Does MDF flex?

[Highlander]

In any structural analysis, every component is considered separately, WRT its role in the whole structure.

Correction: in any human structural analysis every component is usually considered separately (to simplify the problem at hand). Software assembles element stiffness matrices into a global structure stiffness matrix and solves the whole.

If the shelves were fastened to the sides and they were allowed to sag, the sides would pull in and if more weight is on one side, it will rack...

Only if the connection between the shelf and sides is continuous. That is not the case here.

The moment of inertia is resisted more when a rib is added to the edge, perpendicular to the direction of the force.

It is not the moment of inertia that is resisted. It is flexure that is resisted by the moment of inertia.

If two ribs are added- one at the rear and one at the front, it wouldn't sag much at all and if it's important for them to not be seen, they can be moved toward the center, but not less than 1/3 of the width apart.

Why not less than 1/3rd?

Crap...now I don't know if my shelf will flex or not...

See what you started?!

 

[gmichael]

No wonder they call us nerds.

 

[Rickster71]

Ever notice the way threads evolve?
OP has a question.
The question gets answered.
The OP's issue is resolved, and/or the project is completed.
Then the dead horse gets beat Ad infinitum.

 

[highfigh]

"Correction: in any human structural analysis every component is usually considered separately (to simplify the problem at hand). Software assembles element stiffness matrices into a global structure stiffness matrix and solves the whole."

So, what else analyzes structures, besides humans?

"Only if the connection between the shelf and sides is continuous. That is not the case here."

That's why I mentioned it. In this case, if the shelf deflects too much, it will bypass one of the supports. Then, if it springs back in time, it'll jam against the support and....err......LOOK AT WHAT YOU HAVE DONE!!!!!!!!!!!!!!!!!!!!!

"It is not the moment of inertia that is resisted. It is flexure that is resisted by the moment of inertia."

You're right, except it's rotation that's resisted- flexure is caused by the inability of the material or member to remain rigid. However, "there is strength in flexibility", as my professors used to tell us.

"Why not less than 1/3rd?"

Because I said so. No, it was just a generalization, really. If the two ribs are at one edge and we assume somewhat even weight distribution, the opposite edge can flex. If they're in the middle, it would be possible for both outer edges to flex if the weight is concentrated mid-span. I guess they could be located 25% in from the front and back.

 

[highfigh]

Ever notice the way threads evolve?
OP has a question.
The question gets answered.
The OP's issue is resolved, and/or the project is completed.
Then the dead horse gets beat Ad infinitum.

You got a problem with that?

 

[Highlander]

So, what else analyzes structures, besides humans?

Computers.

You're right, except it's rotation that's resisted...

I disagree. Take, for example, the case of a simply supported member under the action of a vertical load. The member will bend over its length and its ends will rotate at the supports. The supports are simple and therefore rotation there cannot be resisted. The degree to which the member bends, on the other hand, is resisted - by the moment of inertia. Bending and flexure are synonomous.

 

[highfigh]

"Computers."

On their own? Why would they care?



"I disagree. Take, for example, the case of a simply supported member under the action of a vertical load. The member will bend over its length and its ends will rotate at the supports. The supports are simple and therefore rotation there cannot be resisted. The degree to which the member bends, on the other hand, is resisted - by the moment of inertia. Bending and flexure are synonomous."

The reason it bends/flexes, and the reason it can't rotate, is the same- it's supported at each end.

I'd bet that this would be a lot easier to discuss with the ability to make real-time diagrams.

 

[ParadigmDawg]

I was able to effectively change the structural coefficient by calculating a maximum vertical displacement of 1/12" assuming conservative load and support conditions according to the Young's modulus values.

In the end; the lip resisted flexing in the vertical direction because of its orientation and because there is more structure to it in that direction (on end).

That and the fact that I accidentally made the spine the exact height that it rests on the bottom of the credenza base at the same time it loads the pins.

 

[lsiberian]

If you needed to borrow clamps you could have just asked. I got corner ones and bar ones. I also got a lot of wood(though the rain got to some of it thanks to that pop up shower the other day.

 

[Highlander]

So, what else analyzes structures, besides humans?

Computers.

On their own? Why would they care?

I think you know what I mean.

There is nothing wrong in claiming that computers analyse structures. The human element involves making judgements based on an assessment of the results of an analysis. At no point have I claimed that computers perform the latter.

I'd bet that this would be a lot easier to discuss with the ability to make real-time diagrams.

No doubt. We will just have to agree to disagree.

 

[gmichael]

And now the horse can rest in pieces.

 

[ParadigmDawg]

And now the horse can rest in pieces.

Now I'm confused...so does MDF flex?

 

[gmichael]

Now I'm confused...so does MDF flex?

Everything flexes to some extent. MDF included.

 

[ParadigmDawg]

Everything flexes to some extent. MDF included.

Can you explain the global structure stiffness matrix to me then?

Here is the end result in case anyone cares at this point.

It looks fine now and I am done jacking with it.

 

[gmichael]

Can you explain the global structure stiffness matrix to me then?

No! I'm more of a big picture guy. I let my techs work out the details.

Here is the end result in case anyone cares at this point.

It looks fine now and I am done jacking with it.

Looking good. Great job.

 

[Rickster71]

Everything flexes to some extent. MDF included.

Explain what you mean by everything?
To what extent do you mean, "To some extent"?
I question your meaning of the word, "Flexes"



OK I'm done

 

[gmichael]

Explain what you mean by everything?
To what extent do you mean, "To some extent"?
I question your meaning of the word, "Flexes"



OK I'm done

I think you're right. I've never seen air flex, although that might be what keeps plains from falling.

 

[sawzalot]

Can you explain the global structure stiffness matrix to me then?

Here is the end result in case anyone cares at this point.

It looks fine now and I am done jacking with it.

I care, and yes it does look fine.Now send me your dimensions for the center channel, the base of your display and I guess I'll just have to build you a non-flexing shelf assembly that could go on the top shelf bringing the center back up to the top shelf with the display sitting on top of the build, that will allow you to put that shelf you just doctored up back in the center of the opening for added equipment , just a thought

 

[lsiberian]

I care, and yes it does look fine.Now send me your dimensions for the center channel, the base of your display and I guess I'll just have to build you a non-flexing shelf assembly that could go on the top shelf bringing the center back up to the top shelf with the display sitting on top of the build, that will allow you to put that shelf you just doctored up back in the center of the opening for added equipment , just a thought

That sounds much simpler than getting my speakers to be airtight. Every project I always get screwed by the extra 2 inches in a foot. Who came up with that. Why can't everything just be metric?

 

[gmichael]

That sounds much simpler than getting my speakers to be airtight. Every project I always get screwed by the extra 2 inches in a foot. Who came up with that. Why can't everything just be metric?

Everything is an extra 2 inches longer over here.