Can someone explain the structural steel beam designation LB21x120?

Question

There are no W,S or M shapes that are 21x120. Is this a custom designation? I cannot find a reference to LB in the Manual of Steel Construction.

Answer 1

Strange, generally the second number is weight in pounds per foot...the closest standard size I'm aware of is a W21 X 101, the "LB"? I don't know... could be a W with a larger flange width and web thickness than W21 X 101.

Edit: There is a W21 X 122..just found it.

Answer 2

E=mc2

Answer 3

It is what is called a "historical shape" as designated by AISC and probably dates back to the earlier 20th century (1920s).  It is a beam 21 inches deep by 120 lbs/linear foot.

Answer 4

Never heard of it.  Must be some old obsolete shape, or a custom shape.