Curved Bridge with a Swing - Woodworking Talk - Woodworkers Forum
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post #1 of 32 Old 06-17-2018, 11:54 AM Thread Starter
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Curved Bridge with a Swing

I am looking to build a curved bridge on an outdoor playground using Pressure Treated Pine. The bridge will be 10ft long with a 6in rise in the curve. This bridge will have a tire swing hanging from the middle giving it more load forces than just a normal footpath only bridge. The load of the swing will be at a single point in the center so I want to make sure the center beam is strong enough to handle the forces.


If building a normal "straight" swing beam I would just use a 4x6 or 6x6 (a 4x4 isn't strong enough for the load of the swing).

Will option 1 work or is option 2 the better way to go?....

  1. Cut a 2x12-10' in the shape of the curve making it 6" high resulting in a curved 2x6-10' beam. I would put a single beam on each side of the bridge and glue three of these 2x6s together to create the center beam where the swing load will be. This will essentially give me a 6x6-10' center beam but I'm not sure if cutting the wood that way makes it lose too much of its strength properties or if gluing three together makes up for that loss.

  2. Create a built-up laminated beam by gluing/fastening multiple thinner sheets together with a curved jig.

Thanks for any advice you can give to this noob!!! :-)

It is basically this...

Last edited by zakird81; 06-17-2018 at 02:13 PM.
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post #2 of 32 Old 06-17-2018, 12:36 PM
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I think cutting an arch from a 2 x 10 would weaken it too much. If you could live with the underside straight and only arch the top for the pathway, that might work.

The laminated beam should work if you can get good adhesion for the glue. Maybe someone else can weigh in on glueing pressure treated lumber.
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post #3 of 32 Old 06-17-2018, 02:28 PM
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I think you would be better with the 2x12's. Provided that you pick your boards very carefully. I also believe that, regardless of the width of this bridge( usually just as important in the calculation as the span) you have some joists tying the center to the outer rims. Ideally, they would be notched together, as well as fastened.

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post #4 of 32 Old 06-17-2018, 04:17 PM
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Really if you want any real longevity out of the bridge it should have a steel frame and you could cover it to where it would hide the metal. The arch could be constructed from a series of angles and welded together. Then you could make the wooden part that showed with a flowing curve to it. For a walk bridge that short a 4" I-Beam would be more than enough. I believe they run about 2 1/2" wide so it would be small enough you could miter with a miter box. That would just leave welding it if you have a welder or know someone.
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post #5 of 32 Old 06-17-2018, 04:58 PM
where's my table saw?
 
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Make a segmented laminated arch

Imagine this is your plank 12" X 36" X 1 1/12" thick. You have established the arch based on the span of the bridge and the rise from end to end in the center. Make a pattern of that arch and mark it on as many sections of plank it will take to make an arch with 3 slabs or 4.5 " thick.



The arch is drawn on the tips of the ends of the plank and then that portion is cut away using a jig saw, BUT save the cut off portion! Glue the cut off portion back on the bottom of the first part, flipping it over such that it forms the same arch on the top and the bottom. Now you have one segment of the completed arch. It will take many segments to form the complete arch. You will glue them together, staggering the glue joints on the centers of each segment making the arch 3 layers thick.

To establish the arch, put a stake in the ground and tie a masons line to it, while scribing a line on the ground, walking out in 10 ft increments until you like how it looks. Place a cardboard on the ground and then trace the arch on the cardboard. that will become your template mentioned above. A more graceful arch will be flatter than a highly curved arch and will look better. A high arch will also be more difficult to walk up, but the kids probably won't care.

A graceful arch:


A high centered arch;

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; 06-17-2018 at 06:51 PM.
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post #6 of 32 Old 06-17-2018, 05:25 PM Thread Starter
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Thanks all for your comments. Here are two drawings of the plan I think I'm going with

Black = a 2x12-10'
Yellow = Cutout from the 2x12
Blue = 4x6-6'
Light Grey = 2x4s (a couple bolted together)
Green = Decking boards



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post #7 of 32 Old 06-17-2018, 06:59 PM
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You had better use a .......

A 2" X 12" isn't 12" it's about 11 1/4", so your drawing won't be correct!
You will need a 2" X 14" to make that as drawn. You can still use my method of gluing the cut off back onto the bottom of the other piece\, which will gain you about twice the height of the arch.

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)
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post #8 of 32 Old 06-17-2018, 07:24 PM Thread Starter
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Quote:
Originally Posted by woodnthings View Post
A 2" X 12" isn't 12" it's about 11 1/4", so your drawing won't be correct!
You will need a 2" X 14" to make that as drawn. You can still use my method of gluing the cut off back onto the bottom of the other piece\, which will gain you about twice the height of the arch.
Fair statement... I should have clarified in my original posts that I'm aware these are the "common" dimensions and not the "Actual" dimensions. That said, will my plan in the drawings still work (with the understanding that the actual dimension will be closer to having a 5.5" beam rather than a 6" beam.

Last edited by zakird81; 06-17-2018 at 07:30 PM.
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post #9 of 32 Old 06-17-2018, 07:34 PM
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Probably ....

Quote:
Originally Posted by zakird81 View Post
Fair statement... I should have clarified in my original posts that I'm aware these are the "common" dimensions and not the "Actual" dimensions. That said, will my plan in the drawings still work (with the understanding that the actual dimension will be closer to having a 5.5" beam rather than a 6" beam.
A 2" X 6" is 5 1/2" , so you in effect will have 5 - 2" X 6"s supporting the bridge. You can double up the ends if you feel that won't be strong enough. Only an on site test will tell you for certain. Better to overbuild, than to have regrets later.

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)
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post #10 of 32 Old 06-17-2018, 09:25 PM
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The picture really helps. I would double each side to make the arch, so each side would basically be 3” thick X 6” wide of treated Pine. I would install the 6”X6” beam perpendicular to the two sides so the weight of the swing is distributed equally from the two arches or 6” total.
Very nice project.

If you don't have time to do it right the first time, when will you have time to do it over?
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post #11 of 32 Old 06-17-2018, 09:59 PM
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post #12 of 32 Old 06-17-2018, 11:00 PM Thread Starter
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Quote:
Originally Posted by Bob Willing View Post
LOL, as I'm sure you figured out from the photo in my original post, they were my inspiration. Got $24k I can have to just buy it from them??!!
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post #13 of 32 Old 06-17-2018, 11:22 PM Thread Starter
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Quote:
Originally Posted by Toolman50 View Post
The picture really helps. I would double each side to make the arch, so each side would basically be 3” thick X 6” wide of treated Pine. I would install the 6”X6” beam perpendicular to the two sides so the weight of the swing is distributed equally from the two arches or 6” total.
Very nice project.
Thanks! Just to make sure I fully understand what you are saying, do these drawings show what you were explaining? For simplicity, I'm using "common" dimensions (but I realize the actual dimensions will be slightly smaller in the end).

Red represents the support wall the bridge will be on.
Black dot represents the point where the tire swing attaches to the bridge.

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post #14 of 32 Old 06-17-2018, 11:45 PM
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Quote:
Originally Posted by zakird81 View Post
Thanks! Just to make sure I fully understand what you are saying, do these drawings show what you were explaining? For simplicity, I'm using "common" dimensions (but I realize the actual dimensions will be slightly smaller in the end).

Red represents the support wall the bridge will be on.
Black dot represents the point where the tire swing attaches to the bridge.

Yes the first “suggested method” is what I had in mind.
It will be strong.

If you don't have time to do it right the first time, when will you have time to do it over?
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post #15 of 32 Old 06-18-2018, 02:15 AM
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You might be leaving it out for simplicity sake but, there needs to be more bridging between the rims to lock it together along the length, or it's going to sway like the swing will...

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post #16 of 32 Old 06-18-2018, 02:40 AM
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I think it may be cheaper to buy the plans than a truck load of lumber to over build that arch. 😊

Wise men speak because they have something to say; fools because they have to say something -Plato

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post #17 of 32 Old 06-18-2018, 10:16 AM
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Quote:
Originally Posted by zakird81 View Post
LOL, as I'm sure you figured out from the photo in my original post, they were my inspiration. Got $24k I can have to just buy it from them??!!
Just get the thickest treated ply and reinforce any one of the above designs for the bridge. It all depends on what you feel safe with your kids. Use the ply to sandwich the boards. Use your imagination!!
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post #18 of 32 Old 06-19-2018, 01:10 PM
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So far nothing has been said about the difference between the strength of a straight beam and an arched beam, perhaps do a bit of research into that. Looking at the photo I am wondering if the arch is in three sections joined at the two uprights which would allow using narrower material.

Wise men speak because they have something to say; fools because they have to say something -Plato

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post #19 of 32 Old 06-19-2018, 02:44 PM
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I think it will be weak toward the ends of the arches where there is exposed end grain (circled in red) . You could reinforce it with plywood, but at that point, I think an arch laminated from 1 x 3's would be less work and be a lot stronger. With kids playing above and below, I'd over engineer it.
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post #20 of 32 Old 06-19-2018, 03:54 PM Thread Starter
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Quote:
Originally Posted by Quickstep View Post
I think it will be weak toward the ends of the arches where there is exposed end grain (circled in red) . You could reinforce it with plywood, but at that point, I think an arch laminated from 1 x 3's would be less work and be a lot stronger. With kids playing above and below, I'd over engineer it.
Yes, I see your point about the weak points. For simplicity sake I didn't explain the full reason why I need to design the bridge with the ends being straight but I think I need to give you all the full picture. I will basically be building what you see in this picture (two castles connected together with a bridge). However, I want the bridge itself to be arched.

For safety, I know I cannot simply span the bridge from the 2nd floor of one castle to the other since the 2nd floor extends about 16" from the wall of the first floor. I need for the bridge to be resting on the wall of the 1st floor. So, my plan was to build the bridge with the first 16" being straight (hidden under the decking of the 2nd floor) and then arching.

At this point I think I'm still leaning toward ToolMan50's suggestion and will cut the arched shape out of 2x12s and then bolt them together. Anyone think that is a BAD idea? I'm not worried about the load from walking on the bridge. I'm confident this design can handle that. I'm more concerned about the supporting load from the tire swing below.







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