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Curved Bridge with Swing

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Forum topic by zakird81 posted 06-17-2018 03:49 PM 518 views 0 times favorited 15 replies Add to Favorites Watch
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zakird81

6 posts in 184 days


06-17-2018 03:49 PM

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 4×6 or 6×6 (a 4×4 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 2×12-10’ in the shape of the curve making it 6” high resulting in a curved 2×6-10’ beam. I would put a single beam on each side of the bridge and glue three of these 2×6s together to create the center beam where the swing load will be. This will essentially give me a 6×6-10’ 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 and screwing multiple thinner sheets together with a curved jig.

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

It is basically this…


15 replies so far

View HerbC's profile

HerbC

1790 posts in 3065 days


#1 posted 06-17-2018 04:23 PM

Gluing the members together in this situation will not be a good long term solution. The pieces will expand and contract in the changing outdoor environment and the glue joints will fail. Mechanical fastners will be much more stable in the long run and if properly designed would perhaps provide the ability to “tighten up” the joints after the structure has weathered the environment for a while…

Good Luck and…

Be Careful!

Herb

-- Herb, Florida - Here's why I close most messages with "Be Careful!" http://lumberjocks.com/HerbC/blog/17090

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zakird81

6 posts in 184 days


#2 posted 06-17-2018 04:52 PM

Herb,

When you say mechanical fasteners do you mean screwing the sheets (in addition to gluing)? If so, that was my original intention. I just forgot to mention it in my original post. However, I’m still unsure if all that is necessary or if option 1 will work just fine (which is definitely the easier option but I want to do whichever will last longer between the two options).

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HerbC

1790 posts in 3065 days


#3 posted 06-17-2018 05:09 PM

I would use through fastners, such as bolts with washers and nuts rather than screws. And I think the glue would be ineffective in this application.

Maybe someone with more experience and engineering background will chime in and help with better info…

Herb

-- Herb, Florida - Here's why I close most messages with "Be Careful!" http://lumberjocks.com/HerbC/blog/17090

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sras

4944 posts in 3335 days


#4 posted 06-17-2018 05:28 PM

If I were doing this, I would create a curved beam with 3 or 4 layers of 2×8’s. Each layer would be made up of 2 or 3 pieces. Stagger the joints & make sure they are tight. Then cut the curve from the resulting stack. A picture might help…

And use lots of nails or screws.

-- Steve - Impatience is Expensive

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BroncoBrian

854 posts in 2164 days


#5 posted 06-17-2018 05:35 PM

Not sure how much the 2×12 would move when you start cutting into it.

I thought about doing that and went with a wobble bridge instead. The best part of the playground.

http://lumberjocks.com/projects/144506

I used 4×4 Redwood with a cable about 3 inches from each end and secured them on each end.

-- A severed foot is the ultimate stocking stuffer.

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zakird81

6 posts in 184 days


#6 posted 06-17-2018 08:23 PM

Thanks all for your comments. Here are two drawings of the plan I think I’m going with.

Black = a 2×12-10’
Yellow = Cutout from the 2×12
Blue = 4×6-6’
Light Grey = 2×4s (a couple bolted together)
Green = Decking boards

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zakird81

6 posts in 184 days


#7 posted 06-18-2018 02:21 AM

Received the following suggestion from someone. Thoughts…?

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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BroncoBrian

854 posts in 2164 days


#8 posted 06-18-2018 02:56 AM

There is no overkill!

Tire swings are heavy and 3 kids pile on them at one time. The twisting and swinging will put a lot of pressure on that bridge. The shape that makes it string vertically will be a weakness in every other direction.

-- A severed foot is the ultimate stocking stuffer.

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Woodknack

12431 posts in 2586 days


#9 posted 06-18-2018 03:26 AM

To answer point #1, height matters more than width. That’s why as load increases, we go from 2×8 to 2×10 to 2×12. Obviously width will add some strength too but I don’t know the break even point. A swing bounces up and down creating a dynamic load which is higher than just the weight of the bridge, swing, and kids. Could be each side of the bridge in the photo is acting like a truss, making it stronger than it appears.

-- Rick M, http://thewoodknack.blogspot.com/

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sras

4944 posts in 3335 days


#10 posted 06-18-2018 04:16 PM

A few thoughts for you:

1. You could skip cutting out the underside of the center beam in your original design. A straight edge in the middle shouldn’t distract from the curve of the bridge and leaving the material will make the beam stiffer.

2. Adding blocking between the center beam and the outer ones will help share the load.

3. When you add the boards to form the sides you could extend the curvature of the lower arc rather than following the straight sections at each end (hope that makes sense).

-- Steve - Impatience is Expensive

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zakird81

6 posts in 184 days


#11 posted 06-19-2018 06:50 PM

Here’s the latest design based upon various suggestions. Also including a photo of the play structure itself so you get an idea of what I’m trying to do here.

View bdresch's profile

bdresch

148 posts in 1814 days


#12 posted 06-19-2018 07:32 PM

6” seems thin for this. At our local park there is a 6×12 timber supporting the tire swing and it has bowed quite a bit over the years. It’s a pretty good sized tire swing that gets abused by big kids but you get the idea.

Also the way you are cutting the 2×12 is going to give you less strength than a straight 2×6 because you are disturbing the grain.

If you are set on 6” for the bridge deck I would do some sort of truss or suspension system to support the load.

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JADobson

1290 posts in 2317 days


#13 posted 06-19-2018 07:41 PM

Forgive my very crude drawing but you could avoid cutting across the grain by making the frames square and adding the curve over top of it.

A piece of plywood with a curve cut out could get tacked to the sides to complete the illusion.

-- No craft is very far from the line beyond which is magic. -- Lord Dunsany — Instagram @grailwoodworks

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zakird81

6 posts in 184 days


#14 posted 06-19-2018 10:34 PM

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sras

4944 posts in 3335 days


#15 posted 06-20-2018 02:18 AM

That’s looking pretty good!

-- Steve - Impatience is Expensive

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