Kumar IronUpdated
In this guide
Parts of an I beam
- Flanges: the two horizontal plates at the top and bottom. They take most of the bending stress.
- Web: the vertical plate joining the flanges. It takes most of the shear and holds the flanges apart.
- Depth: the overall height of the section. In Indian sizes the number in the name is the depth in mm.
- Flange width and thicknesses: listed in the section table for each size.
Why the I shape works
When a beam bends, the top gets squeezed and the bottom gets stretched. The material near the middle does little work. An I section puts most of the steel at the top and bottom, where it is needed, and keeps a thin web in the middle. You get a stiff, strong beam with less steel, so less weight and less cost than a solid bar of the same strength.
The trade off is that an I beam is strong when loaded on its deep axis and much weaker sideways. That is why it must be placed upright and often braced against twisting.
What ISMB means
ISMB stands for Indian Standard Medium weight Beam. The dimensions and weights are listed in IS 808, and the steel itself is usually to IS 2062. ISMB 200, for example, is 200 mm deep. Other series include ISLB (light), ISWB (wide flange) and ISJB (junior beams), but ISMB is the one most commonly stocked.
Sizes run from ISMB 100 up to ISMB 600. For houses, shops and small sheds, ISMB 100 to 300 cover most needs. Larger sizes are for heavy industrial and bridge work.
Common uses by size
| Size range | Typical use |
|---|---|
| ISMB 100 to 150 | Jack arch roof joists, small lintels, loft supports |
| ISMB 175 to 250 | Mezzanine floor beams, wide shop openings, shed main beams |
| ISMB 300 to 400 | Industrial sheds, crane runways, heavy floor beams |
| ISMB 450 to 600 | Large industrial frames and bridges |
The right size depends on span, load and support conditions. Always get it from a structural engineer. For kg per metre of every size, use our ISMB weight chart.
I beam vs channel vs angle
- I beam: best for spanning and carrying heavy loads in bending.
- Channel (C section): good for frames, purlins and edges where one flat face is useful.
- Angle (L section): good for bracing, small frames, and light trusses.
Frequently asked questions
- What is the weight of ISMB 150 and ISMB 200?
- Weights vary by size and are listed in IS 808. See our ISMB I beam weight chart for kg per metre and per 6 m or 12 m length of every size.
- Is an I beam the same as an RSJ or joist?
- In everyday shop language, yes. An RSJ or MS joist is a hot rolled I section, which in Indian sizes is usually an ISMB.
- Which way up should an I beam go?
- With the web vertical and the flanges top and bottom. Laid flat, it loses most of its strength.