What is steel design?

The area of steel design is majorly concerned with the structural engineering domain, which is a specialization of civil engineering. Steel design is often known as structural steel design, whose primary concern is to design steel structures. The primary structure involves the designing and construction of frames using structural steel as the material. Residential buildings, houses, bridges, complexes, tall buildings, industrial warehouses, aircraft frames, frames of ships, and stadiums are some of the examples of structures that use steel framing.

Importance of steel in design

Steel is much lighter than concrete and has an excellent modulus of elasticity. It offers good resistance to loads and moments. Steel also minimizes foundation preparation and offers a larger floor area. Going for a concrete structure requires reinforcement as concrete is weak in resisting tensile loads and also they require lots of maintenance. For steel, material properties are well known and precise, as indicated in the steel construction manual, they allow accurate analysis and predictions. The steel construction manuals are standard data sheets that contain the mechanical properties, stress values, and design guidelines. An engineer uses a steel construction manual to undertake various steel design and fabrication processes. The design rules regarding steel are also clear and specific, and there are ample resources such as instruments and software to support the design process using steel.

Structural engineering and steel

Structural engineering is a specialized branch of civil engineering. The word, "structure" signifies individual rigid members or bodies that are interconnected with each other to form a single system. This single system has its characteristics and degree of freedom. A structure is a combination of bodies in space that supports loads and resists deformations and moments. Steel structures have specific cross-sectional areas, chemical compositions, and mechanical properties. The amount of load, deformation, and moments it can withstand are determined by performing structural analysis.

Steel members can be readily used as beams and columns, just like concrete systems, but an accurate structural analysis should be done regarding the permissible stresses, deflections, and buckling associated with the steel members before they are used for framing purposes.

Factors to be considered by a structural engineer to carry out steel design:

• Cost and construction speed
• Structural strength, longevity, and durability
• Building height
• Flexibility in future expansion and alterations
• Resistance to critical parameters such as corrosion, fire, earthquake, wind forces, and other environmental influences
• Economy and need of special structural arrangements for steel frame construction like brackets, braces, joists, etc.

Steel design methods

Allowable strength design (ASD)

This method is also known as working stress design (WSD). The primary principle in the ASD considers that the stresses developed in the structural members do not exceed the elastic limit of the members. This method only considers the elastic strength of the material and does not consider the plastic and strain hardening conditions. Moreover, the factor of safety is not implemented in the design, which considers the allowable stress of the members, due to which the analysis produces unsafe results. Important parameters such as serviceability limits like deflection, creep, buckling, cracking, and so on are absent in the process, which means, even if the member can perform safely under certain conditions, its potential is left unutilized.

Load and resistance factor design (LRFD)

The material's ultimate strength is considered in design using LFRD, this covers the material's elastic limit, plastic deformation, and strain hardening characteristics, hence LFRD provides safe and accurate results. The main methodology used in this process is that the strength or resistance parameters of the materials are scaled down to certain values, while the load parameters are scaled up to certain values. The strength reduction factors are considered based on the level of confidence and probability of predictions of the strength of the material. For instance, strength reduction factors for concrete are higher than steel, as, for steel, the strength values can be accurately predicted, and its values are precisely included in the steel construction manuals. Regarding load factors, loads that are much unpredictable and have more risk parameters are assigned higher magnitudes. For instance, dead loads and static loads are assigned lower values, while live loads like wind loads and seismic loads are assigned much higher values. Moreover, LFRD considers the serviceability factors and is widely used in designing steel structures.

Steel framing with I-beam

I-beams are mostly used in steel framing structures, they are sometimes known as H-beam. I-beams are regarded to provide the strongest strength and durability as compared to beams having other cross-sections. I-beams generally have two parts, the flange and the web. The horizontal component is known as a web, while the vertical component is known as a flange. The below figure shows the parts of an I-beam.

I-beams are excellent in carrying shear loads and bending loads and are indicated by the Euler-Bernoulli beam theory. The wider flanges help in distributing the bending stresses induced, thus reducing the stress magnitudes. I-beams are widely used in structural engineering applications and many civil engineering constructions. Some of the points that make the I-beams distinct from the other beams are:

• They have a high moment of inertia
• Highly prevents vibrations
• They are easily fabricable
• Can bear high loads
• Uniform weight distribution
• Flexibility in size and dimensions

But it should be noted that due to minimum material in the lateral direction of the I-beam, these beams are inefficient in carrying torsional loads. For such conditions, hollow beams having circular cross-sections perform the best.

Context and Applications

This topic is taught in many undergraduate and postgraduate degree courses like:

• Bachelors of Technology (Civil Engineering)
• Bachelors of Technology (Mechanical Engineering)
• Master of Technology (Civil Engineering)
• Master of Technology (Materials Engineering)
• Master of Technology (Building Construction)

Practice Problems

Q 1. What is the full form LRFD?

1. Load resistance frame design
2. Low resistance and frame deformation
3. Load resistance factor design
4. Load and resistance factor design

Answer: Option d

Explanation: The full form of LFRD is load and resistance factor design.

Q 2. What is the limitation of the I-beam?

1. I-beams are weak in carrying shear loads.
2. I-beams are weak in carrying bending loads.
3. I-beams are weak in carrying both shear loads and bending loads.
4. I-beams are weak in carrying torsional loads.

Answer: Option d

Explanation: I-beams are unsuitable for carrying torsional loads due to minimum material on the transverse side.

Q 3. For which of the following loads, a high load factor is considered in LFRD?

1. Dead loads
2. Dead and static loads
3. Live loads and high magnitude loads
4. All of these

Answer: Option c

Explanation: For high magnitude loads and live loads such as seismic loads and wind loads, a high load factor is considered in the LFRD method of steel design.

Q 4. Which of the following parameter is not considered in the ASD method steel design?

1. Serviceability parameter
2. Plastic deformation and strain hardening characteristics
3. Both a and b
4. Elastic limit

Answer: Option c

Explanation: For designing steel using the ASD method, the serviceability parameter along with plastic deformation and strain hardening characteristics of steel are not considered..

Q 5. Which of the following is/are the advantages of using I-beam over conventional cross-section beams?

1. They have a low moment of inertia
2. They have a high moment of inertia
3. They resist high magnitude of vibrations
4. Both b and c

Answer: Option d

Explanation: High moment of inertia and vibration resistance, are two of the distinct properties that separate the I-beam from other conventional beams having other cross-sections.