This educational application supplements, but does not replace, the official AASHTO LRFD Bridge Design Specifications, applicable state DOT manuals, project specifications, and professional engineering judgment.
Formula & symbol library
Quick reference
Every equation used in the published chapters, with its variable legend and AASHTO reference. Consult the licensed AASHTO LRFD Bridge Design Specifications for the full provision and any applicability limits.
AASHTO LRFD §1.3.2— LRFD design inequality
Σ ηi · γi · Qi ≤ φ · Rn
(1.3.2.1-1)- η_i
- load modifier (ductility · redundancy · importance)
- γ_i
- load factor
- Q_i
- force effect [kip, kip-ft]
- φ
- resistance factor
- R_n
- nominal resistance
AASHTO LRFD §3.6.1.2 / §3.6.2— HL-93 envelope
Lenv = m · [(1 + IM) · max(Ltruck, Ltandem) + Llane]
- L_env
- live-load force effect (per lane, before distribution)
- m
- multiple-presence factor
- IM
- dynamic load allowance (33% typ., 15% fatigue, 75% deck joint)
AASHTO LRFD §4.6.2.2.2b-1— DF, interior girder, moment, 1 lane
DF = 0.06 + (S/14)^0.4 · (S/L)^0.3 · [Kg / (12·L·ts³)]^0.1
- S
- girder spacing [ft]
- L
- span length [ft]
- t_s
- deck thickness [in]
- K_g
- long. stiffness param. [in⁴]
AASHTO LRFD §3.6.1.2.4— design lane load moment
Mlane = w · L² / 8 Vlane = w · L / 2 w = 0.64 klf
AASHTO LRFD §3.4.1— Strength I
MStr-I = 1.25·MDC + 1.50·MDW + 1.75·MLL+IM