STEEL

ISLB (Indian Standard Light Beam)

Indian Standard Light Weight Beam

Also calledislbislb beamindian standard light beamlight beam
Related on InfraLens
CODES
IS 808
TOOLS
steel table
Definition

ISLB (Indian Standard Light Beam) is a family of hot-rolled steel I-sections lighter than ISMB (Medium Beam) of the same depth. Per IS 808:1989 + IS 2062:2011, ISLB has thinner flanges and web — making it lighter (15-30% less weight per metre) and more economical for light-load applications. ISLB designations: ISLB <depth>, e.g., ISLB 200 (200 mm depth, 17.6 kg/m vs ISMB 200's 25.4 kg/m).

Key ISLB sizes: ISLB 100 (8.0 kg/m, Ixx = 158 cm⁴), ISLB 150 (12.6 kg/m, Ixx = 530 cm⁴), ISLB 200 (17.6 kg/m, Ixx = 1339 cm⁴), ISLB 250 (24.0 kg/m, Ixx = 2950 cm⁴), ISLB 300 (32.8 kg/m, Ixx = 5660 cm⁴), ISLB 350 (41.4 kg/m, Ixx = 9560 cm⁴), ISLB 400 (51.7 kg/m, Ixx = 14,500 cm⁴), ISLB 600 (105.7 kg/m, Ixx = 50,500 cm⁴). Compared to ISMB: ISLB has 30-35% lower weight at the same depth, making it suitable for lighter-load applications. ISLB is used for: secondary roof beams, mezzanine framing, light-industrial structures, residential steel framing, and specialty applications where weight matters.

Where used
  • Light-load secondary beams in steel buildings
  • Mezzanine floor framing
  • Light-industrial structure frames
  • Residential steel construction
  • Specialty applications where steel weight matters
Acceptance / threshold
Per IS 808:1989 + IS 2062:2011: dimensions and tolerances per Tables; mass per metre verified by weighing; mill test certificate for material grade. Pre-erection inspection for plumb, sweep, surface condition.
Site example
Site reality: a Hyderabad residential project used ISLB 150 instead of ISMB 200 for secondary floor beams. Weight saving: 17.6 vs 25.4 = 7.8 kg/m × 24 beams × 6 m length = 1,123 kg = ₹0.65 lakh material savings on a small project. Always evaluate ISLB vs ISMB based on actual load — for light secondary beams, ISLB is more economical without compromising structural adequacy.
Frequently asked
What is ISLB?
ISLB (Indian Standard Light Beam) is a family of hot-rolled steel I-sections lighter than ISMB (Medium Beam) of the same depth. Per IS 808:1989. ISLB has thinner flanges and web — 15-30% less weight per metre. Used for light-load applications, mezzanine floors, secondary beams.
What is the difference between ISLB and ISMB?
Same depth designation (ISLB 200 and ISMB 200 are both 200 mm depth). ISLB has thinner flanges and web → 30-35% lower weight (17.6 vs 25.4 kg/m for the 200 mm size). ISLB has lower section modulus and moment of inertia, suitable for lighter loads. ISMB is preferred for primary structural members and higher-load applications.
When is ISLB used instead of ISMB?
ISLB is used when the design load is below ISMB's range — typical for light-industrial, residential, mezzanine, secondary beams. For primary structural members, ISMB is preferred. Selection criteria: design moment + shear vs section capacity, with ISLB chosen if adequate. Cost saving from lower steel weight justifies the choice for non-critical applications.
Related steel terms
TMT Steel Bar (Fe415/Fe500/Fe550)
Thermo Mechanically Treated bars per IS 1786. Fe500 most common.
Lap Length
Length of overlap to transfer force between bars (40-60d typical)
Development Length / Anchorage
Length of bar embedment to develop full strength
Bar Bending Schedule (BBS)
Schedule listing bar shapes, sizes, lengths per IS 2502
Rebar Weight Calculation
Weight = D²/162.2 kg/m where D is dia in mm
Structural Steel Sections
ISMB/ISLB/ISWB/ISHB beams, ISMC channels, ISA angles per IS 808
ISMB (Indian Standard Medium Beam)
Indian Standard Medium Weight Beam — most common I-section
ISWB (Indian Standard Wide Flange Beam)
Wide flange beam — better for bending