Selecting Seismic Dispositions from the Calculation Options dialog for footings (for the defined load of a seismic nature) includes the requirements placed on foundations due to the seismic effects.
- Steel classes and concrete load capacity according to PS 92/11.21 and 11/22
- Additional verification of foundation stability for sliding and lifting [ref. V. Davidovici, 'Formulaire du beton arme' vol. 2].
To set load combinations of a seismic nature, a rule defined in art. A.3.3,22 of the BAEL 91 code is used:
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Foundation Sliding Condition
See Rules of foundation calculations: sliding.
Foundation Lifting Condition
This is a condition resulting from the properties of the seismic load whose action has alternating sign characteristics. In specific situations, it may lead to lifting the foundation from the ground.
The only successful method of defense against foundation lifting is increasing the dimensions of the foundation itself. This causes an increase of its weight and the weight of the soil on top of it. Additionally, it is possible to include the cooperation of the soil using its cohesion or the internal friction angle.
When entering the following signage and keeping in mind the above assumptions, you obtain:
- For non-cohesive soil.
- for cohesive soil:
where:
W - Lifting force
G - Footing weight + weight of soil on top
Po - Total horizontal soil pressure, passive, acting on the vertical, side surface
tg(φ)- Friction coefficient
C - Cohesion (1 - 3 T/m 2)
A - Vertical surface above the footing outline.