This is a hypothetical Live Load Model proposed by AASHTO for analysis of bridges. Reason for proposing this live load model is to prescribe a set of loads such that it produces extreme load effect approximately same as that produced by the exclusion vehicles. Exclusion vehicles were the vehicles above the legal limit but due to grand fathering provision in the state they were allowed to operate routinely.
It has 3 basic Live Loads for bridges called HL-93 Loading (where H stands for highway and L stands for Loading, developed in 1993).
- Design Truck
- Design Tandem
- Design Lane
1. Design Truck:
It is commonly called as HS-20 44 (where H stands for highway, S for semi-trailer, 20 TON weight of the tractor (1st two axles) and was proposed in 1994)
2. Design Tandem:
It consists of two axles weighing 25 kips (110 KN) each spaced at 4 ft (1.2 m)
3. Design Lane:
It consists of uniformly distributed load of .64 kip/ft (9.3 N/mm) and assumed to occupy 10 ft (3 m) transversely.
HL-93 is the maximum of the two:
- Design Tandem + Design Lane
- Design Truck + Design Lane