Resist bending forces produced by the ground, for which armatures will be provided on its underside.
• Adapt to possible terrain movements.
• To withstand the aggressions of the terrain and water and its pressure, if any.

Types of foundation
The choice of foundation depends on the mechanical characteristics of the terrain, such as its cohesion, its internal friction angle, the position of the water table and also the magnitude of the existing loads. From all these data the bearing capacity is calculated, which together with the homogeneity of the terrain suggest using a different kind of foundation.
There are two main types of foundation: direct and deep.
Direct foundations:
They are those that lie in the superficial or shallow layers of the soil, because they have sufficient bearing capacity or because they are constructions of secondary importance and relatively light. In this type of foundation, the load is distributed in a horizontal support plane.
Whenever possible, surface foundations are used, as they are the least costly and simplest type of foundation.
a) Isolated shoes
The insulated shoes are a type of surface foundation that serves as the basis of specific structural elements such as the pillars; so that this shoe widens the support surface until the ground supports the load that transmits it. The term isolated shoe is because it is used to seat a single pillar, hence the name isolated. It is the simplest type of shoe, although when the bending moment at the base of the abutment is excessive they are not suitable and instead combined shoes or running shoes in which more than one abutment should be used.
 
b) Combined or running shoes
This type of foundation is used when the insulated shoes are very close or even overlap. The causes that give rise to this situation are several: the proximity of the pillars, the existence of strong concentrated loads that can give rise to high differential seats, the scarce resistant capacity of the terrain or the presence of discontinuities in this one. If the number of pillars it supports is less than three they are called combined and otherwise run. They are also used to support load bearing walls (load walls or earth retaining walls) whether or not they have built-in supports in which case the width of the shoe can be variable.
 
c) Foundation slabs.
A foundation slab is a floating plate resting directly on the ground. The slab foundation is used as an extreme case of the previous ones when the surface occupied by the slabs or by the slab represents a high percentage of the total surface. The slab can be solid, lightened or have special reinforcements to improve the resistance to punching under the individual supports (called pedestals if they are on the slab and reinforcements if they are under it) or by lines (ribs).

In particular, these types of foundations may also be employed when "compensated" foundations are designed. In them, the design of the building includes the existence of basements so that the weight of the excavated land equals approximately the total weight of the building; the slab uniformly distributes the stresses across the surface and in this case the seats that are expected are reduced. If the building is distributed in several zones of different height, provision should be made for proportional distribution of basements as well as structural joints.

Deep Foundations:
They rely on the shear force between the ground and the foundation to withstand the loads applied, or more precisely on the vertical friction between the foundation and the ground. That is why they must be deeper, in order to provide a large area on which to distribute an effort large enough to support the load. Some methods used in deep foundations are:
Piles: They are slender foundations that sink (prefabricated piles of displacement) or construct in a cavity previously open in the land (piles of extraction executed in situ). Formerly they were of wood, until in the years 1940 began to be used the concrete. The piles have three parts: tip, shaft and stem or support. Their mode of work depends on the nature of the terrain and the depth to which a resistant stratum is found.

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