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Bending Properties of Wooden Crossarms

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					Bending Properties of
 Wooden Crossarms
           H. M. Barnes
 Mississippi Forest Products Laboratory
       Mississippi State University
          J. E. Winandy
   USDA Forest Products Laboratory
           Objectives
• Compare the mechanical properties of
  Douglas-fir crossarms to Southern pine
  crossarms at two commonly used sizes
           Objectives
• Compare the mechanical properties of
  Douglas-fir crossarms to Southern pine
  crossarms at two commonly used sizes

• Compare the influence of boxed-heart
  and juvenile wood in two commonly used
  crossarm species and sizes
      Specifications
•   ANSI 05.3
•   Edison Electric TD 90 (D.fir)
•   Edison Electric TD 91 (S.Pine)
•   REA Bulletin 1728H-701
•   Timber Products Guideline
       Crossarms tested
•   Douglas-fir 3-1/2 x 4-1/2 inches
•   Douglas-fir 3-3/4 x 4-3/4 inches
•   Southern pine 3-1/2 x 4-1/2 inches
•   Southern pine 3-3/4 x 4-3/4 inches
  Materials Source
• Douglas-fir
  – State of Washington

• Southern pine
   – State of Georgia
          Crossarms Tested
Species      Size       Number    Boxed-
                         tested   Heart
 D.fir      3.5 x 4.5      26       0

           3.75x4.75      31        0

S.pine      3.5 x 4.5     34       31

           3.75x4.75      31       14
         Physical Properties
Species      Size       Specific   MC Mean
                        Gravity     (range)
 D.fir      3.5 x 4.5    0.55      20 (15-28)

           3.75x4.75     0.54      21 (16-28)

S.pine      3.5 x 4.5    0.69      21 (15-25)

           3.75x4.75     0.69      18 (13-22)
      Mechanical Testing
• ASTM D-198
• Four-point loading @1/3-points
          (^ v v ^ )
• Span-to-depth: ~17:1 (Span = 61-inch)
• Time to failure: 6-10 minutes
Douglas-fir, Low SG
Douglas-fir, Low SG
Douglas-fir, Medium SG
Douglas-fir, High SG
Southern pine, Low SG
Southern pine, Medium SG
Southern pine, High SG
MOE Moisture Adjustment

                    1.86-0.024(Mp)
 Egreen = Emc *
                    1.86-0.024(mc)



Mp = Point where mechanical properties
     begin to change from green to dry
                        (from Wood Handbook)
MOR Moisture Adjustment

 Pgreen = Pmc * (Rdry/green) ((12-mc)/(Mp –
 12))


Rdry/green = P12/Pgreen   (from Wood Handbook)
                                            MOE
                       100%

                       90%

                       80%
Cumulative frequency




                       70%

                       60%                                                    DF1
                       50%                                                    DF2
                                                                              SP1
                       40%                                                    SP2
                       30%

                       20%

                       10%

                        0%
                              0.5   1           1.5             2             2.5   3
                                        Modulus of Elasticity (million psi)
                                          MOR
                       100%
                       90%
                       80%
Cumulative frequency




                       70%
                       60%                                             DF1
                       50%                                             DF2
                                                                       SP1
                       40%
                                                                       SP2
                       30%
                       20%
                       10%
                        0%
                              4   5   6    7      8      9     10     11     12   13
                                          Modulus of rupture (kpsi)
                                             FSPL
                       100%
                       90%
                       80%
Cumulative frequency




                       70%
                       60%                                                   DF1
                       50%                                                   DF2
                                                                             SP1
                       40%
                                                                             SP2
                       30%
                       20%
                       10%
                        0%
                              3   4          5           6          7            8   9
                                      Fiber stess at proportional limit (kpsi)
      3.5 x 4.5-inch Crossarms
Property   S.Pine    D.fir   p-value
            ______________
 MOE       1.81      1.89    0.2220
           ____      ____
 MOR       9175      7015    0.0001
           ____      ____
 FSPL      5116      6298    0.0001
    3.75 x 4.75-inch Crossarms
Property   S.Pine    D.fir   p-value
            ______________
 MOE       1.87      1.95    0.2127
           ____      ____
 MOR       9291      7156    0.0010
           ____      ____
 FSPL      6063      5363    0.0049
               Summary
• Density of S.Pine greater than D.Fir
               Summary
• Density of S.Pine greater than D.Fir
• MOE equivalent
               Summary
• Density of S.Pine greater than D.Fir
• MOE equivalent
• MOR of S.Pine greater than D.Fir
                 Summary
•   Density of S.Pine greater than D.Fir
•   MOE equivalent
•   MOR of S.Pine greater than D.Fir
•   FSPL equivalent
                  Summary
•   Density of S.Pine greater than D.Fir
•   MOE equivalent
•   MOR of S.Pine greater than D.Fir
•   FSPL equivalent
•   Juvenile wood:
      SP1 w/80% = SP2 w/40% = DF w/no pith
       but MOE, MOR, & FSPL similar
        Cooperators
• Mike Johnson &Greg Campbell
     Cox Industries
     Orangeburg SC
• Lon J. Siebert
     Renewable Resources Assoc.
     Lithonia, GA
• Buddy Peshoff
     Southwest Timber Lab
     Houston, TX

				
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posted:12/18/2011
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