What are the weights allowed per axel on a tractor trailer?

The weights can actually vary by state, and what the vehicle is plated for. The general rule of thumb is 34,000 lbs. on a set of tandems, 20,000 lbs. on a single drive or trailer axle, and the steer axle... 12,000 is generally stated, but it's actually dependent on the weight rating of your tires and how much surface area contacts the road... it is possible for an 80k combination to run up to 20,000 lbs. legally on the steer axle, provided the axle and tires are rated for it - most are not, except on heavy haulers and vocational trucks.