KSI is an abbreviation for kilopounds per square inch, or 1,000 pounds per square inch. It is a measure of the tensile strength of steel.
Tensile strength is the amount of stress a material can undergo before breaking. It refers specifically to stress induced by pulling or stretching; compression stress is measured with a different value. Measuring tensile strength is a vital part of materials science, particularly when steel is to be used for projects that impact the safety of numerous people. Sufficient tensile strength is vital in architecture because the structural integrity of buildings with steel frames depends upon it.