Opinions on strength of stand

[tenohfive]

I've finally got around to doing something I should have done months ago - building a stand for a 3ft fish tank I've got that will be my goldies new home (until I find a cheap 4 ft tank anyway.)

I've got a total of 32ft of 2x2 inch thick timber (soon to be varnished) and some 18mm MDF (again, to be varnished or failing that painted to waterproof.)

The stand will be holding in the region of 150kg when fully loaded, so I've got a quick question regarding the construction. The diagram below is my initial plan - done in paint, but you should get the idea. Cut to length legs and cross bracing, nail in place then add the sheet of MDF on top and nail that to the top of the legs.

Will the MDF cope with the weight okay, and if not how would I be best to brace it?

• edit: forgot the sodding diagram:

Edited by tenohfive on Sunday 9th August 17:06

[mgtony]

Another set of rails at the top of the legs,under the MDF should do the trick.
If it still feels wobbly, screw a piece of MDF on the back, down as far as the cross rail.Will look ok if its up agaist a wall.

Edited by mgtony on Sunday 9th August 17:14

[motco]

If the fish tank is capable of standing on the four corners, i.e. it's a rectangular prism which could stand on a brick at each corner without the water weight bursting through the bottom, then the MDF is not involved in the strength of the frame. The small square areas on each corner will be in pure compression and MDF is man enough to take that if it's kept dry I'd have thought. Only if the bottom of the tank is a flexible membrane like a thick polythene film would the water be a 'live' load that stressed the MDF in flexure.

Edited to say that I would be a bit worried about the potential for lozenging of the frame. You should diagonal braces across the rectangular sides to stop it all collapsing sideways. A sheet of MDF fixed to the rear and sides of the frame by nailing all the way around plus some PVA glue will help.

Edited by motco on Sunday 9th August 17:42

[tenohfive]

Think I understood most of that. Tank is solid glass on all 5 sides, and between the bottom of the tank and the MDF will be 1 inch thick polystyrene if that makes any difference.

So diagonal cross bracing it is. One brace front, one brace back and going in different directions?

[motco]

If the glass on the bottom is strong enough to take the weight of the water then the MDF board on top, with or without polystyrene, will carry almost no load except at the squares immediately above the corner columns of the frame. Cross bracing should be applied at both sides and faces but it is likely that doing the back and the sides will suffice and leaving the front open for apearance's sake won't seriously weaken the structure.

[tenohfive]

Appearance isn't an issue, its not in the house (apparently I'm only allowed two tanks in the house according to SWMBO) so I'll probably do all round.

Very much a novice question I know, but will the diagonal cross braces have to be cut the shape and fit between the legs, or (ignoring aesthetics) would it be okay to nail them to the outside of the legs?

[Busamav]

I would avoid the use of MDF on anything that may be exposed to moisture from possible condensation, it is also lousy at getting a fixing of any use, could you use ply ?

I also think the frame needs something to stop it just collapsing like a scissor jack.

A sheet of faced ply to the back would have me sleeping a little better

[headcase]

Personaly it looks a sound design to me, i would screw and glue intead of nailing, make sure you countersink the screws a few mm. If your painting it then just fill the screw heads with filler, sand and paint or if your staining it (wich may look crap with an MDF top) then create some sanding dust and mix it with some glue, you can then use that as a filler that will be the same colour as your wood

The strenght of your stand is all down to your joinery skills, if you make all nice clean cuts and put it together properly and allow the glue to dry properly before use then you wont need any of the cross bracing or extra pieces of mdf as previously mentioned.

[Simpo Two]

Busamav said:

I also think the frame needs something to stop it just collapsing like a scissor jack.

A sheet of faced ply to the back would have me sleeping a little better

Or diagonal bracing instead of all rectilinear.

[tenohfive]

Sticking with MDF because its cheap. Well, not cheap - I got more than enough to do the stand with for free. Agreed, if it gets wet it turns to cardboard, so I'll be varnishing it and if that doesn't work painting over the top - I know that works, as I've built a previous stand entirely with MDF.

Whilst I appreciate the comments about aesthetics, it really doesn't matter to me - the tank will be in the garage.

My joinery skills are frankly piss poor, so I'd imagine it will need extra strengthening. Would I be better off not bothering with the horizontal cross bracing and going for either diagonal cross bracing or stick some sheet MDF to the outside on three sides?

[Simpo Two]

tenohfive said:

Would I be better off not bothering with the horizontal cross bracing and going for either diagonal cross bracing or stick some sheet MDF to the outside on three sides?

As joinery skills are poor, sheeting three sides would be easier to do than diagonals, and even stronger. Or maybe stay with Plan A and add big brackets in the corners?

[tenohfive]

Sounds good to me. Will get to work cutting MDF to shape today, and will pop out and buy some decent screws.

[Simpo Two]

tenohfive said:

Sounds good to me. Will get to work cutting MDF to shape today, and will pop out and buy some decent screws.

Good man. If you don't know this already, you'll get much better screwed joints if you drill a clearance hole through the outer part of MDF, and what I call a biting hole (the same dimeter as the core part of the screw) in the inner part. Then when you tighten the screw it pulls the two parts togther tightly. Countersink for a nice neat finsh.

[eps]

Busamav said:

I would avoid the use of MDF on anything that may be exposed to moisture from possible condensation, it is also lousy at getting a fixing of any use, could you use ply ?

I also think the frame needs something to stop it just collapsing like a scissor jack.

A sheet of faced ply to the back would have me sleeping a little better

+1 I was going to suggest the same, MDF is heavy and not to be used near water, if any gets onto it, it will compromise it's strength.

ETA : what are the dims of the tank it will hold? I'd do some rough calcs to work out the volume of water and therefore the load..

Edited by eps on Monday 10th August 15:45

[sublimatica]

When you've built the stand, climb onto it with a friend and see if you feel confident in its strength. Better you fall off it than the stand collapses under an expensive tank of fish.

[Busamav]

headcase said:

put it together properly and allow the glue to dry properly before use then you wont need any of the cross bracing or extra pieces of mdf as previously mentioned.

a brave person to promise that , any applied force on the side of that framing whilst it is carrying 150kg will have the lot fold like a pack of cards .

As for countersinking mdf, and you will not get a good positive fixing in or through mdf.

To the OP , if aesthetics are of no concern , a sheet of ply across the back will be a perfect guarantee against collapse.

Edited by Busamav on Monday 10th August 17:56

[motco]

sublimatica said:

When you've built the stand, climb onto it with a friend and see if you feel confident in its strength. Better you fall off it than the stand collapses under an expensive tank of fish.

Err, I wouldn't do that! A person standing on the mdf top is a totally different load from a rigid tank, the mass of which will be carried on the four corner columns. A person - a 'live load' in more ways than one, will stress the mdf as a membrane and may well burst through it.

[sublimatica]

motco said:

sublimatica said:

When you've built the stand, climb onto it with a friend and see if you feel confident in its strength. Better you fall off it than the stand collapses under an expensive tank of fish.

Err, I wouldn't do that! A person standing on the mdf top is a totally different load from a rigid tank, the mass of which will be carried on the four corner columns. A person - a 'live load' in more ways than one, will stress the mdf as a membrane and may well burst through it.

Perhaps it's just me being over-cautious, but if I were making such a stand I'd want to test it by standing on it myself, even if that meant the top surface needed beefing up.

I wouldn't trust a fish tank supported on four corners, although I admit I know nothing about glass tanks - perhaps they're designed for such a support?

[motco]

sublimatica, I believe so, especially in this day and age of 'elf 'n' safty.

[King Herald]

headcase said:

The strenght of your stand is all down to your joinery skills, if you make all nice clean cuts and put it together properly and allow the glue to dry properly before use then you wont need any of the cross bracing or extra pieces of mdf as previously mentioned.

Yes, maybe, with properly jointed hardwood, but I'm guessing this is an amateur construction using B&Q's finest kiln dried pine.

I'd say use the MDF to skin the back and two sides, then use the cupboard created for storing fish foods and stuff inside.