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I have a 2000mm x 1400mm x 15 mm MDF panel (green) which I want to use on a to be constructed high plane for a bed and some space around. The depth should be 1600mm. The room is not rectangular which is why there's a width in the back (3150mm) and one in the front (2730mm). The construction should carry 600kg which seems sane for a bed and activities happening in it. The construction is made from wood (solid strucutre wood/german Konstruktionsvollholz).

I calculated the dimensions for the beams (red) to be 60mm x 200mm (to carry 300kg each over 3150mm) based on values from static tables. Now I'd like to know how to figure out the number of joists (pink) to support the MDF panel which depends on the load bearing of the MDF between the joists. The dimension of the joists/crossbars is 60mm x 100mm since that's the smallest normed dimension which I can obtain locally. The number should be minimal in order to save money (ca. 10 € per joist for me) and be just optimal.

My idea is that for n pink joists I can divide the capacity of 600kg by n and look a value up in a static table for the distance between joists for the MDF which I didn't find after a search engine search. I could then calculate the dimensions of the joists by dividing 600kg/n for the load balance of each joist and then search for a max. span value in the static table.

The space left of the MDF plate (supported by cyan joists) will be filled with OSB or MDF whose capacity I can then calculate based on the same recipe.

The connection between the red beams and pink joists is realized with shoes which aren't in the image. I assume they have no influence on the problem. The space between the cyan and red parts are based on my laziness in drawing/programming the image, they stand for matching connections.

The frame will be connected to wall/hold in position by the surrounding walls which can't bear loads, but will prevent any horizontal and diagonal movement.

enter image description here

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  • Save money? We're talking a couple bucks apiece. Really, it depends on the bed. Some box springs don't require any crossbars. I'd use 5 or 6. 3/4" MDF is pretty stout.
    – isherwood
    May 1 '17 at 16:42
  • I have to get them piece by piece and they should be optically appealing, so it's around 10 € per joist for me (which is enough to raise the question for me). I'll just put the matrace on the MDF which works fine in my experience.
    – Karl Richter
    May 1 '17 at 16:58
  • @isherwood I realize that this Q&A is much more straighforward than I expected (I'm more of a recipe/teach a man how to fish type), but I just edited to present the concrete project rather than the abstract problem. Have a look, your inputs have been appreciated so far. It'd be appreciated (not only by me, I guess), if you could go more into detail about the reason for the solution. I trust your judgement if you say x or y joists and would probably just go with it, but I'm curious as well.
    – Karl Richter
    May 1 '17 at 18:17
  • Well, the crossbars won't do much good if they're too thin and installed flat. They'll just sag along with the MDF. Then the size is important in determining the right quantity. I'm not a numbers person, but I have extensive experience which I can call on. Let me do some metric/English conversion and I'll put something together.
    – isherwood
    May 1 '17 at 18:35
  • @isherwood Agreed. The minimum of KVH crossbars is 60mm x 100mm. According to the linked static table 3 would be sufficient since they carry 200 kg over a span of 1,79m. however that'd mean that the MDF panel had to span 1,60m x 1m where I'd probably bend, so I need more crossbars.
    – Karl Richter
    May 1 '17 at 19:42
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Residential floors in Europe are designed to cope with (by default) either 2.0kN/m2 or a 2kN point load. [Reference: EN 1991-1-1 2002 Tables 6.1 and 6.2].

You are proposing 600kg (= 6kN) over 2m by 1.4m, giving 2.15kN/m2. Considering a normal bed sits on a floor, I'd say we don't need to design for higher loads than a normal floor.

18mm thick MDF floorboards are allowed to span 450mm; 22mm thick MDF floorboards are allowed to span 610mm. [Reference: southern-timber.co.uk] These spans are limited by deflection criteria rather than capacity criteria.

The deflection limit gives a proportional relationship between depth3 and span2. (I'll spare you the engineering equations unless you really care.) Assuming that 15mm MDF is still deflection-governed, and strength doesn't start to dominate, this gives a limiting span of 340mm.

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