Steel Section Types Guide — Which Section to Use When?

Indian Standard steel sections come in 8 types, each designed for specific structural applications. This guide covers ISMB, ISLB, ISWB, ISHB, ISJB, ISMC, ISA (equal), and ISA (unequal) sections as defined in IS 808:2021. Choosing the right section type is crucial for structural efficiency, cost optimization, and code compliance.

Quick Selection Guide

ISMB — Indian Standard Medium Flange Beam

Sizes: ISMB 100 to ISMB 600 (14 sizes)
Weight range: 11.5 to 122.6 kg/m
Flange width: Narrow to medium (75–210mm)

When to Use ISMB

• Primary floor beams in residential and commercial buildings
• Lintels over door and window openings (ISMB 150–250)
• Crane girders for light-duty cranes (ISMB 300–500)
• Portal frames for industrial sheds
• Bridge stringers for short spans

Key Properties

ISMB sections have a good depth-to-width ratio, making them efficient for resisting bending about the major axis. The narrow flanges keep weight down but provide limited lateral stiffness — use lateral bracing for long spans.

ISLB — Indian Standard Light Beam

Sizes: ISLB 100 to ISLB 600 (16 sizes)
Weight range: 7.9 to 89.9 kg/m
Flange width: Narrow (75–200mm)

When to Use ISLB

• Secondary beams supporting floor slabs
• Door/window lintels in masonry walls
• Light-duty roof purlins
• Where weight saving matters more than high strength
• Walkway supports and platforms

ISMB vs ISLB

For the same depth, ISLB is 20–30% lighter than ISMB but has a lower moment of inertia. Example: ISMB 200 weighs 22.9 kg/m with Ixx = 1,999 cm⁴, while ISLB 200 weighs 18.2 kg/m with Ixx = 1,836 cm⁴. Choose ISLB when the lighter section still meets your deflection and stress requirements.

ISWB — Indian Standard Wide Flange Beam

Sizes: ISWB 150 to ISWB 600 (12 sizes)
Weight range: 17.0 to 133.7 kg/m
Flange width: Wide (100–250mm)

When to Use ISWB

• Crane girders — wider flanges resist lateral forces from crane travel
• Long-span beams where lateral bracing is difficult
• Transfer beams carrying heavy loads
• Columns with biaxial bending (alternative to ISHB)

ISWB vs ISMB

ISWB sections have wider flanges than ISMB for the same depth, providing significantly higher Iyy (minor axis) values. Example: ISWB 300 has Iyy = 1,618 cm⁴ vs ISMB 300 with only 454 cm⁴ — over 3.5× more lateral stiffness.

ISHB — Indian Standard Heavy Beam (Column Section)

Sizes: ISHB 150 to ISHB 450 (8 sizes)
Weight range: 34.6 to 87.2 kg/m
Flange width: Wide, nearly equal to depth (150–250mm)

When to Use ISHB

• Steel columns — designed specifically for axial loads with biaxial bending
• Heavy-duty columns in multi-storey frames
• Struts with significant compression loads
• Where both axes need comparable stiffness

Key Properties

ISHB sections have a nearly square cross-section (width ≈ depth), providing high Iyy values. ISHB 250 has ryy = 6.74 cm vs ISMB 250 with ryy = 2.65 cm, meaning ISHB is 2.5× more resistant to minor-axis buckling — critical for column design per IS 800.

ISJB — Indian Standard Junior Beam

Sizes: ISJB 150 to ISJB 225 (4 sizes)
Weight range: 7.1 to 12.8 kg/m
Flange width: Very narrow (50–80mm)

When to Use ISJB

• Light lintels over small openings
• Small-span beams with minimal loads
• Stiffeners in plate girders
• Secondary framing in industrial structures

ISJB is the lightest I-section available. Use only when loads are truly minimal — for anything beyond light partitions, ISLB is a safer choice.

ISMC — Indian Standard Medium Channel

Sizes: ISMC 75 to ISMC 400 (11 sizes)
Weight range: 7.1 to 49.4 kg/m
Shape: C-section (flanges on one side only)

When to Use ISMC

• Roof purlins — bolted to rafter top flanges
• Wall girts — horizontal supports for cladding
• Staircase stringers — C-shape hides connections
• Bracings in industrial structures
• Built-up sections — two channels back-to-back as beams/columns
• Window/door frames in industrial buildings

Key Properties

Channel sections have an unsymmetric cross-section about the minor axis, meaning they tend to twist under load. For beams, either use them in pairs (back-to-back) or ensure adequate lateral bracing. Shear centre is located outside the web, unlike I-sections.

ISA Equal Angle

Sizes: ISA 20×20×3 to ISA 200×200×18 (18 sizes)
Weight range: 0.8 to 54.1 kg/m
Shape: L-section with equal legs

When to Use Equal Angles

• Truss members — tension and compression members in roof trusses
• Bracings — diagonal and horizontal bracing in steel frames
• Connection angles — connecting beams to columns
• Lintels — supporting masonry above openings (with plate)
• Transmission towers — primary members (ISA 90×90 and above)

ISA Unequal Angle

Sizes: ISA 30×20×3 to ISA 200×150×12 (19 sizes)
Weight range: 1.1 to 31.6 kg/m
Shape: L-section with unequal legs

When to Use Unequal Angles

• Truss members with different connection requirements on each leg
• Stair stringers — longer leg vertical for depth
• Purlin cleats — connecting purlins to rafters
• Edge angles — longer leg along the loaded direction

IS 808 Section Summary

Decision Flowchart

1. Is it a beam (bending member)?
   • Heavy loads → ISMB or ISWB
   • Light loads → ISLB or ISJB
   • Need lateral stiffness → ISWB
2. Is it a column (compression member)?
   • Biaxial bending → ISHB
   • Uniaxial bending → ISMB or back-to-back ISMC
3. Is it a purlin, girt, or stringer? → ISMC
4. Is it a truss member or bracing? → ISA (equal or unequal)
5. Is it a connection element? → ISA (equal for symmetric, unequal for asymmetric)

Related Standards

• IS 808:2021 — Dimensions and properties of hot rolled steel sections
• IS 800:2007 — General construction in steel — Code of practice
• IS 2062:2011 — Hot rolled steel for general structural purposes
• IS 802:2015 — Use of structural steel in transmission towers