Foundation slabs

Foundation slabs transfer structural loads to the soil below. They are made of reinforced concrete and are usually constructed on a lean or blinding concrete layer. Thermal insulation can also be placed under the slab. For this purpose, however, only insulation materials may be used that have been awarded a general building inspectorate approval by the “Institut für Bautechnik”, DIBt.

In the case of groundwater, a construction method of so-called waterproof concrete is required. The adjoining rising walls must also be made of waterproof concrete so that a watertight basin is created. This requires reinforcement to be designed in accordance with the static requirements. In the transition area between the base and the walls and at the corners created, joint tapes or expansion joint tapes are used to seal all construction joints so that they are impermeable to water.

In order to take into account the interaction between foundation slab and the soil and to be able to calculate the pressure on the underlying soil and the resulting settlements, foundation slabs can be analyzed using the modulus of subgrade reaction method and the constrained modulus method.

The computer program GGU-SLAB allows analyses of elastically bedded slabs using both the subgrade reaction method and the constrained modulus method applying the finite element method.

The subgrade reaction method represents a linear-elastic stress-deformation behavior of the soil. The flexurally rigid foundation lying on the ground is bedded with springs representing the yielding soil. The subsoil is replaced by individual springs that are decoupled from each other:

Each spring is loaded or compressed by the stresses acting on them. No shear strength of the soil is assumed, so that the influence of neighboring pressure on the underlying soil layers is not taken into consideration. Thus, the contribution to carry loads of the surrounding soil is not covered by this method.

The subgrade reaction method and its implementation in the computer program GGU-SLAB are discussed in detail in the glossary entry "Calculation of settlements with the subgrade reaction method”, and the use of the program is shown in a video.

The constrained modulus method can be used to correctly represent the interaction between the subsoil and the building. With the calculations according to the constrained modulus method, the base stresses are determined iteratively for which the bending of the flexurally stiff foundation equals the settlement depression of the foundation base.

The settlement at a given location depends not only on the pressure on the underlying soil acting directly there, but also on the stresses at neighboring locations and thus on the total load area.

The settlement depression thus also corresponds to the bending of the foundation, cf. the following figure.

Thus, in contrast to the subgrade reaction method, the shear strength of the soil is taken into consideration, so that the settlement depression also extends beyond the loaded base area and the acting pressures on the underlying soil cause settlements in the elastic half-space on which the calculations are based.

The constrained modulus method and its implementation in the computer program GGU-SLAB are described in detail in the glossary entry "Calculation of settlements with the constrained modulus method”, and the use of the program is shown in a video.