Bolt foundation

Point foundation is a type of foundation in which individual, independent foundation elements (foundations) are concentrated beneath columns, piers, or point loads in the ground. Unlike strip foundations (linear under walls) or foundation slabs (area-wide), the point foundation concentrates the load at a defined point and transfers it to the load-bearing subsoil. It is a cost-effective solution when only individual loads need to be supported and the building site has sufficient bearing capacity.

Applications and Types

Point foundations are primarily used where steel or reinforced concrete frame structures with individual columns are erected-typically for industrial buildings, office buildings, carports, and patio roofs. In residential construction, they are frequently used for terrace pillars, freestanding wooden posts, and garden sheds. Depending on the size of the structure and the soil class, point foundations are constructed as reinforced concrete blocks or as simple concrete structures without reinforcement (compacted concrete, for light loads). The dimensions are determined by the load to be supported and the soil parameters from the soil investigation report.

For particularly heavy point loads or unfavorable soil conditions, point foundations are constructed as sleeve foundations (with a concrete sleeve for the column passage) or as pile cap slabs over bored piles. These special forms are frequently found in industrial and commercial construction, less commonly in private residential construction. For temporary or light applications-such as garden pavilions or tool sheds-there are prefabricated steel screw foundations that are screwed into the ground without excavation or concrete and also follow the principle of the point foundation.

Structural and Soil Mechanics Requirements

The design of a point foundation is carried out in accordance with the Eurocodes (DIN EN 1997 - Geotechnics, DIN EN 1992 - Concrete Structures). The decisive factors are the allowable soil pressure (kN/m²), which is derived from the soil investigation report, and the load transfer from the structural framework. In the case of soft or settlement-prone soils-such as peaty or fill-rich layers-isolated footings are insufficient; in such cases, pile foundations or foundation slabs are used. In frost-prone locations, the foundation must extend at least to the frost-free depth (80 cm in Bavaria, often 100 cm in practice).

When designing multiple point foundations for a structural system-such as the four corner supports of a patio roof-the structural engineer must not only design each individual foundation but also check the settlement differences between the foundations. If adjacent foundations settle to different degrees, constraint forces arise in the structural system that can lead to cracks or deformations. A uniform design of all point foundations of a structure for comparable soil pressures minimizes this risk.

Point Foundations in Older Buildings and Renovation Projects

When renovating older buildings, one frequently encounters inadequately dimensioned or weathered point foundations beneath old wooden supports, chimneys, or extensions. Cracks in adjacent structural elements may indicate settlement of such a foundation. A geotechnical investigation is recommended prior to renovation to assess the load-bearing capacity of the existing foundation. If necessary, the foundation can be underpinning (underpinning) or supplemented with new pile foundations.

Typical signs of damage associated with inadequate point foundations include settlement cracks in walls radiating out from the foundation, leaning columns, or sagging balcony structures. These issues can be remedied in the early stages with relatively little effort; however, if ignored for a long time, settlement can progressively worsen and necessitate a complete replacement of the foundation.

A particular problem with historic point foundations made of natural stone or brick masonry is their lack of tensile strength: these materials absorb compressive forces well but fail under shear and tensile forces-for example, when wind or earthquakes introduce horizontal forces into the supports. When renovating such foundations, reinforcement with a concrete jacket or steel connections is often necessary.

Distinction from Other Foundation Types

The point foundation differs from related foundation types in its specific application. The strip foundation runs in a linear fashion beneath walls and distributes the load along a line. The foundation slab (floor slab) distributes the entire building load over a wide area and is suitable for weak soils, as it minimizes ground pressure. Pile foundations bridge poor soil layers and transfer loads to stable depths. The isolated foundation is the most economical solution when only a few concentrated individual loads need to be supported and the soil has sufficient bearing capacity-so it always depends on the specific construction situation.

Practical Tip for Homeowners in Nuremberg and Franconia

The subsoil in the Nuremberg metropolitan region is geologically diverse: sandstone subsoil in urban areas usually offers good bearing capacity, while in valley locations (e.g., along the Pegnitz or Regnitz rivers), softer layers may occur that require particularly careful foundation design. In the districts of Erlangen-Höchstadt and Fürth, there are also areas with loess soil, which loses load-bearing capacity when saturated with water and places special demands on foundation design.

Anyone planning a carport, a patio, or a garden extension should consult a structural engineer regarding the required foundation depth and size before undertaking any work themselves. We recommend obtaining a soil investigation report for all structural changes involving supports and foundations-even for seemingly simple projects like a freestanding carport. The cost, typically ranging from 500 to 1,500 euros, is money well spent and helps avoid costly repairs later on.

Frequently Asked Questions

Do I need a building permit for a point foundation?

That depends on the size and intended use of the planned structure. A small garden shed on point foundations below the threshold for permit exemption (75 m³ of enclosed space in Bavaria) often does not require a permit. As soon as the building is larger or there is a change in use, a building permit is required, which must also include details regarding the foundation.

How deep does a point foundation need to be?

In Bavaria, the bottom edge of the foundation must be at least 80 cm deep to prevent frost damage. Structural requirements may necessitate a greater depth if the load-bearing soil layers lie deeper. In any case, the soil beneath the foundation must be load-bearing in its natural state-any fill material must be excavated.

Can I build a point foundation myself?

For very small, low-load applications (e.g., a carport with few supports), building it yourself is generally possible. However, it is recommended that you have at least the dimensions and reinforcement approved by a structural engineer to avoid liability risks.

How much does a point foundation cost?

The cost of a reinforced concrete point foundation in residential construction ranges from 300 to 1,500 euros per unit, depending on size, depth, and local labor costs, including excavation, formwork, reinforcement, and concrete. Prefabricated steel screw foundations are cheaper and quicker to install, but are only suitable for lighter loads.

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