This document discusses the concept of kern in pre-stressed concrete design. The kern is the region where compressive loads can be applied without causing tensile stresses. It is widely used in designing pre-stressed beams, footings, and dams. The document shows how the location of a compressive force (C) relative to the kern affects the stress distribution - forces within the kern cause only compression, while those outside can cause tension as well. Examples are given for beam and footing sections, explaining how kern limits the maximum compressive load before tension occurs.