This is a foundation design question and that is not my area of expertise. The basic concept is to spread the compression load to the soil under the grillage by way of a bearing pressure. The uplift load is taken by the fill above the grillage.

The grillage foundation is for lightly loaded leg angles like those for tangent towers with small wires and short spans. I would look at foundation design books, but as far as I know, they have not been used for quite a while.

_____________________________________
I have been called "A storehouse of worthless information" many times.

Thank you for your answer transmissiontowers,

Where I work, they have design one steel grillage foundition in the last 20 years (but I couldn't find calculations), and in the other company I have worked more or less the same. This kind of grillage was designed and manufactured for Cuba and for Costa Rica.

I know is quite unusual, but I want to be ready if we need to design in the future. I have clear the idea of who calculate this foundation but I would find an example solved.

To have the area of the grillage in contact with soil should be Sigma admissible of soil = F/Area angles. But for uplift load is the same than a concrete pier? Is the same idea than in concrete pier?