What is the difference between Inventory and Operating rating?

The Inventory rating uses a live-load factor of 1.75 and reflects loads a bridge can carry indefinitely; the Operating rating uses 1.35 and reflects the maximum permissible load, giving a higher rating factor.

What does a rating factor below 1.0 mean?

A rating factor less than 1.0 means the factored live-load demand exceeds available capacity at that level — the bridge may require load posting, restriction, or strengthening.

LRFR Load Rating

AASHTO MBE Preliminary estimate

Inventory and Operating rating factors from the AASHTO LRFR rating equation — available capacity against factored dead- and live-load demand.

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

Method & references

The AASHTO LRFR rating factor (Manual for Bridge Evaluation, §6A.4) compares available capacity against factored dead- and live-load demand:

RF = ( C − γ_DC·DC − γ_DW·DW ) / ( γ_LL·(LL + IM) )

C = φ_c · φ_s · φR_n ( with φ_c·φ_s ≥ 0.85 )

C — capacity, the factored resistance reduced by the condition factor φ_c and system factor φ_s
γ_DC = 1.25, γ_DW = 1.50 — Strength I dead-load factors
γ_LL = 1.75 (Inventory) or 1.35 (Operating)

RF ≥ 1.0 means the member is adequate for that live load. The Inventory level reflects indefinite everyday loading; the higher Operating level reflects the maximum permissible load. Multiply RF by the rating vehicle's gross weight to express the rating in tons. LL + IM here is the per-girder live-load effect — combine the HL-93 tool with the distribution- factor tool to obtain it.

Reference: AASHTO Manual for Bridge Evaluation, §6A. Verify against the governing edition.