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Wood load strength

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  • Talmoor
    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
    Message 1 of 9 , Mar 1, 2009
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      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
    • James Winkler
      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
      Message 2 of 9 , Mar 1, 2009
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        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@...
        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


      • 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 3 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 4 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 5 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 6 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 7 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 8 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 9 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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