Cargo securement is a fascinating topic with many angles to consider. You have math. The basic principles of physics also come into play. Anyone who does it for a living has to be cognizant of things like working load limits (WLL) and break strength.
Speaking of WLL and break strength, do you know which of the two ratings is more important for tie-downs? If you said WLL, congratulations. You are absolutely correct. Even though a tie-down’s break strength can be several times higher than its WLL, the latter number is far more important.
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The Basics of Break Strength
Break strength is measured as the total amount of force it takes to break a strap or chain. Think of it as similar to tensile strength. A tie-down strap with a break strength of 500 lbs. would be expected to suffer catastrophic failure if subjected to that amount of weight even for a short time.
Product manufacturers might mark their tie-down straps with a break strength number. But because WLL is what really matters, some do not bother. The big takeaway here is to not rely on break strength when determining how many tie-downs to use. Instead, WLL is what counts.
The Basics of WLL
Working load limit is a bit more difficult to understand because it’s not absolute. The makers of the Rollercam brand of camp straps describes WLL as the safe load capacity of a strap. It is a representation of the amount of weight a strap can safely handle under normal circumstances, and for extended periods of time – all without the risk of failure.
Going back to the previous example of a tie-down strap with a 500 lb. break strength, its WLL might be only 250 lbs. It can safely secure a 250 lb. load over an extended period of time without issue.
The difference between break strength and WLL is often expressed in something known as the ‘safety factor’. In addition, the safety factor is often represented as a ratio. A safety factor of 4:1 indicates that a product’s break strength is four times higher than its WLL. It matters because the safety factor accounts for a buffer of sorts.
Preventing Failure From Unexpected Forces
When choosing tie-down straps, it is best to plan according to WLL. Let’s say you are hauling something that weighs 500 lbs. Your best bet is to use enough tie-down straps to give you a total WLL of 1000 lbs. Doubling your WLL gives you a buffer. In addition, there is more buffer created by the product’s safety factor.
Why do you want this buffer? Because there are unexpected forces involved in transporting cargo. Under normal circumstances, you expect a smooth and uneventful ride. But unexpected things can happen. You may have to break suddenly to avoid an animal that runs in front of your vehicle. You may hit a large pothole. You need that buffer so that your straps do not fail due to the significant, but temporary force generated by the unexpected event.
WLL and Surface Area
One last thing to consider is the surface area you are working with when tying cargo down. Technically, a single strap with a WLL of 1000 lbs. would be sufficient for securing a 500-pound load. But if that load has a large surface area to contend with, you may be better off with two 500 lb. straps instead of a single strap rated at 1000 lbs.
While you work all of that out in your head, remember that break strength and WLL are two different things. WLL is what matters when calculating your straps.
