When the stem is pinned to the footing:

1. Consider the lateral load that applies to the stem-only.

2. Determine the top reaction by summing moments about the stem-footing joint.

3. Now consider the total lateral load applied to the stem and footing.

4. Determine the slab reaction by sum of forces in the horizontal direction.

When the stem is fixed to the footing:

1. Consider the lateral load that applies to the stem-only.

2. Determine the shear at the bottom of the stem by integrating the load over the full height of the stem and applying formulas for a propped cantilever.

3. Determine the top reaction by sum of forces in the horizontal direction.

4. Determine the slab reaction by summing shear at bottom of stem with active pressure on heel and on key.