AEP-3.12.1.5 military load classification of bridges, ferries, rafts and vehicles

What it is

AEP-3.12.1.5 is a NATO Allied Engineering Publication that fixes a single method for computing the Military Load Classification (MLC) of bridges, military ferries and rafts, including their landing stages, and of military vehicles (clause 1.1). Nations agree that NATO forces will use this method (clause 1.2), and the agreement itself is recorded in STANAG 2021, the cover this document sits under. Partner Nations and Mediterranean Dialogue nations are encouraged to use the same method but are not committed to it, and the document is explicit that its conditions "are not intended to govern existing or future civil bridge design or construction" (clause 1.2). Edition B, Version 1 was promulgated on 26 November 2024 and superseded Edition A, Version 1.

The MLC itself is a comparative number, not a weight. The document is blunt about it: "THE MILITARY LOAD CLASSIFICATION NUMBER IS ONLY A NUMBER, IT DOES NOT REPRESENT THE MASS OF THE VEHICLE" (clause 1.3, para 1). Vehicles and bridges are rated against the same reference set, thirty-two hypothetical vehicles, sixteen tracked and sixteen wheeled, spanning MLC 4 to MLC 150 (clause 2.1), using the same bending-moment and shear-force curves (Annex C) as the common yardstick.

The MLC number and the crossing rule

The document's basic crossing rule is that a vehicle may cross only where its own MLC does not exceed the bridge's, ferry's or raft's; where that is not the case, the document sets out span- and vehicle-specific analysis and exceptional-conditions overrides rather than a blanket prohibition (clause 1.3, para 2; clause 4.1.10; Annex H). A resulting MLC above 150 is reported as "MLC 150+", a value below 4 is calculated against the MLC 4 curve, and a calculated value is always rounded to the nearest whole number (clause 2.5).

Classifying a vehicle

A vehicle's permanent MLC is normally set by the standard (analytical) method: plotting its calculated bending moments and shear forces, at a fixed 30.50 m convoy spacing, against the Annex C curves, and taking the nominal MLC as the highest value across all spans from 1 m to 100 m (clause 3.2, paras 1-3). Vehicles narrower than the equivalent hypothetical vehicle get a width correction that increases the rated MLC (clause 3.2, para 5; Annex D). The document names reference software, VMLC and NAVMLC, as the recommended means to determine a rating "under the responsibility of the national official authorities" (clause 3.2, para 9). A separate expedient (temporary) method lets a vehicle's laden mass be multiplied by a fixed coefficient that varies by the unit the mass is expressed in; the document is explicit that the result "cannot be marked on the vehicle" (clause 3.3, paras 1-2). Vehicle combinations, trailers, hybrid vehicles and lightly loaded vehicles each have their own rule (clause 3.4).

Rating a bridge, ferry or raft

Only the rating for "Normal Crossing" conditions may be permanently posted on a bridge. The document also describes "Caution Crossing" and "Risk Crossing" as tactical or emergency alternatives in Annex H, available at a controlled speed of no more than 5 km/h and, for fixed bridges, one vehicle per structurally independent span, but it does not itself set a posted rating for either, leaving the decision to "appropriate command levels" (clause 4.1.2). All bridges are rated for both wheeled and tracked vehicles, with a minimum 30.50 m longitudinal convoy spacing and, for two-way traffic, a minimum 0.50 m adjacent spacing between vehicles (clauses 4.1.3, 4.1.7). Civilian fixed bridges can have a permanent MLC fixed by the bridge owner from a complete, reliable set of bridge data (clause 4.2.1); the document sets out five categories of assessment methodology and states that only the more detailed analytical methods, not the reduced or correlation-based ones, are suited to a permanent rating (clause 4.2.5). A damaged bridge is rated Green, Amber or Red depending on the extent of damage to its critical load-carrying members (clause 4.2.6).

Floating bridges, rafts and ferries

Military fixed and floating bridges, rafts and ferries are normally rated as part of a design and test programme rather than assessed after the fact (clause 4.3.1). Floating equipment carries its own criteria: both strength and hydrostatic/hydrodynamic stability must be checked, with the lower of the two figures governing the rating (clause 4.3.5.3), against standard current speeds the document sets for construction and normal use versus survival conditions (clause 4.3.5.4), and against a table of minimum freeboard by floating-support type (clause 4.3.5.5). A raft or ferry may also carry a maximum MLC representing several simultaneous vehicles, in addition to its single-vehicle nominal MLC (clause 4.3.5.5). Bridges supplied by private industry and advertised "on the open world market with a standard NATO MLC" may be given a temporary rating, but only once the company has warranted the MLC or offered supporting test and analytical data to the satisfaction of the national authorities concerned (clause 4.3.4).

Marking (Annex J)

Marking a bridge, raft or vehicle with its classification is compulsory on in-service vehicles, on military-constructed bridges, rafts and ferries during training and operations, and on all operationally essential bridges in a NATO Joint Operations Area; marking existing civilian infrastructure in peacetime is left to national discretion (clause J.4). The Annex fixes sign shapes, minimum sizes and colours for circular bridge and raft signs and rectangular restriction signs, and for front and side vehicle signs, and exempts vehicles of 3 tonnes gross weight or less and small baggage trailers from the marking requirement (Annex J.6, para 6).

Who it binds, and how

AEP-3.12.1.5 does not address a compliance supplier or a management-system auditor. Its own terms centre on the national authorities responsible for determining and verifying a vehicle's MLC, and on personnel "sufficiently qualified in bridge assessment", who alone are permitted to use span-dependent MLC curves or examine a specific vehicle/bridge crossing (clause 3.2, para 4; clause 4.1.10). It reaches two narrower groups directly: an equipment designer, since selected parts of the agreement "may also be used as a basis for the design of military equipment" (clause 1.1, para 3), and the private company advertising a bridge under a NATO MLC described above. A nation binds itself by ratifying the STANAG 2021 agreement.

How it is evaluated

There is no certification scheme attached to AEP-3.12.1.5, and it names no accredited body or notified body: "no specific method will be imposed through this document" for the safety concept a rating depends on (clause 4.1.5, para 6). A vehicle's permanent MLC is calculated under the responsibility of the national official authorities (clause 3.2, para 9); a military bridge's permanent MLC is assigned once it passes a country's own in-service acceptance criteria, verified by calculation and normally by load testing (clause 4.3.3); and a private company's bridge earns even a temporary rating only once the national authorities are satisfied (clause 4.3.4). What gets checked afterwards is the posted rating itself, the Annex J signs a crossing commander compares against a vehicle's own MLC before a normal-conditions crossing.

Getting the document

NATO publishes AEP-3.12.1.5 through the NATO Standardization Document Database at no charge. ComplyTrain does not sell it or host a copy; the NSDD listing for AEP-3.12.1.5 is the source. It is covered by STANAG 2021, the agreement recording nations' commitment to use it.

Standards it references

• AMovP-01Background