When entering the footing size on the footing definition tab for a spread footing RC Pier has a box you can check that reads "compute forces at bottom of footing". When this box is checked the bearing pressures and shear forces are higher than if this box is left unchecked. What is the purpose of this box and why is it changing the design?
If the option “Compute forces at bottom of footing" under Footing Definition is unchecked then the P , Mxx and Mzz values are calculated at the Top of the Footing.
If the option “Compute forces at bottom of footing" under Footing Definition is Checked then the P , Mxx and Mzz values are calculated at the Bottom of the Footing.
Then the final Mxx and Mzz values are calculated as follows.
At the bottom of footing Fz and Fx values are generated.
Then the Mxx Total = Mxx + ( Fz x Depth of footing).
Mzz Total = Mzz + ( Fx x Depth of footing).
Please see the attached jpeg files for more information.