Bent laminate ? for the pros - Woodworking Talk - Woodworkers Forum
 
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post #1 of 6 Old 10-14-2009, 07:43 PM Thread Starter
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Bent laminate ? for the pros

If you caught my other thread you already know that I am planning a bent laminate project. If you haven't the short version is this....covered bench swing with seating for 3 adults, the bent laminate portion is the load bearing 'A' frames on each end.

In order to make sure this swing is structurally sound I want to build the laminate thick enough to hold 3 large adults. I am pretty sure I have decided on using quarter sawn white oak because of stability and rot resistance, plus plyability and low cost. Each laminate will be approx 4" wide by 1/8" thick (I will resaw thinner slices if 1/8" won't bend enough) by what ever length I figure out I need. I will probably end up cutting the width from 4" down to about 3.5" in order to clean up the edges.....unless I get lucky and they all stay straight and even. I was thinking of a final thickness of 5/8" but I am not sure if I should increase it to a full 1". I want to figure this out so I can calculate how thick of a board I need to start with before resawing all the laminates. The wood will be a local pickup, but if I can pick up alll the wood at one time I can avoid having to run back for an extra piece and risk that piece not matching the grain/color of the original.
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post #2 of 6 Old 10-14-2009, 08:54 PM
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Quote:
Originally Posted by Kender View Post
I was thinking of a final thickness of 5/8" but I am not sure if I should increase it to a full 1".

Is this your question? Posting a picture of some kind would help to see what you have in mind.




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post #3 of 6 Old 10-14-2009, 09:50 PM Thread Starter
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Originally Posted by cabinetman View Post
Is this your question? Posting a picture of some kind would help to see what you have in mind.





I haven't seen any pics of anything similar and I am not even sure if I have anything on the computer that would let me draw it. I could draw something up and then scan it as a pic. I know I can do that much. The bench portion is probably going to be copied from another set of plans and the measurements just lengthened to meet my needs (with added supports if I need to lengthen too much) but since the bench is going to be cut from solid wood I don't have to guess.

I am almost thinking that the 5/8" would be thick enough, it would kinda be like using a 3" wide strip of 3/4" (almost) plywood for the 6ft tall legs. I just don't want to overbuild it as I tend to do. The more I work with wood the more I find that it is much sturdier than I have given it credit for. I guess I could build it to the 5/8" and if it seems a little weak I could fiberglass the structural parts.....the same way strip built kayaks are glassed to add strength.

Input is welcome, and if I can get a chance to draw this idea up then I will post it.

Thanks
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post #4 of 6 Old 10-15-2009, 05:50 AM
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Look at some other swings

Quote:....In order to make sure this swing is structurally sound I want to build the laminate thick enough to hold 3 large adults. I am pretty sure I have decided on using quarter sawn white oak because of stability and rot resistance, plus plyability and low cost. Each laminate will be approx 4" wide by 1/8" thick (I will resaw thinner slices if 1/8" won't bend enough) by what ever length I figure out I need. I will probably end up cutting the width from 4" down to about 3.5" in order to clean up the edges.....unless I get lucky and they all stay straight and even. I was thinking of a final thickness of 5/8" but I am not sure if I should increase it to a full 1". I want to figure this out so I can calculate how thick of a board I need to start with before resawing all the laminates. ....[/quote]

The legs on my outdoor swing, an "A" frame design, are made from 4 x 4's, and this swing is wide enough for 2 large adults. The loads on a swing are dynamic and change directions. I don't think a 5/8" laminate is enough. I'd be looking at 2" or so. The other issue is how to support the horizontal beam to which the swings are attached. Again my swing uses a 4 x 6 for 2 adults.
If you hang the beam under the laminations any holes for attachments may weaken the structure. If you top load the beam and secure it this may be stronger, keeping in mind the dynamic forces. Is the beam to be a laminate? metal? A sketch would be great, so advice would fit the application here. bill

The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)

Last edited by woodnthings; 10-15-2009 at 05:53 AM.
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post #5 of 6 Old 10-15-2009, 08:45 AM
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Kender, you might be okay with 5/8" if I understand your design. I'd probably still build it to at least a full inch though. You're not really going to add that much weight or size from an aesthetic perspective, but the added strength with the extra lam will be tremendously beneficial. For 3 adults, let's just average 600 pounds (assuming relatively large males) as a static load. Centripetal force is F=(mv^2)/r. For a 600 pound load (let's use weight as mass for this example even though that's not quite right) and a height of 5 feet for the length of the swing arm (pivot point of support to bottom of swing seat), if I did my math right, that makes a load of approximately 4735 pounds generated by the swing for a velocity that would make a full circle with the swing in 5 seconds. Centripetal force isn't the right calculation for this because it assumes a constant velocity, which a swing doesn't have. Calculation of the torque is probably a better formula, but I don't have that handy at the moment. Either way, it gives you an idea of the types of forces that can easily be generated by a mass in motion, though.

I'm assuming your design would put the 5/8" perpindicular to the downward load (IE in the horizontal plane of the swing) and the 4" measurement is parallel to the motion (IE in the verticle plane of the swing). As such, the 5/8" is really more a factor when considering the lateral motion of the swing (side to side as you sit in the swing). With appropriate bracing along the bottom I don't see why that would be a big issue, because laminated wood is stronger than a single slab and (if I'm correct in my assumptions of your design) you're not really dealing with the major forces of the swing with this particular dimension.

All that said, I'd either bump it up a little to the full inch, or glass it as you suggested. Never hurts to over-engineer a little bit.

Last edited by frankp; 10-15-2009 at 08:48 AM.
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post #6 of 6 Old 10-15-2009, 09:58 AM Thread Starter
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Frankp: When I say I over engineer it's to the point of using 2x4 frame and 3/4 ply covering the frame for a simple work bench. I didn't think a 1" thickness would hurt the appearence but since I have the go big or go home tendency I wanted to be sure.

To answer the other ?'s. The top support beam will either be a 4x4 or 4x6 and attached on top of the 'A' frame. I also will use angled braces to connect the side frames to the top beam for lateral support. At the point where the beam touches the frame I plan on making the frame a little wider so I can bolt it together while maintaining a decent amount of undamaged wood.

The more I think about it the more I like the idea of fiberglassing the finnished swing. The added strength plus weather resistance opens me up for an easier build from the glue type to using onlays rather than inlays. I am going to have to draw this up as soon as I get a chance so you guys can chime in easier.
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