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Fundamentals: Plywood I-Beam: Not all things work

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  1.  
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      CommentAuthorSteamyTea
    • CommentTimeMar 14th 2014
     
    Decided to see if I could make my own curved I-Beam and then see how strong it was. Making it was pretty easy. To test it I decided to jack the car up and then lower it onto two beams separated by a bit of 9mm OSB. I initially measured the height (131mm), started to lower the car down, found that the OSB was failing, jacked it up again, moved it a bit over so the wheel was about the I-Beam and lowered it again.

    I videoed it.
    http://youtu.be/OuiuKs4S_BY

    Not the greatest experiment, but this method has potential as I think it was the ply that delaminated before my adhesive did.
    I have made a strait beam and shall see what I can do with that when it has fully cured.
  2.  
    Says the video is private

    hope you have uploaded the right video... :)
    •  
      CommentAuthorSteamyTea
    • CommentTimeMar 14th 2014
     
    Should work now
  4.  
    This is an experimental beam for strawbale insulation infill , made from pallets and OSB. It didnt work as well as we hoped.
    •  
      CommentAuthorSteamyTea
    • CommentTimeMar 14th 2014
     
    What went wrong with it?
  6.  
    <blockquote><cite>Posted By: SteamyTea</cite>Should work now</blockquote>


    Cool. At 3min 11 sec there is a small puff of smoke that comes out from under the beam...
    •  
      CommentAuthorSteamyTea
    • CommentTimeMar 14th 2014
     
    You have good eyesight. Wonder what it was.
  8.  
    Controlled demolition explosive charge is the first thing that comes to mind.


    I bet if you did double ply curved box beam it would be much stronger.
    •  
      CommentAuthorSteamyTea
    • CommentTimeMar 14th 2014
     
    Yes, it was pretty flimsy what I did, just using up some off-cuts.
    I could jump on it OK, but half a tonne of car is a bit different.
    When I get some more off-cut I may try and make a longer one that is a bit deeper than 80mm and glue instead of screw a better bit of ply on the top instead of OSB.
    •  
      CommentAuthorfostertom
    • CommentTimeMar 14th 2014
     
    I like all the neighbours scarpering - 'he's at it again'.
    •  
      CommentAuthorSteamyTea
    • CommentTimeMar 14th 2014
     
    One did
    • CommentAuthorEd Davies
    • CommentTimeMar 14th 2014
     
    Would some blocking at the ends where there's a point load onto the ground have helped?
    •  
      CommentAuthorSteamyTea
    • CommentTimeMar 14th 2014
     
    Probably, it was starting to twist.
    The beam was 80mm deep, 5.5mm thick web with 20mm by 16.5mm 'ears'.
    I have no idea how to calculate this, all those moments if inertia and rotations perpendicular to an axis. Its too much for a foggy Friday.
    • CommentAuthorSeret
    • CommentTimeMar 14th 2014 edited
     
    Posted By: SteamyTea
    I have no idea how to calculate this, all those moments if inertia and rotations perpendicular to an axis. Its too much for a foggy Friday.


    It's actually pretty easy, it's a statics problem so you don't need to worry about things like rotation. What you want is a thing called the "engineers bending equation", since you're dealing with simple three point bending. You'll need to calculate the second moment of area, but that's easy for an I beam. You're using a composite construction, so you'll also have to calculate/guesstimate the strength from the volume fractions of the components.

    What might complicate things is your method of construction to get that curve? Presumably you're gluing things while they're under load, so you've got intrinsic stresses to take into account. If you know the strength you can calculate those from what would be required to maintain the strain (ie: curvature) you want.

    Also, plywood is anisotropic isn't it? I don't work in construction, wood isn't really my thing. It might also be handy to have some better close ups of the points where it's failed, and details of how the beam is put together.
    • CommentAuthorSeret
    • CommentTimeMar 14th 2014
     
    Posted By: Ed DaviesWould some blocking at the ends where there's a point load onto the ground have helped?


    Even blocking one end would make the calculations easy TBH.
    •  
      CommentAuthorSteamyTea
    • CommentTimeMar 14th 2014 edited
     
    It is made from cheap WBP 3-ply. 5.5mm thick.
    The main web is 80mm by 5.5mm.
    The ears (no idea what they should be called) are made from three layers of ply 20mm wide by 5.5mm thick.
    They are stuck on the side of the web 'end on' so that they stick out 20mm.
    I have made a strait beam as well that I can do some tests on. Maybe with bricks rather than cars this time.

    Yes ply is anisotropic.
    OSB is isotropic in 2 dimensions I think.

    The failure seemed to start in the ply web (not the bit I glued) but soon spread and then the twisting set in. I was going to jack the car back up but noticed that the useless jack was not vertical anymore (had a car fall of a jack and it makes you jump). So just decided to carry on to destruction (did my apprenticeship in a company that made testing machines).
    • CommentAuthorEd Davies
    • CommentTimeMar 14th 2014 edited
     
    Posted By: SteamyTeaThe ears (no idea what they should be called)…
    Flanges. At least that's what the JJI-Joists Technical Manual calls them and some other sources I've seen, too.

    The smallest JJI joist is 145 mm deep (vs your 80), has flanges 47 mm wide (vs 45.5) and 45 mm deep (vs 16.5) so the exposed bit of flange web is 65 mm (vs 47). The flanges are C24 softwood and the web is 9 mm OSB. The web only goes into a routed slot in the flange about 10 mm or so (not a specified dimension) so there's plenty of flange around the end of the web. There's a special machine which feeds the web into the slot in the flange so it's a very tight fit but I can't visualize quite how that works. Glued as well, I assume.
  18.  
    "They are stuck on the side of the web 'end on' so that they stick out 20mm."

    I think that probably why it failed, there wasnt enough tensile reinforcement from the flanges.
    I reckon that puff of dust at 3m11sec was tensile failure of the plywood.
    • CommentAuthordickster
    • CommentTimeMar 14th 2014
     
    Why a curved beam with a massive load on it?. If it were for a roof, it would have a relatively light load spread across all the beams. Good experiment though.

    I made a load of non I beam single plane tapered and curved beams longer than 8 foot, so laminated out of 1/8th ply for a quarter of a hemisphere shed roof, worked an absolute treat.
    •  
      CommentAuthorSteamyTea
    • CommentTimeMar 14th 2014
     
    I was interested in just how strong it would be.
    • CommentAuthoralexc
    • CommentTimeMar 16th 2014
     
    I have been thinking about these. Some time later this year, i have a little project for hanging bikes from a ceiling.
    I'd use a router (with a guide for a very straight line) and but a groove down the centre of the flange. Then, glue into groove and place web in groove. Perhaps put on some buttreses along the line of join to give greater contact area(thus stength)
    I'd guess you lose strength in the flange if your using two pieces as per drawings above.

    I saw somewhere composite beams should be judged at 85% strength, not sure though.
    •  
      CommentAuthordjh
    • CommentTimeMar 16th 2014
     
    Posted By: SteamyTeaYes ply is anisotropic.
    OSB is isotropic in 2 dimensions I think.

    No OSB is anisotropic as well. It has three layers; the centre 'core' layer has a different orientation and very often a different binder/resin to the outer layers. But at least with OSB the properties are pretty similar over the whole sheet, unlike ply with voids! Still, the design numbers take that into account.

    Stress tensors at dawn, chaps!
    •  
      CommentAuthorSteamyTea
    • CommentTimeMar 16th 2014 edited
     
    Posted By: djhStress tensors at dawn, chaps!
    Thankfully my dawn is half our later :bigsmile:
    •  
      CommentAuthorSteamyTea
    • CommentTimeMar 19th 2014
     
    I have been looking at my ply beam and noticed that it is predominately 3 ply,s, but has a very thin veneer on the outer faces.
    This means that I got the grain the wrong way around.

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