What is the most terrifying thing in this person's room?

[tribdog]

you are the most stubborn person I've ever met. the cardboard back is structually very important. there has to be a way to save face here. Even Gokul agreed with me. it's okay to blow me off, but c'mon we are talking Gokul here.

 

[Gokul43201]

On those cheap bookcases? That flimsy piece of cardboard really doesn't do much.

I disagree. The cardboard doesn't do anything to withstand stresses that make it bend into a "C" or "S" shape, but it's several orders of magnitude stiffer (the moment of inertia normal to the plane being about 10⁴ times larger than those about the in-plane axes) with respect to stresses that try to shear the board along its plane.

Here...grab a sheet of cardboard by its top and bottom edges. Now try and pull the bottom edge to the left while pulling the top edge to the right (try to deform the rectangle into a parallelogram). You can't do it. Without the backing board, the bookshelf is weakest with respect to this mode of failure (assuming there's no L-brackets beneath the shelves). With the backing board, it no longer is.

Moral : There's a real, structural reason that the backing board is so thin - the moment of inertia comes from the dimensions normal to the thickness (it's the same reason a pipe is nearly as stiff as a similar looking rod).

 

[Moonbear]

Here...grab a sheet of cardboard by its top and bottom edges. Now try and pull the bottom edge to the left while pulling the top edge to the right (try to deform the rectangle into a parallelogram). You can't do it. Without the backing board, the bookshelf is weakest with respect to this mode of failure (assuming there's no L-brackets beneath the shelves). With the backing board, it no longer is.

I would agree, in theory, with a sturdier backing, it would help, but my experience with those bookshelves tells me that in practice, it doesn't work that way. The reason is that the nails holding the cardboard backing to the bookcase pop off or tear through the cardboard too easily (because of the narrow boards used, the nails all are placed within about 3 mm of the edge of the cardboard, and it really is very thin cardboard). In other words, no matter how strong the cardboard itself is against such deformation, the failure is the attachment point of the nails.

What provides the support that prevents deformation into a "parallelogram" is that one shelf that's installed backward that is screwed into place in the center. There is always one fixed shelf in addition to the top and bottom of the case, and that's what that shelf is doing, is providing the support against the whole thing deforming into parallelogram.

 

[tribdog]

that fixed shelf won't stop the parallelograming of the bookcase. you'll just have two parallelograms, or one big one divided in half. pretend there are hinges instead of screws on the shelves it folds the same with or without the middle shelf.

 

[tribdog]

I've built hundreds of these things and there is always a collapse danger until that backing is put on. I've never come out on top in an argument with you and I"m not giving up on this. Acknowledge me as your superior and it'll all be over. Submit. Resistance is unpossible.

 

[Moonbear]

that fixed shelf won't stop the parallelograming of the bookcase. you'll just have two parallelograms, or one big one divided in half. pretend there are hinges instead of screws on the shelves it folds the same with or without the middle shelf.

But they aren't hinges, that's what makes it more stable. Though, that's not to say these things are particularly stable anyway. Quite simply, don't try pushing them from the side to move them, they aren't constructed to withstand it.

 

[DocToxyn]

Sorry Moonbear, I got to go with tribdog and Gokul on this one. I've had numerous kit furniture like this and the backing is key. I'll admit that once the cardboard/masonite-type material is compromised at the nail holes, it's weaker, but when installed correctly, all the nails work together to support the structure (under reasonable loads). Another point that I hope the manufaturer made is to nail into the back of the shelves as well as the perimeter box, thus the nails are not only at the edge of the backing but also distributed across the material.

Penguino, I think the laminate is part of the "wood", which is really a compressed composite of chips and glue and such. Sometimes you can find it at hardware/lumber stores under the name 'melamine'.

 

[Moonbear]

Sorry Moonbear, I got to go with tribdog and Gokul on this one. I've had numerous kit furniture like this and the backing is key. I'll admit that once the cardboard/masonite-type material is compromised at the nail holes, it's weaker, but when installed correctly, all the nails work together to support the structure (under reasonable loads).

Yeah, but now tribdog is going to take any concession on my part as letting him be on top, and, well, you can understand why I'm just going to have to be unreasonably stubborn, right?

 

[Gokul43201]

...as letting him be on top...

I'm sure tribdog's not picky that way !

 

[Moonbear]

I'm sure tribdog's not picky that way !

:shy: I was afraid that might be the response.

Alright, alright, alright, I went around wiggling bookcases. I really never noticed a difference before. Then again, in my defense (sort of), I think I was referring more to not-so-reasonable loads...like when you're trying to push a full bookcase a few feet to the right to make room for something else. Anyone buying it?

 

[Gokul43201]

...like when you're trying to push a full bookcase a few feet to the right to make room for something else.

And you're not using at least something like this ?

http://www.fimaks.com/images/forklift-big.jpg

 

[Moonbear]

Okay, maybe not completely full...then again, full enough that the base stayed put and the top moved, and, well, like I said, the bookshelf itself doesn't hurt much, but all the books sure do. :shy: I have a tendency to overdo things in the lifting department. I just rearranged my home office (so I can actually use it now that I have a new computer), and moved three bookcases around without bothering to remove the books...they aren't those tall 5-shelf ones though, just 3-shelf bookcases. One doesn't have much on it, so no big deal, one has photo albums and some paperback books, so again, not that big of a deal. The third one is filled with old textbooks. But, I didn't just push it sideways, I sort of "walked" it across the room...lift one side, rotate, lift the other side, rotate, etc. The bookcase and I both survived, though I've had sore muscles for a few days. It doesn't work that well with those tall ones though, and you only have to topple one once to not try it again.

 

[tribdog]

Say it. I want to hear you say it.

 

[Moonbear]

Say it. I want to hear you say it.

You already told me I'm stubborn.

 

[tribdog]

It would kill you wouldn't it. That's alright. You don't have to say anything.

 

[Moonbear]

It would kill you wouldn't it.

It might. I've never tried letting you be right before. I'm afraid of what might happen.

That's alright. You don't have to say anything.

Does that mean I can stay on top?

 

[tribdog]

If you think you have the stamina