I-Beam Shape Choices: I, S, W, M or H?

When is an I-Beam not really an “I” Beam?  While it’s common to use the generic term “I-Beam” for any beam with an I shape section, there are actually variations with different letter designations.  And, there are advantages for the various shapes in certain situations — which is why they exist.

We started this discussion about beams of various types in the article “Beam Shapes To Build With“, but there is always more.  This article goes into more detail for the I style beams.

“I-Beam” Shape Differences

First off, I need to point out that beam nomenclature often depends on the country you’re in.  The info here is typical USA standards.  Second, I am not the beam guru, so I will not cover all the types or specifics.  The focus here is general knowledge for DIY projects like trailers and gantry cranes.

Classic I-Beam S ShapeThe classic standard (S) profile “I-Beam” shape has tapered flanges that are thick in the middle near the web, then thinner at the periphery.  This variation in thickness we call the slope or taper.  These beams also include a generous radius in the transition.

Wide Flange BeamThe other generic designation is an “H” profile which has a more blocky section.  They have a consistent thickness on the flanges and usually not a lot of radius — especially at the flange ends.

While these are typical references, the “I” and “H” often interchange, so be careful with the terms.

“W” or Wide Flange beams, and Narrow Flange beams are in the family of “H” beams, along with some other designations like UC, UB, HP, M, etc..  These group because the overall shape is similar.  — Though all the letters can get confusing.

Anyway, we won’t go into that detail in this article.  (Read more detail on Wikipedia or do a Google search.)

For our purpose, the flanges are the difference — with a taper for the standard I-Beam shape, and rectangular for the others.  In practical DIY, the standard I-Beam supports loads on the flanges better, while H-Beams are easier for bolting (by not dealing with the slope).

When ordering a beam, I suggest describing it as “Wide Flange”, “Narrow Flange”, “Standard I-Beam”, etc., rather than simply H or I.  Use the actual beam dimensions and weight per foot to verify the right one, not just the beam name.

Practical Uses of Each I-Beam Shape

Why would someone choose one shape over another?  Here are some examples from our project plans here at Mechanical Elements.  Since most DIYers don’t build skyscrapers or road bridges over the freeway, we’ll focus our attention in 2 areas where we have I-Beams in our plans:  Gantry Cranes and in some long Trailer Frames.

The key reason for I-Beams is strength to weight ratio.  That is bending strength superiority, because I-beam shapes are not very good for torsion loads and not extra awesome for shear strength.  In DIY, that offers some nice advantages when we need long, strong beams, that won’t bend.

For A Gantry Crane

For a gantry crane, the standard I-beam shape has the nice benefit of extra strength for the flanges.  At the intersection of the vertical and horizontal, the thick area along with the radius helps distribute stress from flange loading — like from a crane trolley, for instance.  It’s the taper along with the radius that distributes the load nicely to the rest of the beam.  One big advantage of the standard I-Beam shape.

The standard I-beam shape gives a little more strength for the weight.  That’s why the special shape exists.

Alternatively, if we choose an H shape of about the same size for a crane, there is additional stress.  Since the transition from web to flange is more abrupt, the load capacity is a little less.  (It does not mean the beam is not strong — there are many factors in that comparison.)

Our Gantry Crane plans do specify a standard I-Beam shape in the design.  While it is certainly possible to change to other shapes, the strength also changes some.

For A Trailer Frame

Narrow Flange I-Beam ShapeWhen considering trailers, the S, H and W sections work nicely because trailers usually don’t have the intermediate point loads on the flanges.  More often, the loading distribution is over a longer distance, or mounted directly to the web.  In our trailers that use I-Beams, we often specify one of the more squared up shapes.  (Not always, because the more optimized standard I-Beam shape has some nice strength to weight benefits.)

From an engineering perspective, the H style beams will sometimes offer a better platform to build on.  They can be thinner material, for the same height, and they have many more options for width and thickness for each height.  For example, in the 12″ height, they come in 19 different arrangements from 14 lbs/ft to 120 lbs/ft.  They also span the range from 11.91″ tall to 13.12″ tall with flanges from 3.97″ width to 12.75″ width.  Lots of choices.

With trailers, usually weight and stiffness are the driving factors, which means a taller, narrower beam is often the desired choice.  In other cases, like for a car hauler trailer, an I-Beam design allows a lower deck with more strength.

If you look under some trailers — like RV’s particularly — you might even find fabricated I-Beams.  These are not really “I-Beams”, rather they consist of 3 pieces of flat steel welded together in an I shape.  They do function like an I-Beam, however.

Defining and Ordering I-Beams

For most materials, the size designation is something like Width x Height x Material Thickness.  That is certainly true with Square and Rectangular tube.  Also, Angle iron is Leg Length x Leg Length x Thickness.  Even round stock and tube are referenced by the physical diameter (and wall thickness) dimensions.

On the other hand, I-Beam and C-Channel usually have a single height dimension, then a weight.  S 6″ x 12.5 lbs  or  S6x12.5  for example.  Here the S6 designates the Standard I-Beam shape profile at 6″ tall.  12.5 is the weight (steel) in lbs per foot of length.  All the other dimensions like width of the flanges, thicknesses of web and flanges, are baked into the standard for this beam at 12.5 lbs/ft.  You must look it up to know that beam is 3.33″ wide.

I-Beams with Labels

From this standpoint I-Beams are a little more difficult to work with.  For example, if we initially choose a 4″x 2″x 1/8″ wall rectangular tube, then later decide we should use a 3/16″ wall for more strength, it’s easy to substitute.  The outside dims are the same, so other areas of the design don’t change.  If, on the other hand, we decide to use the S6x12.5 beam, then later decide on the next heavier size, it is more than just a thickness difference.  S6x17.25, is wider at 3.56″ width.  The extra 0.23″ might not matter, but sometimes it does.  Especially, when both the width and height change – as with many “H” style beams.  This is just something to be aware of when designing with I-Beams.

Bolting To I-Beams

One of the complaints about standard I-Beams is bolting.  The beam shape is optimized for carrying a vertical bending load — which is wonderful — but, bolts don’t sit as well on the tapered flanges.

The taper, as shown in the image above, has an angle defined in a few different ways.  I’ve looked for a good definition, but this is not a popular dimension to show.  Furthermore, I found one reference saying 7.14° but that doesn’t seem right.  Most often it is given as 1:6 or “2 in 12”.  Sometimes as 16 2/3% which is consistent.  9.46° is the conversion.

A lot of times the slope doesn’t really matter.  For instance, with our Gantry Cranes, we just bolt through anyway.  Use a flat washer both on top and below so that will take the scratches.  Not a big deal since the joint loads are not really in tension or shear.

If the taper is a concern, there are ways to conquer it.

Taper WashersFirst, use special washers made for it.  See the image.  Tapered Washers have the same slope, so when put the other way, they make a surface parallel to the beam flange outer face.  They are usually available, but sometimes it’s just luck if your local bolt store has the right size or not.  We use them for both C-Channel and I-Beams.

Another option is to make your own taper washers.  Simply cut a small chunk of the beam flange from excess material, then drill a hole and use it like a taper washer.  It’s more work than just buying them, but in a pinch this works too.

Finally, an example of clamping on the beam taper is shown for our I-Beam Connector Clamp.  Plans are free with the code.  Here’s the article about the clamp.

Stress And Loads

It’s worth a small dive into stresses and loading with I-Beams in this conversation.  We won’t go too deep.

The one point to make, when doing traditional calculations for wide flange beams, once deflection becomes a constraining factor (like in a long trailer or a long gantry crane beam), you must compensate for flanges that deflect differently than the rest of the beam.  It’s something that comes out in FEA, but it is usually ignored in traditional calculations.  Basically, the wider the flanges are with respect to the height, the more it becomes a factor. Minimal as it is, it can be really important for broad spans.

We don’t see this in the traditional I-beam shape, because the ratio of height to width, and because of the thickness change from the web to the periphery.

Always More . . .

Beam Clamp Free PlansOf course, there is always more to learn about nearly every topic.  If this one sparks your interest, keep searching.  While there is good tech info around, it’s not super easy to find.

And if you like analysis, we have an intro article on calculating beam loading for trailers.

In the meantime I’ll leave you with an image of the I-Beam clamp assembly (shown as an end view for easy understanding).  You can see the taper to taper connection pretty easy.

Good luck with all your I-Beam Projects.

 

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