psp steel

Steel Structures

1. Introduction

  • The design of a building consists of:

    • Functional Design: Planning a building for requirements, including ventilation, lighting, etc.

    • Structural Design: Proportioning various elements to safely transfer loads to the ground and minimize material use.

  • Materials used for load transfer:

    • Asbestos sheets, tiles, bricks, concrete, steel, aluminum, and R.C.C. (Reinforced Cement Concrete).

    • Main modern structures comprise R.C.C. or steel; composite construction of steel and concrete is used for tall structures.

2. Common Steel Structures

  • Steel is favored for its high strength-to-weight ratio, enabling large column-free spaces. Common uses include:

    1. Roof trusses (factories, auditoriums, etc.)

    2. Industrial elements (trussed bents, crane girders)

    3. Railway and bus station canopies

    4. Domes in auditoriums and stadiums

    5. Bridges (plate girder and truss)

    6. Transmission towers

    7. Water tanks

    8. Chimneys

3. Advantages and Disadvantages of Steel Structures

Advantages

  1. High strength-to-weight ratio allows for smaller elements, conserving space and enhancing aesthetics.

  2. Quality and Durability: Steel is durable and maintains quality standards.

  3. Construction Speed: Pre-fabricated standard sections minimize onsite delays.

  4. Reinforcements: Structures can easily be upgraded with additional steel sections.

  5. Transportability: Quick dismantling/transportation via bolted connections.

  6. Resistance: Proper joints yield waterproof and gas-resistant structures, useful for water tanks.

  7. Reusability: Steel materials can be reused.

Disadvantages

  1. Corrosion: Steel is prone to rust, necessitating corrosion protection.

  2. High Maintenance: Regular painting and maintenance increase costs.

  3. Material Cost: Steel can be expensive compared to other construction materials.

4. Types of Steel

  • Steel is primarily an alloy of iron and carbon. Various elements enhance specific properties:

    • Carbon and manganese increase tensile strength but reduce ductility.

    • Excess sulfur or phosphorus causes brittleness and affects weldability.

    • Chromium and nickel enhance corrosion resistance and heat resistance.

  • Main types:

    • Structural steels: Used for frameworks (mild steel, high tensile steel).

5. Properties of Structural Steel

Physical Properties

  • Unit Mass: 7850 kg/m³

  • Modulus of Elasticity (E): 2.0 N/mm²

  • Poisson's Ratio (u): 0.3

  • Modulus of Rigidity (G): 0.769 x 102 N/mm²

Mechanical Properties

Includes yield stress, tensile strength, elongation percentages, and notch toughness; determined mostly through tensile tests according to IS standards.

6. Rolled Steel Sections

  • Steel sections are rolled in mills; they come in various geometrical shapes and sizes. Types include:

    1. I-sections

    2. Channel sections

    3. Angle sections

    4. Tee sections

    5. Bars

    6. Tubes

    7. Plates

    8. Flats

7. Special Considerations in Steel Design

Size and Shape

  • Design must accommodate available standard shapes and sizes from steel mills.

Buckling Considerations

  • Compression members may buckle due to their slender size; lateral buckling is a consideration in beam design.

  • Codes specify ineffective parts for buckling.

Minimum Thickness

  • Corrosion resistance requires minimum thickness guidelines, especially for exposed members:

    • 6 mm if accessible for cleaning

    • 8 mm if not accessible.

Connection Design

  • Essential connection types:

    1. Riveted

    2. Bolted

    3. Welded

8. Loads on Structures

  • Commonly classified loads include:

    1. Dead Loads (DL): Permanent construction weight

    2. Imposed Loads (IL): Live loads, crane loads, etc., which vary over time

    3. Wind Loads (WL)

    4. Earthquake Loads (EL)

    5. Erection Loads (ER)

    6. Accidental Loads (AL)

9. Design Combinations and Analysis

  • Load combinations must ensure safety and economy. Recommended combinations include various configurations of DL, IL, EL, and others based on their probabilities of simultaneous occurrence.

  • Structural analysis methods include elastic, plastic, advanced, and dynamic analysis.

10. Design Philosophy

  • The objective of design:

    1. Safety: Structure should withstand expected loads.

    2. Serviceability: Should remain functional during and post construction.

    3. Durability: Capacity to sustain loads over time.

    4. Resistance to Risks: Adequate safeguards against exceptional events.

  • Design methods: WSM, ULD, and LSM.

11. Bolted Connections

  • Steel structures require effective design of connections to join members and sections securely. Types include:

    1. Riveted: Involves heating rivets to join plates.

    2. Bolted: Uses metal pins with nuts; categories include unfinished, finished, and HSFG bolts.

    3. HSFG bolts: Provide a stronger grip and are used for high-dynamic load connections.

12. Tables and Forms

  • Tables and classifications for bolt dimensions, strengths, connections, etc., according to IS standards provide vital information for design efficiency.

13. Limit State Design Principles

  • Limit states consider the entire structure's serviceability, stability, strength, and adaptability vs. expected service conditions and behaviors over time.