Bending Strength Of Aluminum Vs Wood?

Discussion in 'Boat Design' started by jdworld, Dec 31, 2009.

  1. jdworld
    Joined: Nov 2009
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    jdworld Junior Member

    Anyone know? If you have a wood 2 x 4 supported each end spanning 9' alongside an aluminum tube 2x4 (1/8" walls?) spanning 9' and you stood on each one at about mid span, which one is going flex and/or fail more?

    Also - if anyone can point me to a place online that has SIMPLE aluminum span charts that would be great - have had no luck with my googling.

    thanks!
     
  2. Guest62110524

    Guest62110524 Previous Member

    you mean a piece of square section, ? depends on the alloy and the timber, if you have a peice of jarrah, oak, , or other heavy timber, it,ll be far stronger than the a low tensile alloy, if you have piece of radiata pine and stood in the centre it would break, the alloy would not, of it was a 6000 series t6, it would spring, so many variables
     
  3. DCockey
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    DCockey Senior Member

    Strength or Stiffness ????? Which are you intereted in? They are fundamentally different.

    In engneering terms strength means the amount of load which can be withstood before failing, and failure frequently means permanent deformation. Frequently when someone talks about strength they are actually interested in stiffness. Stiffness is the ration of deformation to load, and unless otherwise indicated is assumed to be in the elastic range. Elastic means the material returns to its original shape when the load is removed. Most materials and configurations are also behave linerally in the elastic range, meaning the amount of deformation doubles if the load is doubled, etc.
     
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  4. Guest62110524

    Guest62110524 Previous Member

    yes well put, sometimes pushing frames, the alloy has better spring than mild steel, nowhere near as soggy , one thing, standing on a piece of alloy it may flex, but , but if say i was 5 stories up walking across it, , i would feel far safer than on pine, which could break with no warning at all
    Simple test, just load it up,
    coarse there are pc programmes that (test) beams , in fact there are free ones to download
     
  5. jdworld
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    jdworld Junior Member

    yeah, i'm mainly talking about "stiffness" - how springy the alum would be vs the wood. Picture the floor joists for the top deck of a houseboat for example. I have about a 9' span. I would love to keep the thickness to 4" for appearance/weight reasons. I'm pretty familiar with wood, and a Douglas Fir wood 2x4 with maybe 1/2" plywood decking would not span that far without feeling very springy underfoot, and may infact fail. So what about a 2x4x.125" aluminum tube? Softer, stiffer, same? what about .25"?
     
  6. Guest62110524

    Guest62110524 Previous Member

    a
    alloy even 6063 t5 is ok, but spacing? depends on the decking, 1/2 I would space the beams 300mm centres but if you went, three layers 6mm, space 500--600
    or if you had big loads would go 150x50 x3, 6mm is too heavy
     
  7. steele m.a.
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    steele m.a. Designer/Engineer

    1100 Aluminum is pure aluminum , soft , bendable , not load carrying .
    2117 Alloy has copper/zinc - Stronger , stiffer .
    Extruded , or wrought aluminum , can be like cast iron , brittle , breaking
    when flexed .
    markalfredsteele@yahoo.ca
     
  8. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    There is no such "span chart" as there is no free ride when it comes to developing a beam into a structural element. There are countless calculations that are required and you haven't provided even the remotest amount to begin with. In short, there isn't a short cut, to engineer a load path. Specifics can be dealt with, but we need to know what you're trying to do.
     
  9. narwhal
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    narwhal Junior Member

    Without getting into calculations, one way you could get greater stiffness with a 4" thick member is to increase its width, say going from a 2x4 on edge to a 3x4 or a 4x4.

    Still more effective, if you are only concerned with the height of the joist at the top of the wall and can stand some camber in the top deck is to take say a 2x8 and radius the top chord into a fair curve, so that you have a 2x8 at the center, which is the point of maximum bending stress.

    Significant improvements in bending stress may also be made by adding a flange to the bottom chord of your joist. You might try a load test with an ordinary 2x4 and a 2x4 with a 1x2 glued and screwed flat to its bottom chord. Since the store-bought 2x4 is actually 1-1/2x3-1/2, and the 1x2 is actually 3/4" thick, you'll end up with a member 4-1/4" tall.

    The roof deck glued and screwed to the top chord of the joists will provide the lateral stiffness you need if you're using 2x4s. If your load tests indicate you need more than the 2x4, and you decide to use a wider member or the cambered joist suggested above, you may need bridging between joists at the bottom chord of the joists. This bridging can be as simple as a 2x4 nailed between alternate joists at their centers, or one continuous 2x4 with rounded corners which can double as a hand-hold in rough weather.
     
  10. PAR
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    PAR Yacht Designer/Builder

    We build boats here Narwhal, not 3 bed room, 2 bath Cape Cods, in platform framing techniques.

    I think what Jdworld is looking for is a quick estimate of something, comparing a wooden 2x4 to an aluminum 2x4. What he hasn't mentioned are the important things, like what kind of wood, how it's cut, moisture content, etc. or what kind of aluminum, welded, extruded, solid, hollow, what alloy mix, etc., so a comparison can't be made without clarification of the details.

    He has mentioned it's a hollow 2x4 beam with and 1/8" or 1/4" wall, but that's it on the aluminum side. I honestly don't think he realizes how complicated an issue this could be.
     
  11. Guest62110524

    Guest62110524 Previous Member

    actually he has paul, all sections he talks abt are extruded the tensile and yield easily obtainable
    But i just posted what we do, we use 130x50 x3 mm, box section, which is structural no matter what grade, thats what is made for . Like crane booms, jib, drilling it will not weaken it For our soles, in deck saloon the spacing is 800, so we can lift engines out, then over, ply foam ply, makes it all stiff and sound effective
    If the poster can put up a dwg would help Another light weight way would be, glue up a ply box section Whatever its an easy (problem)
    once there is a dwg, you dont need math degree to know what the structure could be
    but here is beamboy a free calc programme
    http://www.engineering.com/tabid/74/Default.aspx
     
  12. apex1

    apex1 Guest

    No actually he has´nt! You are talking about extruded al, he did´nt.
     
  13. narwhal
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    narwhal Junior Member

    Very true, and I'm often guilty of extending the definition of a problem past its original intent. Still, the reference to upper deck joists, whether of wood or aluminum, speaks to framing techniques used by carpenters on both boats and houses, so there is room for cross-pollination of knowledge here.
     
  14. jdworld
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    jdworld Junior Member


    Gee funny because there are several span charts out there for wood and they work great for quickie prelim analysis.

    Let's try this again......and i know it tends to fry some circuitry, but forget the variables and absolutely required infinite calculations for a second:

    If you take an off the shelf 8' long wood 2x4 from Home Depot, support it at both ends, lay it on edge, and stand on it in the middle and bounce around alittle.....is it going to feel MORE SPRINGY or LESS SPRINGY than an off the shelf 8' 2X4 extruded aluminum tube with 1/8" wall thickness???

    What about the same comparison for 1/4" wall thickness??
     

  15. Stefan H
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    Stefan H Junior Member

    I guess aluminium is lighter and stiffer but might bend under too heavy load while the wood either flex back or break. Although there are different kinds of aluminium for different applications. It depends on what are you building.
     
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