Foam Sandwich Staircase/Bureau

Discussion in 'Fiberglass and Composite Boat Building' started by fallguy, Dec 30, 2017.

  1. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    So, I want to build a 4 step staircase that is 20" wide by 10" deep each step and 10" high for each step. I'd like to build the steps so there is a dresser/bureau underneath. The bottom step would be open and the next two steps would be dresser drawers underneath. I'd like to build the steps from corecell and of course, the drawers and drawerfaces as well. The only question I have is how to build the steps strong enough to avoid deflection over time.

    Does anyone know how to do a deflection calculation for this sort of thing?

    So, to be clear.

    Design load is 250# on center of span.
    Span is 20" with supports on ends.
    Depth of span is 10", unsupported on the depth
    Product is 12mm corecell. I can obviously laminate two panels together for 1", or three for 1.5", or add glass on either side, etc.

    I think I need to design the steps for a deflection of 1/16th of an inch or say 0.065" to 0.100"max.

    If this is sounding complicated, just consider a flat panel of corecell 20"W x 10" deep, supported on each 20" end with center loading of a man's foot at 250#.

    What I don't want is for the steps to sag overtime and make the drawers become unworkable. That said, I will build 1/8" clearance into the drawers, and I would be tabbing the steps to a stair stringer also made from core, but the stringer would be connected to the bottom of the boat and a bulkhead on the back. I suppose there could be some sag or distortion there as well, so any advice about the stair stringer would be great, but after all built up; seems like a 12mm stringer with glass 18oz each side would be plenty.

    Thanks.
     
  2. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    A suitable deflection equation is: Deflection for a simple beam with concentrated load at the center = (Pl^3)/(48eI)

    P is the load, in your case 250 #, l is the length, in this case 20 inches, e is the elastic modulous of the material involved, I is the mass moment of inertia for the section.

    Your step is going to need some depth and it will need a considerable amount of
    FRP on the bottom and top. Think of it as an I beam. The value of e for such a composite is a little bit chancy to estimate. We could do the calculations in a day or two but you would be far better to use something simple like a 2 by 10 or 2 by 12 plank of pine, oak, ash, or similar wood.

    You can be pretty certain that the wooden 2 by, if only 20 inches between supports, will hold the 250 pound load without any difficulty, and with precious little deflection.

    Run the numbers for the wood.....as follows. Use 1.2 times ten to the sixth for the e factor..... do the calculation for I like this......I= (bd^3)/12
    you get about 2.67 for the value of I of a 1.5 by 9.5 plank....thus (250 x 8000)/(48 x 1.2 x10^6 x 2.67).... you get something like one sixty fourth inch deflection...

    We could run the numbers for a composite foam glass structure if we could only make a reasonable guess about the elastic modulous of the glass laminate. In this case we would treat the calculation for I as if the structure consisted of two rectangles spaced some distance apart. We would be forced to assume the the foam holding the glass rectangles apart would be sufficiently durable as to hold the glass rectangles apart reliably and without moving relative to one another. The foam would add practically nothing to the beam strength.

    Ignore all that fun and games with the numbers, and make up some samples of the foam/glass step and find out what you need by trial and error. Hint: the farther apart the glass rectangles the better your I value will become. That implies that the step should be thick like 1.5 or 2 inches, maybe more. The bottom rectangle will need to be hell for strong. It will be in tension mode. The proportion of glass to resin will be one of the deciding factors as well as how perfectly your laminates are made.

    I apologize for having attacked your question with too many words.
     
  3. Tungsten
    Joined: Nov 2011
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    Tungsten Senior Member

    I'm a kitchen guy,the drawer glides i use are under mount soft close.Each side is adjustable up down so i make a clearance of at least 1/4".You do loose height in the drawer because of the glides.Another option is side mount but you'll have to incorporate an adjustable drawer front with an oblong insert so you can adjust over time.
    the foam with the right amount of glass on the underside won't deflect with that short of span.Do some tests you'll be surprised how stiff it is.The drawers and gables i would just do marine ply for simplicity,cover with a foam front to match the stairs.Alternatively you could infuse a 4x8 sheet then cut out on table saw just like ply,use a tongue and grove joint and rabbit a bottom in epoxy.But that sure sounds like a lot of work.
     
  4. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Well the stringer in corecell M will be lighter. So that is my start. Side mounted soft close pulls... There must be some boats that use core for steps. Seems strange I can’t plug a formula for my goal and get a result. Of course, I don’t know the formula.

    Why does this seem like an Rx question?
     
    Last edited: Dec 30, 2017
  5. jorgepease
    Joined: Feb 2012
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    jorgepease Senior Member

    Go with the thicker core for the step and a heavier laminate for the stringer... or consider ... If you are screwing slides into corecell, that is going to fail before your step does, use Coosa high density foam for stringers will be better choice, stiffer and will hold screws better.
     
  6. fallguy
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    fallguy Senior Member

    Can I use high density corecell 12mm? I have it in 12mm. Then screw length is limited to 10mm, but might work.
     
  7. jorgepease
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    jorgepease Senior Member

    I've never worked with high density corecell but I imagine yes, especially if you use some 5200 on the screws as there won't be much load on them.
     
  8. Tungsten
    Joined: Nov 2011
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    Tungsten Senior Member

    I have some A 1200 Corecell, by itself it wont hold screws.
    If you built a box that fits into the space from ply then the top of the box sits under your foam step,this makes it even stiffer.If I'm understanding what your trying to do.
     
  9. fallguy
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    fallguy Senior Member

    The point of using corecell is to build the stairs/dresser ultralight.
     
  10. AusShipwright
    Joined: Aug 2016
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    AusShipwright Junior Member

    Use plywood in place of foam where the glides will mount to. Only need to be the width of the glide and the rest can be foam. You’ll also need hard spots for whatever catch mechanism you’re using.

    Also instead of drawers, have you considered hatches in the steps instead?
     
  11. fallguy
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    fallguy Senior Member

    Some good and some bad there Aus.

    The plywood in place of foam on the draw mounts is good.

    Hatches? The steps are 4x by 10" deep by 10" high. A open hatch would result in the bottom opening being 30" deep (the bottom step will not be a drawer and be open underneath for shoes. It would be very difficult to reach back in 30" to get pants. A couple of nice drawers would be awesome here. First one would be 24" slides, next one up would be 18" slides, and the top would just be an open hatch with a lip that would fit an overnite case, etc.
     
  12. AusShipwright
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    AusShipwright Junior Member

    Just a thought without knowing the particulars in the drawers.

    I still believe a single hatch over steps 2 and 3 would give more space below for much less work.
     

  13. fallguy
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    fallguy Senior Member

    Oh, under the top step will be a hatch, but the 30” drawer and 20” drawers will be drawers.
     
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