Bending Strength of Edge Loaded Plywood

Discussion in 'Boat Design' started by Chuck Losness, Dec 24, 2015.

  1. Chuck Losness
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    Chuck Losness Senior Member

    One of the things that I have been researching off and on is the bending strength of edge loaded plywood. Every study that I have found except for one you tube video is testing the plywood being panel loaded. The one you tube video did not provide what their test results were. Just showed a video of the actual test. Can anyone point me to a study of edge loaded plywood?
    Thanks
    Chuck
     
  2. upchurchmr
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    upchurchmr Senior Member

    What do you mean by edge loaded?
    Can you make a sketch?

    I don't even know what you mean by panel loaded.
     
  3. Chuck Losness
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    Chuck Losness Senior Member

    Here is a sketch, Basically when you are panel loading the load is across the face of the plywood. When you are edge loading the load is against the edge of the plywood.
     

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  4. SamSam
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    SamSam Senior Member

    You might try looking in 'plywood beam specifications' or something like that, construction type stuff. Maybe here http://www.fpl.fs.fed.us/ or the various plywood organizations, maybe this place http://www.apawood.org/resource-library .

    I kind of doubt there would be much about single thicknesses of edgewise ply bending strength, like a 'beam' 1' x 8' x 3/4" or something. It would be no good for anything in that configuration as it would buckle too easily, unless it was combined with other materials to make an 'I' beam type thing or multiple laminations of plywood that would overcome that weakness, such as an 8" x8" beam set so the glue joints are vertical.

    I think it might be called lateral-torsional buckling https://en.wikipedia.org/wiki/Buckling#Lateral-torsional_buckling
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    You may find the attached of good use


    Merry Christmas ya'all too
     

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  6. Barry
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    Barry Senior Member

  7. SukiSolo
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    SukiSolo Senior Member

    One other thing in addition to the excellent resources pointed to above, when in doubt test a sample. As most ply is rotary peeled, the exact properties may be slightly 'off' from using a data table for grain orientation. A small known sample testing would give real data.

    Normal practice is to use addiional battens in high load areas (normally vertical if panel horizontal). Sometimes full trussing, or additional load spreaders (battens of solid timber, or epoxy fillets) top and bottom making a reinforced I beam. Possible to use lightening holes too with this latter mode.
     
  8. Barry
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    Barry Senior Member

    "Normal practice is to use additional battens in high load areas (NORMALLY VERTICAL? if panel horizontal). "

    If you are building a plywood beam, I am not sure how vertical battens will help with either strength or rigidity. Can you explain

    "Sometimes full trussing, or additional load spreaders (battens of solid timber, or epoxy fillets) top and bottom making a reinforced I beam. Possible to use lightening holes too with this latter "

    Cutting lightening holes in a plywood I-beam configuration , similar to the ones used in floor joists requires some engineering calculations

    In say a 2 x 12 foot solid floor joist, you can cut holes near the support ends but have to limit the holes cut in the middle. Ie at the ends where shear is significant, the 2 x 12 solid joist can support the loads with holes cut through.
    (depending on the hole size of course)

    In an engineered floor joist with 1 1/2 x 3 inch upper and lower chords with a plywood/waferboard web, you cannot cut the same hole near the support areas due to the lack of shear strength in this area. The plywood I beam weak spot so to speak is in the support area.
     
  9. SukiSolo
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    SukiSolo Senior Member

    Depends on the 'problem' the OP has. For dealing with support from say the hog to king plank on a high load rig (or similar) these verticals help, even if only to keep the ply on edge. The OP maybe engineering a floor - I don't know, it is not specified. Hence a little more choice in approaching an engineered solution.

    I agree completely in finding a suitable engineered solution. Sometimes ply on edge ie enough, sometimes it is not, there is a range of possibilities. Without further information on the 'problem' all we can do is advise on the multiple ways that may be used, in this case with edge loaded ply as the main element.
     
  10. upchurchmr
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    upchurchmr Senior Member

    If the illustration is just for general explanation, not the exact loading, and you had effectively a vertical sided boat causing the sides to be in bending in a seaway, then vertical battens would help if you have a buckling issue (thin plywood).
     
  11. Chuck Losness
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    Chuck Losness Senior Member

    I didn't specify any particular application because I don't have anything in mind at this time. Sometimes I just research stuff for the knowledge that I gain and store it away. I am retired with time to burn. In long bygone times it seemed like people would engage in research just to see what they might discover. These days it seems like everything is money driven. Thanks for providing the above links. Most of them I have already read. The Canadian plywood design link was new to me and did have a formula for calculating the bending strength of edge loaded plywood if the bending strength for panel loading is known. This was for douglas fir plywood. I don't know if the given formula would be accurate for other types of plywood.
    I apologize if I am using incorrect terminology. I am not an engineer.
    I have thought about trying some simple experiments but think that the crude measurements that I could take would be subject to such gross error that anything I came up with would be meaningless.
    I have found some interesting studies. I learned from one APA study that for a given thickness 3 ply plywood is stronger in the direction of the face grains and weaker in the cross direction then say 5 or 7 ply which has more equal strength in all directions but lesser total strength.
    In another study involving adding a layer of carbon fiber cloth beneath the face plys increased the panel loaded bending strength by an average of 33%. In this study the failure always occurred in the cross ply immediately under the carbon fiber. I wondered why they never eliminated that cross ply in their testing. My intuition tells me that eliminating the failing cross ply would lead to an even greater increase in the bending strength. That is just a guess on my part.
     
  12. Russell Brown
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    Russell Brown Junior Member

    Your question piqued my curiosity as we machine gunwales out of 25 mm Okoume for our nesting dinghy kits (PT Watercraft) and the gunwales are loaded the way you are curious about. The way we make these gunwales strong enough is to cap the faces. The inboard face is capped by the hull skin, but the outboard face is capped with unidirectional and woven glass. The plywood has a fair amount of strength, but it is probably less than half of an equivalent sized piece of lumber. It is however an excellent core material if the faces are capped.
    Plywood does work beautifully for our purpose as the parts can be machined into their designed shape instead of laminating these shapes.
     
  13. SukiSolo
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    SukiSolo Senior Member

    It is also worth considering other angles in the ply make up. Most ply is 90 deg but I have made 45 deg and narrower for custom applications. Uneven numbers of plies are also possible and can be stable if you are careful. Even straight multiple laminate is useful sometimes to achieve bending with no residual stress.
    It tends to 'move' less than heat bent stuff, unless the latter is very thin, in my experience.

    Have also used ply with aluminium sheets as part of the layup to help with compression stress. The bonding adhesive and sheet preparation is important with this material, though it can be done successfully.
     
  14. Barry
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    Barry Senior Member

    A most interesting comment as most failures of beams are in the tension elements. ( tensile stresses caused by bending)

    What example can you give where you had aluminum glued? to plywood to help with compression stress.

    My first thought would be is that if you had an aluminum sheet glued to the plywood, for a simple beam in bending (edge loaded) as the OP's diagram and initial comments were, would be that the aluminum would take the bulk of the load, due to its stiffer modulus of elasticity and hence the plywood would never really get loaded up anywhere to its allowable limit, making it's weight inefficient. Inefficient within this context would be an increase in weight due to the plywood for a small increase in the beam strength due to the plywood.

    The OP's diagram shows that his interpretation of edge loading is a simple beam in bending and my comments are directed at this situation.
     

  15. SukiSolo
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    SukiSolo Senior Member

    Helping to take the compression of bolting a rowing rigger through a racing shell....
     
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