Does Tie Down Angle Change WLL?

You are correct. They seem to be applying lifting principles to securing devices.

In lifting the load (force) is the constant and the angle of the device has an effect on the tension the strap sees.

In securing the tension is determined by the winch and the load (force) the straps exerts is determined by the angle.

 

Email sent.

 

So, got a response.

Here's my email:

I believe your article contains incorrect information (article URL: What is the Impact of Tie Down Angle?). You appear to be confusing "applied vertical force" with WLL. The WLL is a fixed number supplied by the manufacturer and will not change, regardless of the angle or manner of attachment. The force applied to the load certainly will change depending on the angle or manner of attachment.​

And, their response:

The article comes from the Web, Sling and tie-down Association (WSTDA) and can be found in The standard T-1 - Recommended Standard Specification for Web Tie Downs.

4.5.1 Effect of Angle - Example: If using the tie downs at 30 degrees the effective downward pressure is reduced to 50% of the vertical strap assembly efficiency. Multiply the working load limit (WLL) by 50% to the reduced effectiveness of the tie down and add additional tie down necessary to secure the load properly. The effectiveness examples pictured below are the only true of indirect tie downs.

I believe we are missing the headings for Tie Down Angle and Effective Downward Pressure (Percentage) on the web page

I have attached the standard for your review.​

Thank you to Cargo Equipment Corporation for responding promptly.

 

In the crane picking world, spreading your rigging out to extreme angles lessens the working load limit of the rigging. For example if you pick a 40 foot beam in the center, the rigging is considered 100%. If you spread the rigging out to the ends of the beam it is reduce by over half depending on how long the rigging is. The flatter out your go, the less WLL you will have for your pick.

 

If they removed the majority of "WLL", and replaced it with "Effective Downward Pressure", I would agree with them.

It doesn't matter what you rig... or how you rig it. Whether you are lifting, or chaining down... WLL will NOT change. Working load limits are what they are. Your example of spreading the rigging out to the ends of the beam does NOT change the "set in stone" WLL or Ultimate Breaking Strength. You have simply changed the forces on your equipment, making you reach the WLL limit sooner. A 10,000 lb breaking strength strap, will break at 10,000 lbs... it doesn't matter if you are lifting 10,000 lbs directly with it, or if its leverage doing it.

I'm not trying to argue with you, because I do believe that you are correct for the most part about what you are saying, except you are calling an apple an orange. Your last comment, tells me that you do understand it... "... the less WLL you will have for your pick" Meaning part of your working load limit is already being used in a different direction.

 

Everyone has seen some of these 'people' hauling machinery around and the angles of their securement is all wrong. (chains stretched too far out, etc.) WE know their load won't stay on the trailer if it rolls, but you can't explain it to THEM.

 

This is due to your center of gravity not how much WLL the equipment has.

 

Rigging angles have nothing to due with the center of gravity. The farther your rigging is from vertical the more tension it will see to pick the same weight.

 

Is this what you're talking about?