question: W-shape versus "American standard shape (S-Shape)" and flange tapering in genera

[fryed911]

I just can't seem to find any answers on my own to the following basic question:

Why were the flanges tapered on the S-shape? Why has the W-shape essentially replaced it in modern construction?
I believe modern channels are still tapered. Again, why?

 

[Lesali]

I don't know the history but S shapes are used in cranes and trollies. Here the load is applied directly to the bottom flange. The S shapes are better suited for this use having a tapered flange, the section modulus of the flange increases with the bending moment.

 

[dik]

I think the earlier S shapes (I shapes) were rolled using rollers that 'backed off'... this means that as they moved away, the beams were rolled to the same depth, only the web and flange thickness changed.

As rolling mills evolved, there was movement of both the web rollers and the flange rollers and all dimensions could be varied for the most effective section.

Early cranes accommodated the 'bevelled' flanges and still do... because cranes often have high shear loads, the thicker web is a bit of an advantage. You can easily get equipment that works well with W sections as well as S sections.

Dik

 

[hokie66]

dik is correct. When rolling tapered flange sections, the internal geometry of the flange stays the same, but the web gets thicker with the heavier shapes, thus the width of the member is wider.

Whether "modern" channels are tapered or not depends on where you are, I think. In Australia, we have parallel flange channels (PFC) for the bigger ones, and tapered flange channels (TFC) for the little ones.