According to my cheat book for 'C' section channels:
Size Web Section Modulus
3"x6# .36" 1.38
3"x5# .26 1.24
3"x4.1# .17" 1.10
4"x7.25# .32 2.29
4"x5.4# .18 1.93
----------------------------
6"x8.2# .20 4.38
8"x11.5# .22" 8.14

Stength is proportional to section modulus so take whatever tables you
have and work from there. Generally, 3" is 1/3rd as strong as 6" and
1/5th as strong as 8". 4" is half as strong as 6" and 1/4th as strong as
8" YMMV.

Cheers.


Grant Erwin wrote:
My buddy wants to build some ramps to load a skid steer onto the back
of a flatbed. He plans to build 2 ramps, each of which is built like a
ladder. The side rails will be steel channel and the cross pieces will
be steel angle, all hot rolled, welded. He has asked my help in figuring
how light he can build it. I figure his Bobcat weighs around 4000 pounds.
I own a Ryerson Steel data book which has allowable uniform loads for
steel channels but it doesn't go down to the 4" or 3" channel he's
considering. I can't find any useful specs online, not because I haven't
googled, but because I get way too much wrong information or because I
can't think of the smart way to limit or phrase the search string.

In particular, I want to know the allowable uniform load in ksi of 4" and
3" channel, each the lightest channel made (approximately 3/16") with a
span of eight feet.

Anyone?

Grant Erwin
Kirkland, Washington