Strip footings are commonly used as foundations of load-bearing walls. The footing usually has twice the width as the load bearing wall, sometimes it is even wider. The width as well as the type of reinforcement are depending on the bearing capacity of the foundation soil.
The basic program for design and analysis strip footings under load-bearing wall is GEO5 Spread Footing program.
For analysis of strip footing under columns is better to use a GEO5 Beam program.
Engineering Manual - Settlement of a spread footing
Online Help - Program Spread Footing
Online Help - Program Beam
Video Tutorial - Introduction to Spread Footing
Video Tutorial - Introduction to Beam