3. Definition of forces applied to foundations

The ductility principle is based on the fact that, during a given earthquake, the displacements obtained when the structure undergoes plastic deformation are of the same order as those obtained from an elastic analysis (isodisplacement hypothesis). It is then possible to opt for plastic (non-linear) behavior by dividing the forces obtained from the elastic analysis by a coefficient q, known as the "behavior" coefficient, the value of which depends on the element's capacity to deform in the plastic domain. When q is between 1 and 1.5, the structure is designed to remain in the limited post-elastic domain.

In common cases, the load descent is determined by considering supports locked in translation in all three directions, using q = 1.5. In this case, the design engineer is asked to supply the force torsorsors at the head of the foundation in all directions, calculated...