Results 1 to 28 of 28

Thread: Wood spring-back problem on Gothic arch bow shed

  1. #1
    Join Date
    Feb 2017
    Location
    Scotland
    Posts
    27

    Default Wood spring-back problem on Gothic arch bow shed

    Hello there.

    I am trying to make a Gothic arch greenhouse (The classic Gothic arch is known as the quinto acuto or "pointed fifth").


    The arcs are 4/5 of the span. The centres of the arcs are inside the span of the arch.



    I have created a jig to make a sample arch. Everything was fine until I removed the arch from the jig. I did not wait at all for the glue to dry thinking the screws would hold the arch. But this was not the case, spring-back action is occurred. In this case spring-back was really high because I did not clamp the wood together properly and wait for the glue to dry. If I try to do again, I think the spring-back will be minimal.


    My question is: How can I draw the jig taking into account of spring-back action? I am looking for a rough approximation. It does not have to be a perfect calculation.

    I am located in Scotland and could not find the proper timber for this job. I have used this timber:

    http://www.rembrand.co/products.php?...-treated-4.88m

    it is: 3 x 2 ( 38 x 63 mm) CLS treated 4.88m
    I have used 2 of them to make one arch. I also used the same timber to put in the middle (15cm blocks every 30cm and at the each end 30cm blocks).

    My jig:







    Spring-back:




  2. #2
    Join Date
    Nov 2006
    Posts
    7,899

    Default Re: Wood spring-back problem on Gothic arch bow shed

    I''ve never seen a bowshed framed with such heavy timbers. That's your problem. Can you rip them in half edgewise?

  3. #3
    Join Date
    Feb 2017
    Location
    Scotland
    Posts
    27

    Default Re: Wood spring-back problem on Gothic arch bow shed

    I am not good with woods... Do you mean the depth or the width?

  4. #4
    Join Date
    Nov 2006
    Posts
    7,899

    Default Re: Wood spring-back problem on Gothic arch bow shed

    Quote Originally Posted by baris22 View Post
    I am not good with woods... Do you mean the depth or the width?
    The depth, the pieces are about twice as thick as they need to be.

  5. #5
    Join Date
    Apr 1999
    Location
    Hyannis, MA, USA
    Posts
    42,510

    Default Re: Wood spring-back problem on Gothic arch bow shed

    A gothic arch is not a bowshed and neither is a parabolic arch. The parabolic arch makes for manageable strain vectors at the wall/roof join. The bowshed is a single structure, no roof/wall issues. The gothic arch has pretty proportions but the weight of the roof puts a serious spreading strain on the walls. This arch may be small enough that the wall construction by itself can absorb the outward thrust but I doubt it. (Remember all those flying buttresses? They are not decorative.) If you've not engineered this issue, now's a good time to pause and think it through.

    Spring-back can be influenced by how long you leave the thing in the mold. If you're interested in moving fast, do a test piece with reusable screw holes in the stringers and new blocks test to test as needed. Just keep making a sharper mold till it stays close enough. When you start making the real thing, through-bolt both stringers right through the spacer blocks and use glue to really keep the joints from slipping about.

    G'luck

  6. #6
    Join Date
    Feb 2017
    Location
    Scotland
    Posts
    27

    Default Re: Wood spring-back problem on Gothic arch bow shed

    Quote Originally Posted by Jim Ledger View Post
    The depth, the pieces are about twice as thick as they need to be.
    What about these timbers?
    it is: 25mm depth by 38mm width.

    http://www.rembrand.co/products.php?...le-batten-4.8m

  7. #7
    Join Date
    Jan 2000
    Location
    Portland, Maine
    Posts
    14,592

    Default Re: Wood spring-back problem on Gothic arch bow shed

    Are there any sawmills near you? Maybe even small one man type outfits? Then you could get some fresh cut green wood. That's what I used for my bow roof shed. Rough-sawn green red pine. Cheap as chips. I got enough 1"x4" to build the whole thing for $130.


  8. #8
    Join Date
    Feb 2017
    Location
    Scotland
    Posts
    27

    Default Re: Wood spring-back problem on Gothic arch bow shed

    Quote Originally Posted by StevenBauer View Post
    Are there any sawmills near you? Maybe even small one man type outfits? Then you could get some fresh cut green wood. That's what I used for my bow roof shed. Rough-sawn green red pine. Cheap as chips. I got enough 1"x4" to build the whole thing for $130.
    That looks great. I will look into sawmills.

  9. #9
    Join Date
    Jul 2013
    Location
    Aquitaine
    Posts
    700

    Default Re: Wood spring-back problem on Gothic arch bow shed

    On top of the other's thoughts:
    If you bandsawed the timbers into half the thickness, then tried again with your mould blocks moved to a tighter curve. The more laminātes, and thinner ones, the less springback occurs. Try relocating the blocks by half the amount of the current springback.

  10. #10
    Join Date
    Dec 2001
    Location
    Too far inland.
    Posts
    7,597

    Default Re: Wood spring-back problem on Gothic arch bow shed

    Those who've commented on the thickness of the timber are correct.

    Also, your blocking between the chords needs to be deeper. (the chords need to be spaced farther apart). Turn the blocking on edge, like in Stephen's photo in post #7.
    A boatless inlander, searching for the meaning of life-aground.

  11. #11
    Join Date
    Nov 2011
    Posts
    3,718

    Default Re: Wood spring-back problem on Gothic arch bow shed

    This worked perfectly. 1x4 with short (3.5") pieces of 2x4 on 12" centers between with glue and decking screws. Your blocks are too long. Since they are straight and don't conform to the curve you need to keep them short.








  12. #12
    Join Date
    Feb 2017
    Location
    Scotland
    Posts
    27

    Default Re: Wood spring-back problem on Gothic arch bow shed

    Quote Originally Posted by Figment View Post
    Those who've commented on the thickness of the timber are correct.

    Also, your blocking between the chords needs to be deeper. (the chords need to be spaced farther apart). Turn the blocking on edge, like in Stephen's photo in post #7.
    Just to clarify... Do you mean the depth of the blocks I used between the two arches? At the moment the timber I have used for the blocks are exactly the same as the ones I used for the arches. The depth is 38mm, wideness is 63mm and it is 15cm long. Should the depth be more than 38mm?

  13. #13
    Join Date
    Nov 2011
    Posts
    3,718

    Default Re: Wood spring-back problem on Gothic arch bow shed

    Quote Originally Posted by StevenBauer View Post
    Are there any sawmills near you? Maybe even small one man type outfits? Then you could get some fresh cut green wood. That's what I used for my bow roof shed. Rough-sawn green red pine. Cheap as chips. I got enough 1"x4" to build the whole thing for $130.

    Hey Steven, got any more pics of what's aft of that that Viking bow?

  14. #14
    Join Date
    Dec 2003
    Location
    Portland, Oregon
    Posts
    41,373

    Default Re: Wood spring-back problem on Gothic arch bow shed

    I'd agree that reducing the thickness of your flanges and separating them as much as possible (increasing the size - i.e. 'height' in the sketch below) of your web, given your material constraints, would be the two most important steps.

    What you are running into is the marvelous magic of beam strength. By that, I mean that increasing or decreasing the thickness of your beam will change the stiffness by the cube of the change. That is - if you doubled the overall height of one of the illustrated beams (h X 2)... the stiffness increases by the cube of the change (h X 23). So doubling the height would mean increasing the stiffness 8 times.

    The same dynamic comes into play with the thickness (height) of your flange members. By reducing that dimension by 1/2, you will reduce the rigidity of that component (and, to the point of your problem, the propensity for springback) by a factor of 8.

    And you can see that, likewise, increasing the width of your web spacers (height in the sketch) will also effect the overall rigidity of the final structure substantially.

    And DO let your glue dry before you take your trusses out of the jig. What adhesive are you using? And what plans are you working from? Stimson's?



    Beyond that - modifying your assembly jig to build in just a tiny bit of over-bending will allow for some springback. This part doesn't have to be precise, and it doesn't have to be much... if you've allowed your glue to dry, and if you reduce your flange thickness (height in the sketch). Also... when you go to assemble your shed, the beams will flex a small amount... allowing for adjustments in the field.

    Of course a real engineer can explain all the variations, caveats, and modifiers that would yield true precision in such a calculation - but the basic guideline of beam stiffness increasing as a cube of the increase in height is a very good approximation for most of what we do.
    Last edited by David G; 02-28-2017 at 10:35 AM.
    David G
    Harbor Woodworks
    http://www.harborwoodworking.com/boat.html

    "It was a Sunday morning and Goddard gave thanks that there were still places where one could worship in temples not made by human hands." -- L. F. Herreshoff (The Compleat Cruiser)

  15. #15
    Join Date
    Dec 2001
    Location
    Too far inland.
    Posts
    7,597

    Default Re: Wood spring-back problem on Gothic arch bow shed

    Quote Originally Posted by baris22 View Post
    Just to clarify... Do you mean the depth of the blocks I used between the two arches? At the moment the timber I have used for the blocks are exactly the same as the ones I used for the arches. The depth is 38mm, wideness is 63mm and it is 15cm long. Should the depth be more than 38mm?

    Yes, orient the spacer blocks (web of the beam) the other way, so that you have 63mm separation between the flanges.

    I also agree with Gib that your spacer blocks are too long. 10-12cm should be adequate. (except for those at the ends, which look correct in your photo)
    A boatless inlander, searching for the meaning of life-aground.

  16. #16
    Join Date
    Sep 2016
    Location
    Rochester, NY
    Posts
    82

    Default Re: Wood spring-back problem on Gothic arch bow shed

    Hi Baris, We swapped messages on YouTube about this project.

    check out Walt's build, he has lots of pictures to help explain what you are trying to do.

    http://forum.woodenboat.com/showthre...=bow+roof+shed
    A ship is safe in the harbor, but that's not what ships are for.

    http://www.seadreamerproject.com
    http://www.youtube.com/c/SixPointsWoodWorks

  17. #17
    Join Date
    Jul 2011
    Location
    Lindstrom, MN
    Posts
    1,661

    Default Re: Wood spring-back problem on Gothic arch bow shed

    It looks like your arch drawing from this site: http://www.codesmiths.com/shed/works...ues/arches.htm

    You might want to look at this plan from the U of TN: EDIT 8/26/17 plans listed, but no longer linked bioengr.ag.utk.edu/extension/extpubs/Plans/6298.pdf
    New source, LSU same plan http://www.lsuagcenter.com/~/media/s...greenhouse.pdf
    Anther link http://www.builditsolar.com/Projects...ArchGH6298.pdf

    I would agree with your basic geometry, i.e. the circular arch. I don't agree with Ian's first paragraph at all. He usually provides good advice, but not this time. There was a thread a few years ago where the shed builder had problems with the 19mm thick wood breaking, and I think that the circular arch would have helped reduce the stress. The typical bow shed plans have a tighter radius in part of the arch that is harder on the wood, and only lowers the peak of the roof a foot or so. Steven Bauer's shed looks more like your plan to me.

    As stated before, your boards are twice as thick and more than 8 times as stiff as the 1x2s (19x38mm) boards that are more commonly used. The spring back is unavoidable. The boards are pressure treated, and while the web site says that they are kiln dried, the fact that you didn't break them makes me think that they are soaking wet, which is typical of pressure treated lumber around here. Truth in advertising. The wood was indeed kiln dried before they saturated it with water with the CLS treatment. Wet lumber is a good thing for a bend like that, so I wouldn't complain. The only way that you are going to hold the curvature is to tack a board or run a cable from end to end to keep them from straightening out, not that I think it is a good idea. In the end, ripping them down to 19x63 would be a good idea. A table saw would work well, but the length will take some room and a bit of help.

    The webs, I agree, should be deeper. The U of TN plans show 2x4s on edge where you have 2x3 on the flat. The total depth of the 1x2s spaced with 2x4s on edge is 127mm where your 3 38 mm thick boards are 114mm and much heavier, so I would not be too concerned, but I would still turn the 2x3s (38x63) on edge.
    Last edited by MN Dave; 08-26-2017 at 12:25 AM.
    Management is the art of counting beans. Leadership is the art of making every being count. --Joe Finch

  18. #18
    Join Date
    Feb 2017
    Location
    Scotland
    Posts
    27

    Default Re: Wood spring-back problem on Gothic arch bow shed

    By the way, I have never done this kind of job before. I want to build a greenhouse that will be strong enough and won`t cost a lot of money.
    I think I will be Ok. with the help I get in here.


    Quote Originally Posted by Gib Etheridge View Post
    This worked perfectly. 1x4 with short (3.5") pieces of 2x4 on 12" centers between with glue and decking screws. Your blocks are too long. Since they are straight and don't conform to the curve you need to keep them short.
    That is amazing. I can make more than one arch at a time with this setup.

    Quote Originally Posted by David G View Post
    I'd agree that reducing the thickness of your flanges and separating them as much as possible (increasing the size - i.e. 'height' in the sketch below) of your web, given your material constraints, would be the two most important steps.
    Thanks David, very informative.

    Quote Originally Posted by scott2640 View Post
    Hi Baris, We swapped messages on YouTube about this project.

    check out Walt's build, he has lots of pictures to help explain what you are trying to do.

    http://forum.woodenboat.com/showthre...=bow+roof+shed
    Thanks Scot.
    Last edited by baris22; 02-28-2017 at 03:31 PM.

  19. #19
    Join Date
    Jan 2000
    Location
    Portland, Maine
    Posts
    14,592

    Default Re: Wood spring-back problem on Gothic arch bow shed

    Quote Originally Posted by Gib Etheridge View Post
    Hey Steven, got any more pics of what's aft of that that Viking bow?
    I posted lots of pics here when my son built it back when he was in high school. It's an Iain Oughtred ELF.

    http://forum.woodenboat.com/showthre...-Elf-is-turned!

    When I built the shed I figured I'd need 14' boards. But the lumber yard was out of 14s so they sent 16s. That's why the shed is so tall, I thought "why cut 'em?"

    ]


    My form was much simpler than Gibs was. I made a 16' long table out of two sheets of osb on a couple of 2x4s on some sawhorses. I think I just had a clamping block at each end and one in the middle. I glued and screwed the blocks but removed them from the jig right away so I could make the next one. They are a full 1"x4", not 3/4"x3 1/2".
    Last edited by StevenBauer; 02-28-2017 at 11:13 PM.

  20. #20
    Join Date
    Nov 2011
    Posts
    3,718

    Default Re: Wood spring-back problem on Gothic arch bow shed

    Thank you Steven.

  21. #21
    Join Date
    Dec 2011
    Location
    Onancock, VA
    Posts
    842

    Default Re: Wood spring-back problem on Gothic arch bow shed

    I had almost no springback to my surprise. Maybe 3/4 ". I did laminate the outside and inside layer.

    http://forum.woodenboat.com/showthre...hlight=bowshed

  22. #22
    Join Date
    Jun 2012
    Location
    Salem, MA
    Posts
    128

    Default Re: Wood spring-back problem on Gothic arch bow shed

    Not directly applicable here, but the science is very good to know about.

    Just for the record - from the free-on-line Gougeon Boatbuilding Bible page 92 give a formula to use for calculating springback for a laminated part (as for a tiller or deck beams)

    http://www.westsystem.com/ss/assets/...k%20061205.pdf

    Couldn't get the file upload here to work, so you'll need to get the free Gougeon book for yourself - a good thing
    Cheers,

  23. #23
    Join Date
    Apr 2012
    Posts
    192

    Default Re: Wood spring-back problem on Gothic arch bow shed

    http://shamrockaquaponicfarm.com/wp-...e-Plan-Set.pdf

    Plans for a greenhouse that will withstand 160mph winds. Same source that Walt used, other, smaller sizes available for a price. This set can be downloaded free. Can be made shorter or longer by changing the number of arches.

  24. #24
    Join Date
    Sep 2016
    Location
    Rochester, NY
    Posts
    82

    Default Re: Wood spring-back problem on Gothic arch bow shed

    Those are the plans that I used as well.



    Quote Originally Posted by ulav8r View Post
    http://shamrockaquaponicfarm.com/wp-...e-Plan-Set.pdf

    Plans for a greenhouse that will withstand 160mph winds. Same source that Walt used, other, smaller sizes available for a price. This set can be downloaded free. Can be made shorter or longer by changing the number of arches.
    A ship is safe in the harbor, but that's not what ships are for.

    http://www.seadreamerproject.com
    http://www.youtube.com/c/SixPointsWoodWorks

  25. #25
    Join Date
    Feb 2017
    Location
    Scotland
    Posts
    27

    Default Re: Wood spring-back problem on Gothic arch bow shed

    I went to a sawmill today and got a quote for the whole greenhouse project. I showed him all the plans I have and the images of the project. He suggested that I should use larch timber. He said he can provide me with wet or air dried strips. The final strip will be 25mm, 70mm, 4.88 meters.

    This will cost me £4. It is much cheaper than what I have used for my trial. I think I will open a new topic for this project to document it from start to finish.

    Can you please advice me whether I should get it wet or air dried? Can you also let me know if the dimensions of the blocks I am planning to use are proper.

    Thanks for the advice.


  26. #26
    Join Date
    Jan 2000
    Location
    Portland, Maine
    Posts
    14,592

    Default Re: Wood spring-back problem on Gothic arch bow shed

    I was happy with the wet wood I got for my shed. Except for one thing. The weight! I drove out to the yard to pick it up and the yard guy looked at the rack on my truck and shook his head. I said I'd put that much wood on the rack before and he said "this wood was a tree the day before yesterday." And he was right. We tried to put half of it on the truck with his forklift and I thought the rack would collapse. So I went back in the office to see about delivery. There was no charge and it was in my driveway when I got home.

  27. #27
    Join Date
    Jul 2011
    Location
    Lindstrom, MN
    Posts
    1,661

    Default Re: Wood spring-back problem on Gothic arch bow shed

    Quote Originally Posted by baris22 View Post
    I went to a sawmill today and got a quote for the whole greenhouse project. I showed him all the plans I have and the images of the project. He suggested that I should use larch timber. He said he can provide me with wet or air dried strips. The final strip will be 25mm, 70mm, 4.88 meters.

    This will cost me £4. It is much cheaper than what I have used for my trial. I think I will open a new topic for this project to document it from start to finish.

    Can you please advice me whether I should get it wet or air dried? Can you also let me know if the dimensions of the blocks I am planning to use are proper.

    Thanks for the advice.
    Ask them about bending the larch. Wet wood will not be as stiff and will generally bend more without breaking and with less spring back. The glue will be slower to set and it will be heavier. If you can get samples of wet and dry it might be instructive. Dry wood is considerably stronger and stiffer than wet wood. All I can say about the dimensions is that they seem reasonable to this unqualified sidewalk supervisor.

    Larch sounds like a good choice. http://www.wood-database.com/european-larch/
    Management is the art of counting beans. Leadership is the art of making every being count. --Joe Finch

  28. #28
    Join Date
    Apr 2017
    Location
    BurnsLake BC. Canada
    Posts
    1

    Default Re: Wood spring-back problem on Gothic arch bow shed

    Kiln Dried wood is very springy. Best to use wood with no less than 20% moisture. Local band saw cut is a good choice for bending.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •