Load Rating

What It Means

Load rating is the quantitative engineering evaluation of a bridge's live-load capacity, performed under the AASHTO Manual for Bridge Evaluation (MBE) using one of three methodologies: Allowable Stress Rating (ASR, legacy), Load Factor Rating (LFR, transitional), or Load and Resistance Factor Rating (LRFR, current standard for all new ratings since 2013). The analysis produces a Rating Factor (RF) for specific rating vehicles at two stress levels: Inventory (equivalent to design stress levels, can be sustained indefinitely) and Operating (maximum permissible under controlled conditions). A Rating Factor of 1.0 means the bridge can exactly carry the rating vehicle at the specified stress level; above 1.0 means additional capacity exists; below 1.0 means the bridge cannot safely carry that vehicle and must be posted, permitted, strengthened, or replaced. Standard rating vehicles include HS-20 (the historical 36-ton AASHTO design truck), HL-93 (the current AASHTO LRFD design load combination), and the Legal Loads representing typical state legal trucks (Type 3, Type 3S2, Type 3-3). Some states also rate for Specialized Hauling Vehicles (SHV) and Emergency Vehicles (EV). Load rating must be updated when bridge condition changes, when rating methodology changes, or on a schedule not exceeding five years under the 2022 NBIS. Load rating interacts directly with the NBI: Item 63 captures Operating Rating Method, Item 64 captures Operating Rating, Item 65 captures Inventory Rating Method, and Item 66 captures Inventory Rating. The Inventory Rating in metric tons is what most state DOTs use to establish posting thresholds. Load rating is performed by licensed professional engineers, typically at the state DOT bridge office or through consultant contracts.

Frequently Asked Questions