What is the HL-93 live load?

HL-93 is the AASHTO LRFD highway design live load: the greater of a design truck or design tandem, combined with a 0.64 kip/ft design lane load.

Does the dynamic load allowance apply to the lane load?

No. The dynamic load allowance (IM, typically 33%) applies only to the design truck or tandem, never to the design lane load.

AASHTO HL-93 Live Load

AASHTO LRFD Preliminary estimate

Maximum highway live-load moment and shear per design lane on a simple span — design truck vs. tandem plus lane load, with the dynamic load allowance applied correctly.

Results update as you type. These are educational estimates — confirm against the governing code and a licensed engineer before design use.

Method & references

The HL-93 highway live load (AASHTO LRFD §3.6.1.2) is the greater of the design truck or the design tandem, plus the design lane load:

Design truck — 8, 32, 32 kip axles; 14 ft to the first rear axle, 14–30 ft variable to the last (the HS-20 truck)
Design tandem — two 25-kip axles, 4 ft apart
Design lane load — 0.64 kip/ft, uniform

LL + IM = max(M_truck, M_tandem) · (1 + IM) + M_lane

The dynamic load allowance IM (§3.6.2) — 33% for strength and service limit states, 15% for fatigue — applies to the truck or tandem only, never to the lane load. Results are per design lane. To get per-girder demand, apply the live-load distribution factors and multiple-presence factors of AASHTO LRFD §4.6.2 / §3.6.1.1.

Reference: AASHTO LRFD Bridge Design Specifications, §3.6. Verify against the governing edition.