Working Load Limits and Aggregate Tie-Down Requirements
Federal cargo securement standards under 49 CFR Part 393, Subpart I, are based on the concept that cargo must be secured against the forces that can act on it during normal and emergency driving. The regulations quantify minimum securement through the Working Load Limit (WLL) and aggregate WLL calculations.
Working Load Limit (WLL): the maximum load that a tie-down device may carry in each direction it acts. Every chain, strap, rope, or other tie-down device is rated by the manufacturer with a WLL stamped or labeled on the device. The WLL is typically about one-third of the device's breaking strength — a significant safety margin. Drivers must use devices whose WLL is rated appropriately for the load being carried.
The aggregate WLL is the total holding capacity of all tie-down devices applied to the cargo in each direction. Federal regulations require the aggregate WLL to be at least half the weight of the cargo being secured in the forward direction and at least half in the rearward and lateral directions. This is the fundamental mathematical requirement: if a piece of equipment weighs 40,000 lbs, the tie-downs must provide at least 20,000 lbs aggregate WLL forward, 20,000 lbs rearward, and 20,000 lbs laterally.
The minimum number of tie-downs required is also specified regardless of WLL calculation: cargo that is 5 feet or less in length and 1,100 lbs or less: 1 tie-down. Cargo 5 feet or less in length over 1,100 lbs: 2 tie-downs. Cargo more than 5 feet but not more than 10 feet: 2 tie-downs minimum. Cargo over 10 feet: 2 tie-downs for the first 10 feet, plus one additional tie-down for each additional 10 feet or fraction thereof. These minimums may require additional tie-downs to satisfy the aggregate WLL requirement.
Anchor points on trailers are rated and must be used within their rated capacity. Damaged anchor points, bent tie-down rings, cracked floor rings, or corroded chain hookups must be taken out of service. Tie-downs must be tightened to remove slack and must be protected from abrasion at any contact points with cargo edges. Using corner protectors (edge guards) prevents straps from being cut by sharp cargo edges and preserves strap WLL.