"I Beam" design

Discussion in 'Boat Design' started by Spiv, May 30, 2008.

  1. Spiv
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    Spiv Ancient Mariner

    I searched he forum and found two discussions, one on Crossbeam construction, the other on beam scantling for Catamarans. Both very interesting.
    Some links to beam calculators and some research are no longer working.
    In any case I could not find the info I need:

    How to properly design and calculate stresses on a Composite "I Beam"?

    From my memories of college, I'd say important consideration would be:
    1. Relationship of flanges/web width,
    2. orientation of fiber: how many layers of uni, how many of 45/45,
    3. Thickness of web, use of core to widen it and reduce twisting,
    4. Use of e-glass and carbon,
    5. Influence of resin: epoxy v vinylester,
    6. any other factor I am not aware.
    Being able to compare such composite beams with metallic ones will of course be very interesting.

    Thanks in advance to anybody that can make a contribution.
     
  2. rwatson
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    rwatson Senior Member

    "I" beams are a usefull structural component on buildings that need square, edge support and vertical loading.

    But my simple understanding on beams for cats etc is that box shapes, triangular sections and round sections provide much better strength (in all directions) and anti-twist than the I beam shape
     
  3. bobg3723
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    bobg3723 Senior Member

    Hi Spiv,
    From my observartions as a struct. steel ironworker, I can say that
    the capacity for an I-beam girder or open web truss to carry a dead load is due in part to its form area, and that is proportional to the depth of the girder's web or the distance from top to bottom chords of a truss. Wider flanges reduce translation horizontally or rotationally about its gyrus, but that moment is typically checked by employing lateral cross braces between the I beam and another I beam or a single diagonal strut from the bottom flange of one beam to the top flange of the other providing there is a horizontal connecting member between the two from flange to flange, like a floor. An I beam or truss allowed to translate horizontally or rotationally weakens the load bearing capacity, and is amplified by a point load bearing on the top flange. So some means of preventing that is vital and usally involves using lateral struts, floors or cross beams, especially so in dynamic environments like hurricane and earthquake zones. Short beams lengths and short beam depths obviously don't require these measures, but long undamped I-beams "wobble" horizontally very easy and can get set into resonance quickly.

    I've seen trailer sailors with folding I-beam crossbeams, but they were relatively short in length as they only connected between a central pod latererally to the hulls. Lateral X-bracing and bracing diagonal struts (knee struts) are the key for lighter I- beam structures instead of widening the flanges if you're using long spans or deep webs.

    I'm not an engineer, just my observation in the field so its best to investigate this further. I would recommend Fundamentals of Structural Anaylysis by M. Leet and C. Uang. Save a buck and get a used second edition. Its an easy read and includes of a review of statics which is the standard prequesite to this textbook. The student webpage also provides a beam calculator you can download to work out the books practice problems.

    I hope this thread gets some good feedback cause I'm tryin to figure out crossbeams too! :D

    Cheers,
    Bob
     
  4. Spiv
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    Spiv Ancient Mariner

    Thank you rwatson, I can understand that, I might use box section to strenghten my new cat, however I am still very interested in understanding Composite "I" beams.
     
  5. Spiv
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    Spiv Ancient Mariner

    Interesting point, one more thing to keep in mind.
    I know that there is ample data for metal beams out there, but composite is different. So many variables, I have not been able to find any yet, that's why I think with the combined knowledge on this forum we can get somewhere.
     
  6. bobg3723
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    bobg3723 Senior Member

    From my personal collection, I would recommend Materials Selection in Mechanical Design, 2nd Ed. by M. Ashby. ISBN:0750643579

    You'll find a lot of answers to a whole gamut of questions, as in chapter 8 on shape.-- Case study 8.4 "Floor joists: wood or steel?
    Paragaphs start out with sentences like "Consider stiffness first." then (and this I like) guides you through selecting appropriate factors in the provided tables to plug into supplied equations for a variety of materials in well illustrated charts of overlapping material envelopes of characteristic merit, such as stiffness, density, and modulus. Its an engineers reference of all sorts, and gives brief overviews of the basis for the structural anaylysis behind the case studies, with lots of informative graphics to illustrate the point. It has the formulas, but doesn't bog you down with rigorous proofs dense with calculus.

    Here's a link to a summary from Google Book Search.
    http://books.google.com/books?id=t7JRAAAAMAAJ&q=isbn 0-7506-4357-9&dq=isbn 0-7506-4357-9&pgis=1

    Amazon Books has a Search Inside feature on the book. http://www.amazon.com/Materials-Sel...79/ref=sr_11_1?ie=UTF8&qid=1212180296&sr=11-1

    Highly recommended for your composites focus,

    Regards,
    Bob
     
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  7. bobg3723
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    bobg3723 Senior Member

  8. petereng
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    petereng Senior Member

    Spiv,
    I beams are not efficent for torsional applications of which some crossbeams are. But to answer your request: (I've answered out of order)
    1) epoxy vs vinyl ester. Not much difference structurally, the VE shrinks which can change the shape of your structure
    2) For a bending application you require about 70% UD and 30% DB. For a torsional application you require all DB
    3) Web width, the wider the better but at some point they buckle. No rule of thumb here as composites have low shear stiffness, but if you made it similar proportions to steel sections I'd expext it to work
    4) You can't change the geometry to prevent twisting, its purely a function that it is not a closed form eg a circular or rectangular tube
    5) Eglass will be about 1/3 the stiffness of a carbon laminate of the same construction. very good glass laminates will be about 30GPa stiffness, carbon will be about 90GPa stiffness designed for a bending application. Aluminium is 70GPa stiffness.
    6) Overall this comes down to the quality of the laminate. Hand laminates can be at less then 50% resin weight ratio or about 30% fibre volume. Very good laminates are at 60% fibre volume and 25% resin by weight ratios.

    Infused laminates provide the best structural performance and poor hand laminates can be very disappointing.

    To properly design a lmainte you need to know its ply properties and use classic laminate anayisis. This is very complicated. Its easier to use beam theory and gross laminate properties for the less technically oriented. Composite beams deflect more then metal beams of the same size and stiffness due to their low shear stiffness. So if you know the laminate properties and use the beam theorems include the shear deflection calculation which is usually left out as metals have high shear stiffness so shear deflection is usually considered negligable.

    Regards Peter
     
  9. MikeJohns
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    MikeJohns Senior Member

    Following on


    An I beam is not a good cross section for this application, also be careful applying civil engineering methods to boat structures their loads are not dynamic and are better defined.

    Elastic stability/instability is a subject in it's own right for isotropic materials. Composites can be a nightmare I think I would far rather use an alloy structural member in this application.

    Composites have some poor structural failure modes particulalry with high numbers of moderate stress cycles. The load definitions can be very hard unless someone has vigorously load celled similar structures in a variety of sea states. Almost inevitably the stresses are higher than you would have analysed.

    Some insurance is already built into an isotropic material for that unexpected stress and yield is much more forgiving than rupture.

    Analysing composites accurately !
    We gave up on the concept of FEA with composites because we saw such a huge discrepency between theoretical and actual strength for anything other than very simple laminates. I'm also not that keen on the s-n curves for FRP for light weight structural apps (CF-epoxy is much better). By the time you took the weakest test samples applied your FOS for fatigue the member would always be well over the weight of an alloy equivalent. Then there's matrix aging to consider.

    Petereng
    You are probably more expert on this.

    Cheers
     
  10. petereng
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    petereng Senior Member

    Although an I beam is not "ideal" there are many tris that use I and U sections for their beams. I have designed and analysed beams for a couple of clients. The main reason for using them is that they are easier to build. A hollow section would be lighter but the tooling is more difficult.

    It could be argued that a controlled torsionally compliant beam is better then a torsionally stiff one! I've been involved with two cats where the beams have been on bearings to allow the hulls to rise and fall independantly as they travel over waves, in this way the energy lost usually in raising the boat is not lost.

    If the beam is straight and a suitable aluminium extrusion exists its no doubt that this would be the cheapest way to go. But if you want to design a curved beam to get it up from the water or to integrate the design better with the ama then composites are the go.

    The loads on an ama are quite straight forward to calculate. Its up to the designer and the client to determine the design service factors to use for the beam.

    I do FEA and laminate testing quite often and have been doing it for many years. I do linear, linear buckling and non-linear analysis regularly. As long as the client does adequate testing FEA is a viable and accurate method.

    There is no S-N curve for composites as the fatigue mechanism in composites is totally different to metals. Although it is used, as it is a standard approach to the problem, it needs to be understood what information the S-N data is providing. Matrix ageing is usually a function of temperature cycling and water ingress. These need to be considered in the design stage and a suitable resin selected.

    This all sounds so complicated yet home builders have been building successful composite boats and planes for decades.

    If you can build good quality laminates there is no reason why the result would not be stronger and lighter then aluminium. For a Tri beam this would be the case in glass or carbon. To produce a very good laminate requires vacuum bagging or infusion processes. Its difficult to get good compressive properties using hand fabrication methods. This is because there is not enough fibres packed into hand built laminates to support each other. Once the fibre volume ratio gets over 50% the fibres are well supported and typically the compresive strength is as good as the tensile strength.

    It would seem the Mike has had poor experiences with composites. But if you keep working with them and get to understand them its a superior material to use. There is currently no reason to have metal on a boat!

    Regards Peter S
     
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  11. bobg3723
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    bobg3723 Senior Member

    Add me to the list thanking everyone here for answers to thoughts I also have on material considerations. Most excellent. :) When I get my head sufficiently grounded around statics, beam analysis, and mechanics of materials of crossbeams, I intend on starting a new thread there.

    In regards to composites, I feel that the cross section shape will be dictated by the conditions it is expected to perform. On catamarans beams, where my focus lies, a monocouqe center pod accomodation would easly transition into an short stubby I-beam at the corners laterally inbetween the pod and hulls for trailer sailer type cats of that scale. It would seem to me as a laymen, myself with no formal background in engineering of course, that it is in these light displacement boats where slight changes in increasing section form area would have significant percentage of negative impact on displacement capacity and lightweight composites should be looked into if one has the expertise in doing so, which I don't posses unfortunately.

    I'm more comfortable with metal, of course. :)

    best regards
    BobG
     
  12. Spiv
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    Spiv Ancient Mariner

    Hi everyone,
    it seems that most are focusing their attention on the suitability or not of "I" beams to multihulls.
    I deliberately did not open the thread in the multihull forum because I am interested in understanding how to design composite "I" Beams.

    I can not see any reason why one would want to use an I Beam as a cat crossbeam, when you can actually make it oval and curved like most are.
    Tri also use specifically designed beams to keep them together.

    I would like to see some formulas that would help design I Beams, so that beams could be made in quantity, like for metal beams, and one can just go and buy what he wants to make floors or reinforcements anywhere in a boat that they might be beneficial.

    For instance Peter suggested 70%UD and 30%DB, I suppose for the Flanges and 100%DB for the Web. Do I understand it right?

    Now, if we put a core in the web, how wide should it be (before we turn it into a box)?

    For the actual manufacture, I would expect at least vacuum bagging, but more usually infused and always post cured; or even the use of prepregs.
     
  13. bobg3723
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    bobg3723 Senior Member

    Sorry about the confusion Stefano. Somehow your search for beam scantlings for catamarans in your opening thread had a subliminal effect. I simply keyed off the preceding thougts....so I'll pass the blame to RWatson for hijacking your thread.:p

    Q: Relationship of flanges/web width.
    A: That depends on the bending, torsion loads your application will see and stiffness and strength required to resist failure. See the book on materials I mentioned. It talks about that and works through the calculations in its case studes.

    Q: orientation of fiber: how many layers of uni, how many of 45/45,
    A: Can someone with the knowledge address this?

    Q: Thickness of web, use of core to widen it and reduce twisting,
    A: Thats a statics and structural engineering problem and requires indertminate beam analysis. I THINK :confused: I'll browse through my text by Leet on any case studies and see what I can come up on anything close to describing that problem.

    Q: Use of e-glass and carbon,
    A: See the book on how they stack up.

    Q: Influence of resin: epoxy v vinylester,
    A: Some else take this?

    Q: any other factor I am not aware.
    A: See the book. Many factors for you to consider.

    You need to talk to a manufacturer and describe your goals and he can best work with you on making any adjustments to your requirements and scheduling, and with an engineer putting his stamp of approval on your endeavor. That being said, if you simply want to understand more about what goes into the calculation of beams (I-beams for floor support), then I'll sheepishly bow out and let the experts weigh in on that. :eek: But that book I suggested is a good text for case studies for developing your strategy to weigh the many possibilties you might have to consider and things you haven't considered.
    The book also has those forumulas for stiffnes, strength, bending and torsion for I-beams measured against a baseline round bar. In their Charts and Material Properties for Composites, they cover the figures of merit for:
    Carbon fiber reinforced fiber, glass fiber reinforced polymer, Kevlar fiber reinforced polymer. That and, wood, alloys, etc.

    So taking the figures of merit and applying it the the formulas for the various shapes under the four loading conditions, you get an idea of each and their comparative strength. It's not a cookbook on HOW to make these structures, only how to arrive at calculating their performance given the material and shape.

    I hope you find your solution,
    Best regards,
    BobG
     
  14. rwatson
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    rwatson Senior Member

    Lets see - post a 'beam calculation' inquiry in the 'Boat Design Forum', in the same sentence as Catamarans??

    The beams would be used for ....... building masts ????

    Not so much a hijacking as being led astray methinks.

    Last time I help a little old lady across the road - when she is actually waiting for a bus!
     

  15. bobg3723
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    bobg3723 Senior Member

    I'm just giving you sheet, RWatson. :D I'm sure those old ladies see you as their savior. ;)

    But I too want to know about composites. But for long crossbeams, ya cant beat aluminum for a simple and known quantity.

    Regards,
    Bob
     
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