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    Structural / Bridges

    Bridge Load Rating

    The AASHTO rating factor: how many times the design live load a member can actually carry after its own dead load is deducted. Below 1.0 the bridge needs a posting; the inventory and operating levels bracket the answer.

    Inventory rating0.95Operating rating1.23Net capacity2650 kN·m
    Inventory rating
    0.95
    Post or strengthen
    Operating rating
    1.23
    Absolute limit
    Net capacity
    2650 kN·m
    After dead load
    Live load with impact
    1596 kN·m
    33% IM
    Rating against live load
    RF = (φRn − 1.25DC − 1.50DW) / (1.75(LL + IM)) at the inventory level.
    Rating against live load
    Live load (kN·m)Inventory RFOperating RF
    6001.902.46
    9001.271.64
    12000.951.23
    16000.710.92
    20000.570.74
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    Choose a quick result view, a scaled comparison, or the complete diagnostic set.
    Inventory rating against live load effect
    The rating falls as 1/LL — doubling the demand halves the factor.
    Inventory rating against live load effect0.0000.4260.8511.281.702.13600600900900120012001600160020002000Bridge Load Rating
    Canonical equations
    The exact relationships implemented by this engine.

    RF = (φRn − γDC·DC − γDW·DW)/(γLL(LL + IM)) · AASHTO MBE.

    γLL = 1.75 · The design check.

    γLL = 1.35 · The absolute limit.

    engineering methodology

    A clear path from inputs to results.

    The AASHTO rating factor: how many times the design live load a member can actually carry after its own dead load is deducted. Below 1.0 the bridge needs a posting; the inventory and operating levels bracket the answer.

    Step-by-step methodWorked exampleAssumptions explained

    How to use

    1. Set member capacity φrn (kn·m).
    2. Set dead load effect dc (kn·m).
    3. Set wearing surface dw (kn·m).
    4. Set live load effect ll (kn·m).
    5. Read the headline metrics, then check the table for the detail.

    The formula

    RF=(φRn−γDC⋅DC−γDW⋅DW)/(γLL(LL+IM))RF = (φRn − γDC·DC − γDW·DW)/(γLL(LL + IM))

    Reference: AASHTO MBE load rating: RF = (φRn − γDC·DC − γDW·DW)/(γLL(LL+IM)).

    Worked example

    At the default inputs

    Inputs
    • Member capacity φRn (kN·m): 4000; Dead load effect DC (kN·m): 900; Wearing surface DW (kN·m): 150; Live load effect LL (kN·m): 1200; Dynamic allowance IM (%): 33
    Calculation path
    1. 1

      Inventory rating: 0.95 · Operating rating: 1.23 · Net capacity: 2650 kN·m · Live load with impact: 1596 kN·m

    Result

    Inventory rating: 0.95 · Operating rating: 1.23 · Net capacity: 2650 kN·m

    Interpreting the result

    The inventory rating is 0.95, so the member carries 0.95 times the design live load.

    Assumptions & limitations

    • One member, one load effect.
    • No distribution factor or multiple presence.

    Frequently asked questions

    What does a rating below 1.0 mean?

    The bridge cannot carry the design live load and needs a posting or strengthening.

    Why are there two levels?

    Inventory is the design check at 1.75; operating is the absolute limit at 1.35.

    Methodology last reviewed 2026-10-22. Results are informational and not financial or engineering advice — see our disclaimer.