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Strength of Laminated Cedar

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Forum topic by NotThatJeff posted 09-27-2020 03:35 PM 346 views 0 times favorited 13 replies Add to Favorites Watch
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NotThatJeff

4 posts in 32 days


09-27-2020 03:35 PM

Hi All,

Brand new member here.

I am building a yurt (a heavy duty, Mongolian style tent with a wooden frame), and one of the pieces that is still very much un-decided is the central roof ring where the rafter poles fit (slightly loosely) into mortices. Here is a picture of a very nice, lightweight design built up of laminated 1/8” plywood:

And here is a (very heavy!) design, much more along traditional Mongolian lines:

I’m thinking of a hybrid between the two, stronger than the first, lighter than the second. My idea is to cut Western Red Cedar into thin strips 4-ish inches wide and cold laminate them (glue and clamps, no steam) into a 4-foot diameter circle, where the total thickness of the layers is about 3 inches. (This would then get cut down some, so the cross section is not a simple rectangle. And I’ll probably do something simpler than the traditional interior, maybe with just 4 spokes.)

I love the look, strength, light weight, and bendability of cedar. Oh, and the smell! One shortcoming of cedar, however, is that it is very splitty: a teeny, tiny split can instantly shoot along the grain. For this project, a split along the grain would be disastrous.

One the one hand, I feel like laminated cedar should be much less prone to splitting than the solid wood as many different layers would be weak in different places.. OTOH, cedar tends to be very straight grained, and all the layers will tend to have their (weak) grain lines in parallel: there’s nothing strong anywhere to prevent a split.

Does anybody have a sense for how strong (split-resistant) laminated cedar is?

Is there anything unobtrusive I could put between layers that would add strength? (I saw a boat-builder use some kind of cotton fiber added to epoxy; I think he said it wouldn’t work for wood glue, which is what I plan to use.)

Alternatively, is there another (inexpensive, light, easily bendable) wood I could use in alternating layers with cedar? Something that would add cross-grain strength and not contrast too much?

Another idea: Would it be crazy to use cedar oriented at 90 degrees on alternating layers? (I’m picturing taking a whole bunch of short cedar strips, taping them together edgewise on one side, then applying glue to the other side and wrapping/clamping them around the previous layer…)

Thanks for all your ideas!


13 replies so far

View Lazyman's profile

Lazyman

5864 posts in 2270 days


#1 posted 09-27-2020 04:50 PM

I don’t think that orienting alternating layers at 90° will add any strength. I would think that the strength of a bent laminated piece comes largely from the fact that the grain runs down the entire length, plus as you bend them, rotated strips are likely to break on the grain lines. You may want to find a source for air dried or even green boards. Kiln drying sets the lignin and makes them less flexible. White oak is probably a better choice for bending than the WRC. WRC is pretty brittle in general and will be more prone to breaking. Regardless, you want to look for boards with very strain grain. Any grain that runs diagonally will be prone to split as you bend it. Also, with a 4’ diameter ring, the length around is about 12.5’ so unless you get strips that long, you’ll have to piece each layer from multiple pieces and you may get gaps if you are not careful. Getting enough 12’ strips with straight grain will be tough so I would plan on shorter strips anyway.

Personally, I would go with the segmented approach, similar to how a wagon wheel is made. I think that it will be easier to make. For one thing, you can cut the mortises before assembly. With the glue-lam approach, you have to cut all of the mortices after it is glued and trimmed to round. It probably does not need to be as massive as the example above so you should be able to lighten it up considerably.

Sounds like a fun project.

-- Nathan, TX -- Hire the lazy man. He may not do as much work but that's because he will find a better way.

View LesB's profile

LesB

2622 posts in 4326 days


#2 posted 09-27-2020 05:02 PM

First question is what type of cedar are you talking about?
Western Red Cedar is easily bendable in thin strips and I do not consider it “splitty”.

Have you considered using solid Cedar with 8 sections (like a school clock) and joined with mortise and tenon joints then cut into a circle on the outside where the ribs fit in. The inside could actually be left straight. That seems to be how the one in the lower picture was made. It would be a lot less work and no messy glue and clamping.

-- Les B, Oregon

View drsurfrat's profile

drsurfrat

152 posts in 69 days


#3 posted 09-27-2020 05:23 PM

I understand your concern for cedar splitting, that’s how they make shingles… maybe WRC is not as bad.

Hickory might be a good choice to laminate w cedar, it has interlocking grain, and is one of the traditional woods to bend. I bet you could use it at 1:3 w the cedar and still get the split resistance you desire.
https://www.rockler.com/wood-species-guide

-- Mike (near Boston) ... Laziness is the mother of invention, necessity is the mother of exhaustion - me

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Aj2

3414 posts in 2681 days


#4 posted 09-27-2020 05:29 PM

Laminated ring is a very good way to make that ring. I think the success of your work will depend a lot on your skills and the quality of the wood you choose.
Western red cedar is really in the cypress family. I’m thinking there’s better choices then WRC.Alaskan yellow cedar would be the top pick. It’s very very stable easy to resaw glues great bends fantastic. And smells great.
There’s another cypress that grow in port orford Oregon it’s expensive and clear stock will be nearly impossible to find.
Another cypress that grows in the swamps of south like Louisiana I forget the name. But I’ve had the pleasure of using it and remember it almost waxy with oils.

Here’s a look at some AYC that I have 50+ rings per inch is very common I’ve had some upwards 70+

Good Luck

-- Aj

View Loren's profile

Loren

10712 posts in 4530 days


#5 posted 09-27-2020 05:56 PM

You may need to engage in the tedium and/or expense of not only resawing the cedar thin enough to bend on that radius (3/16” would be my guess but you may find 1/4” or even thicker will do it), but also perhaps of thickness sanding. I have bend guitar sides from cedar and it takes a careful approach, heat and moisture and a thickness of about 3mm or less.

I know steam bending is intimidating, but it’s really a good way to go.

Consider perhaps as well, bending plywood is sold. You could add cedar skins to top and bottom to imitate the look of the second example.

All I’m saying is that laminating is costly in terms of not only wasted material but also time.

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Loren

10712 posts in 4530 days


#6 posted 09-27-2020 06:20 PM

One thing you might try is steam bending small sections of 1/2” or thinner cedar (other woods may bend easier) and then effectively brick-lay them into concentric rings. This will speed up laminating by using thicker boards and reduce waste and time lost resawing.

Imagine taking an embroidery hoop, cutting the two parts into segments and then gluing it all back together by overlaying the cut sections. By steam bending this way a smaller steam box can be used and a less elaborate jig to bend the parts, perhaps even a “wedge” jig used by boatbuilders.

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Loren

10712 posts in 4530 days


#7 posted 09-27-2020 09:09 PM

I should add that laminated wood is generally intensely strong. It’s basically plywood. I wouldn’t be concerned about strength unless you approach the lamination for example using pieces that are too thick to glue up tightly, or without enough clamps.

For that matter, do you have a lot of clamps? Even using bending plywood or really thin wood you’ll need to consider clamping. It can be done with rope and wedges in the traditional Asian style I suppose but I’ve never tried it.

View Madmark2's profile

Madmark2

1714 posts in 1471 days


#8 posted 09-27-2020 09:30 PM

15’ band clamps

-- The hump with the stump and the pump!

View Loren's profile

Loren

10712 posts in 4530 days


#9 posted 09-27-2020 10:08 PM

^^^ certainly worth testing.

Every time I’ve laminated “plywood” I’ve cranked the pressure with clamps every few inches because otherwise I had gaps in the layers. If the edges are going to be covered up, why bother laminating in the first place? It would still be strong but any gaps will be hard to ignore. On a table apron gaps could be filled for curious fingers, but looking up at the focal point of a yurt, I think one would want a real clean look.

View Lazyman's profile

Lazyman

5864 posts in 2270 days


#10 posted 09-28-2020 02:53 AM

One other thing to remember if you’ve never done a laminated glue up…it’s pretty difficult to keep the lamination straight while you are clamping. They tend to slide and you will inevitably have some that stick out above or below the edge so you will have to plane the top and bottom edges smooth afterwards which means you may need to make the laminations slightly wider than your target width.

-- Nathan, TX -- Hire the lazy man. He may not do as much work but that's because he will find a better way.

View NotThatJeff's profile

NotThatJeff

4 posts in 32 days


#11 posted 09-29-2020 08:49 PM

OP here.

Wow a lot of replies! (Work got crazy and I couldn’t respond right away…)

First of all I must admit that as someone with a degree in mathematics, I am embarrassed to admit that I hadn’t realized the strips would be 12+ feet long. SMH. (In my defense, I had originally meant for the circle to be 3’ in diameter…)

As far as wood choices, I was inclined to go with cedar b/c of light weight and matching the wood I’m using for the roof rafters. Although I got it from someone selling it as “cedar” fencing posts/rails, there’s quite a few variations in color, including some (rather dark brown) rather nice wood that doesn’t matching any pics or descriptions of cedar I’ve seen online. I mentioned Western Red Cedar in my post b/c I assume that’s what the bulk of my stock is based on pics and the fact that I’m in the West.

Another big factor in this project is frugal-bragging rights. I got 200 pounds of wool for free, got my “cedar” for about 1/3 of Lowe’s price, etc… So I’d like to come up with a solution that looks awesome but doesn’t involve paying big bucks at the fancy-schmancy hardwood store.

The ring in the lower example was made from pine 2x lumber, and was carved and painted over in the traditional (very ornate) Mongolian style. I’m not against painted pine, but it seems like it would clash with the amazing exposed cedar colors/grain of the rafters and the (mandatory, so it would seem…) Shou sugi ban (burnt) wall slats. Actually, just typing that gave me the idea of burning the roof ring as well. Must think some more…

As for technique, I really like gluing up laminations. For the wall slats I’m gluing up 2 layers of 1/4” thickness sawn from big-box store Douglas Fir 2×4’s. So many knots that it would be hard to find 8’ of usable solid 1/2” thick material. But laminating two layers together (with no knots lining up) makes the slats pretty strong. I made a curved clamping jig for the job and I use very closely spaced wooden wedges as clamps. One a day or so and I’ll be done by Spring. It’s a very satisfying process.

[One of my favorite jokes is rather long and involved, but the punch line is “What’s time to a pig?” I’m doing this purely as a hobby so the value of the time I’m putting into it is 0. As long as I enjoy the process, it’s all good.]

As for steam bending I’ve looked into it, but it seems somewhat more “un-relaxed” than cold lamination. Plus I live at about 6000 feet, so I don’t know if I can get the lignin up to the right temperature.

The “standard” way to make the roof ring is by gluing up blocks in an 8-sided arrangement. If thin (2x dimensional) stock is used, there may be 3 such layers. That’s my plan A, but this post was to explore a plan B.

I definitely don’t like the idea of a veneer on top of something else. Looks great at first, but after some banging around (this piece will get banged around) it looks pretty awful, IMO.

View Foghorn's profile

Foghorn

571 posts in 269 days


#12 posted 09-30-2020 01:41 AM



Laminated ring is a very good way to make that ring. I think the success of your work will depend a lot on your skills and the quality of the wood you choose.
Western red cedar is really in the cypress family. I’m thinking there’s better choices then WRC.Alaskan yellow cedar would be the top pick. It’s very very stable easy to resaw glues great bends fantastic. And smells great.
There’s another cypress that grow in port orford Oregon it’s expensive and clear stock will be nearly impossible to find.
Another cypress that grows in the swamps of south like Louisiana I forget the name. But I’ve had the pleasure of using it and remember it almost waxy with oils.

Here’s a look at some AYC that I have 50+ rings per inch is very common I’ve had some upwards 70+

Good Luck

- Aj2


Agree that yellow cedar bends way easier and is way tougher and more split resistant than WRC. Good luck. Nice project.

-- Darrel

View Lazyman's profile

Lazyman

5864 posts in 2270 days


#13 posted 09-30-2020 01:28 PM

Your mention of knots in the DF reminded me to mention that any knots in the cedar are going to make it tough to bend without breaking so you will want to cut out any knots from your strips. A knot won’t bend very well. What dimensions are the cedar boards you are going to be cutting the strips from? You may find that it is easier to work with strips that are about half the circumference. As Lauren alluded to, you really need each strip to be a consistent thickness so running them through a thickness sander might be necessary or perhaps handing planning them to make sure that there aren’t any humps that will mess up the glue up or make the ring out of round. It can be pretty tough getting a perfectly flat strips off the saw, especially with long boards. I would try cutting a few practice strips to see how difficult it is to get a flat strip and to see how thin they need to be for the radius you are bending. Grain that runs diagonally through the thin dimension may cause it to break if the radius is too small or the strip is too thick.

Not trying to be a naysayer—just thinking out loud.

-- Nathan, TX -- Hire the lazy man. He may not do as much work but that's because he will find a better way.

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