Structural Steel | Types and Grades

Structural steel is a popular construction material widespread in its applications as small as road rails to massive applications like high-rise building construction. It is considered an inevitable building material due to its fabrication versatility and structural strength. 

This article gives you a deeper insight into the features, properties, and types or grades of structural steel used worldwide in construction. 

What is Structural Steel?

Structural steel is a type of steel that is optimized for use in building construction. They are differentiated from other steel grades that are used in the manufacture of engineering tools, appliances, etc. 

Structural steel is used for the construction of structural members like footing, columns, beams, trusses, etc. Structural steel is rolled into a variety of shapes and sizes in the rolling mill.

Structural steel is a category of steel with a carbon content of not more than 2.1% and not less than 0.6 %. Structural steel is generally specified based on the appropriate industry standards like ASTM, BSI, ISO, etc. In most cases, the standards provide basic requirements like chemical composition and tensile properties of structural steels.

Chemical Composition of Structural Steel

The chemical elements present in structural steel contribute to the major properties and types of structural steel. Some of the important chemical elements in structural steel are mentioned in the table-1 below.

The chemical composition of certain elements for certain ASTM structural steel grades is mentioned for better clarity. 

Table-1: Chemical Elements in Structural Steel[Ref: Design of Steel Structures MIT Department of Civil and Environmental Engineering Spring Semester, 1999]

ELEMENT	PERCENTAGE COMPOSITION	PROPERTIES
Carbon( C )	0.15 To 0.30 %	Higher Strength & lower ductility
Manganese (Mn)	0.50 to 1.70 %	Necessary for the process of hot rolling of steel when combined with oxygen and sulfur.
Aluminum (Al)		Act as deoxidizers & provide more fine-grained crystalline microstructure.
Chromium (Cr)	Smaller amounts	Corrosion resistance occurs in combination with nickel and copper.
Columbium (Cb)	ASTM A572 Grade Type 1 and 3	Strength-enhancing elements and some corrosion resistance influence. Cb is an important element in HSLA-type structural steel
Copper (Cu)	Not less than 0.2 percent	Primary corrosion-resistant element. The primary component is grades A242 and A441.
Molybdenum (Mo)	0.08 to 0.25% for grades of A588 steels 0.15 to 0.65 % for grades of A514	Increase strength at high temperatures and improve corrosion resistance.
Nickel (Ni)	0.30 – 1.50 % in ASTM A514 grades 0.25 to 1.25 % in ASTM A588 grades	Improve low-temperature behavior of the material by improving fracture toughness
Phosphorus (P) & Sulfur (S)	Undesirable in structural steel Less than 0.04 to 0.05 percent.	Increase internal segregation in the steel matrix. Decrease ductility and detrimental to ductility
Silicon (Si)	Less than 0.40 percent	Principal deoxidizers for structural steel. Used for semi and fully-killed steels.
Vanadium (V)	0.02 to 0.15 % for ASTM A572 & A588 0.03 to 0.08 % for ASTM A514	Develop a finer crystalline microstructure and increase fracture toughness.
Other Elements	Alloys like boron, nitrogen, and titanium	Enhance the specific material performance

Primary Types of Structural Steel

The primary classification of structural steels is done based on the chemical composition explained in the table. They are:

1. Carbon Steel or Mild Structural Steel
2. High-Strength Low Alloy (HSLA) Steel
3. High-Strength Quenched and Tempered Alloy Steels (Q &T)

1. Carbon Steel or Mild Structural Steel

These are structural steels with a primary composition of carbon and manganese in addition to iron. Carbon steels are usually the least expensive with average adequate strength and strength characteristics, hence the most widely used grades. ASTM grade A36 is a prominent carbon steel used that has a minimum yield stress of 36 ksi.

2. High-Strength-Low Alloy Steels (HSLA)

HSLA steels are structural steel with higher strength ranging between 42 to 65 ksi. They use small amounts of additional chemical elements to impart higher strength and properties. The two common HSLA steel are ASTM grade A572 and A588.

3. High-Strength Quenched and Tempered Alloy  (Q &T) Steels

Q & T steels are steels with a higher strength level between 90 and 100 ksi, which is achieved through heat treatment. The only ASTM grade Q&T steel available is A514 in plate form up to 6 inches.

Grades of Structural Steel

The grades of structural steel vary with the country standards followed. Here, we will discuss the different grades followed in Europe, America, and India.

1. Grades of Structural Steels as per European Standard

Most steels used throughout Europe must comply with the European standard EN 10025, which is governed by the European Committee for Iron and Steel Standardization (ECISS), a subset of the European Committee for Standardization (CEN). The grades are expressed starting with S representing structural steel.

Among the numerous European grades of Steel, the most commonly used are S195, S235, S275, S355, S40, S460, etc. Among these S235, S275, and S355 are used in major in Europe.

2. Grades of Structural Steel as per American Standard ASTM International

The structural steel used for building construction in the US is based on the specifications provided by ASTM International. The grades are represented by A which identifies alloys. The common ASTM standard used for structural steel are:

Carbon Steel	High strength low alloy steels	Corrosion resistant high strength low alloy steel	Quenched and tempered alloy steels	Forged Steel
A36 – structural shapes and plate. A53  – structural pipe and tubing. A500 – structural pipe and tubing. A501 – structural pipe and tubing. A529 – structural shapes and plate. A1085 – structural pipe and tubing.	A441 – structural shapes and plates (Superseded by A572) A572 – structural shapes and plates. A618 – structural pipe and tubing. A992 – Possible applications are W or S I-Beams. A913 – Quenched and Self Tempered (QST) W shapes. A270 – structural shapes and plates.	A243 – structural shapes and plates. A588 – structural shapes and plates.	A514 – structural shapes and plates. A517 – boilers and pressure vessels. Eglin steel – Inexpensive aerospace and weaponry items.	A668 – Steel Forgings

3. Grades of Structural Steel as per Bureau of Indian Standard BIS

The Bureau of Indian Standards (BIS) classifies structural steel into different categories based on the ultimate or yield strength. Structural steels are as per the codes:

1. IS: 226
2. IS: 2062-HOT ROLLED MEDIUM AND HIGH TENSILE STRUCTURAL STEEL — SPECIFICATION
3. IS: 3502- STANDARD FOR STEEL CHEQUERED PLATES
4. IS: 1977
5. IS: 961
6. IS:8500

Advantages of Structural Steel

The main advantages of employing steel as a structural material are:

1. High strength
2. The high density of steel makes the structure water and gas tight
3. Provide long service life
4. Structural steel can be employed in prefabricated members
5. Readily dismantled and replaced

Disadvantages of  Structural Steel

The main disadvantages of steel as a structural material are:

1. Expensive
2. Low fire resistance
3. Susceptible to corrosion

Also Read: Iron in Construction- Pig Iron, Wrought Iron & Cast Iron