Buckling vs Racking

[dalecyr]

After thinking about this for a couple of weeks,
I realized I do not know how to figure this out.

The application is a rack for a small pickup to carry long stock.

If, in side view, the rack is a rectangle,
resistance to both racking and buckling is provided via the welds at the corners.

Pickup Rack - simple side.jpg

Intuitively, to reduce the change of racking, a brace would be added like this:
Pickup Rack - with corner to corner brace.jpg
It would seem, intuitively, that this configuration would not resist buckling very well though.
The columns could still buckle with little resistance.

So, intuitively, to reduce the change of buckling,
we need to reduce the column height.
Such a configuration would look like this:
(left side only is shown for clarity)

Pickup Rack - with corner to middle brace.jpg

Engineering is always a series of compromises and trade-offs.
But I am having trouble discerning where the trade-off is in this last example.
It seems that putting a brace half way up the column
would reduce the change of both buckling and racking.

What am I missing?

[jboggs]

Dale,
Funny how we all think differently. This forum really shows that. Your question is a good example. I can't imagine a structure suitable for use in a small pickup that would even have the potential of buckling under load. If it did, I would think you would have lots of other problems. Racking? Maybe. Buckling? No. On the other hand I would probably lose sleep over some detail that you would find a no-brainer.

My thoughts on your diagrams above - The brace in the third diagram does stiffen the left column against buckling, in that in effectively makes the length exposed to buckling shorter. But it does nothing for the right column in that regard. I always look at loading as force vectors. Buckling = vertical load. Racking = horizontal. To resist racking in your third diagram you are effectively converting the left column into a beam. By that I mean that the horizontal racking force it is resisting is perpendicular to its axis - like a beam. Beam formulas would apply.

In my view, racking is something you resolve with geometry, force vectors, and joint design. Buckling is best addressed by looking at the cross section and stiffness of the member itself. Also, for a structure I would expect to see on a small pickup buckling would be of MUCH less concern than racking.

My thoughts for what they're worth.
Joe

[dalecyr]

I see now that my drawings were too complex to isolate the specific question I had.
They should not have shown the top or right side members.

And although we always want to know the application,
so that we can warn against 'un-thought of' concerns
it is irrelevant to the question at hand.

So then:
Assuming a load placed on top of the left side column;
assuming the load can apply a force vertically and horizontally;

If I understand you correctly, the following statements can be made:
1. It is true that a brace as shown in the second drawing will reduce the chance of racking, but not (significantly) buckling.
2. It is true that a brace as shown in the third drawing will reduce the chance of both buckling and racking.

(The application of these principals will come into play later.)

Right?

[jboggs]

With qualifications, yes.
If buckling is your only concern, diagram 3 is best.
If racking is your only concern, diagram 2 is best.

For what its worth, I've never seen a situation where buckling was a concern while racking was not.

[PinkertonD]

I would instantly fire someone who presented a design even remotely approaching that of #3. In fact, I would
probably punch him in the nose before firing and I am NOT a man prone to physical violence.

Are you kidding me? Let's transfer load into the middle of a free-standing support column!

[dalecyr]

Anybody know of another forum where a guy can discuss and learn about principals of engineering without being ridiculed?

[PinkertonD]

Dale, this forum is just fine for you to learn, but you do not seem to want to listen and that gets tedious fast when stuff is repeated and ignored.

Buckling can only happen under one of two conditions.
1: You under-designed it for the intended loads.
2: The load exceeded the intended design load.

JB had already said, "Racking? Maybe. Buckling? No"

You continually try to negotiate the Engineering principles so you can stick with your idea of how the thing should be built. You seem to ignore 90% of the Professional advice given. Then when the project is completed you step back and make a statement, "It does not have that "quality piece of machinery" feel to it. And I do not know why, or how to fix it. " You seem to say that with a great deal of surprise, having abandoned 90% of the 300-years of collective wisdom of the regular Professional contributors to this forum.

Do you recall the base for your pickup and the quad and how many iterations we went through and still you did it pretty much your way. That is OK to do it your way, but don't be surprised if it doesn't turn out as you had hoped. Let me adjust an old maxim, " a Camel is a Horse designed by Engineering negotiation."

So, please stick around and ask and learn, but "learn" IS the operative. An artist can paint a tree in any of a zillion ways and they will all be OK and correct. An Engineer can design a tree in probably two ways max. Personally I would only do it one way, but I say "two" to allow for the reality that I do not know everything.

[dalecyr]

Dave:

I realize now that I made two mistakes in this thread.
I gave far too much information.
I should have titled it "Questions about Bracing".

The information that this might relate to a pickup rack was irrelevant
to the question of how bracing placement affects racking.

Joe's comments related to, specifically, a pickup rack. ("Racking? Maybe. Buckling? No")
While I appreciate and understand that answer,
(and it is particularly true for the size stock I will be using)
it made me realize that I had not phrased my question correctly.

But he did guide me with enough information so that I can figure out how to calculate
how much the difference in bracing placement affects the racking component. (Thanks Joe)

That was the question, and the answer, I was looking for.
This thread was complete, and essentially closed.

That is why I was surprised by your vitriolic, personal attack.
I am disappointed that the moderators allow that kind of post to stand.