What is curb weight on semi tractors?

It depends on the truck and whether you're meaning just the truck or the truck/trailer combo. Most trucks are in the 18,000-20,000 lb range but can vary anywhere from 15,000-25,000 lbs. Truck/trailer combos also vary depending on the specific truck and trailer but most are in the 32,000-35,000 lb range but can vary from 25,000-50,000 lbs.