Steel Specifications


The term ‘steel specification’ is very closely related to ‘steel standards’ and the terms are often used interchangeably. The standards specify strict parameters or boundaries on the finished steel’s chemical composition and/or mechanical properties. (JIS, ASTM, AISI, etc.) Some of the typical mechanical properties and tolerances that may be specified are minimum tensile strength, minimum yield strength, minimum ductility, and toughness.
Other material properties may not appear in a specification, yet are critical in building design; the most important such property is perhaps the elastic modulus or stiffness. The reason that this does not appear in specifications because there is little variability amongst the various steels. The following are the general characteristics of the steel standards recommended as the base specifications for galvanized slit coil used in FRAMECAD machines:


The following are the key property of Structural Steel and their definitions, also each is discussed with relevance to how each characteristic affects how steel performs in structural applications.



The elasticity of the steel is shown by the elongation which takes place in direct proportion to the load. Elasticity is important in structural steel because it indicates the level amount the steel can deform ‘elastically’ and then return to its original shape once the force or stress is removed.
Using steel with high elasticity values allow the completed steel structure to “move and flex without permanent structural damage”. Materials are considered to flex ‘elastically’ until they reach their Yield Point.


Yield Strength (also Yield Point) 

In practice, this is the material property of greatest importance for characterizing a particular steel grade, commonly referred to as Yield Strength (Fy). Yield strength is measured in ksi or MPa. The yield strength of a material is the material's‘ elastic limit’, and is defined in engineering and materials science as “the level of stress at which a material begins to deform plastically”. Beyond this ‘elastic limit’, some fraction of the deformation will be permanent and non-reversible, a structural component with permanent deformation is considered ‘damaged’ and may be deemed ‘unsafe’ or ‘unsound’. 

“For structural applications, plastic deformation is unacceptable because it means that the structure has been damaged permanently - Yield Strength is therefore used as the design limitation of steel.”


Tensile Strength (or Ultimate Strength)

After the Yield Point, ductile materials will undergo a period of ‘strain hardening’, in which the stress point (or tensile strength) increases again with the increasing strain until at a certain point the steel will typically begin to ‘neck’. Necking is when the materials cross-sections start to contract rapidly. The Tensile Strength is the maximum stress a material can withstand while being stretched or deformed before ‘necking’ takes place. When necking is substantial it causes the stress-strain curve to curve downwards - this is because the curve measures the ‘Engineering Stress’ which is calculated using the original cross-section area and does not take into account the ‘necking’ or ‘narrowing’ of the sample cross-section. The ‘True Stress’ curve shows the increasing ‘hardness’ of the steel taking into account the proportionate change in the cross-section during necking.



Ultimate Failure


The Ultimate Failure describes the breaking point of the material. In Structural Steel this is typical ‘fracture’. The fracture of material occurs when either an internal or external crack elongates the width or length of the material, ultimately resulting in a complete break of the material.