Foundation (construction)

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The foundation is the constructive and static formation of the transition between the structure and the ground . The most important task of the foundation is to absorb loads from the structure and to pass them on to the subsoil without the resulting compression of the soil leading to disadvantages for the structure or the environment. In the case of very tall and slim buildings, horizontal forces from wind pressure can also occur.

The foundation area corresponds to the outline of the floor slab or the outer edges of the strip foundations ; the foundation level is sometimes also used .

In order to ensure the stability of a structure in the long term, various factors must be taken into account:

  • the absorption of snow and dead weight as well as horizontal forces from wind, and possibly water, earth pressure and vibrations
  • the permissible soil pressure in order to limit any subsidence to a harmless level
  • the load distribution, in order to avoid different settlement of different parts of the building on possibly inhomogeneous subsoil
  • avoiding or dealing with the uplifts and subsidence associated with a freeze-thaw cycle
  • if necessary, securing cavities (cellars) located below the surface of the earth against buoyancy from groundwater, seepage or floodwater
  • the durability of the building materials used under changing humidity conditions

If the foundation area is below the surface of the earth, a construction pit is dug. From a certain depth, space must be provided for the work area next to the foundation area. An excavation pit that has not been built up ends with an embankment all around , while a built-in excavation pit is secured against earth breakage by a sheet pile wall , shotcrete or similar measures . The work area will be backfilled after the foundation and the basement have been completed.

The depth of the foundation is to be chosen so that no movements of the subsoil due to frost effects or drying out (shrinkage cracks) of the upper soil layers are to be expected. The depth of frost to be considered varies with the local climate and is often assumed to be 80 cm in Germany.

The foundation can be designed as a continuous base plate, strip foundation or as a point foundation . If the subsoil is soft, it may be necessary to have a deep foundation using piles driven in or made of reinforced concrete . The load distribution can alternatively be improved by stone packings, wooden sleepers, beams or, if necessary, by replacing the floor .

In parlance, the term base encompasses both the underground parts of the foundation as well as the technical - or only optical - parts of the base of the structure . In bridge construction one speaks of substructure . The term foundation was originally Germanized by Philipp von Zesen with the expression " foundation stone ".

Founding relationships

The foundation conditions are determined before the construction of a structure. In the case of larger construction projects, a soil report is drawn up which, in particular, evaluates the settlement behavior and the load-bearing capacity of the subsoil. The exploration is usually done through core drilling , pile driving tests and prospecting .

Types of foundation

Historic foundings

Historical foundations differ from modern foundations in terms of the material used, the construction method and the utilization. All foundations before 1920 are considered historical. As a rule, they have low tensile strength and flexural strength.

Wooden foundations

Foundation with a wooden pile grid for strong walls (from the Handbuch der Holzustersauten, Böhm, Springer 1911)

Foundation structures made of wood can consist of sleepers (horizontal) and piles (vertical). From Roman antiquity to the early 20th century, sleeper grate foundations were common on soft ground, with several sleepers placed next to each other along the wall and connected by shorter cross sleepers. Only at the corners of the building were the thresholds laid crosswise on top of one another. The spaces in between were filled with gravel , building rubble or clay , and first unmoled masonry was placed above and above mortar masonry.

Stone foundations

Foundation and wall made of large field stones from a sheep pen in the museum village of Cloppenburg

A distinction is made between strip foundations ("banquets"), rectangular point foundations (such as under pillars) and surface foundations, which had to be designed as inverted vaults until the introduction of reinforced concrete. Stone foundations were widened towards the base by stepping the masonry in order to improve the load distribution.

Stone foundations mostly consisted of a packing layer in the lower area , in which layers of larger stones were filled with gravel in the spaces between them. Dense rock was preferably used for the packing layer in order to prevent the capillary rise in soil moisture. In addition, barriers made of slate or (from around 1800) made of tar paper could be provided above .

Today's foundations: shallow and deep foundations

Under a flat foundation is in the construction a form of establishing understood, in which the structural loads on horizontal surfaces in the base to be initiated. It also helps to distinguish the following types: individual foundations, strip foundations , floor plates and pans (as a variant of the base plate). The main building materials used are concrete , fiber- reinforced concrete and reinforced concrete .

Deep foundations , on the other hand, consist of vertical components that transfer the structural loads to the subsurface largely through friction : piles , wells and caissons , diaphragm walls , milled walls , high-pressure floor reinforcement . Wood , concrete, steel , fiber concrete, reinforced concrete and mortar are used as building materials .

Type and execution of foundations

Single foundations for a warehouse made of precast concrete parts
Workers pour concrete into a floor slab
  • Point or single foundations are usually built for individual supports or isolated components such as chimneys and the like.
  • Strip foundations are most common in Germany. They take on the loads of the load-bearing walls erected on them, while non-load-bearing interior walls are usually erected directly on the floor slab. Their width is often twice the walls standing on them; the exact dimensions and any reinforcement result from the load-bearing capacity of the subsoil. The foundations are often made in concrete strength class C20 / 25 or C25 / 30.
  • Raft foundations or sole plates are used when single or strip foundations are not economical because of high building loads. It can be more economical to use a base plate even with low loads, as the workload may be less. The entire floor slab is then used as a foundation slab . A foundation slab is always reinforced on the top and bottom. It often protrudes laterally over the outer edge of the (basement) walls. Before concreting the foundation slab, a thin blinding layer of lean concrete and / or a solid PE film is placed on the bottom of the construction pit so that the reinforcement cannot move during concreting and the concrete does not mix with the subsoil. Lately, foundation slabs made of steel fiber concrete have been increasingly manufactured , especially in residential building . The advantage is the much simpler production, plus the price advantage compared to conventional reinforced concrete slabs.
  • Basement tubs are required when there is pressing water, i. H. in the event of a possible rise in the water level above the foundation base, e.g. B. in the vicinity of bodies of water or at very high groundwater levels or strata water . Depending on the design, a distinction is made between white tubs and black tubs . White tubs are made of waterproof concrete (waterproof concrete ). In the case of black tubs, a conventional foundation is sealed from the outside using bitumen or bitumen sheeting. In the case of basement tubs, the rising building must have sufficient weight in all construction stages so that the tub does not float, or provision must be made for flooding in good time. Flooding can only be viewed as a measure in an emergency. Normally, the groundwater is lowered to such an extent that there is sufficient buoyancy security in every construction stage. A prominent example of structural damage caused by disregarding this rule is the Schürmann building in Bonn .
  • Physical foundations : With this type of foundation, the load is distributed through pressure.
  • Rigid foundations : With this type of foundation, the load is distributed through pressure and tension inside the foundation.

Load-bearing behavior of foundation slabs

Above, the moment curve is shown in a reinforced concrete floor slab. From the middle of the field to the support, the torque initially drops to zero, and then increases sharply again with the opposite sign. The structure of the foundation is shown schematically in the middle. Below this is the (greatly simplified) course of the soil pressure that is established in the form of an upwardly directed surface load. At the very bottom there is the graph of the course of the transverse force (or the shear stress ). Since both the moment and the shear force are highest in the foundation below the columns, the foundation is made stronger there.

Floor slabs that transfer structural loads are usually made of reinforced concrete . Without the use of reinforcing steel, individual foundations have a load spread angle of around 45 degrees. In order to introduce high loads into the subsoil, correspondingly large foundation heights are necessary in order not to exceed the limit stress of the soil. In contrast to unreinforced foundations, reinforced floor slabs can be subjected to bending loads. This means that loads with relatively thin component thicknesses can be distributed over large areas and thus the stresses on the ground can be reduced. The floor slab acts like an inverted ceiling, and the static reinforcement is arranged in the field on the upper side and below the load application points on the lower side. Modern industrial buildings have high individual loads because of their supporting structure, which consists mainly of columns. The reinforcement under the columns in the floor slab is therefore often very tight, and reinforcement steel is often used in several layers with diameters of up to 30 mm. If the shear and punching forces in the vicinity of the support exceed the plate resistance, either appropriate reinforcement, usually vertical stirrups, must be arranged or the plate thickness must be increased locally. For economic reasons, the latter method is predominantly chosen.

The use of steel fiber concrete , which can also absorb the loads from walls and columns, significantly simplifies the overall system . Watertight connections can also be made with this type of construction, provided that there are no impermissibly wide cracks.

more details

The frost case , a capillary- breaking layer , is usually installed under the base plate . Moisture rising from below cannot penetrate to the base plate. Here, however, the vapor pressure must be taken into account, which is present as a load case through these layers up to the lower edge of the floor slab and thus also dampens a floor slab. This load case becomes a problem when basement rooms are used for residential purposes. Appropriate planning requirements for the quality of the floor slab and its other floor structures must be observed.

Foundation earth electrodes are also installed in the base plate , which serve as potential equalization for the entire electrical installation .

Thermal floor panels

Permanently pressure-resistant insulation panels or a layer of foam glass can be laid under the foundation plate. These insulation panels must be highly pressure-resistant and permanently withstand the load case of pressing and non-pressing groundwater for the entire service life of the building. In order to exclude thermal bridges in uninsulated foundations, this layer must be extended to the outer edges of the floor slabs. This inevitably leads to a separation between the foundations and the frost curtain as well as different settlement behavior within the foundation level. This problem is avoided in the case of glass gravel insulation, since after installation the glass gravel layer of this area is compressed by using vibrating plates. There is no need for a frost apron through foundations because the glass gravel layer protrudes by approx. 60 cm at the side. There are no thermal bridges if the front edge of the sole is insulated down to the glass gravel layer. Uninsulated foundations, on which the floor slab, which is only insulated between the foundations, rests, represent an avoidable thermal bridge which, due to its overall length, takes up a considerable amount. Even if these can mathematically lead to compliance with the EnEV due to the possibility of a balanced consideration of the thermal insulation certificate, they represent a violation of the principle of avoiding thermal bridges. In any case, the dew point must be calculated so that the surface temperature does not lead to condensation within the The base of the outer walls leads to the base plate.

Currently (2011), 20–40 cm thick reinforced concrete slabs are planned for load transfer. The structural engineer must calculate the exact thickness in collaboration with a geologist. If a foundation slab is made, you can do without a screed in the basement, but then you have no option of installing a vapor barrier as an upper seal. Such solutions can therefore not be regarded as suitable for living spaces, especially if further floor structures such as parquet floors would be applied directly to the concrete floor.

Screeds need around six weeks to dry. Cement screed and reinforced concrete slabs achieve the intended strength after 28 days. Underfloor heating can also be implemented as concrete core temperature control if the option of quickly adjusting the temperature is not necessary. Additional expansion joints may be necessary for a crack-free design. If the floor slab is used to control the temperature of the building, the thermal insulation must be carried out with particular care.

The vertical outer surfaces of floor slabs or those of cellars ("cellar outer walls") are now insulated with perimeter insulation in accordance with the Energy Saving Ordinance.

See also