ISMB (Indian Standard Medium Beam)
Indian Standard Medium Weight Beam — most common I-section
ISMB (Indian Standard Medium Beam) is the dominant family of hot-rolled I-section steel beams used in Indian construction. Standardised in IS 808:1989 with sections from ISMB 100 (100 mm depth) up to ISMB 600 (600 mm depth), ISMB beams are the workhorse for moment-resisting framing in steel buildings, industrial sheds, mezzanine floors, and bridge girders. ISMB designation reads: ISMB <depth in mm>, e.g., ISMB 200 has overall depth 200 mm with corresponding flange width 100 mm and weight 25.4 kg/m.
Key ISMB properties (IS 808 Table 1): ISMB 100 (8.9 kg/m, Ixx = 257 cm⁴), ISMB 150 (14.9 kg/m, Ixx = 726 cm⁴), ISMB 200 (25.4 kg/m, Ixx = 2235 cm⁴), ISMB 250 (37.3 kg/m, Ixx = 5131 cm⁴), ISMB 300 (44.2 kg/m, Ixx = 8603 cm⁴), ISMB 400 (61.6 kg/m, Ixx = 20458 cm⁴), ISMB 500 (86.9 kg/m, Ixx = 45218 cm⁴), ISMB 600 (122.6 kg/m, Ixx = 91813 cm⁴). For comparison, ISMB has thicker flanges and web than ISLB (Light Beam) of the same depth, and ISWB (Wide-Flange Beam) has wider flanges than ISMB at the same depth.
Design per IS 800:2007 Section 8 covers flexure, shear, lateral-torsional buckling, and deflection of ISMB beams. Lateral-torsional buckling is the most common governing limit state for unrestrained ISMB beams — the top flange must be laterally supported (by slab, secondary beams, or bracing) at intervals satisfying Cl. 8.2.2 effective length criteria; otherwise the moment capacity is significantly reduced. For continuous floors, ISMB beams supported by composite slab construction (with shear studs per IS 11384) achieve nearly full plastic moment capacity due to the slab's lateral restraint.
- Floor beams in steel-framed multi-storey buildings
- Industrial mezzanine floors and platform structures
- Roof girders for medium-span PEB structures
- Bridge stringers — IRC 24 secondary girders
- Crane runway beams in factories (often ISMB 300-500)