Introduction to Steel in Civil Engineering

Introduction to Steel

• Steel is a critical material in civil engineering.
• Steel is an alloy of iron and carbon, often with other elements to enhance properties.
• Steel is approximately 95% iron.
• Steel typically contains 1-2% carbon.
• Other trace elements, like manganese and nickel, are also present.

Learning Objectives

• Understand structural steel types.
• Discuss stress-strain relationships.
• Highlight the advantages and disadvantages of steel.
• Explore fatigue failure.
• Discuss brittle fracture and fire performance.

What is Steel?

• Steel's composition includes iron and carbon.
• Other elements are added to optimize steel properties.

Properties of Steel

• High strength
• Ductility
• Malleability
• Resistance to wear makes it suitable for structural applications.

Types of Steel

• Low Carbon (Mild) Steel
• Medium Carbon Steel
• High Carbon (Tool) Steel
• Cast Iron

Low Carbon Steel

• Contains 0.05-0.32% carbon.
• Used for nails, screws and structural components.
• Offers high ductility and weldability.
• Lower corrosion resistance, requiring coatings.

Medium Carbon Steel

• Contains 0.35-0.50% carbon.
• Offers increased strength and hardness.
• Lower ductility and malleability compared to low carbon steel.
• Suitable for structural steel and rails.

High Carbon Steel

• Contains 0.55-1.50% carbon.
• Very hard, but less ductile.
• Used for hand tools, chisels, and saw blades.

Cast Iron

• Contains 2-4% carbon.
• Hard, brittle and strong under compression.
• Used in engine blocks and machine parts.

Structural Steel Applications

• Multi-story building skeletons
• Industrial buildings
• Transmission towers
• Railway bridges
• Reinforced concrete rebar

Advantages of Steel

• High strength-to-weight ratio.
• Lightweight, aiding transportation.
• Long lifespan with minimal maintenance.
• Ductility provides failure warnings.
• Easy to alter or extend.
• Fast construction.
• High scrap value.
• Reusable after demolition.

Disadvantages of Steel

• Susceptibility to corrosion (requires coating).
• Needs fireproofing treatment.
• Fatigue reduces strength under repeated stress.

Stress-Strain Relationship

• Steel deforms elastically before yielding.
• Plastic deformation becomes permanent beyond the yield point.

Grades of Steel

• Common grades include S275, S355, and S460.
• Yield strength ranges from 275-460 N/mm².
• Grade indicates the strength of the steel.

Fatigue in Steel

• Repeated stress leads to gradual cracking and failure.
• Avoid abrupt changes in section to prevent fatigue failure.
• Bolted joints are preferable to welded joints to minimize fatigue.

Brittle Fracture

• Occurs at low temperatures or stress concentrations.
• Avoid abrupt changes in design.
• Choose steel grades resistant to brittle failure.

Fire Performance

• Steel loses strength at high temperatures.
• Yield stress drops by ~30% at 550°C.
• Use fireproofing measures like coatings or concrete encasing.

Corrosion of Steel

• Corrosion occurs in the presence of moisture or oxygen.
• Protection methods include galvanization, paint coatings, and specialized alloys.

Carbon Steel

• Carbon is the primary alloying element.
• Higher carbon content increases strength, but reduces ductility.

Alloy Steel

• Contains additional elements like chromium, nickel, and molybdenum.
• Improves toughness, hardness, and wear resistance.

Tool Steel

• Known for hardness and ability to hold an edge.
• Used in cutting tools and dies.

Stainless Steel

• Contains more than 10.5% chromium.
• Resistant to rust and staining.
• Used in kitchens, architecture, and medical tools.

Structural Steel Design

• Based on yield strength, ductility/weldability, and impact resistance.

Hot Rolled Sections

• Examples include universal beams (UB), channels, and tubes.
• Used in large frameworks.

Cold Rolled Sections

• Examples include corrugated steel roofs and wall panels.
• Ligher and easier to handle than hot rolled sections.

Fabricated Sections

• Built-up sections like I-beams and box girders are used for high-strength applications.

Future of Steel

• Emerging trends include advanced alloys, sustainable practices (recycling), and smart materials for adaptive construction.

Conclusion

• Steel is crucial in modern construction.
• It balances versatility, strength, and sustainability.

This quiz explores the critical material of steel, its composition, types, and mechanical properties relevant to civil engineering. You will learn about stress-strain relationships, advantages and disadvantages, and issues like fatigue failure and brittle fracture. Test your knowledge on the different types of steel and their applications.