Yield Point
The stress at which steel stops springing back and starts to stretch for good.
Quick Answer
Yield point is the stress at which a material stops behaving elastically and begins to deform permanently. Structural steel is specified by its minimum yield strength, expressed in ksi, which means kips per square inch, where one ksi equals 1,000 psi. Designers use that value to size members and set allowable stresses.
The Full Picture
When you load steel, it stretches in proportion to the force and springs back when the load is removed. That elastic range ends at the yield point. Past it, the steel deforms permanently, and the member may not return to its original shape. Engineers design so that ordinary service loads keep members below yield. Some mild steels show a clear upper yield point followed by a lower one on the stress-strain curve, and design commonly relies on the lower value as a conservative basis.
For steels without a clear yield point, such as many high-strength grades, the yield strength is defined with an offset method, often at 0.2 percent permanent strain. The result is reported as yield strength. In everyday construction language, yield point and yield strength are used almost interchangeably: the number on the drawings is the minimum specified yield strength of the grade. Yield strength differs from tensile strength, which is the higher stress the steel can reach before it fractures.
The unit is the ksi, short for kips per square inch. A kip is 1,000 pounds, so 1 ksi equals 1,000 psi. Grades are often named by yield strength: Grade 60 reinforcing bar has a specified minimum yield of 60 ksi, and A36 steel has 36 ksi. Wide-flange shapes under ASTM A992 are commonly specified at 50 ksi. In metric terms, 1 ksi is about 6.895 MPa, so 50 ksi is about 345 MPa. Confirm the grade and its yield on the structural general notes rather than assuming it.
For contractors, yield strength is a specification and substitution issue more than a quantity item. The grade on the drawings sets what the supplier must furnish, and a substitute with a lower yield, or a different grade of rebar or plate, needs the engineer's approval. Mill test reports document the actual yield of the delivered steel. Price and availability can differ by grade, so an estimator checks the notes and schedules to see which grade applies to each member.
Real Examples
Common Misconceptions
People assume: Yield point is where the steel breaks.
Actually: Yield is where permanent deformation begins. Fracture happens at a higher stress and strain, after the steel has stretched, which is why steel gives warning before failure.
People assume: KSI is a type of steel.
Actually: KSI is a unit of stress, kips per square inch. Grades such as 50 ksi describe the minimum yield strength, and the same unit is also used for concrete strength and other stresses.
People assume: A higher yield grade can replace a lower one with no review.
Actually: Higher strength can change ductility, welding requirements and connection behavior, and the design may have assumed specific properties. Substitutions need the engineer's approval.
Frequently Asked Questions
What does ksi mean in construction?
Ksi means kips per square inch, a unit of stress. One kip is 1,000 pounds, so 1 ksi equals 1,000 psi. It is used for the yield and tensile strength of steel and for concrete strength in some contexts.
What is the difference between yield strength and tensile strength?
Yield strength is the stress at which permanent deformation begins. Tensile strength, also called ultimate strength, is the highest stress the material reaches before it fractures. Tensile strength is higher than yield for structural steel.
What is the yield strength of common structural steel?
A36 is specified with a minimum yield of 36 ksi, and A992 wide-flange shapes are commonly specified at 50 ksi. Rebar Grade 60 has a minimum yield of 60 ksi. Always confirm the grade on the project documents.
Where is the yield point shown in the drawings?
Typically in the structural general notes and material specifications, listed as the ASTM designation and grade for each product, with the minimum yield. Schedules and details may repeat it for plates and connection material.