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Subsoil model

The subsoil model is a part of members with type "Member on subsoil". It's a spring support along the whole member length. Subsoil acts in the local coordinate system of member and can be described as a row of springs with given stiffness that support the member. The subsoil usually acts in the direction of local axis 3 (direction of gravity), however, also subsoil in the direction of local axis 2 may be defined (Fin 3S).

Subsoil is considered as springs that act both in compression and tension. As this model usually does not correspond to the real conditions, the appearance of tensile stress should be checked.

The spring subsoil substitutes outer supports of the structure and contact stress is the substitution of joint reactions. Outer supports have to secure only movements perpendicular to the subsoil direction.

This model of subsoil provides accurate results for symmetric cross-sections without any rotation (concrete rectangle, steel RHS etc.). Rotated or unsymmetrical cross-section don't meet all assumptions of the model, results should be considered as approximate in these cases. However, such results usually describe the behaviour of the structure in a sufficient way.

Subsoil characteristics

The subsoil stiffness is characterized by two constants C₁ and C₂. The constant C₁ it the typical spring stiffness in the direction of the corresponding axis, the constant C₂ acts like the shear connection between particular springs C₁. The model contains also shear effects at the beginning or end of the member. It means, that the subsoil exists also in front of the member beginning and behind the member end and affects the structure. Consideration of these effects can be switched on by the user. It should be used only in cases, where the subsoil isn't affected by another member or structure (neither in the member direction, nor in the perpendicular direction).

Next parameter is the width of contact between structure and member. As a default, contact width is considered as the corresponding dimension of the cross-section, however, arbitrary value may be specified.

The fundamental parameters of the subsoil are constants C₁ and C₂, however, they aren't known very often. The values can be calculated by the software. This calculation is based on subsoil parameters E_def (the deformation modulus), ν (The Poisson's ratio) and h_d (the depth of deformation zone).

The deformation modulus E_def is the value usually obtained from in situ measurements. Sometimes, only the oedometric modulus of deformation E_oed is available. Following expression describes the relation between these two values:

The depth of deformation zone describes the subsoil depth, that is deformed by the beam. This value is given as a ratio of the deformation zone depth h_d and beam width b. For example, ratio 3 means the deformation zone depth h_d=3b . Recommended values are between 1.5 and 5.0.