1. Introduction of Steel Structures (as per IS 800:2007)

Steel has been used as a construction material for over 150 years, and its role in modern infrastructure continues to grow. Steel structures refer to load-bearing frameworks made from structural steel members – beams, columns, trusses, frames, and plates – that are designed to support and transfer loads safely to the foundation.

In India, the Bureau of Indian Standards (BIS) published IS 800:2007 – the third revision of the code – to align Indian practice with international standards, particularly Eurocode 3. This code adopts the Limit State Method (LSM) of design, replacing the older Working Stress Method (WSM) that was the foundation of IS 800:1984.

Figure 1: Typical steel framed structure showing beams, columns, and bracing as per IS 800:2007 design principles.

Why IS 800:2007 Matters

• It brings Indian steel design in line with global best practices.
• Introduces the Limit State Method for more rational and economical design.
• Covers a wide range of structures: buildings, bridges, towers, industrial sheds, and more.
• Provides detailed guidance on connections, fatigue, fire resistance, and durability.
• Supersedes IS 800:1984, though both codes are still referenced in examination syllabi.

Scope of IS 800:2007

IS 800:2007 applies to general construction in steel. It covers the design, fabrication, and erection of steel structures using hot-rolled sections, cold-formed sections, hollow sections, plates, and other structural steel products. The code addresses:

• Structural analysis and design
• Tension, compression, bending, and combined loading
• Connections (bolted, welded, and pinned)
• Stability (local and overall buckling)
• Fatigue and fracture
• Fire and corrosion resistance

2. Structural Steel – Definition and Uses

Structural steel is a category of steel used as a construction material for making structural or load-bearing components. It is characterized by a specific shape or cross section, along with defined chemical composition and mechanical properties. Structural steel is produced in various standard shapes known as sections or profiles.

Chemical Composition

Structural steel is primarily composed of iron (Fe) with controlled amounts of carbon (C) and other alloying elements. As per IS 2062:2011 (the material standard referenced by IS 800:2007), the key elements include:

Common Uses of Structural Steel

• Buildings: Multi-storey frames, industrial sheds, warehouses
• Bridges: Girder bridges, truss bridges, cable-stayed bridges
• Towers: Transmission towers, telecom towers, wind turbine towers
• Industrial Structures: Crane girders, gantry girders, bunkers, silos
• Offshore Structures: Oil platforms, marine jetties
• Special Structures: Sports stadia, exhibition halls, airport terminals

Figure 2: Common applications of structural steel in civil engineering – from bridges and multi-storey buildings to industrial towers.

Grades of Structural Steel (IS 2062:2011)

IS 800:2007 primarily refers to IS 2062 for structural steel. The commonly used grades are:

3. Properties of Structural Steel

Understanding the properties of structural steel is fundamental to good structural design. These properties are broadly classified into mechanical properties and physical properties.

3.1 Mechanical Properties of Structural Steel

Figure 3: Idealized stress-strain curve for structural steel – showing proportional limit, yield plateau, strain hardening, and ultimate stress.

The stress-strain curve of structural steel has distinct zones:

• Elastic Zone (O–A): Stress is proportional to strain (Hooke’s Law applies). The slope is Young’s Modulus (E = 200 GPa).
• Yield Point (A–B): Stress remains nearly constant while strain increases significantly. Upper yield point and lower yield point are observed.
• Plastic / Strain Hardening Zone (B–C): After yielding, the material can sustain additional load before ultimate failure.
• Necking and Fracture (C–D): Cross-sectional area reduces locally; fracture occurs.

3.2 Physical Properties of Structural Steel

• Colour: Silvery-grey (fresh cut); brownish when oxidized/rusted
• Melting Point: Approximately 1370–1510°C (varies with composition)
• Thermal Conductivity: ~50 W/m·K – steel is a good conductor of heat, which affects fire resistance design
• Electrical Conductivity: Steel is electrically conductive – earthing requirements in structures
• Magnetic Properties: Structural steel is ferromagnetic
• Hardness: Brinell Hardness Number (BHN) ≈ 120–180 for mild steel
• Weldability: Good weldability for low-carbon grades; decreases with increasing carbon equivalent (CE)

3.3 Ductility

Ductility is one of the most important properties of structural steel. It is the ability of the material to undergo large plastic deformations before fracture. Structural steel typically exhibits:

• Minimum elongation of 23% (for Grade E250 on 200 mm gauge length)
• This ductility ensures warning before failure, redistribution of stresses, and suitability for earthquake-resistant design

4. Advantages of Steel as a Structural Material

Figure 4: Key advantages of structural steel making it a preferred material for modern construction.

4.1 High Strength-to-Weight Ratio

Steel has a very high strength per unit weight compared to concrete or masonry. A steel member can carry the same load as a much heavier concrete member, resulting in lighter foundations and longer spans. This is especially beneficial in high-rise buildings and long-span bridges.

4.2 Uniformity and Homogeneity

Unlike concrete (which varies with mix design and curing), steel is produced under controlled conditions. Its properties are uniform throughout the section and predictable. This makes structural calculations more reliable.

4.3 Ductility and Toughness

Steel can undergo significant plastic deformation before fracture. This ductility provides visible warning before failure and allows redistribution of stresses, making steel structures inherently safer under overload conditions. Steel structures also perform well under seismic (earthquake) loading.

4.4 Speed of Construction

Steel members are prefabricated in the factory and erected quickly on site. This reduces construction time significantly compared to reinforced concrete (RC) construction. Fast-track projects, like industrial buildings or airports, often prefer steel for this reason.

4.5 Adaptability and Ease of Fabrication

Steel can be cut, welded, drilled, and bent to any required shape. Connections can be made efficiently using bolts or welds. Modifications, extensions, and alterations to existing steel structures are relatively straightforward.

4.6 Reusability and Sustainability

Steel is 100% recyclable. Structural steel sections can be dismantled and reused in other projects. Even scrap steel can be melted and recast. This makes steel one of the most sustainable structural materials available.

4.7 Long Life Span

With proper protective coatings (painting, galvanizing, weathering steel grades), steel structures can last well over 50–100 years with minimal maintenance.

4.8 Aesthetic Appeal

Exposed steel structures can be architecturally striking. The Eiffel Tower, Sydney Harbour Bridge, and countless modern stadia demonstrate the aesthetic potential of structural steel.

Summary Table – Advantages

5. Disadvantages of Steel Structures

Despite its many advantages, structural steel has certain limitations that engineers must account for during design and detailing.

5.1 Susceptibility to Corrosion

Steel rusts when exposed to moisture and oxygen. Corrosion reduces the cross-section of members, weakening the structure over time. This necessitates protective coatings (paints, zinc galvanizing) and regular maintenance, adding to lifecycle costs – particularly in coastal, industrial, or humid environments.

5.2 Poor Fire Resistance

Structural steel loses strength rapidly at elevated temperatures. At around 300°C, yield strength begins to drop; by 550–600°C, strength reduces to about 60% of the ambient value. This requires fire protection (intumescent paint, concrete casing, spray-on fire protection) to meet building code requirements.

5.3 Buckling Susceptibility

Slender steel members (columns, beams, plates) are prone to buckling under compressive and bending loads. This requires careful proportioning and often increases the size of members beyond what pure strength requirements would dictate, reducing material efficiency.

5.4 High Initial Cost

The cost of structural steel per unit weight is significantly higher than concrete. Although overall project costs can be comparable or lower (due to speed and lighter foundations), initial material costs can be a barrier in cost-sensitive projects.

5.5 Fatigue Under Cyclic Loading

Steel structures subjected to repeated or fluctuating loads (bridges, crane girders, offshore platforms) can fail due to fatigue at stress levels well below the ultimate tensile strength. Careful detailing of connections and stress concentration points is essential.

5.6 Need for Skilled Labour and Equipment

Fabrication and erection of steel structures require skilled welders, fabricators, and specialised equipment (cranes for erection, welding machines). Availability of skilled labour can be a constraint in remote or developing regions.

5.7 Maintenance Requirements

Steel structures, if not properly protected, require periodic repainting and inspection. This is an ongoing maintenance cost over the life of the structure.

6. Aluminium as a Structural Material – Comparison with Steel

While steel dominates structural engineering, aluminium alloys are increasingly used in specific applications where their unique properties offer advantages. Understanding the comparison between aluminium and steel helps engineers make informed material choices.

Figure 5: Comparative chart of structural properties of steel vs aluminium – key parameters for engineering selection.

Key Properties of Structural Aluminium Alloys

• Density: ~2700 kg/m³ (about one-third that of steel at 7850 kg/m³)
• Modulus of Elasticity: ~70 GPa (about one-third of steel’s 200 GPa)
• Yield Strength: 100–500 MPa depending on alloy and temper
• Corrosion Resistance: Excellent – aluminium forms a natural oxide layer that is self-protecting
• Thermal Expansion: ~23 × 10⁻⁶/°C (almost double that of steel)
• Melting Point: ~660°C (significantly lower than steel)

Detailed Comparison: Steel vs Aluminium

When to Choose Aluminium over Steel

• When weight reduction is critical (e.g., vehicle bodies, aircraft, portable structures)
• In corrosive environments where maintenance-free service is desired (marine applications, chemical plants)
• For architectural cladding, curtain walls, and roofing in buildings
• For footbridges and pedestrian bridges where lightness is an advantage
• When low magnetic permeability is required (e.g., MRI facilities, electrical installations)

Note: IS 800:2007 is specifically for steel structures. Aluminium structural design in India refers to IS 8147 (Code of Practice for Use of Aluminium Alloys in Structures).

7. Rolled Structural Steel Sections

Structural steel is available in a wide range of standard cross-sectional shapes, known as rolled sections, produced by hot-rolling steel billets through shaped rollers. These sections are standardised and catalogued in IS 808 (Dimensions for Hot Rolled Steel Beam, Column, Channel, and Angle Sections).

Figure 6: Standard rolled structural steel sections as per IS 808 – I-beam, T-section, channel (C), angle (L), and hollow sections.

7.1 I-Section (Universal Beam / ISMB / ISWB / ISLB)

The I-section (also called H-section or wide-flange section) is the most widely used structural section. It consists of two flanges connected by a vertical web.

• ISMB: Indian Standard Medium Weight Beam
• ISLB: Indian Standard Light Weight Beam
• ISWB: Indian Standard Wide Flange Beam
• ISHB: Indian Standard Heavy Beam (for columns)
• ISSC: Indian Standard Special Channel (for crane girders)

Applications: Beams, girders, rafter purlins in roof trusses, columns in multi-storey buildings

Why preferred? The I-shape is structurally efficient for bending – material is concentrated in the flanges where bending stresses are highest, while the web resists shear.

7.2 T-Section (ISNT / ISLT / ISST)

• ISNT: Indian Standard Normal Tee (cut from ISMB)
• ISLT: Indian Standard Light Tee
• ISST: Indian Standard Short-legged Tee

Applications: Top chord and bottom chord of trusses, stiffeners, built-up sections, composite construction

7.3 Channel Section (ISMC / ISLC)

Channel sections have one web and two flanges on the same side, giving a C-shaped profile.

• ISMC: Indian Standard Medium Weight Channel
• ISLC: Indian Standard Light Weight Channel

Applications: Purlins, girts, bracings, side rails, stringer beams, built-up sections

Note: Channel sections have a shear centre outside the web; they are susceptible to twisting under transverse loads unless properly restrained.

7.4 Angle Section (ISA)

Angle sections (L-shaped) are available in equal-legged and unequal-legged variants.

• ISA (Equal angles): e.g., ISA 100×100×10 (leg × leg × thickness)
• ISA (Unequal angles): e.g., ISA 150×75×10

Applications: Truss members (web and chord members), bracing systems, lacing and battening of columns, cleats and gusset plates

7.5 Hollow Sections (SHS / RHS / CHS)

• SHS: Square Hollow Section
• RHS: Rectangular Hollow Section
• CHS: Circular Hollow Section (pipe sections)

Applications: Trusses (tubular trusses), columns, space frames, architectural exposed structures

Advantages: High torsional stiffness, uniform strength in all directions, aesthetically pleasing, closed section minimises corrosion exposure

7.6 Plate Sections and Built-Up Sections

When standard rolled sections are insufficient for the required span or load, built-up sections (also called plate girders or fabricated sections) are used. These are welded together from plates to form custom I-sections, box sections, or other profiles.

Figure 7: Visual representation of major rolled steel sections used in structural design per IS 800:2007 – I, T, channel, angle, and hollow profiles.

8. Convention for Member Axes (as per IS 800:2007)

A consistent axis convention is essential for describing forces, moments, deflections, and section properties in structural analysis and design. IS 800:2007 (Clause 1.3) establishes the standard axis convention for structural steel members.

Figure 8: Standard member axis convention as per IS 800:2007 – x-x (major axis), y-y (minor axis), and z-z (longitudinal axis) for an I-section.

Axis Definitions

Important Notes on Axis Convention

• For symmetric sections (ISMB, ISHB, ISMC), the centroidal axes y-y and z-z are also the principal axes.
• For angle sections (ISA), the centroidal axes are not principal axes. The principal axes u-u and v-v are inclined at an angle to the geometric axes, and section properties (Iᵤᵤ, Iᵥᵥ) are different from Iyy and Izz.
• Major axis bending (about y-y for I-sections) has a higher section modulus and is the primary loading plane for beams.
• Shear centre location is critical for sections like channels and Z-sections that are susceptible to twisting when loaded transversely.
• IS 800:2007 uses the right-hand coordinate system with positive directions as per the standard convention.

Section Properties Defined with Respect to Axes

• I_yy – Second moment of area about y-y axis (major axis for I-sections)
• I_zz – Second moment of area about z-z axis (minor axis for I-sections)
• r_yy = √(Iyy/A) – Radius of gyration about major axis
• r_zz = √(Izz/A) – Radius of gyration about minor axis
• Z_e – Elastic section modulus
• Z_p – Plastic section modulus (used in Limit State Design)

9. Important Recommendations of IS 800:1984

IS 800:1984 was the second revision of the Indian Standard for General Construction in Steel, and it remained the primary design code in India for over two decades until it was superseded by IS 800:2007. It was based on the Working Stress Method (WSM), also called the Allowable Stress Design (ASD) method.

9.1 Design Philosophy – Working Stress Method (WSM)

In WSM, the actual stresses in a structure under working (service) loads must not exceed certain permissible (allowable) stresses. The permissible stress is obtained by dividing the characteristic material strength (yield stress or ultimate stress) by a Factor of Safety (FOS).

• Permissible tensile stress = f_y / FOS ≈ 0.6 × f_y
• Permissible compressive stress in bending = 0.66 × f_y
• Permissible shear stress = 0.45 × f_y

9.2 Key Provisions of IS 800:1984

• Material: The code specified use of Grade Fe 410 W steel (mild steel) as per IS 2062 as the primary structural material. Yield stress f_y = 250 MPa, UTS = 410 MPa.
• Tension Members: Net effective sectional area for tension = gross area – area of holes; efficiency of connection considered via shear lag factor. Permissible tensile stress = 0.6 f_y.
• Compression Members: Design based on permissible compressive stress, which was a function of slenderness ratio (KL/r). Maximum permissible slenderness ratio for main members = 180; for bracings = 250.
• Beams (Flexural Members): Permissible bending stress = 0.66 f_y for laterally restrained beams; reduced for laterally unrestrained beams based on elastic critical moment.
• Connections – Bolts: Permissible shear stress in bolts = 100 MPa (for ordinary bolts); bearing stress and tension provisions also specified.
• Connections – Welds: Permissible stress in weld = 0.4 × UTS of weld metal (approximately 165 MPa for ordinary welds).
• Lacing and Battening of Built-up Columns: Detailed rules for spacing, angle, and slenderness of lacing bars and battens.
• Gantry Girders: Additional dynamic load factors for crane girders; restrictions on deflection.
• Serviceability: Deflection limits – span/325 for beams, span/240 for purlins carrying AC sheet roofing.

9.3 Limitations of IS 800:1984

• Does not account for variability in loads and material strengths probabilistically.
• No distinction between different failure modes (yielding vs. fracture vs. buckling).
• Does not provide rational guidance on fatigue, fire design, and dynamic loads.
• Not aligned with international codes (Eurocode 3, AISC), limiting global competitiveness.
• WSM can be overly conservative in some cases, leading to heavier and uneconomical designs.

10. Important Recommendations of IS 800:2007

IS 800:2007 represents a landmark shift in Indian structural steel design. It adopts the Limit State Method (LSM), bringing India in line with modern international codes. The third revision was published in December 2007 and introduced a comprehensive, probabilistic framework for design.

Figure 9: Comparison of design philosophy – IS 800:1984 (Working Stress Method) vs IS 800:2007 (Limit State Method).

10.1 Design Philosophy – Limit State Method (LSM)

Limit State Design considers two categories of limit states:

• Ultimate Limit States (ULS): Conditions leading to collapse or structural failure – strength, stability, fracture, fatigue, progressive collapse.
• Serviceability Limit States (SLS): Conditions making the structure unfit for use – excessive deflections, vibration, deformation, and cracking.

Design is achieved by ensuring that the design action (factored loads) does not exceed the design resistance (factored material strength) at any limit state.

10.2 Partial Safety Factors

IS 800:2007 uses partial safety factors for both loads and materials:

10.3 Section Classification (IS 800:2007, Clause 3.7)

One of the most important new concepts in IS 800:2007 is the classification of cross-sections based on their susceptibility to local buckling before reaching the plastic moment capacity:

• Class 1 – Plastic: Can form a plastic hinge with sufficient rotation capacity for plastic analysis. (b/t ≤ 9.4ε for flanges, d/tw ≤ 84ε for webs)
• Class 2 – Compact: Can develop full plastic moment but local buckling limits rotation capacity. Not suitable for plastic analysis.
• Class 3 – Semi-compact: Can reach the yield moment but local buckling prevents full plastification of the cross-section.
• Class 4 – Slender: Local buckling occurs before yielding; requires effective width or effective section approach.

Where ε = √(250/f_y) is the yield stress ratio.

10.4 Tension Member Design

• Design strength governed by three modes: yielding of gross section, rupture of net section, and block shear.
• Yielding: T_dg = f_y × A_g / γ_m0
• Rupture: T_dn = 0.9 × f_u × A_n / γ_m1 (with shear lag factor)
• Block shear: T_db – combination of shear and tension failure paths
• Design strength T_d = min(T_dg, T_dn, T_db)

10.5 Compression Member Design (Columns)

• Design based on multiple column curves (a, b, c, d) depending on section type, axis of buckling, and fabrication method – a major advancement over IS 800:1984.
• Non-dimensional slenderness: λ̄ = √(f_y/f_cc)
• Imperfection factor (α) depends on the column curve.
• Design compressive strength: P_d = f_cd × A
• Maximum slenderness ratios: main members = 180, bracings = 250 (same as IS 800:1984)

10.6 Beam Design (Flexural Members)

• Laterally restrained beams: Design bending strength = Z_p × f_y / γ_m0 (for Class 1 and 2 sections)
• Laterally unrestrained beams: Reduction for lateral-torsional buckling using non-dimensional slenderness λ̄_LT.
• Shear capacity: V_d = f_yw × A_w / (√3 × γ_m0)
• Serviceability: Deflection limits – span/300 (live load), span/500 (superimposed dead load + live load)

10.7 Connection Design

• Bolted connections: High Strength Friction Grip (HSFG) bolts preferred; ordinary bolts (4.6, 8.8, 10.9 grades) also covered. Slip resistance and bearing failure considered.
• Welded connections: Fillet welds and butt welds; throat area as basis for strength.
• Minimum edge distance, end distance, and spacing clearly specified.

10.8 Other Significant Provisions

• Fatigue Design (Chapter 13): Detail categories, S-N curves, and cumulative damage rules – new to IS 800:2007.
• Fire Design (Chapter 15): Structural fire design using load reduction factors and temperature-dependent material properties.
• Durability (Chapter 15): Protective coating systems, exposure categories, and maintenance guidelines.
• Fabrication and Erection (Chapter 12): Tolerances, workmanship, and quality control requirements.
• Plastic Analysis (Chapter 4): Permitted for Class 1 sections – new provision enabling more efficient design.

Figure 10: Overview of IS 800:2007 chapter structure covering design, connections, fatigue, fire resistance, and fabrication.

❓ Frequently Asked Questions (FAQ)

Conclusion

Steel structures are a cornerstone of modern civil engineering, offering unmatched combinations of strength, ductility, speed of construction, and versatility. IS 800:2007, the current Indian Standard for steel structure design, represents a significant advancement over its predecessor – moving from a conservative working stress approach to a rational, probabilistic limit state design framework.

For civil engineering students and practicing engineers, a thorough understanding of IS 800:2007 is essential. Key takeaways from this article include:

• Structural steel is defined by its chemical composition and mechanical properties, with Grade E250 (Fe 410) being most commonly used.
• The stress-strain curve of steel reveals its ductile behaviour – a critical safety advantage.
• IS 800:2007 uses Limit State Method with partial safety factors, section classification, and multiple column curves.
• Rolled sections (ISMB, ISMC, ISA, SHS, etc.) provide standardised shapes that can be selected from IS 808 tables.
• The axis convention (x-x, y-y, z-z, u-u, v-v) is fundamental to correctly interpreting section properties and analysis results.
• Aluminium offers competitive advantages over steel in specific applications but cannot replace steel for most structural applications due to lower stiffness and higher cost.

Whether you are preparing for GATE, university exams, or professional practice, mastering these fundamentals of steel structure design will serve as a solid foundation for advanced topics like connections, plate girders, industrial buildings, and multi-storey frames.

Figure 11: Summary overview of IS 800:2007 – India’s comprehensive code for steel structure design covering materials, design methods, sections, connections, and special topics.