Tie-Down Math: Aggregate WLL Worked Examples
The securement rules are two numbers and a comparison, but rules read differently than they rig. Here are three ordinary loads worked all the way through, the way you would run them standing at the trailer.
The two rules, restated in one breath
Count: one tie-down for short light items (5 feet or less, 1,100 lb or less); two for anything up to 10 feet; add one per additional 10 feet or fraction. Strength: the aggregate Working Load Limit — every tie-down's WLL added together — must be at least half the cargo weight. Both rules must pass; the worse one governs. (Full detail in the DOT count guide, and what WLL itself means in the WLL guide.)
Example one: a 3,000 lb mini excavator, 7 feet long
Count rule: longer than 5 feet, under 10 → two tie-downs minimum. Strength rule: half of 3,000 is 1,500 lb of required aggregate WLL. Two 2" ratchet straps in the 3,300 lb class give an aggregate over 6,600 — both rules pass with margin. But equipment gets a better answer: four points of contact, one at each corner, the way the vehicle tie-down guide lays out. The minimums are floors; machines with tracks and hydraulics deserve the four-point treatment regardless of what the floor allows.
Example two: a 24-foot lumber bundle, 4,400 lb
Count rule: two for the first 10 feet, one for the next 10, one for the final 4 (a fraction of 10 still counts) → four tie-downs. Strength rule: half of 4,400 is 2,200 lb aggregate — four straps in the 3,000 lb class laugh at that. Here the count rule governs, not strength: this is the load where people run three straps that are each strong enough and still ride illegal, because length sets the count no matter how burly each strap is. Corner protection where webbing crosses the bundle edges is part of the legal package too (edge guide).
Example three: a 16,000 lb dozer on a lowboy
Strength rule first this time: half of 16,000 is 8,000 lb of required aggregate. Four 3/8" Grade 70 chains at 6,600 lb WLL each give 26,400 — comfortably past it — while four 5/16" chains at 4,700 give 18,800, also passing. Count rule: heavy equipment over 10,000 lb rides on four-point independent securement. Both sizes pass on paper; the 3/8" package is the standard answer because margin is cheap insurance on iron that heavy (the chain size guide has the full lineup). Note what does not matter here: the binders and hooks each carry ratings too, and the lowest number in each chain's path is the one you add.
The habit
Length → count. Weight ÷ 2 → aggregate. Add your labels, compare, and re-tension fifty miles in. If the labels are the problem — mystery straps with no tags — the fix is gear that carries its rating plainly, off the shelf or built to your spec with the governing WLL shown before you buy.
Always rig within the rated Working Load Limit. Inspect straps and chain before each use and remove them from service if cut, gouged, excessively worn, corroded, or deformed.