HL-93 is a type of theoretical vehicular loading proposed by AASHTO in 1993. It is used as the design loading for highway structures in USA and other countries where AASHTO code is followed. AASHTO HL-93 vehicular live load is a combination of three different loads,

1. HL-93 Design Truck (formerly, HS20-44 Truck)
2. HL-93 Design Tandem (formerly, Alternate Military)

Details of application and calculation of Design Vehicular Live Load is explained in “AASHTO Bridge Design Specifications Cl 3.6.1.2”.

## HL-93 Design Truck

Design Truck consists of three axles, front and two rear axles with front axle weighing 8kip (35 kN) and two rear axles weighing 32kip (145 kN). The distance between front and rear axle is 14’ (4.3m) and that of two rear axles can be varied between 14’ (4.3m) to 30’ (9.0m) to obtain the worst design force. The tyre to tyre distance in any axle is 6’ (1.8m).

Generally, Center of truck wheels must be at least 2’ (600mm) from the edge of a design lane and 1’ (300mm) from the deck overhang. Minimum tail to nose distance between two successive trucks in a lane is 50’ (15m).

For Truck and Tandem, the design contact area of tyre is assumed to be a single rectangle of with 20” (510mm) and length 10” (250mm). The tyre pressure is assumed to be uniform over the contact area.

## HL-93 Design Tandem

HL-93 Design Tandem consist of twin axles spaced 4’ (1.2m) apart, weight of each axle is 25kip (110 kN). The distance between the tyres in an axle is 6’ (1.8m).

To obtain maximum negative moments, a pair of tandems should be considered, spaced at 8.0m to 12.0m along with design lane load to produce worse hogging effect. Code doesn’t specify maximum number of tandems that can be considered in a lane, nor it does explicitly specify minimum tandem to tandem distance.

The design lane load shall consist of a load of 0.64 klf (9.3 kN/m) uniformly distributed in the longitudinal direction. In transverse direction, the design lane load shall be assumed to be uniformly distributed over a 10.0’ (3m) width.

Design vehicle and lane loads should be applied in such a way that extreme force effect is obtained for design. In case of multiple lanes, Multiple Lane factors described in the code shall be considered.

Impact factor or dynamic load allowance is applied only for trucks and tandems, not for lane loading. The governing design force shall be taken as the larger of the following:

1. The effect of the design tandem + the design lane load;
2. The effect of one design truck (HL93) + the effect of the design lane load;
3. For hogging moment between inflection points, 90% of the effect of 2 nos. design trucks (HL93 with 4.3m axle spacing) spaced at a minimum of 15m. combined with 90% of the design lane load.