Live-Load Distribution Factors

AASHTO LRFD §4.6.2 Preliminary estimate

Per-girder live-load distribution factors for moment and shear — the piece that turns a per-lane HL-93 effect into the demand on a single girder.

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

Method & references

For a concrete deck on steel or prestressed-concrete I/bulb-tee girders (AASHTO LRFD §4.6.2.2 cross-section types (a), (e), (k)), the approximate live-load distribution factors are:

Interior moment, 1 lane: 0.06 + (S/14)^0.4(S/L)^0.3(K_g/12Lt_s³)^0.1
Interior moment, 2+ lanes: 0.075 + (S/9.5)^0.6(S/L)^0.2(K_g/12Lt_s³)^0.1
Interior shear, 1 lane: 0.36 + S/25 · 2+ lanes: 0.2 + S/12 − (S/35)²

K_g = n(I + A·e_g²) is the longitudinal stiffness parameter (n = modular ratio, e_g = girder-to-deck centroid distance). Exterior girders use a multiplier on the 2+-lane interior factor: e = 0.77 + d_e/9.1 for moment, 0.6 + d_e/10 for shear.

The factors are in lane units (the multiple-presence factor is already included) and apply to a single girder. Multiply the per-lane HL-93 moment or shear by the factor to get per-girder demand. The equations apply within set limits (3.5 ≤ S ≤ 16 ft, 20 ≤ L ≤ 240 ft, 4.5 ≤ t_s ≤ 12 in, N_b ≥ 4); outside them, use the lever rule or refined analysis.

Reference: AASHTO LRFD Bridge Design Specifications, §4.6.2.2, Tables 4.6.2.2.2b-1 and 4.6.2.2.3a-1. Verify against the governing edition.