Weight limits?

I work construction and I’ve had some discussion with fellow drivers about how much we can legally haul. Most drivers have said the limit is 80k gross for a tractor with a dual axle trailer or 84k-85k (depending on who I asked) if the trailer is a tri-axle, or again depending on who I asked, a dual axle trailer with tags. My understanding, and please correct me if I’m wrong, is that the limit is 80k gross regardless of axle configuration unless one has an overweight permit. Also, I suppose it is possible that because dump trailers are shorter than typical freight trailers (around 38-42 ft) one could gross out under 80k but still be over on the ‘trailer bridge’ or any other axle combination though I have no idea if this is likely since I’ve only ever been able to get an overall weight on my truck at the gravel pits and don’t drive out of state.

I know there are some differences state to state but for starters I'm only looking for information that applies to driving on an interstate. Who's right or are we all wrong?

 

Thanks for the reply!

According to http://ops.fhwa.dot.gov/freight/sw/overview/index.htm

Federal commercial vehicle maximum standards on the Interstate Highway System are:
Single Axle: 20,000 pounds
Tandem Axle: 34,000 pounds
Gross Vehicle Weight: 80,000 pounds

According to http://vsw.fhwa.dot.gov/qa/qa.jsp?category=23 CFR 658.17

What is the definition of a single axle?

It is one or more axles not more than 40 inches apart. If two axles are less than 40 inches apart, they are considered to be a single axle.

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May States limit the maximum weight of steering axles on the Interstate System to less than 20,000 pounds?

They may limit the weight to 20,000 pounds or the axle weight rating established by the manufacturer, whichever is lower. ​

Kind of confusing since it seems to define steers as 'Single Axle' but doesn't say anything about how many tires per axle. It would also give 84k gross but I guess you would be ok if one set of axles was under the limit.

Regardless, am I correct in assuming a tri-axle or tag axle equipped trailer in a tractor-trailer would still be limited to 80k? Would it also be reasonable to assume that with a short 40ft trailer that 80k gross could still be over weight on the trailer bridge or drive axles?