Structural lamination question

Discussion in 'Materials' started by Fr33dom, Nov 12, 2010.

  1. Fr33dom
    Joined: Nov 2010
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    Location: ohio

    Fr33dom New Member

    Hello All-
    I have a question about a wood lamination. Its not boat-related, but this is the highest level knowledge base in matters of glue-ups I can find, so I hope it's okay to post here.

    I'm building an open twisting column for a sculpture, which needs to support a load from the top. I plan to use 1/16" thick ash, 12 layers total, with west system epoxy.

    I'm testing the project with rough 1:5 scale models, and the challenge at this point is in obtaining the needed rigidity. Although the piece has the form of a spring, I want to minimize how much it behaves like one. As I add more layers (along 'A') it gets stronger, and as I use wider strips (along 'B'), it builds even more strength. But I want to keep the form looking as 'open' as possible.

    My questions:
    -If I were to add layers of glass or carbon between the wood layers, would I get an increase in strength (against force 'C')?
    -Is there another adhesive I could be using to make this more rigid?

    Any help is GREATLY appreciated- thank you!
     

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  2. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Do you have to use wood? Even restrained with epoxy and thin laminations, wood's physical properties will still play a major role in the project dynamics. As you've likely found wood, in this arrangement is pretty flexible.

    Yes, carbon will help, but not as much as you'd think because the fibers, though in compression have a plenty of room to move in this helix configuration, but I'll add using other materials than wood will offer a quicker, probably thinner laminate for the stiffness required. I'd be inclined to do some testing with other materials, such as pure fabric laminates. I also don't think the resin system is going to back much difference, but vinylester may be an option.
     
  3. fg1inc

    fg1inc Guest

    Fr33dom, I'm involved in a similar sculpture and would like to discuss it via PM, if that's OK with you. Thanks.
     
  4. Landlubber
    Joined: Jun 2007
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    Landlubber Senior Member

    ...may I suggest that you use structural fibres and laminate wood th the outside pieces to achieve the "look" that you desire...that should solve the problem...but remember that you are constructing a spring in fact, so compression WILL happen.
     
  5. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    You could have a metal core.
     
  6. Petros
    Joined: Oct 2007
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    Petros Senior Member

    Fr33dom,

    As an engineer I do mostly structural analysis for a living and I can help you with this design. In fact, all materials act like a spring, regardless of their shape. I have designed and analyzed many unusual structures, including similar ones as this.

    Your design can be made to work, it is just a matter of making it stiff enough. Structurally it is two eccentrically loaded slender columns, not a very efficient structure, but of course this is a sculpture that needs to carry some load. Do you know the design load it has to carry? If not PM me and send me the building plans and I can determine what the building code required design loads should be.

    This design would be much stiffer if you can joint the two spirals with a connector at say every foot or so along the length of each member. This can be a small metal tie, 1/4" dia rod or similar. Painted to match this would hardly be noticeable and still look light.

    Carbon laminated between the layers would make it a lot stiffer, but perhaps not enough, depending on the required load capacity. Another idea that might be more practical is to use metal square tubes, bent into the two spirals, and than laminate over this with a wood veneer.

    There are code required slenderness ratios that we have to meet if this is going to part of the building primary structure. We can get away from this if we can demonstrate by testing that it meets the required capacity and more importantly, the maximum allowed deflection limits. I suspect that the deflection limit is going to be the controlling factor in the design.

    I have software that we can use to determine the load bearing capacity of the design, but I will need some more details of the design, column height, spiral dia. number of spirals, etc. We can limit the total column height by using a solid base and cap that reduces the effective length of the spiral column. Say solid six inches top and bottom caps would reduce the effective length by a whole foot.

    As speculated, the adhesive is not going to affect the load capacity much. The properties of the base material, and the dimensions, are the issues that matter. Laminating carbon between the layers makes the material much much stiffer, and might be enough to make it work. But I can tell you right now that the cross sections of the spirals are much too small. We have to protect the column elements from Euler buckling, which is a function of the length to cross sectional area ratio (smaller is better), the elastic modulus of the material (stiffer is better), the eccentric loading (less is better, that means a smaller dia. spiral), total free column length (less is better of course) and total load.

    By tying the two slender spiral columns together as noted above, you have reduced the tendency for column to buckle by about 100 fold or more (a guess based on experience). It is like making some thin sticks into a stiff truss. Otherwise the spirals will have to get much larger in cross sectional area. Or made from steel.

    If you want a lightweight spiral sculptor, a more practical way to do accomplish this would be to install a structural beam or header, or some other way, to carry the loads, and the spiral sculpture would be just decorative and appear to hold up the roof. You are really asking a wood laminate to do quite a lot in this application, it will not have much capacity as designed. As you can see the relationship between load bearing capacity on an unual shape is quite complex. That is why do not see columns like this.

    Also, the model might give you some practical information, but you will not be able to determine the load bearing capacity of a full sized column. the scaling factor is very complicated, it is a funtion of the cube of the dia, a fourth power of cross sectional area, and a number of other factors. As a practical matter I would not depend on this messy of a scaling relationship to make a full sized column. We have an imperfect understanding of all the factors that affect slender colunm buckling, so larger safety factors are always required, and you want to do this on a complicated shape too. I would say the theoretical Euler buckling equations are only a guide at best.

    Good luck.
     
  7. apex1

    apex1 Guest

    In addition to all the valid comments and arguments of my peers, given above, let me just add:
    You did not execute the laminate properly, as it seems, judging from your picture.

    The laminate would not separate at the glue line if done right. It would shear the wood fibre instead.

    So, if your double helix HAS to be wooden, you should try again with a perfect lamination.

    Regards
    Richard
     
  8. Fr33dom
    Joined: Nov 2010
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    Fr33dom New Member

    Thank you very much guys- wow there is a LOT of helpful information and ideas here. Noted on the gaps in my scale models and I’ll certainly improve the lamination quality as I refine my studies.

    There are a number of viable options here. The open question still in my mind is how much help would layers of carbon between wood add, with apex and petros perhaps having slightly differing opinions on the matter (of course it’s also hard to discuss without more quantifiable details).

    I'd like to do things without additional structural support, or involving metalwork, so I'll try adding some layers of carbon into the next scale model and see if there is a noticeable difference in rigidity at 1:5 scale, and then I'll move into full scale tests and see how things are working.

    Petros thanks so much for your offer to help and I may take you up on your engineering expertise if I get stuck down the line if the offer still stands. For now I'll be plowing forward in the shop and see what direct feedback I get.

    I will post the results of my next round of studies (in 4-8 weeks time) for those interested. THANK YOU ALL!
     

  9. variverrunner
    Joined: Dec 2009
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    Location: New York, USA

    variverrunner Junior Member

    another idea

    Fr33dom,

    I have done a lot of work with epoxied thin veneers. All of the above advice makes perfect sense but I suggest trying:

    Jumping to a 3 or 4 foot tall vacuum bagged full scale mock-up using
    uni-directional carbon fiber between your outer wood veneer. MAS Epoxy makes a very slow mix for use in Florida (they did a few ago). If your hand laying up you will probably need all the time you can get.

    Test the strength with sand bags (alla FLW @ the Johnson Wax building) or a hydraulic jack w/ a load cell up against a door header or whatever. Be careful not to break the door frame.

    PVC pipe wet sanded to 400 or 600 grit, waxed, and PVA'd work well as a mandrel. Throe the cured piece out in your Ohio snow and the part" should drop right off.

    If standard pipes sizes don't fit your sculpture you might beef up the o.d. pipe size with sections of pattern makers wax sheets. They are available in 1/16" increments andare pretty inexpensive. I think I got mine from Grainger Industrial Supply or maybe MSC?

    If you get stuck, feel free to PM me.

    Best of luck with you Art.

    Allan
     
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