Structural - shelf unit

[andrewm]

I am about to build a large shelf unit from 19mm MDF. It will be about 2300mm high by 1100mm wide by 600mm deep. The intention was to have adjustable shelves on supports pushed into each side panel.

There will be a back panel to give some rigidity but I have started to wonder whether I should have a fixed shelf about halfway up to provide some structural strength and stop the sides spreading and depositing all the shelves in a pile at the bottom. I would rather not because it will limit flexibility on shelf placement but would rather do so if it will keep me out of trouble.

What does the team think?

Andrew

 

[StevieB]

Hi,

Really depends what is going to go on the shelves I guess. The back sheet will certainly help. Two other options I can think of (depending on where its going!) are to screw the sides to the wall it will go up against using a right angled bracket (I assume as its MDF you are not making a signature furniture piece for the dining room) and second, rather than sitting the shelves on supports, if you use a lug that goes up into the shelf this should grip the shelves and prevent the sides pulling outwards. Its a little difficult to describe, but imagine a cup hook as a right angle rather than a cup, screw the end into the side and drill a hole in the shelf for the hook part. It will be mostly hidden when the shelf sits over it.

Cheers,

Steve.

 

[Anonymous]

Andrew,

With a unit of that size (to use old money, it's around 7' high, 3' wide and 2' deep, you're going to have to think very carefully about the material sagging or bowing under any kind of load.

I'm not a pro, and don't have particularly wide experience, but I have made a few shelves out of various sheet goods, and these sizes would worry me for a few reasons:

1 - The width of 3' per shelf will sag without loading unless you add some other kind of supports underneath them. Typical things that are done are adding struts underneath them or a deep front edge to increase ridgiity. However, with this depth, and depending upon what you're going to keep on the shelves, I'd be looking to make them a lot more rigid than just ply.
As an example, there's an 800mm wide bookshelf next to me, full of woodworking mags. It's made from veneered 19mm chipboard and has sagged by about half an inch in the middle. If it was any wider, it would have sagged even more.

2 - The depth of the shelves, unsupported in their width, seems too much.

3 - The height of them, as you've spotted, is going to mean they're liable to bow outwards under their own weight. This is normally overcome with a fixed shelf roughly half way up. I don't see any easy way around this, short of fixing to the wall as per StevieB's suggestion.

I'd be inclined to reduce the width of your shelves either by making two in the same space, or reducing the width of the cabinet overall. The alternative would be to increase the thickness of each shelf to cover it. If you look at commercially available furniture made from similar materials, you'll get some idea of the dimensions that work and also how often a support is needed. Kitchens and bedrooms are good examples - typically with 600 or 800mm spans and heights of similar sizes. I'm sure that's not just down to kitchen design, but is based on practical considerations.

Cheers,

AG

 

[andrewm]

Okay, a few more details.

The unit in question is, in effect, the interior of a wardrobe. It is in a bedroom, the spare room in fact, and will be behind a sliding door. The size is dictated by that of the available alcove next to the chimney breast.

However, it be used for general storage, not clothes as there will be another run of wardrobes in the main bedroom. It will also NOT be used for books. I had considered reducing the shelf width and the equivalent in the other room will have just that but I was worried that for general storage there were items in boxes more than 600mm long that wouldn't fit.

I don't envisage anything on the shelves being that heavy but have the option to fit a solid strip on the front edge if I encounter problems. The original intention was to screw into the wall either side but I was wondering if I could avoid doing this.

As SteveB commented its not a signature piece but is in Maple veneer with a solid maple edge to the uprights so should look reasonable for a bedroom. Other pieces in the room will be made to match when a get a round tuit.

Thanks for your input guys.

Andrew

 

[johnelliott]

If you have the equipment, this is how to solve your problems- first, stiffen the shelves by adding a two inch strip to the front of each shelf. Mount the strip so that the top is flush with the shelf and the rest projects below.
Stiffen the front edge of the carcase by similarly fitting pilasters, maybe 3 inchs wide. If you do this then you probably won't need to fix a shelf
John

 

[Midnight]

There will be a back panel to give some rigidity but I have started to wonder whether I should have a fixed shelf about halfway up to provide some structural strength and stop the sides spreading and depositing all the shelves in a pile at the bottom. I would rather not because it will limit flexibility on shelf placement but would rather do so if it will keep me out of trouble.


Andrew

Andrew.
I'l go along with the guys recommendations re stiffening the shelves with a front brace. To stiffen the sides you could use the same principle, edging the casework with a face frame. Sounds like an interesting project.