The calculation is based on the evaluation of the rigid body equilibrium (EQU) of the foundation, considered infinitely rigid (both flexurally and extensionally), with respect to rotation around the sides (edges) of the footprint polygon (base) of the polygonal foundation on the lean concrete.
To achieve equilibrium against the overturning moment around each side, the sides are considered as a fixed linear hinge. All moments (overturning and stabilizing) are projected onto the vertical plane orthogonal to the individual base sides, excluding any soil reaction force (which, in a rigid rotation limit state, is indeed free from contact with the foundation base).
Consequently, the software checks if, for each side of the polygon, the ratio between the stabilizing moment and the overturning moment Mstab/Mrib is greater than the partial safety factor ϒR = 1.15$ (New NTC 2018/Eurocode). Note: This completes the last column of Table 6.5.I of NTC 2008, which did not contemplate overturning verification, allowing ϒR = 1.0$ until the new regulations were issued.
GEOMETRY AND FOUNDATION WEIGHT
The foundation contact polygon can be assigned as Rectangular, Polygonal (up to 24 sides) inscribed in a circle of any radius, or Generic Polygonal (up to 24 sides). In the case of a circular footing, assigning a 24-sided polygon provides a sufficient approximation.
In GeoStru Foundation Overturning Verification, the foundation height refers to the vertical distance between the upper extrados—where concentrated loads (normal force, moments, and shears) transmitted by the superstructure (columns, piles, wind towers, etc.) are applied—and the horizontal support plane of the foundation footprint on the lean concrete.
The foundation can have a simple parallelepiped or stepped shape, provided the steps share the same plan centroid as the footprint base. The total weight of the foundation (including any permanent surcharges) must be calculated separately by the user and is always applied by the program at the centroid of the base polygon.
The foundation weight is then reduced (automatically during calculation) by the partial coefficient ϒF = 0.9 provided in Table 2.6.I of NTC 2008.
LOADS ON THE UPPER FACE OF THE FOUNDATION
The rigid body hypothesis allows all superstructure actions (un-amplified) to be consolidated into just the stress components N, Mx, My, Vx, Vy (positive moments are counter-clockwise with respect to the reference axes x, y). These components are applied at a single point, freely assigned by the user but generally chosen to coincide with the centroid of the connection section between the superstructure and the upper face of the foundation.
In the software, users must assign these loads and the coordinates of the application point. This point can be defined via coordinates or chosen to coincide with the centroid of the base polygon (calculated automatically).
For each side of the base polygon, the program automatically applies the coefficient ϒF = 0,9 to the normal force N (consisting only of permanent loads; positive if directed downwards) and the coefficient ϒF = 1,5 to the remaining variable actions, projecting the moment and shear components onto the plane perpendicular to the side itself.
In the case of wind towers or piles, the manufacturer typically provides only the resultant (always positive) on the foundation’s upper face for moments and loads, not amplified by partial coefficients. In this case, the checkbox “Moments and stresses from all directions” must be selected. The calculation will ensure that for every side, the projection of moments and shears always corresponds directly and simultaneously (conservatively) to the assigned values of Mx and Vy.
RESULTS
The software provides the overturning moment and the stabilizing moment relative to the foundation base side that produces the lowest safety factor against overturning among all sides. The verification is positive if this coefficient is not lower than 1.15 (according to Eurocode/NTC).