Will Your Knee-Out Work?

By David H. Glabe, P.E. / March 1, 2012

A description of the proper use of a knee-out.

There are two issues that need to be addressed when considering the use of a knee-out in your scaffold.  The first issue involves the stability of the scaffold while the second issue involves leg loading.  Stability can be a real problem if the base width of the scaffold that the knee-out is attached to is narrow compared to the size of the knee-out.  While “off the shelf” knee-outs normally do not exceed 45 inches in the horizontal dimension, a knee-out can be any size you want—if you know how to design and construct it.  Let’s say you have a scaffold that has a base width dimension of 5 feet.  You decide to install a knee-out on the outside leg of the scaffold that happens to be 7 feet, measured horizontally.  If you don’t have enough weight in the base scaffold, the whole thing will fall over.  Of course, the clever, or not so clever, scaffold erector assumes the weight of the scaffold will be the “counterweight” for the knee-out.  Imagine what happens if the knee-out gets loaded up with plank and materials that weigh more than the scaffold equipment or, better yet, somebody decides to dismantle the base scaffold first before dismantling the knee-out.  The dismantling may not take as long as you thought!

Knee-outs have a direct impact on the leg to which it is attached.  Assuming that an upper scaffold leg is supported by the knee-out and built up from there, there are two types of forces that the supporting scaffold leg must support, vertical forces and horizontal forces.  The vertical loads from the knee-out are transferred into the supporting leg and presumably down to the scaffold foundation.  The connection to the leg at the top of the knee-out has to resist a horizontal force that wants to pull the leg outward while the bottom connection of the knee-out wants to push the leg inward.

Since supported scaffold legs, normally a steel tube for frame, systems, and tube & clamp supported scaffolds, can handle vertical/axial loads efficiently, the vertical force is no big deal as long as the total load of the knee-out vertical load and the supporting scaffold leg do not exceed the allowable load for the supporting leg.  Remember, the supporting leg is basically holding up two legs, and the loads on those two legs.

The horizontal forces are a little trickier.  Round tubes can handle vertical loads well but do a really lousy job of handling horizontal loads, exactly the horizontal forces/loads that a knee-out applies to a round tube.  What is a designer to do?  Well, the qualified designer knows that bracing is required to transfer the imposed loads properly so none of the scaffold components are overloaded.  This load transfer can be achieved in a variety of ways.  The first requirement is to install a horizontal member at the knee-out connection so that at least two legs are connected horizontally.  Then a vertical diagonal is required to transfer the load down to the next runner location, typically one frame down if it is a frame scaffold, and 6’-6” or so if it is a systems scaffold or tube & clamp scaffold. This process is repeated until either the vertical legs can handle and disperse the horizontal loads to other legs, or you have transferred the loads through the bracing down to the foundation.  How do you know when that occurs?  Well, there are two ways; analyze the scaffold or try it out and see if it bends!  I strongly recommend the analysis method rather than the guess method—workers’ lives are at stake here.

Another issue that comes up, and is usually not considered by erectors guessing and “winging it” involves the diagonal member that transfers the leg load supported by the knee-out to the supporting leg.  If that member is installed at a 45 degree angle, the force/load in the member is almost 1.5 times (1.404 to be exact) more than the leg load it is supporting.  And since this diagonal member is in compression, it also must be braced when the length exceeds its ability to support the expected load.  This is the stuff of qualified designers, typically qualified engineers who can develop an appropriate design for the specific situation.

If you can’t ascertain the loads the knee-out is subjected to, if you cannot calculate the horizontal forces applied to the supporting scaffold, if you cannot figure out how to transfer the applied loads so the scaffold can handle them, don’t guess at it; leave it to the experts to design it for you.

Tags: Scaffolding clamp horizontal forces knee-out leg-loading Resources scaffold scaffold stability tube vertical forces

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David H. Glabe, P.E.

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