Curved Bridge with a Swing

[zakird81]

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...

[daddo]

This may not work for you, but it keeps the woods strength and you need only cut the 2x12 in an arch, which also adds to the strength of the 4x6 as it is bolted to the 2x12.

I never ended up using this plan I made- decided to cut costs and go flat. Maybe someone has a better idea.

[zakird81]

Daddo - I was originally going to do that (straight bottom and curved top) but it doesn't give the effect I am looking for. So, now I'm looking to curve both the top and bottom.

[Cecil]

Daddo - I was originally going to do that (straight bottom and curved top) but it doesn't give the effect I am looking for. So, now I'm looking to curve both the top and bottom.

you should be able to hide a steel rod behind the curve.  Thread the ends and cap it with fender washers and nuts.

[mike4244]

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!!! :-)



Since you want a curve top and bottom I would laminate two beams. Make a form ,several short pieces of 
2x4 nailed or screwed to a plank with a 1x4 board glue and nailed to the blocks to give you the 6" curve. Set the form on trestles or a bench. Try to bend 3/4" stock to see if it bends easily. If not try a thinner piece. I would have clamps , panel or subfloor adhesive ready. 
Start by cutting your pieces over size, at least 10" in length and 1/2" in width. 
Bend the first piece and clamp then screw from the back. Start a screw on one end of the next board ,dot subfloor adhesive on the board. Screw one end ,bend and clamp .Screw as you go along. Make sure the screws on this piece do not come thru into the first one. 

Repeat until you get the thickness of the beam you need. Screws can be longer now as long as they don't go thru the first piece .I would think 10" would be more than enough. Check with an engineer, maybe even search online for the size beam needed.

When done unscrew from the form, some spring back will occur. Cut each oversize end to dimension. The width will not be completely flush. Hand plane or belt sand flush on each side.

This is a lot of work since it needs to be done twice. This is the best I could come up with and still have a curve top and bottom.

mike

[ronlaughlin]

Why not cut the curve in a piece of 3/4" plywood,  3 maybe  4 pieces  for each beam?   Laminate the pieces, but stagger  side by side pieces two feet to achieve the ten foot span.

[daddo]

In that case, the upper rails and supports will add to the strength as well via the pickets. Take that into consideration.

[crokett™]

I think that a 2x6 ripped into 1/4" wide or so strips and then glued back together in the shape of a curve as a laminated beam would be stronger than cutting a curve into a wider piece of wood.  It would be a lot more work though.

[JosephP]

I think that a 2x6 ripped into 1/4" wide or so strips and then glued back together in the shape of a curve as a laminated beam would be stronger than cutting a curve into a wider piece of wood.  It would be a lot more work though.

Home-made LVL.  Sounds like a good plan - using an outdoor-rated glue I'd think that would be very strong and look really good too.  The challenge would be, as always, finding straight enough stock to start with.

[daddo]

Home-made LVL.  Sounds like a good plan - using an outdoor-rated glue I'd think that would be very strong and look really good too.  The challenge would be, as always, finding straight enough stock to start with.

 If I were to do it that way, I would use stainless bolts throughout the laminated width spaced at least every 12" for safety.  Constant and long term stress and moisture then drying out for long periods can play havoc on wood and even waterproof glues adhesive to the wood.