English speakers call it an "I-beam" since it looks like the capital letter "I", though occasionally you will hear somebody call it an H-beam. In many European languages, it's a "double-T" beam. In India, it’s an Indian Standard Beam. In Australia, it’s a Universal Beam.
W? That’s because the American Institute of Steel Construction calls it a Wide Flange Beam. The flanges are the two long, flat things on the top and bottom. The AISC has a designation for just about every US manufactured shape, which is published in their “Steel Construction Manual.” That’s the book that engineers use when sizing beams. Here’s a list of the shapes that they provide info on. Each shape looks different (except for pipes and round HSS) and the info provided is sizes and structural properties.
W-ShapesM-Shapes
S-Shapes
HP-Shapes
C-Shapes
MC-Shapes
Angles (L-Shapes)
WT-Shapes
MT-Shapes
ST-Shapes
Rectangular (HSS-Shapes)
Square (HSS-Shapes)
Round (HSS-Shapes)
Pipe
Double Angles
2C-Shapes
2MC-Shapes
W-Shapes w/ Cap Channels
S-Shapes with Cap Channels
Crane Rails
We typically only use W, C, MC, L, WT, HSS and Pipe shapes. If you renovate old buildings, you may see some S shapes and some Crane Rail shapes. And, watch out: M shapes look very similar to W shapes.
Each shape has been engineered for a specific purpose. Each is really good – meaning it has an extremely efficient economy of material - at doing one or two things but probably not so good at other things. For W-Shapes that thing is supporting uniform loads; that is, loads that are evenly distributed along its length. It’s not as good at supporting non-uniform loads or loads that act to twist the beam (torsional loads). Of course, we use it all the time for supporting non-uniform loads. Don’t let that bother you, steel is really strong, and those beams are up-sized to handle the non-uniform load.
So why is the I shape so good as supporting uniform loads? Well, first we found that functionality to be a fact and then we asked the engineers to write an equation for it – not the other way around. That is usually the way it goes: observation then explanation (Einstein was the only guy who did it the other way around).
After years of monkeying around with structural beams – logs, lumber, trusses, steel beams, etc. – humans observed that the horizontal stresses (meaning the stresses that are perpendicular to the direction of gravity) experienced by beams supporting uniform loads are greatest at the extreme top and extreme bottom of the beam and get rapidly less as you get closer to the long centerline of the beam that’s half way between the top and the bottom. (By “observed” I mean “they observed their buildings and bridges fail and fall down.” That’s the most common method of engineering advancement: build, watch it fail, try something else, watch it fail, repeat. Sooner or later the duration between failures gets so long that it’s called success. And then we figure out the maths that best describes that success and use those maths to create future designs. Observation then explanation. Things don’t happen because of the maths, the maths only help us explain and predict what’s happening. Apples don’t fall to the ground because of an equation, the equation only describes the fall. Why is there such a thing as gravity? I don’t know, but, I have a nice equation to help describe it and predict when something will hit the ground after it starts falling.) That long centerline under the uniform load is called the neutral axis. The neutral axis experiences no horizontal stresses – no tension, no compression. Above the neutral axis everything is in compression, below the neutral axis everything is in tension.
So, these same observers said “Hey, let’s try putting most of the material that is experiencing the greatest stress – at the two farthest edges away from the neutral axis – and just connect them with one skinny web and see what happens.” First, they made S -Shapes. That worked so well they founded huge companies that turned into empires, like U.S. Steel and Bethlehem Steel, sold their product and built free public libraries all over the country and a super good music hall in New York with the extra income. As they got better at manufacturing the shapes they made different shapes that were friendlier for the building installers. The W shape is easier for installers than the S shape, even though they look very similar.
The web isn’t doing so much. You can safely drill all sorts of holes through it. Even big holes. Not so much through the flanges. The web is resisting the shear loads (think of scissors cutting a piece of paper, that’s shear), but, steel is super strong and that’s easy for it. And if it needs a little help it’s really easy to put a tiny vertical support in.
So, our I beams are the shape of an I because that shape has the most efficient use of material for performing the desired structural purpose while also being the easiest to work with.
BTW, if the beam has a bit of a torsional load – like a column from above landing to the left of right of the centerline of the web – then a rectangular HSS shape would be best. Rectangular and square HSS shapes are the most efficient shapes for supporting torsional loads.
And stress-skin panels – they’re really just I beams with a really fat and squishy web.
