I-beams are shaped like an I because bending puts the highest stresses at the top and bottom of a beam, so the shape puts most of its steel there, in the flanges, and uses a thin web to hold them apart. That makes an I-beam far stronger and stiffer per kilogram than a solid bar: a W200x27 steel beam is roughly 27 times stiffer in bending than a solid square bar of the same weight. The trade-off is that the shape is only strong in one direction, and it handles twisting poorly.
What happens inside a beam when it bends
Picture a beam spanning between two posts with a floor load on top. It sags slightly. As it does, the top of the beam is squeezed shorter (compression) and the bottom is stretched longer (tension). Somewhere in between is a line that is neither squeezed nor stretched, called the neutral axis. On a symmetrical beam it runs through the middle of the depth.
The stress at any point in the cross-section grows with its distance from the neutral axis. Steel at the very top and bottom works hardest. Steel near the middle does almost nothing to resist bending. So if you have a fixed amount of steel to work with, the smartest place to put it is as far from the middle as you can, and that is exactly what flanges do.
Two section properties capture this:
- Moment of inertia (I) measures how well a shape resists bending deflection. Each bit of area counts in proportion to the square of its distance from the neutral axis, so moving steel outward pays off quickly.
- Section modulus (S) is the moment of inertia divided by the distance to the outer fibre. It sets bending strength: a bigger section modulus means a bigger load before the steel yields.
The I-shape maximizes both for a given weight. For the engineering detail behind these numbers, see W-beam section properties and moment of inertia.
What the flanges and the web each do
An I-beam (in Canada usually a W-shape, or wide-flange beam) has three parts, and each has a job:
- Top flange: carries most of the compression from bending.
- Bottom flange: carries most of the tension.
- Web: the vertical plate that joins them. It holds the flanges at the right distance apart so they work as a pair, and it carries the shear force, which is highest near the supports.
Shear doesn't need much steel, which is why the web can be thin compared with the flanges. On a W200x27, the web is 5.8 mm thick while each flange is 8.4 mm thick and 133 mm wide. Our guide to the parts of a steel beam walks through each dimension and why it matters on site.
Depth is the big lever. Doubling the distance between the flanges, with the same flange size, roughly quadruples the flanges' contribution to stiffness. That is why a deeper beam of the same weight usually spans further, and why beam sizing is often a trade-off between strength and headroom.
Is an I-beam stronger than a solid beam?
It depends what you hold constant. Here is a W200x27 (W8x18), one of the most common house beams in Ontario, compared with solid steel sections. Beam values are from the CISC section tables; the solid sections are simple calculations.
| Section, bent about its strong axis | Mass (kg/m) | Moment of inertia (x 10⁶ mm⁴) | Section modulus (x 10³ mm³) |
|---|---|---|---|
| W200x27 I-beam, 207 mm deep | 26.6 | 25.8 | 249 |
| Solid square bar, same mass (about 58 mm square) | 26.6 | 0.96 | 33 |
| Solid plate on edge, same mass and depth (207 x 16 mm) | 26.6 | 12.1 | 117 |
| Solid block, same outside size (207 x 133 mm) | about 216 | 98.3 | 950 |
What this shows:
- Against a solid bar of the same weight, the I-beam wins easily: about 27 times stiffer and about 7.5 times stronger in bending.
- Against a flat plate of the same depth and weight, it is still about twice as stiff and strong. The plate would also tip and buckle sideways long before reaching that strength.
- A solid block the same size is stronger, but it uses about eight times as much steel. Every extra kilogram has to be bought, handled, lifted and carried by the posts and footings below.
So the honest answer to "why is an I-beam stronger" is that it isn't stronger than everything; it is stronger for its weight. Steel is sold, shipped and installed by weight, and a beam has to carry its own weight too, so strength per kilogram is what matters in real buildings.
Where the I-shape is weak
The same geometry that makes an I-beam efficient in one direction creates limits in others.
Weak axis bending
Turn a W200x27 on its side and its moment of inertia drops from 25.8 to 3.30 x 10⁶ mm⁴, about one-eighth. An I-beam is meant to be loaded through its web, top to bottom. Loads that push sideways, or a beam installed on its side, need a different check entirely.
Twisting
Open shapes like I-beams and channels resist twisting (torsion) poorly. A load hung off the edge of one flange, such as a brick ledge or a balcony bracket on one side, twists the beam. Closed shapes like square and rectangular HSS tubes are much better in torsion, which is one reason they show up as edge beams, mono stringers and posts. See W-beam vs HSS for when each fits.
Sideways buckling
The compression flange of a long beam behaves a bit like a slender column lying on its side. If nothing holds it in place, it can buckle sideways and twist before the steel reaches its full bending strength. This is called lateral torsional buckling. Floor joists, decking or bracing fastened to the top flange usually prevent it; a long beam with nothing attached to it can lose a large share of its capacity. Our post on lateral bracing and unbraced length explains how designers deal with it.
Local buckling and concentrated loads
Thin plates can wrinkle locally under high stress. Rolled shapes are proportioned to limit this, but where a heavy post or concentrated load lands on a beam, or the beam sits on a short bearing, the web may need stiffener plates. Those are designed by the engineer and shown on the shop drawings.
Why most "I-beams" today are wide-flange W-shapes
People say "I-beam" for almost any steel beam, but the original I-beam (now called an S-shape or standard beam) has narrow, tapered flanges. Modern framing mostly uses W-shapes, with wider, parallel flanges. The wider flanges:
- Improve weak axis strength and resistance to sideways buckling.
- Give a flat surface for bolting, welding and seating wood framing or a nailer.
- Make connections simpler and more consistent.
There are also H-shaped W-sections, with flanges about as wide as the beam is deep, that work well as columns because they are closer to equally strong in both directions. Our article on I-beam vs W-beam vs H-beam covers the naming and the practical differences.
What this means when you choose a beam
Understanding the shape helps you have a better conversation with your designer and fabricator:
- Deeper is stiffer. If headroom allows, a deeper, lighter beam often outperforms a shallower, heavier one on the same span.
- Load it the right way up. I-beams are designed to carry load through the web, top flange to bottom flange.
- Brace it. Joists, blocking or decking attached to the compression flange let the beam develop its full strength.
- Watch eccentric loads. A load hung off one side twists an open section; your engineer may add plates, change the detail or choose an HSS.
- Don't cut the flanges. The flanges carry most of the bending, so notching or cutting them, even a little, can remove a big share of the capacity.
How much a particular beam can carry depends on its size, span, support conditions and bracing, which is why the number comes from design rather than a rule of thumb. For a feel for the factors involved, see how much weight a steel beam can hold.
Getting the right beam for your project
We supply and fabricate W-shapes, HSS and channels in our CWB certified shop in Erin, cut to length with plates, holes and connections ready to install, and we deliver across the GTA and Ontario. Engineering services are available for projects that need them, and we install structural steel, including basement beams and posts.
Send your drawings or your designer's beam size and we'll prepare a quote: request a quote, or browse our steel products.
Frequently asked questions
Why are I-beams so strong?
Because the shape puts most of the steel in the flanges, as far from the centre of the beam as possible, where bending stresses are highest. The thin web holds the flanges apart and carries shear. Per kilogram of steel, that gives far more bending strength and stiffness than a solid bar.
Is an I-beam stronger than a solid beam?
For the same weight of steel, yes, by a wide margin when loaded on its strong axis. A W200x27 is about 27 times stiffer in bending than a solid square bar of the same mass. A solid block with the same outside dimensions is stronger still, but it weighs about eight times as much.
Why are steel beams I-shaped instead of square tubes?
For simple bending in one direction, the I-shape is the most efficient common section and its open flanges are easy to bolt, weld and connect to. Square and rectangular tubes (HSS) are better where a member twists, bends in two directions or works as a column.
What is the weakness of an I-beam?
I-beams are weak about their minor axis (sideways), resist twisting poorly, and long unbraced beams can buckle sideways under load. Designers handle this with bracing, connections and by choosing a different shape where twisting governs.
