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Reviewed and updated by ICD - Week 24/2026
Quick summary:
- The key technical metrics of a dunnage bag are its AAR load level, the standard void range, and how it handles non-standard voids.
- AAR level (1-5) rates how much impact force the bag absorbs - the gold standard for transport safety, mandatory for rail.
- Static compression fixes the load initially; dynamic compression pushes it back when it surges in transit.
- The bag only reaches optimal pressure when the void sits within its rated range (typically 10-40 cm, up to a stated max void).
AAR load level: the gold standard for transport safety
The AAR (Association of American Railroads) level is the leading standard, mandatory for dunnage bags used in rail transport and treated as the benchmark for all multimodal container shipping. It rates the bag’s ability to absorb and disperse strong impact forces. For the fundamentals, see what a dunnage bag is.
AAR levels and what they mean
AAR classifies bags into levels 1 to 5 based on the simulated impact (shunting) they withstand in testing:
- Level 1-2: light to medium loads, road or low-impulse sea transport. Level 2 is usually the minimum for normal ocean freight.
- Level 3-5: heavy to very heavy loads, especially rail or international sea transport where impact forces are large.
A shipment of timber to the US once failed because only Level 1 bags were used for rail - the goods shifted and were badly damaged. Always pick an AAR level above the expected risk, especially where rail is involved.
The forces inside a container
| Force | Source | Danger |
|---|---|---|
| Impact | Rail-car coupling (shunting), port handling | Destroys load structure, tears the bag |
| Surge | Hard braking, sudden acceleration of truck or ship | Longitudinal shift, pressure on the doors |
| Sway | Heavy seas, cornering, rough roads | Toppling - the greatest pressure on the bag |
| Vibration | Continuous engine and road vibration | Loosens lashings, gradual load shift |
Standard and non-standard voids
Standard void and the max-void rule
The standard void is the ideal even gap between loads where the bag can inflate and create optimal pressure - usually 10 cm to 40 cm of even width. Each bag size (for example 100 x 180 cm) has a manufacturer-stated maximum void (for example 60 cm). Exceed it and the bag deforms into a sphere, losing dynamic load capacity. Stacking the goods to leave even, easy-to-fill voids is a simple but critical step.
Non-standard voids and the fix
These are uneven or too-large/too-small gaps. For a too-large void, add rigid filler (a wooden pallet, thick cardboard or EPS foam) to bring it within the max void before inflating - do not stack multiple bags to fill one large gap, as that creates weak points. For uneven voids, use bridging: place boards or stiff cardboard on each side to make a flat, stable surface so the bag inflates evenly and transfers force straight to the load. On a machinery shipment with protruding parts, cardboard taped around sharp edges created a flat surface and stopped the kraft bag tearing the moment the ship weighed anchor.
Static and dynamic compression
Static compression is the initial inflated pressure (around 0.2 bar for Level 1) that fixes the load. Dynamic compression is the bag’s response when the load surges: as the goods press in, the bag increases counter-pressure and pushes them back. The whole point of a higher AAR level is a greater ability to absorb dynamic force - which can be 2-3 times the static load. See structure and types for how materials deliver this.
Frequently Asked Questions (FAQ)
What does the AAR level measure?
The impact (shunting) force a bag can absorb. Levels run 1-5; Level 2 is the usual minimum for ocean freight, Level 3+ for heavy or rail loads.
What is the max void?
The largest gap a given bag size can safely fill. Beyond it, the bag over-rounds and loses dynamic capacity.
How do I handle a too-large void?
Add rigid filler (pallet, thick cardboard, EPS foam) to shrink it within the max void before inflating - do not stack bags.
What is bridging?
Placing boards or stiff cardboard across an uneven gap to create a flat surface so the bag inflates evenly and transfers force directly to the load.
Contact ICD Viet Nam
For help matching AAR level and void size to your cargo, contact our team for specifications and a quote.
- Northern Vietnam: 0983 797 186 / 090 345 9186 / 090 5859 186
- Southern Vietnam: 098 6784 186
- Email: sales@icdvietnam.com.vn
- Zalo: Chat on Zalo
