# AASHTO HL-93 Loading Highway Design

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