Since about '83 all states in the US have the same basic laws governing axle weights. Since it was the state I accumulated most of My O-t-R miles in I jumped on their website to get a better answer to Your questions.
And the answer is:......(drum roll please) It depends!
There's Bridge Laws to take into account, axle spacing, from front most to rear most axle, etc. etc. Spacing from tandem axle pair to tandem axle pair.
Here's their link.....
http://www.dot.state.il.us/road/size_weight_bklt.pdf
My personal opinion is that since steer axles only have single tires on each end the tandem steer axles would only be granted 12,000#/axle capacity each unless they're spread more than 9 feet center-to-center. But the most any single axle is allowed is 20,000#. Tandems and Tri's have the same legal weight rating if 9 feet center-to-center or less. If a truck had driven tandems and a non-powered "Cheater Axle" spaced beyond that 9 ft I think the tandems would be allowed 34,000# and the cheater 20,000# more. If two cheaters were available, making the truck a Quad, then 34,000# for both pairs IF the bridge laws were obeyed.
I didn't take all the time necessary to read everything on this website. And I'm sure if You asked a dozen State Cops, DOT Officers, and Scale Masters at the IL weigh stations listed You'd get at LEAST ten different answers to Your questions.
