tube Archives | DH Glabe & Associates

Will Your Knee-Out Work?

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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.

Is it Okay or Not?

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How to determine if the scaffolding you have is in good condition or whether it should be scrapped.

Is your scaffold any good or has it seen better days?  While it would be nice to always use fresh out of the box (so to speak) scaffolding, the reality is quite different.  Scaffold components are used for many years, showing their age with each succeeding season.  How bad can the equipment get before it isn’t any good anymore?  How much rust is permissible?  How many dents and dings can we tolerate before it’s time to pitch it in the dumpster?

Since scaffolding isn’t free, although some suppliers may think they are giving it away, it is important that scaffold parts are not discarded before their useful life has ended.  After all, parts is parts.  On the other hand, it would not be prudent to use scaffolding that cannot function as expected.  What exactly determines whether a scaffold component is no longer useable?  For some components the answer is easy.  If a suspension rope has too many broken wire strands it’s time to throw it out.  If a suspension hoist has worn internal parts, it’s time to replace them.  If the motor doesn’t run, it should be obvious that something needs to be repaired.  But what about more subtle problems such as rust; how much rust is too much?  Should a scaffold frame, for example, be discarded because it has a rusty appearance or should it be kept in inventory, and rented, until the rust goes all the way through the tube wall? To answer those questions, it is best to understand what the hazards are.

Strength, fit, and alignment can be used to determine the worthiness of a scaffold component.  Strength is affected by a variety of factors.  For example, a cracked stirrup on a two point suspended scaffold would not be good.  Is it necessary to say that maintaining scaffold component strength is important to the integrity of the completed scaffold?  Well it is.  Consequently, any condition that affects the strength of the component is not allowed.  So, what precisely gives a scaffold its strength?  Let’s take a look at a supported scaffold, a scaffold that utilizes tubes for vertical support. The material that is used for the tube is one parameter that dictates strength.  Any action that adversely affects the properties of that material is not good.  For example, excessive heat will affect the molecular strength of the steel and therefore must be avoided.  The shape of the tube is important.  Any action that changes the shape of the tube will adversely affect the carrying capacity of the tube and therefore must be avoided.  This includes kinks, bends, distortion, flattening and stretching of the tube.

Decreasing dimensional changes in the tube wall thickness can never be a good thing.  This typically occurs through rust and corrosion although it can also happen through chemical deterioration (such as acids) and through galvanic/electrolytic action.  The question of course is how much corrosion is too much?  It should be obvious that if you can look through the wall of the tube and see daylight on the other side, you have a problem.  Basically the concern is whether the tube has lost too much of its material.  Surface rust is harmless.  Pitted surfaces are another story.  It is difficult to get complete consensus on the amount of pitting a tube can experience because evaluation is rather subjective.  Additionally, scaffold tube wall thickness varies depending on the type of scaffolding.  The wall thickness can be as little as 0.09 inches (2.3 mm or 3/32”) to more than 0.154 inches (3.91 mm or 5/32”).  If you have 1/32” (0.8 mm) pitting on a tube that has a wall thickness of 0.09” (2.3 mm) and you just lost a third of its material.  On the other hand, if you have a tube wall that is ¼” (6.35 mm) thick, you just lost 1/8 of its material.  It looks like the amount of permissible corrosion is based on the dimensional properties of the tube in question.  You may want to consult a metallurgist if a large inventory is under suspicion.

Dents and kinks in tubing decrease the strength of the component.  While a flattened horizontal tube in a scaffold frame will decrease the capacity it isn’t that big a deal if it is not the load bearing top ledger.  On the other hand, if the dent is in the leg or top ledger it can be a problem since it reduces the strength.  It’s tough to determine how much a dent decreases the capacity; don’t take a chance, discard it.  Kinks will most certainly decrease the capacity of a tube besides throwing the tube out of alignment.  Kinks in load bearing members mean discard it unless you have a qualified engineer analyze its worthiness.

Cracked welds are an indication of poor manufacturing or abuse.  In either case the component should be discarded unless a qualified and certified welder makes the repairs.  (Depending on the pay scale, it may be cheaper to get rid of the part.)  Broken welds surely indicate that the component was treated badly unless you bought the item from an unscrupulous dealer in the first place.  In any event, the component must be carefully examined for additional damage that was done to cause the weld to break in the first place.  If there is rust in the crack or break, this may be an indication of a pre-existing condition, which suggests that someone wasn’t taking care of the product and wasn’t examining it very closely in previous inspections.

Speaking of abuse, splits in tubes can never be good.  Splits can occur because of poor manufacturing or because someone tried to fit one leg on top of a larger diameter leg.  In either case, the tube is useless and must be discarded.  Twisted tubing, while rare, will overstress the material and degrade its strength.  Get rid of it.  This goes for twisted angles too.

Overall fit and alignment of members are important.  Taking a scaffold frame as an example, if it is racked (one leg is higher than the other) or warped (it doesn’t lie flat on the floor) you have a problem.  If the frame can be straightened without breaking the welds, go for it.  On the other hand, if you bought an inferior product, it may have been manufactured that way and you are out of luck.  You will never get the scaffold to be “plumb, level and square” as required by the regulations; you should have bought better stuff.

Look for missing appurtenances such as cross brace locks, wedges, pins, chains and bolts.  Make sure the bolts and pins are the correct size and have not been changed to bolts and pins of lesser strength or size.  Inspect critical items very carefully.  If the item is bent, it may also be cracked.  Don’t take chances.

Finally your manufacturer may have guidelines for inspection of their specific products.  It would be worth your while to contact them since they should know their products.  Also, go for technical bulletins that will be of value to you.

Note:  Thanks to SAIA member Alan Kline for his suggestion for this month’s subject.