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  1. #1
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    Question Help with design of large TV cabinet

    Hi All,

    I hope you are well. I am busy designing and building the below TV unit. The carcass as well as main dividing shelf in the middle will be constructed out of 22mm A.White Oak and all drawers out of 16mm oak veneered chipboard (Particle Board). Currently the unit measures 2100(L) x 450mm(W) x 450(H). The middle shelf will be supported by a dado joint and glued in place, while the body of the unit will be biscuit jointed, glued and pocket joined.

    I have a concern that over time the unit may sag in middle, with no support in the cavity underneath the drawers. There is a hidden space as shown in the middle of the cabinet to house electronics where there is a box frame which support and joins the top with the middle shelf which should provide some support.

    Am I being silly in thinking that this may sag over time?
    Attached Images Attached Images

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  3. #2
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    Is the cabinet hanging off a wall ? Or sitting on the floor ?

    If itís on the floor the sag will start the day itís made . And get worse the more you fill the drawers and Center section up .

    If the cabinet is hanging on the wall by a French cleat at the back behind the top section , then that should take care of any sag and be good . Just the lower board , the bottom of your unit will sag a little . Depending on whatís stored in there .

    Is the TV is fitted to the wall ? Not sitting on the cabinet ?

    The easy fix is to divide the lower section up into two or three sections .

  4. #3
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    Am I being silly in thinking that this may sag over time?
    Nope and to be honest I'm surprised the images you've attached haven't started sagging yet. Also, what's the idea with the false drawer fronts each, kinda, half covering the shelf area?

    Just to start with, as auscab has asked, Is the cabinet hanging off a wall ? Or sitting on the floor ?

  5. #4
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    Hi Genesis,

    you may also be interested in this: The Sagulator – WoodBin

    Itís a rough calculator for the sagging of wood (and MDF etc) according to width, thickness etc.

    Brian

  6. #5
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    any reason you can't put a back panel on it?

  7. #6
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    South Africa
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    Default

    Thanks for the reply @auscab.

    It is a floor standing unit, TV mounted to a wall behind the unit, however I need the design to cater for the fact that it may not always be that way so your comments are valid. The client is fairly set on having an open area behind the lower section of the cabinet, I'd like to see whether i can make that happen with placing any support bracing. There is a backing that is being mounted behind the top section 10mm oak veneered ply, may help the situation?

    thanks for your response on this one, much appreciated.

    Quote Originally Posted by auscab View Post
    Is the cabinet hanging off a wall ? Or sitting on the floor ?

    If itís on the floor the sag will start the day itís made . And get worse the more you fill the drawers and Center section up .

    If the cabinet is hanging on the wall by a French cleat at the back behind the top section , then that should take care of any sag and be good . Just the lower board , the bottom of your unit will sag a little . Depending on whatís stored in there .

    Is the TV is fitted to the wall ? Not sitting on the cabinet ?

    The easy fix is to divide the lower section up into two or three sections .

  8. #7
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    Thanks Fekit, The design files attached were just so the client could get an idea of the internals as well as the drawer face design. If the image order is the same as mine then the last image depicts what it would look like fully closed etc. See the response I posted to Auscab, floor standing unit, client has specified an open area beneath the drawers.

  9. #8
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    Thanks Brian, yup I have used that and its a great tool ) The only thing which it doesn't really give is how those calculation are affected when you have two boards braced etc.

    Thew one consideration I am looking at it changing the top of the unit to a 32mm thickness board, which should then also assist in preventing some sagging and create additional rigidity.

    Thanks for Your response )

  10. #9
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    Yup, there is a backing panel, just not depicted\hidden in sketchup (these were the v1 client files used). 10mm oak veneered ply backing board for the rear of the unit.

  11. #10
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    Arrow Some additional support\bracing

    Hi Everyone,

    Thanks to everyone who has commented thus far, all comments have been very useful.

    @auscab>> Your comments regarding the french cleat intrigued me and I realised I'll need to support that top in some way, so, see below images attached...I have placed a 22mm x 50mm stretcher (MARKED "A" + "B") front and back of the top shelf which will be glued and screwed across it's length as well as width joining it to the sides of the cabinet.

    Additionally, I have placed a stretcher of the same dimensions (MARKED "C") underneath the middle shelf\divider just behind the midpoint so as to minimise the chance that it can be seen unless you really want to.

    Thoughts from those are assisting me on this one?

    Once again, thank You for the responses and assistance on this one )
    Attached Images Attached Images

  12. #11
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    Default So, I've reworked the entire design...

    Hey Everyone...

    Gave it some thought, seems it may have been flawed completely, attached redesigned unit below.

    It now consists of 3 independent carcasses that will house two sets of drawers on either side, and then the centre carcass for the electronics etc. These will be tied together with screws to join them, as well as fixed to both top, side and bottom panels. It does add more weight, however, overall I think it increases the load distribution a lot more. All carcasses will be made using 16mm oak veneered chipboard, a 10mm backing will still be used to add additional rigidity too.

    Any thoughts, suggestions, kicks in the ass etc encouraged P

    Appreciate everyones help on this one.

    Screenshot 2021-03-03 at 12.13.11.png

  13. #12
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    Genesis,

    Your fears are, I think, well founded. The thought of those types of spans in particleboard makes my sphincter twitch. Whilst I am no expert I have seen how chipboard and MDF struggle to hold their own weight over time, let alone a load of any significance. If you take a 450 mm wide piece of particleboard that is (say) 600 mm long, and support it at either end, you can almost watch the bugger sag. OK. I exaggerate, but try it. Get such a piece of particleboard, fasten two bits of scrap at either end to hold the centre of the board about an inch off the ground and forget it for a month or two. Then return with a tape measure and figure out how much it has sagged.

    I am a hobbyist, not an expert, but i speak from experience. Nonetheless, you should seek out those who are expert for a more knowledgeable opinion.

    In my study I have a bookcase made of 22 mm surian cedar boards. It is about 1,200 mm wide and 2,100 high, with 7 shelves and a 3 mm ply back. It has held my books for 20 years, and has always been full to overflowing. On seeing your post I looked along the leading edges of the shelves to see how it had held up. They have bowed about 1-2 mm, but if you look at it from the front, this bow is not noticeable.

    One of my favourite quotes is, "Don't give me advice, show me your portfolio" (Taleb). So here goes. I build a fair bit of furniture from lipped MDF/particleboard veneered with 2mm shop cut veneers. Even though the panels are supported by web frames and/or a structural member under the lower panel both front and rear, I have never allowed each span to exceed 500 mm. There is such an entertainment unit in the shop right now. The unsupported spans max out at 500 mm. Even then the veneered top sits on a web frame with seven short cross members and is framed/edged with 45 X 32 mm timber, as noted below.

    IMG20210303190356.jpg

    The structural supports under the cabinet are 32 mm thick and vary from 35 mm deep at the ends to about 85 mm in the centre. To avoid the demon sag (and to keep it lighter), the bottom shelf is a web frame in Surian with seven cross pieces covered with a sheet of 3 mm MDF and then lipped at the front with a 25 X 30 mm piece of camphor laurel. That, sir, will never sag, especially given the supports glued and screwed under it. I hope you can see that bitter experience has taught me to be very careful with MDF and particleboard panels that lie horizontally.

    I don't think your design is the central problem. If you really like the long, low design, and clearly you do ... and if you want it to sit on the floor ... then particleboard is not the material I'd choose, because it sags so badly. MDF suffers a similar fate. This is unfortunate, because particleboard and MDF are so economic, but unless well supported, the stuff sags, just as you feared it would.

  14. #13
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    Quote Originally Posted by JohnSamuel
    ... I don't think your design is the central problem. If you really like the long, low design, and clearly you do ... and if you want it to sit on the floor ... then particleboard is not the material I'd choose, because it sags so badly. MDF suffers a similar fate. This is unfortunate, because particleboard and MDF are so economic, but unless well supported, the stuff sags, just as you feared it would.

    I want to both agree and disagree with John. In my view:
    • Your design is a central problem. The overall width of 2100mm is far too wide and there is no lateral support, and
    • particle board and MDF are essentially crap products with very little resistance to sagging.


    When I was a student I made some bookshelves by stacking particle board shelves on cork blocks. They sagged badly before the end of term.

    Later I made some bookshelves from 18mm pine; they also sagged quickly. Next I made some shelves from 18mm pine but backed it with plywood screwed every 150mm along each shelf. The backs of each shelf remained reasonably straight but the fronts sagged and the books slid off.

    Three lots of bookshelves - all failures. Then I read two significant facts:
    • stiffness of timber increases with the square of its thickness, and
    • the scientific measure of stiffness is the modulus of elasticity, and it is a published figure for most timbers.


    For my next bookshelves - I am married to an academic so we have heeps of bookshelves - I chose the relatively stiff (for a pine) celery top pine with a dry modulus of elasity of 12 GPa and then made the shelves 32mm thick with spans varying from 900 to 1500mm. After 30+ years the 1500mm wide shelves are still straight; no sagging. And I think that the thicker timber looks good.

    Bookshelves.jpg

    In my view, to prevent sagging I think that you will have to either:
    • add two or three vertical supports to your design, or
    • add lateral support the full width of both shelves, or
    • increase the thickness of all shleves substantially (say 40mm ???) and also slect a far stiffer material.

  15. #14
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    Quote Originally Posted by genesis View Post
    Hi Everyone,

    Thanks to everyone who has commented thus far, all comments have been very useful.

    @auscab>> Your comments regarding the french cleat intrigued me and I realised I'll need to support that top in some way, so, see below images attached...I have placed a 22mm x 50mm stretcher (MARKED "A" + "B") front and back of the top shelf which will be glued and screwed across it's length as well as width joining it to the sides of the cabinet.

    Additionally, I have placed a stretcher of the same dimensions (MARKED "C") underneath the middle shelf\divider just behind the midpoint so as to minimise the chance that it can be seen unless you really want to.

    Thoughts from those are assisting me on this one?

    Once again, thank You for the responses and assistance on this one )
    Respectfully it's still going to sag, whatever you do, even in solid wood you would need better vertical support from the bottom up.

    MDF is easy to make work and it won't sag if built right despite the negatives already posted
    _M043292.jpg
    The picture is of my close to ten year old now MDF based AV/TV stand. What you can't see in the picture is a plinth it sits on that is recessed about 120mm al all sides.
    Bottom and middle shelf are just plain 16mm MDF and the top is two sheets laminated to 32mm. Unit is 1800 wide and 500 odd deep.
    The vertical supports are space with air space each side of standard width audio gear.

    So for your open bottom shelf I would add vertical supports that divide the space up for AV gear. with 2100 you would get 3 sections middle, left, right with additional small storage areas each side. Or space the vertical dividers to maximise the support for the drawers above whilst keeping the spacing for AV gear

    Before i have the 75" LED screen on top there was a 60" plasma screen that was nearly twice the weight. The pioneer amp on the bottom is literally frigging heavy, but it sits over the plinth so the weight is distributed. the bow in the photo is a lens artifact not a bow in the mdf.

    I hope this gives you some ideas.

    Cheers
    Phil

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