Foundation

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Venice is perhaps one of the most remarkable examples of the foundation by means of wooden piles.

The foundation is the name given to the set of structural elements of a structure whose mission is to transmit its loads or elements resting on it to the ground, distributing them in such a way that they do not exceed their admissible pressure or produce zonal loads. Since the resistance of the soil is generally less than that of the supporting pillars or walls, the contact area between the soil and the foundation must be proportionally larger than the supported elements, except in very coherent rocky soils.

The foundation is important because it is the group of elements that support the superstructure. The stability of a building depends to a large extent on the type of ground on which it stands.

General information

Cross-sectional view of the foundation of the television tower in Stuttgart.

The foundation is a base of simple or reinforced concrete stone materials. Whenever possible, it is preferable that the foundations are stressed by centered loads, since the eccentric ones can cause differential thrusts.

It will always be sought that the support ground is resistant and, if that is not possible, it will be necessary to look for alternative solutions.

In many cases, the foundations not only transmit compression, but also, through friction and adherence efforts, they even support horizontal and tensile loads, anchoring the building to the ground, if necessary. In addition to their main functions, the foundations have to fulfill other purposes:

Be tough enough not to break by cutter.
To support the bending efforts that produce the ground, for which in general armor will be available on its lower face, which will absorb the tractions.
To accommodate possible field movements.
Support the assaults of the ground and water and its pressure, if any.

Types of foundations

The choice of the type of foundation depends especially on the mechanical characteristics of the ground, such as its cohesion, its internal friction angle, position of the water table and also on the magnitude of the existing loads. Based on all these data, the bearing capacity is calculated, which, together with the homogeneity of the terrain, makes it advisable to use one or another different type of foundation. Whenever possible, shallow foundations are used, since they are the least expensive type of foundation and the simplest to execute. When due to problems with the bearing capacity or its homogeneity it is not possible to use superficial foundations, other types of foundations are valued.

There are two fundamental types of foundations: direct and deep.

Soils and types of foundations

The soil is a fundamental part of the structure, taking into account the different types of soil depends on what type of foundation should be used. And if the ground fails, the structure, residence or building will also fail.

If the ground is made up of rocks, the foundation could be started without any problem since this soil is very resistant, but this situation is very rare, you have to dig until you find a suitable soil for the foundation. Professionals in this case, when excavations are carried out at different types of height, analyze in their laboratory the characteristics of the type of soil according to its level, until reaching the level or foundation plane, which is the perfect level where the soil resists loads. demanded by the building or address.

Foundation level or plane: type of soil suitable for unloading the force of the structure on it.

This data is obtained through a geotechnical report that allows to know at what depth the foundation plane is located and what will be the admissible tension that the ground will have, it means that the weight of the structure will support that soil, if this tension is exceeded It can lead to a break in the ground causing the collapse of the structure.

Another piece of information that allows knowing the soil study is the presence of groundwater, that is, groundwater due to the presence of rain.

Zapata who transmits the effort to a superficial foundation of a bridge beam. The foundation is buried and is not visible in the figure.

It shows where superficial (more cheap) foundations and deep foundations are applied. Many times on bad grounds you must always choose deep foundation, even for low-weight constructions, like a small house.

They are those that rest on 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 on a horizontal support plane.

In important structures, such as bridges, foundations, even shallow ones, are supported deep enough to ensure that damage does not occur. Shallow foundations are classified into:

Cyclops.
Zapatas.
- Insulated Zapatas.
- Zapatas cumshots.
- Combined Zapatas.
The slabs of foundation.

Cyclopean foundations

In cohesive soils where the trench can be made with vertical walls and without landslides, the cyclopean concrete foundation is simple and economical. The procedure for its construction consists of filling the trench with stones of different sizes while pouring the concrete mixture in a 1:3:5 ratio, trying to mix the concrete perfectly with the stones, in such a way that continuity is avoided. in their meetings. Cyclopean concrete is made by adding more or less large stones as the concrete is being poured to save material. Using this system, smaller stone can be used than in concreted masonry foundations. The cyclopean concrete technique consists of throwing the stones from the highest point of the trench onto the mass concrete, which will be deposited on the foundation. Precautions:

Try to keep stones from contacting the wall of the ditch.
Let the stones not be piled up.
Alternate in layers concrete and stones.
Each stone must be completely wrapped by the concrete.

Insulated footings

Insulated footings are a type of superficial foundation that serves as the base of specific structural elements such as pillars; so that this footing expands the support surface until the ground supports the load it transmits without problems. The term isolated footing is due to the fact that it is used to settle a single pillar, hence the name isolated. It is the simplest type of footing, although when the bending moment at the base of the column is excessive they are not suitable and instead combined footings or strip footings must be used on which more than one column rests. The insulated footing does not need to be together because, being embedded in the ground, it is not affected by thermal changes, although in structures it is normal, as well as advisable to put a joint every 30 m approximately. In these cases, the footing is calculated as if only a single pillar rested on it. A variant of the isolated footing appears in buildings with expansion joints and in this case it is called "footing under pillar in fingerboard joint".

Construction of an isolated shoe foundation

In the calculation of the pressures exerted by the shoe it must be taken into account in addition to the weight of the building and the overloads, the weight of the shoe itself and the lands resting on its flights, these last two loads have an adverse effect on the sinking. On the other hand in the calculation of the spine, where the weight of the shoe and the lands on them have a favorable effect. The calculation of the sinking pressure, for small eccentricities ( ${displaystyle e=M_{max }/N_{x}}$ Where ${displaystyle N_{x}}$ is the vertical load on the ground and ${displaystyle M_{max }}$ The following formulas can be used:

$<math alttext="{displaystyle p={begin{cases}{cfrac {N_{x}+P}{A}}left(1+6{cfrac {e}{a}}right)&e/aleq 1/6\{cfrac {N_{x}+P}{A}}{cfrac {4}{3(1-2e/a)}}&1/6<e/ap={Nx+PA(1+6ea)e/a≤ ≤ 1/6Nx+PA43(1− − 2e/a)1/6.e/a.1/2{displaystyle p={begin{cases}{cfrac {N_{x}+P{A}}}}left(1+6{cfrac {e}{a}{a}}{a}}}{right}{eq 1/6}{cfrac {N_{x}{A}{A}{cfrac {4}{4/6}{3(1⁄2}{x}}}{1}}}{<img alt="{displaystyle p={begin{cases}{cfrac {N_{x}+P}{A}}left(1+6{cfrac {e}{a}}right)&e/aleq 1/6\{cfrac {N_{x}+P}{A}}{cfrac {4}{3(1-2e/a)}}&1/6<e/a$

where A is the area of the isolated foot strike and P its weight. To build an isolated footing, the foundations and structures of the buildings located on land of heterogeneous nature, or with discontinuities, must be separated so that the different parts of the building have stable foundations. It is convenient that the building facilities are on the foundation plane, without cutting footings or braces. For all types of footing, its support plane must be embedded 1 dm in the ground layer.

The depth of the support plane is set based on the geotechnical report, without altering the behavior of the ground under the foundation, due to variations in the water table or possible risks due to frost. It is convenient to reach a minimum depth below the surface level of 50 or 80 cm in those areas affected by these variables. In the event that the building has a structural joint with duplicate support (two pillars), a single footing is made for the two supports. It is convenient to use concrete of plastic consistency, with aggregates of size around 40 mm. In execution, and before pouring the concrete, lay out a layer of poor concrete approximately 10 cm thick (cleaning concrete) on the bottom, before placing the reinforcements. the footings have concrete.

Flat footings

The strip footings are used to cement load-bearing walls, or rows of pillars. Structurally, they function as a floating beam that receives separate linear or point loads.

These are foundations of great length compared to their cross section. The continuous footings are indicated as the foundation of a longitudinally continuous structural element, such as a wall, in which we intend the seats on the ground. This type of foundation also acts as bracing, can reduce the pressure on the ground and can bridge defects and heterogeneities in the ground. Another case in which they are useful is when many nearby isolated footings would be required, making it easier to make a strip footing.

Strip footings are normally applied to walls. They can have a rectangular, stepped or tapered section. Its dimensions are in relation to the load that they have to support, the resistance to compression of the material and the admissible pressure on the ground. For practicality, a minimum height of approximately 3 dm is adopted for the concrete foundations. If the heights are greater, they are given a staggered shape taking into account the angle of distribution of the pressures.

In the event that the earth tends to collapse or the foundation must be staggered, formwork will be used. If the foundations are made of rammed concrete, they can be concreted without the need for them.

If the foundation works must be interrupted, it is recommended to cut the vertical joint in steps to achieve a correct connection with the following section. Likewise, placing some armor irons will reinforce this union.

The Continuous Footings are, according to the Technical Building Code (CTE), those footings that include more than three pillars. Thus, he considers them different from combined footings, which are those that include two pillars. This distinction is the subject of debate since a combined footing can perfectly support four pillars.

Combined footings

A combined footing is an element that serves as a foundation for two or more piers. In principle, isolated footings take advantage of the fact that different pillars have different bending moments. If these are combined in a single foundation element, the result can be a more stabilized element and subjected to a lower resultant moment.

Foundation slabs

Main article: Smenting slab

A slab foundation is a floating plate resting directly on the ground. As a slab it is mainly subjected to flexural stresses. The thickness of the slab will be proportional to the bending moments acting on it. The relationship between the thickness of the slab, the bending moments of the plate, the external loads and the elastic properties of the concrete of the slab is given by the following expression:

${displaystyle left[{cfrac {partial ^{2}}{partial x^{2}}}+{cfrac {partial ^{2}}{partial y^{2}}}right](m_{x}+m_{y})=-12(1+nu){frac {q(x,y)+k_{b}w(x,y)}{Eh^{3}}}}$

Where:

{displaystyle m_{x},m_{y},}

moments in the directions x e and.

{displaystyle E,nu ,}

elastic constants of concrete.

{displaystyle q(x,y),}

effective surface load at each point on the upper face of the slab.

{displaystyle k_{b},}

the scale coefficient of the ground under the slab.

{displaystyle w(x,y),}

vertical descent at each point of the slab.

Semi-deep foundations

Main article: Feature of foundation

Excavating one of the foundation wells for a bridge. The concrete tube (concrete) is sinking as it is excavated. In this case it was reached 24 m deep.

Fees of foundation or caissons: They are actually intermediate solutions between the superficial and the deep, so they are sometimes classified as semi-deep. Sometimes these should be done under water, when the river cannot be diverted, in that case they work on pressed chambers.
Brick arches on concrete males or masonry.
Walls of containment under tear: it is not necessary to anchor the wall to the ground.
Micropilotes are a variant based on the same idea of the pilotage, which often constitute a semi-deep foundation.
Compacted Attached Columns/Pilas: Improve the effort-deformation properties of the ground by densifying the soil matrix.

Deep Foundations

They are based on the shear force between the ground and the foundation to support the applied loads, or more precisely on the vertical friction between the foundation and the ground. They need to be located deeper, in order to be able to distribute over a large area an effort large enough to withstand the load. Some methods used in deep foundations are:

Pilots: are elements of slender foundation that are stained (prefabricated 'displacement' pills) or build in a previously open cavity on the ground (extracting pills) executed in situ). They were old wood, until in the 1940s the concrete began to be used.
Screens: it is necessary to anchor the wall to the ground. The CTE classifies them as containment elements.
- isostatic displays: with an anchor line
- hyperstatic displays: two or more anchor lines.

Machine foundations

Unlike building foundations, which are generally subjected to static or quasi-static loads, machinery foundations are frequently subjected to cyclic loading. The existence of cyclic loads make it necessary to consider the vibration service limit state and the ultimate fatigue limit state.

Some types of foundations used for machinery are:

Type block
Type of cells
Walls
Porticadas
With pilots
About elastic support

Picture Gallery of Types of Shallow Foundations

Drawing of a foundation of pilots
A primitive construction with wooden pilots in Switzerland.
A hórreo
Ruins showing the foundation in a house (Davis House) Gardiner, New York
Classical stroke
A house in France (basement).

Additional bibliography

Medina Sánchez, Eduardo. Construction of reinforced concrete structures. Building (2nd ed.). Madrid: Delta Publications. ISBN 84-96477-96-7. Consultation on 13 May 2022.

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