3"x3" steel tube, Structural engineering question

[centerline]

TL;DR Summary

Load capacity in relation to distance from beam support

Hi.. im hoping someone can help solve a problem which I simply dont have enough education to know the symbols required, or numbers needed to run through the engineering myself which would allow me to find the PROPER answers for the strength of a "cantilevered" square steel tube..
The steel alloy is A500b..

I would like to know the safe working load on a static, cantilevered square steel tube that measures 3" x 3" x .188 wall thickness, at 6ft from the fulcrum..

For my own reference It would be nice to have this figure for the 3x3x .188 square steel tube as well as for the same for a .120 wall thickness
AND... the same figures for 2.5"× 2.5" square steel tube, as I am trying to build it as lightweight as possible, but with enough strength to withstand the load, which is about 800lbs.

Im thinking the 3x3x.188 tube will be strong enough without knowing the actual "numbers", but the numbers will allow me to move forward, possibly using lighter material, without doubting what I "think" I know...

Thank you for any help you may be able to offer..

 

[berkeman]

Welcome to PF.

What is the application? What happens if the cantilever beam fails? How can you know that the load will always be static? Are you sure there are no dynamic load variations involved?

 

[Baluncore]

I would like to know the safe working load on a static, cantilevered square steel tube that measures 3" x 3" x .188 wall thickness, at 6ft from the fulcrum.

The way the tube is supported and attached at the fulcrum will be important but has not been specified. That will be a critical point with maximum moment, where the tube might collapse due to the high force on the fulcrum. A bolthole or the heat effected zone of a weld could also greatly reduce the safe load.

 

[centerline]

Thank you for the replies

Here are more specifics...

The fulcrum area will be reinforced with a tapering "shoe" welded to it, to prevent collapse and with gussets to offer a better and stronger grip to the base structure, allowing the stress to be dissapated over a larger area in both the support beam and the structure its welded to..

A "failure" would mean that it bends down and does not return to its original position.
If the engineering numbers and my welding skills are correct, there will be no worry about sudden, catastrophic failure...

The purpose is to make a storage rack for stacking atv/utv vehicles on, using a forklift, 3 high..

My plan so far is to use 2 cantelevered" 3"x3" square steel tubes, with decking to create a "platform", with the actual "center balance" point of the load placed between the cantelevers, at 5'6" from the fulcrum..

It will be a static load, except when loading or unloading the platform...

Knowing the values will allow me to know how much error of safety I have on the cantilevers if the load is placed too far off center..

Thank you..

 

[Lnewqban]

Welcome!
What about using diagonal bracing?
What will the horizontal tubes be welded onto?

 

[berkeman]

The purpose is to make a storage rack for stacking atv/utv vehicles on, using a forklift, 3 high..

Is it in an isolated area with only forklift access so if an ATV falls there is no way it can hurt anybody?

It will be a static load, except when loading or unloading the platform...

Um, yeah. Have you ever driven a forklift?

 

[erobz]

Are you building this for personal use, or for a client?

 

[jrmichler]

The strength of your cantilever beams is only a small part of the problem. Other parts include the joints between the beams and support column, the strength of the support columns, and the interface between the support column and floor.

We need a clear sketch of the entire structure, then we can help guide you through the process of calculating the strength of each component. The structure will be only as strong as the weakest part.

Um, yeah. Have you ever driven a forklift?

Forklifts bang into everything they can bang into, and some things that they cannot. In most cases, the real design criteria is to make it survive forklift drivers.

 

[Baluncore]

I am trying to build it as lightweight as possible, but with enough strength to withstand the load, which is about 800lbs.

The weight and insured value of an ATV is very similar to a barrel of wine. I was given a similar design problem a few weeks ago. A rack that stands against a wall, to support pallet-based barrels of wine, standing three levels above the floor. The original sketch used cantilevered RHS arms, on an angle-iron back frame, bolted to the wall.

I insisted on it having front legs to eliminate the cantilevers, with a fully welded tubular frame for rigidity, and with all legs being bolted to the floor, to prevent a collapse into the workspace. Failure is unacceptable, in any way.

One difficult problem is designing the rack to accept variable size slide-in pallet modules, without them skewing or snagging on the frame, or partly falling through between the pallet supports.

You must expect damage to occur during the recovery from unexpected events.

If you design it to withstand an earthquake, it might survive a tired forklift driver.

 

[centerline]

So with all your concerns that arent relevant to the question I asked, we are off the subject...which is what is the strength (bending modulus) of a properly annchored and supported, cantileverd 3"×3"×.120 steel tube at 7ft from the nearest "well supported and protected" support...
I may have previously used the word fulcrum but it has no movement, as its welded solid with welded inner supports, welded outer supports and gussets welded for added strengh... the tube isnt coming loose at the base, but may bend, which is why im looking for someone smarter than myself to help me out with the answer.

In that respect, when I found this site I was under the assumption this was an engineering forum where engineers and designers helped each other find answers to questions outside their own expertise...

So far it seems more like a "We dont know the answer to your question so we will side step it and just throw out a lot of obvious hypothetical scenarios that surely no one has ever thought of before".... Really!

How or where to find the answer to my question is elusive to me, but the concept of a how a cantilevered beam can be safely secured to a base structure seems so elementary that I was sure I would find someone here who was educated enough in the art of structural engineering that they could give a reliable answer to my cantileverd beam question...
Which, if I had the Answer to the "strength of the beam", I could make absolutely certain it was constructed strong enough during the construction.. I just need to know the bending modulus numbers of it..

Dont get your cart ahead of your horse and you will find life is much easier.. and cheaper

just sayin'....

 

[Baluncore]

An engineer would not build a simple cantilever, because it would be heavier and more expensive than the many alternative design solutions. You need a redundant structure, one that will not suffer a total collapse, as a result of the failure of any one component.

You know the root of the cantilever is important, but you are ignoring all the other more likely modes of injurious failure for the combined structure. If we give you the one number you are demanding, you will go ahead and ignore the safety of the rest of the design, as you are in denial of its primary importance.

A qualified engineer comes with an insurance policy, and a restriction on what they can build without the oversight and approval, of a greater body of engineers. You are clearly not insured as an engineer, so, once it falls on you, and you end up in a wheelchair, someone will sue this forum for having assisted you, in injuring yourself. The only way we can prevent that, is to withhold the only number you think you need, until you realise your limitations and can moderate your naïve trajectory.

I may have previously used the word fulcrum but it has no movement, as its welded solid with welded inner supports, welded outer supports and gussets welded for added strengh... the tube isnt coming loose at the base, but may bend, which is why im looking for someone smarter than myself to help me out with the answer.

Being "heavily built" is poor engineering. Any welds, on the tube side of the fulcrum, will need to be along the corners of the tube. You will need to weld it "solid" as you say, without welding across the tube, since that will generate a heat effected zone, with a lower threshold for failure than the simple cantilevered tube.

You need a low cost, light weight, and safe design, so you need to study "redundancy" in structures, and "plastic" design, where the structure progressively bends as the load is increased, without it ever collapsing, as the simple cantilevered square tube will surely do.

 

[jrmichler]

its welded solid with welded inner supports, welded outer supports and gussets welded for added strengh...

None of which tells us that the overall design is safe. We also have to consider that other people read these threads to learn how to solver similar problems.

We need a clear sketch of the entire structure, then we can help guide you through the process of calculating the strength of each component. The structure will be only as strong as the weakest part.

If your design is what I suspect that it is, the stress at a particular point is three times the stress at the base of the shelf beam. Your structure could be dangerously weak. Safety is very important here at PhysicsForums. I am not the only person here with experience at designing structures. When we ask for information, it's for a reason.

You know the root of the cantilever is important, but you are ignoring all the other more likely modes of injurious failure for the combined structure. If we give you the one number you are demanding, you will go ahead and ignore the safety of the rest of the design, as you are in denial of its primary importance.

This thread is one example of unexpected things that can go wrong: https://www.physicsforums.com/threads/replacing-atv-trail-bridges-cat-259d-skid-steer-impact.971071/.

 

[berkeman]

In that respect, when I found this site I was under the assumption this was an engineering forum where engineers and designers helped each other find answers to questions outside their own expertise...

So far it seems more like a "We dont know the answer to your question so we will side step it and just throw out a lot of obvious hypothetical scenarios that surely no one has ever thought of before".... Really!

There is an explicit prohibition against discussing dangerous activities in the PF Rules (see INFO at the top of the page), and we take safety very seriously here. Too often unqualified people try to use the Internet to help them design things that are dangerous and can get people hurt or killed. It would be irresponsible of us to support such activities.

This thread is now closed. I advise you to meet with your employer and their liability insurance agent to discuss how best to proceed with this design.