Origin / History of MC & Junior Channels?

[BSVBD]

Does anyone know the origin or history of where "MC" channels came from?

What is the difference between "MC" and "C"?

I thought the flanges were different, but, I cannot tell from the AISC steel manual.

What is a "Junior Channel"?

I am aware that the MC12x10.6, 10x8.4, 10x6.5 and 8x8.5 are commonly referred to as "Junior Channel" and "Stair Channel", but why aren't the smaller and equally narrow "C" and "MC" channels labeled as such?

Incidentally, I commonly specify C12x20.7 and various HSS for stair stringers. How would one safely specify any of the aforementioned Junior Channel or even Plate for stair stringers? Is there a diaphragm formula, unity formula or other formula that people are using to specify such members?

Thank you!

 

[racookpe1978]

Classic definitions (back in the old days) declared C shapes as a member rolled with a 16 degree sloped upper and lower flange; the web being vertical and of uniform thickness, but not necessarily the same thickness as the two flanges. (Average of the flange thickness is normally a bit thicker than the web. Outside of the flanges are always exactly perpendicular to the outside of the web, the sloped in the flanges is inside the "C".)

BC is rolled as if from a single plate shape into the final shape, so flange thickness is constant across the flange, and web thickness = flange thickness. MC are also rolled and have parallel webs and flanges, but a MC shape may have thicker flanges and its web. They (MC and BC) are a little bit easier to bolt to through the flanges, since no spacer washer is needed to create a flat bolt surface inside the "C".

Both are designated by height x weight per foot.

I have have not heard of nor read of anybody using the term Junior Channel.