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RE: [MedievalSawdust] Wood load strength

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  • James W. Pratt, Jr.
    For more Specific information look into testing of wood for Airplanes. They have jigs you can make to test the strength of structural pieces and grow rings
    Message 1 of 9 , Mar 1, 2009
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      For more Specific information look into testing of wood for Airplanes.  They have jigs you can make to test the strength of structural pieces and grow rings per inch required for thing like spars and longerons...of course that is mainly for spruce!

       

      James Cunningham

      Still hoping to fly a Pits Special

       


      From: medievalsawdust@yahoogroups.com [mailto: medievalsawdust@yahoogroups.com ] On Behalf Of James Winkler
      Sent: Sunday, March 01, 2009 1:38 PM
      To: medievalsawdust@yahoogroups.com
      Subject: RE: [MedievalSawdust] Wood load strength

       

      There are probably sources for generic information. .. but too much is involved in calculating the angle of load to the grain, the quality of the stick in general, the overall geometry and weight dispursion of the structure in general, etc., etc...  There's also the question of whether you're talking 'loading' or 'shear' strenght...
       
      Inherently, the weakest part of this design is the pivot point at which the legs cross.   the wood is thinest there and you have those pesky grain issues that come into play... not to mention the fact that that is the EXACT point at which all of those "pounds per square inch" are maximized into the the fewest possible square inches....
       
      So... what we're really talking here is the shear strenght of the wood along the grain at this point...  and I don't think your going to find that anywere... at least not specific to whatever stick your using.
       
      Chas.
       


      To: medievalsawdust@ yahoogroups. com
      From: talmoor@yahoo. com
      Date: Sun, 1 Mar 2009 17:54:06 +0000
      Subject: [MedievalSawdust] Wood load strength

      Does anyone have a good site or book for figuring the weight load a
      piece of wood can take? I am making a dantesca chair and wonder how
      much of a limit it can take and if there is any way to strengthen it.

      Alasdair

    • Conal O'hAirt Jim Hart
      There is an option that would do two things.... allow you to use thinner stock making it less expensive and smaller pieces for less waste. ( which might allow
      Message 2 of 9 , Mar 1, 2009
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        There is an option that would do two things....

        allow you to use thinner stock making it less expensive
        and smaller pieces for less waste. ( which might allow
        you to use a more expensive and stronger wood.... )

        and

        allowing you to slightly change the orientation of the grain
        making it a little stronger.


        You could glue two pieces of 3/4" thick together to make 1 1/2"
        thickness..... ( might be a little hard to visualize and to explain )

        but you would still have the weak point at the center pivot....hmm....




         
        Baron Conal O'hAirt / Jim Hart

        Aude Aliquid Dignum
        ' Dare Something Worthy '

      • AlbionWood
        Period problems have period solutions! (quoting Duke Cariadoc) Don t futz around with laminating etc., just figure out how the same issue was treated by the
        Message 3 of 9 , Mar 1, 2009
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          "Period problems have period solutions!"  (quoting Duke Cariadoc)

          Don't futz around with laminating etc., just figure out how the same issue was treated by the people who made the chairs 500 years ago... some of which are still around today... guess they managed to solve this question!

          /rant

          Tim


          Conal O'hAirt Jim Hart wrote:
          There is an option that would do two things....

          allow you to use thinner stock making it less expensive
          and smaller pieces for less waste. ( which might allow
          you to use a more expensive and stronger wood.... )

          and

          allowing you to slightly change the orientation of the grain
          making it a little stronger.


          You could glue two pieces of 3/4" thick together to make 1 1/2"
          thickness..... ( might be a little hard to visualize and to explain )

          but you would still have the weak point at the center pivot....hmm....




           
          Baron Conal O'hAirt / Jim Hart

          Aude Aliquid Dignum
          ' Dare Something Worthy '

        • AlbionWood
          USDA publishes a book of engineering data for wood structures. That would be your best bet. It will give you shear strengths and other values for most commonly
          Message 4 of 9 , Mar 1, 2009
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            USDA publishes a book of engineering data for wood structures.  That would be your best bet.  It will give you shear strengths and other values for most commonly used woods.  You'll have to do the engineering calcs yourself - resolving the stress vectors, calculating bending moments, shear stress, etc, before you can use any of the values in the book.  Anyone who struggled through Strength of Materials in college will tell you that's not a trivial exercise!

            But you can't go too far off by using the same wood and the same dimensions as the original chairs.  Dantescas are pretty strong if you cut that crossing-joint carefully and use good wood.  Avoid red oak or other coarse-grained, easily-split wood.  If you're really concerned for strength, find some elm.

            Cheers,
            Tim




            Talmoor wrote:
            Does anyone have a good site or book for figuring the weight load a 
            piece of wood can take? I am making a dantesca chair and wonder how 
            much of a limit it can take and if there is any way to strengthen it.
            
            
            Alasdair
            
            
            
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          • Bill McNutt
            Generally, my response to the observation: Geeze, that looks flimsy has been 1) Use a denser wood. Oak, Hickory, and even walnut hold up better than pine
            Message 5 of 9 , Mar 1, 2009
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              Generally, my response to the observation: “Geeze, that looks flimsy” has been 1) Use a denser wood.  Oak, Hickory, and even walnut hold up better than pine and poplar.  2) Use MORE wood.  2” thick is stronger than 1” thick.  And, finally, 3, re-inenforce it with metal.

               

              All period solutions.  All add more weight.

               

              Will

               

              From: medievalsawdust@yahoogroups.com [mailto:medievalsawdust@yahoogroups.com] On Behalf Of AlbionWood
              Sent: Sunday, March 01, 2009 6:39 PM
              To: medievalsawdust@yahoogroups.com
              Subject: Re: [MedievalSawdust] Wood load strength

               

              "Period problems have period solutions!"  (quoting Duke Cariadoc)

              Don't futz around with laminating etc., just figure out how the same issue was treated by the people who made the chairs 500 years ago... some of which are still around today... guess they managed to solve this question!

              /rant

              Tim


              Conal O'hAirt Jim Hart wrote:

              There is an option that would do two things....

              allow you to use thinner stock making it less expensive
              and smaller pieces for less waste. ( which might allow
              you to use a more expensive and stronger wood.... )

              and

              allowing you to slightly change the orientation of the grain
              making it a little stronger.


              You could glue two pieces of 3/4" thick together to make 1 1/2"
              thickness..... ( might be a little hard to visualize and to explain )

              but you would still have the weak point at the center pivot....hmm....



               

              Baron Conal O'hAirt / Jim Hart

              Aude Aliquid Dignum
              ' Dare Something Worthy '

               

            • Dave Manley
              Here is a link to Chapter 4 - The Mechanical Properties of Wood from the Wood Handbook Posted by the US Forest Service Website.
              Message 6 of 9 , Mar 2, 2009
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                Here is a link to "Chapter 4 - The Mechanical Properties of Wood" from the Wood Handbook Posted by the US Forest Service Website. 


                this gives the Modulus of Elasticity and Rupture plus a work to maximum load ratio of many different types of wood.

                It may be more info than what you are looking for, but interesting reading for engineering types. :-D
                 
                Dave Manley
                Et in Arcadia ego


              • Alex Haugland
                With the dantesca chair style, much of the strain on the joint can be alleviated by making sure that the legs make good, solid contact to both sides of the
                Message 7 of 9 , Mar 2, 2009
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                  With the dantesca chair style, much of the strain on the joint can be alleviated by making sure that the legs make good, solid contact to both sides of the pivot joint, before the lap joint for the pivot.  That transfers much of the weight and downward force on the chair directly from the arms and seats to the legs without passing it through the pivot joint, which keeps the chair strong.

                  --Alysaundre Weldon d'Ath
                  Barony of Adiantum, An Tir

                  Talmoor wrote:

                  Does anyone have a good site or book for figuring the weight load a
                  piece of wood can take? I am making a dantesca chair and wonder how
                  much of a limit it can take and if there is any way to strengthen it.

                  Alasdair


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