Wood Physical Properties comparison between WRC and DF

Discussion in 'Materials' started by Martell, Jan 8, 2020.

  1. Martell
    Joined: Feb 2018
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    Martell Junior Member

    Hello,

    I am analyzing the physical properties between Western Red Ceder WRC and Douglas Fir DF.
    I intend to substitute DF for WRC. I am not considering in this analysis the rot resistance. I need some support to help understand if my thinking is out of line.

    The source of my data is: The Wood Database https://www.wood-database.com/ I am not affiliated with this site. If I am using the wrong information, or if anyone has a better source that they can share please provide some meaningful links that might help me get closer to the truth.

    My calculations yield the following results:
    Douglas Fir is 39.13% heavier ADW than WRC
    Douglas Fir is 77.14% harder JK than WRC
    Douglas Fir is 66.67% greater MR than WRC
    Douglas Fir is 59.01% greater EM than WRC
    Douglas Fir is 52.41% greater CS than WRC

    If I have a specification for 19 MM - 0.748031 INCHES Western Red Ceder, and I want to normalize the Douglas Fir substitution for weight at 11.5652 MM 0.455323 INCHES. What would be the net effect to the mechanical properties of the material? How would I calculate the net effect of the substitution?

    Best regards for your support.
    AJM


    WRC Common Name(s): Western Red Cedar
    Scientific Name: Thuja plicata
    Distribution: Pacific Northwest United States/Canada
    Tree Size: 165-200 ft (50-60 m) tall, 7-13 ft (2-4 m) trunk diameter
    ADW Average Dried Weight: 23 lbs/ft3 (370 kg/m3)
    SG Specific Gravity (Basic, 12% MC): .31, .37
    JK Janka Hardness: 350 lbf (1,560 N)
    MR Modulus of Rupture: 7,500 lbf/in2 (51.7 MPa)
    EM Elastic Modulus: 1,110,000 lbf/in2 (7.66 GPa)
    CS Crushing Strength: 4,560 lbf/in2 (31.4 MPa)
    S Shrinkage: Radial: 2.4%, Tangential: 5.0%, Volumetric: 6.8%, T/R Ratio: 2.1

    DF Common Name(s): Douglas-Fir
    Scientific Name: Pseudotsuga menziesii
    Distribution: Western North America
    Tree Size: 200-250 ft (60-75 m) tall, 5-6 ft (1.5-2 m) trunk diameter
    ADW Average Dried Weight: 32 lbs/ft3 (510 kg/m3)
    SG Specific Gravity (Basic, 12% MC): .45, .51
    JK Janka Hardness: 620 lbf (2,760 N)
    MR Modulus of Rupture: 12,500 lbf/in2 (86.2 MPa)
    EM Elastic Modulus: 1,765,000 lbf/in2 (12.17 GPa)
    CS Crushing Strength: 6,950 lbf/in2 (47.9 MPa)
    S Shrinkage: Radial: 4.5%, Tangential: 7.3%, Volumetric: 11.6%, T/R Ratio: 1.6

    WRC ADW 23 lbs/ft3
    DF ADW 32 lbs/ft3
    DF-WRC ADW 9 lbs/ft3
    DF > WRC ADW % 39.13%

    WRC JK 350 lbf
    DF JK 620 lbf
    DF-WRC JK 270 lbf
    DF > WRC JK % 77.14%

    WRC MR 7500 lbf/in2
    DF MR 12500 lbf/in2
    DF-WRC MR 5000 lbf/in2
    DF > WRC MR % 66.67%

    WRC EM 1110000 lbf/in2
    DF EM 1765000 lbf/in2
    DF-WRC Em 655000 lbf/in2
    DF > WRC EM % 59.01%

    WRC CS 4560 lbf/in2
    DF CS 6950 lbf/in2
    DF-WRC CS 2390 lbf/in2
    DF > WRC CS % 52.41%
     
  2. gonzo
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    gonzo Senior Member

    Crushing strength is one of the limitations for planking. What is the minimum you need on your design?
     
  3. DCockey
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    DCockey Senior Member

  4. Martell
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    Martell Junior Member

    The requirement specification call for 19 MM - 0.748031 INCHES Western Red Ceder which has a CS Crushing Strength: 4,560 lbf/in2 (31.4 MPa), and the substitution of Douglas Fir has a CS Crushing Strength: 6,950 lbf/in2 (47.9 MPa). The Douglas Fir has a 52.41% greater crushing strength than the WRC.

    One point I do not understand is a have a square inch of 9 MM - 0.748031 INCHES Western Red Ceder at Crushing Strength: 4,560 lbf/in2 and one square inch of Douglas Fir at Crushing Strength: 6,950 lbf/in2 (47.9 MPa).
    sorry for the typo: mm = MM = millimeter
     
  5. gonzo
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    gonzo Senior Member

    The crushing strength is the force necessary to crush the planking perpendicularly to the surface. Methods of construction will also affect your choices. For example, the depth of the hole to countersink a screw is a fix dimension. Thinner planking may not allow for it. Other considerations, are that fir is notorious for checking. Thin planking like you suggest will have probably have large gaps through it in short time.
     
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  6. DCockey
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    DCockey Senior Member

    I assume you want to know the effect on the mechanical properties of the object to be built using the material. The answer to that question depends on the design of the structure, the loading conditions, and the criteria used to assess the results (stiffness, ultimate strength, ???).

    For instance in tension a piece of 11.6 mm DF compared to a piece of 19 mm WRC will have about the same stiffness. But in beam type bending the thicker WRC will have about 2.8 times the bending stiffness of the thinner DF. (Bending stiffness is proportional to the thickness cubed.)
     
  7. fallguy
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    fallguy Senior Member

    Another thing to consider is wrc is very hard to get in good full clears.

    I realize this does not meet your questions goals, but getting an apples comparison only works if you also know the quality of the actual material. My experience with wrc is it is very difficult to get in clear lumber. Of course this is not the direction you intend.

    I have a feeling you are going to need to pay closer attention to Gonzo's advice. Things like screw lengths are greatly affected by swings as great as you intend.

    I would probably be more inclined to operate on nominals and adjust the fir to 5/8" and then see how fastenings and final weight are affected.

    My personal experience may help you. I planked a canoe years ago and spec was 1/4", but I built to 3/16". When it came time to sand and fair, my planking was getting too thin on the chines and I was scared to fair well.

    When you fair a round hull; you will be going far below your assumptions. And you may end up sanding ?0.100"; especially if you have any trouble and then it will really bother you to think you are under 3/8" against a 3/4" spec.

    So I encourage you to avoid the temptation to design down too far. And, it would be better to consider penalties for the weight delta for say each 1/16" thicker planking.

    Also, consider future fairing that may result in more sanding still if you start at 440, end up at 340 and sand more still in 15 years.
     
  8. Martell
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    Martell Junior Member

    Thank you all for you valuable feedback. I am starting the build of my WALLER 1480 catamaran. I am planning the hull strip planking and was looking for some deeper understanding of the question before I reach out to Mike to discuss my plan and plank substitution.

    Fallguy, did not consider deeply how fairing will impact my plank thickness. Thanks for the heads up. Will look at how 5/8 will impact weight.



    L.O.A. 14.68 Mtrs
    L.W.L. 14.00 Mtrs
    Beam 8.00 Mtrs
    Draft 1.28 Mtrs
    Displacement 10,000 Kg
    Hull Length / Beam 10.6:1
    Sail Area 143.5 Squ Mtrs
    Payload (Ave) 2000 Kg
    Payload (Max) 4000 Kg
    Headroom (avg) 2.0 Mtrs
    Bridgedeck clearance 1.00 Mtrs
     
  9. gonzo
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    gonzo Senior Member

    If you can't get red cedar, pine is a better substitute than fir. You should also be ale to get Atlantic white cedar locally.
     
  10. Rumars
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    Rumars Senior Member

    Sorry but what you inquire about is not possible without a complete redesign. The planking is a composite sandwich panel with the strip-plank WRC as a core. Making the core thinner will change the panels properties. Whatever wood you use the thickness must stay at 19mm. You either accept the weight penalty or use a wood in the same weight class as the one it was originally designed for. The Atlantic white cedar is only 1lb/ft3 more heavy then WRC and appropiate.
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    This may help:

    upload_2020-1-9_9-53-45.png
     
  12. fallguy
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    fallguy Senior Member

    I would like to see the engineering rationale for a Doug Fir planking versus a WRC planking not being allowed to be a 1/16th difference, for example. Not so much to support my earlier assertions about not planking down too far, but just simply because it would be nice to know the specific properties that would change too much. And the OP will for sure want to understand precisely what you mean here.

    And, like I said earlier, wrc is very difficult to find in full clears and what will happen a lot for the builder is they will end up with tons of pieces. If he could get a better hull using Douglas fir at say even 11/16 with a moderate weight penalty, what precisely is driving that difference versus 12/16? It is not readily apparent in the data; even using the conservative numbers, although I didn't put a pen to it.

    I would assume some quality of flexural rigidity would be the main concern, but I am asking and don't know.
     
  13. Rumars
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    Rumars Senior Member

    Stiffness mainly, and strenght to a lesser degree, but the design is stiffness driven.
    This is a cored panel with the stress carried by the skins. The core is not considered in the equation, it is there to keep the skins apart. If he emails Waller with the question if he can substitute endgrain balsa or foam of the same thickness for the cedar the answer will most likely be yes, and there is no need to change the skin scantlings. (BTW Martell, what fiberglass weight and type is specified in your plans? I am guessing biax or triax around 600-1200gr/sqm?)
    Calculating an active core, where the cores own strenght and stiffnes is considered is of course possible. It also does not always makes sense because you also have other things to consider like puncture and abrasion and you need a minimal fiberglass thickness for that anyway. You can read more about it in the multihull structure thread, but that is what it comes down to.

    Why not always use foam, build lighter and increase the payload? Well because foam is more expensive then cedar and strip planking with wood can be a simpler build for the amateur when it comes to complex round surfaces.

    I don't know where you buy your cedar but all clear, 20ft lenght, VG or MG lumber is available even in Europe. Anyway, he can buy lower grades and simply cut the knots out, there is no need to have full lenght strips. And at the quantities he requires (including the waste for cuting strips) he will not buy it at the big box store.
     
    Last edited: Jan 8, 2020
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  14. Martell
    Joined: Feb 2018
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    Martell Junior Member


    I am thankful you guys are sharing with me your experience. I assumed that the WRC is being used primarily as a core material. I was also considering how much of the core material was contributing to the overall mechanical properties and wondering how a substitution of great magnitude like I questioned would impact the design. I specifically chose Waller because of his design attitude " designed specifically to fill the role of an ocean crossing family cruiser suitable for world cruising and extended on board living."

    The specification for the exterior of the hull calls out 750 gsm triaxial glass.
     

  15. fallguy
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    fallguy Senior Member

    I, personally, would never go against the designer, so curious why you didn't start there.
     
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