Starting my bridge!!

[ning]

Railing idea

View attachment 789877View attachment 789876

FTFY

 

[Doorman63]

Put a couple angled pieces in that handral and you have essentially built a 40" tall truss....which will be ALOT stronger than just the 2x6 bottom tube. Maybe to the point of needing nothing more

I kinda figured that’s the route to go.

Any particular orientation for the “kickers”?
I’ll most likely notched the ends of the kickers to lap onto the vertical tube. Lots of surface to weld set up like this.

 

[Streetcar]

The railing doesn't have to be that strong. The end supports need to be able to carry whatever weight you're transferirng to the tension rods (easy, that's not much) and the railing needs to be sturdy enough in compression not to buckle. Neither of these are dealing with a multiplied force like the tension rods.

A couple of wood 2x6's is probably sufficient in compression if you can stiffen them, that's why I suggested that if you had more of the 2x6 tube that filling it with concrete would stiffen them. If the end supports are well anchored that would also help, but really this is the lightweight part of the design, just put something between the uprights that'll stay between them.

if the king truss is used above the deck you are making the railing a main structural element. Below the deck the deck is the compression member. You are also inducing a lot of stress in the end post connection. In former career I was lead bridge inspector

 

[LD1]

I kinda figured that’s the route to go.

Any particular orientation for the “kickers”?
I’ll most likely notched the ends of the kickers to lap onto the vertical tube. Lots of surface to weld set up like this.

Hard to say.

Upside down V.....going from bottom of middle post out to the top of the two end posts puts that angle brace in tension. Angles the other way going from bottom outside to top of middle post puts them in compression.

Gut feeling makes me think either will be fine....and the ultimate yield strength of the structure will be limited to localized connections of the thin wall tubing.

 

[Doorman63]

Hard to say.

Upside down V.....going from bottom of middle post out to the top of the two end posts puts that angle brace in tension. Angles the other way going from bottom outside to top of middle post puts them in compression.

Gut feeling makes me think either will be fine....and the ultimate yield strength of the structure will be limited to localized connections of the thin wall tubing.

Crappy mark up I know.
I’d notch the ends of the angle and have then lay on the face of the post. That would give me plenty of surface to weld too

 

[ning]

if the king truss is used above the deck you are making the railing a main structural element. Below the deck the deck is the compression member. You are also inducing a lot of stress in the end post connection. In former career I was lead bridge inspector

The king truss doesn't even have to exist above the deck when using the tension rods. It serves *zero* purpose in the tension system. The only reason to have it in this bridge is as a brace for the top rail itself.

If the end posts were sturdy enough and you could otherwise guarantee that they won't fall over, you don't even need the top rail either; all the tension system needs is for the tension rods to be pulling up and out.

It kinda reminds me of another bridge...

you can see the end posts, which are being kept vertical by tension cables pulling from the ends to the land. Where's the king truss and rail?

In fact, the bridge we're discussing here - with tall end posts and a tension rod pulling up on the middle - is essentially the most basic of suspension bridges. The bridge itself is strong enough not to need suspension every 4" instead it's only suspended from a single point, the middle.

Since the end posts aren't braced otherwise, we need the railing as a structural component, to keep the end posts from collapsing inwards from the horizontal component of the tension. If you set them up some other way (like use a telephone pole sunk way deep, or set up a really long H-brace away from the bridge - which is basically a tension rod arrangement too) so that they won't be pulled towards each other, you can eliminate the top rail too, but it's nice when bridges have that railing to keep an eye on as you drive over the bridge too. Then, the "king truss" is just something that keeps a 20' rail from bowing downwards.

 

[Streetcar]

The king truss doesn't even have to exist above the deck when using the tension rods. It serves *zero* purpose in the tension system. The only reason to have it in this bridge is as a brace for the top rail itself.

If the end posts were sturdy enough and you could otherwise guarantee that they won't fall over, you don't even need the top rail either; all the tension system needs is for the tension rods to be pulling up and out.

It kinda reminds me of another bridge...
View attachment 790069
you can see the end posts, which are being kept vertical by tension cables pulling from the ends to the land. Where's the king truss and rail?

In fact, the bridge we're discussing here - with tall end posts and a tension rod pulling up on the middle - is essentially the most basic of suspension bridges. The bridge itself is strong enough not to need suspension every 4" instead it's only suspended from a single point, the middle.

Since the end posts aren't braced otherwise, we need the railing as a structural component, to keep the end posts from collapsing inwards from the horizontal component of the tension. If you set them up some other way (like use a telephone pole sunk way deep, or set up a really long H-brace away from the bridge - which is basically a tension rod arrangement too) so that they won't be pulled towards each other, you can eliminate the top rail too, but it's nice when bridges have that railing to keep an eye on as you drive over the bridge too. Then, the "king truss" is just something that keeps a 20' rail from bowing downwards.

Yes the bridge can be totally redesigned to get rid of king post, but you are making the structure a lot more complicated and harder to build safely .
in a suspension bridge the towers are in compression and have relatively equal forces acting on each side
the bridge under discussion is truss design not suspension design

 

[ning]

Yes the bridge can be totally redesigned to get rid of king post, but you are making the structure a lot more complicated and harder to build safely .
in a suspension bridge the towers are in compression and have relatively equal forces acting on each side
the bridge under discussion is truss design not suspension design

It's not a re-design; he brought up using a cable for tension half the thread ago; it was refined and calculated out shortly thereafter. Turning the bridge into a tressle is a redesign he doesn't need.

We're not talking about a truss design. The end posts for tension rods are similarly in compression; since they don't have equal forces on each side, that's why there's the rail between them.

Now, if you go making that rail do more work than just compression by adding diagonal trusses between it and the bridge, *that* is some serious complication and I'm not going to touch that with my math. I'd stick with the end posts, a simple beam between them that doubles as a hand-rail or just visual rail, and a couple tension rods. Very simple math there and a tenth the cutting and welding.

 

[Doorman63]

After school on the playground by the swings?


Wondering if I should start a thread for my elevated tiny house when that gets built. (Of course I will)


I’m reading and trying to digest all the stuff you guys post.

My thoughts…..
A “truss” design seems to make the foundation less important.

I’ve got to build some kind of railing so it seems the truss is a 2 for 1 kinda deal.

I’m not afraid to cut/weld/fabricate stuff. Simple is always better though.

Can I eliminate the top tube, replace with angle iron and a wood cap?
If what I’m reading you guys are posting it seems that my be an option.

 

[Streetcar]

After school on the playground by the swings?


Wondering if I should start a thread for my elevated tiny house when that gets built. (Of course I will)


I’m reading and trying to digest all the stuff you guys post.

My thoughts…..
A “truss” design seems to make the foundation less important.

I’ve got to build some kind of railing so it seems the truss is a 2 for 1 kinda deal.

I’m not afraid to cut/weld/fabricate stuff. Simple is always better though.

Can I eliminate the top tube, replace with angle iron and a wood cap?
If what I’m reading you guys are posting it seems that my be an option.

Making the truss into your railing, introduces lateral stability issues that should be accounted for. Can lead to sudden failure of your bridge. These calculations are not read chart and get your member type designs.
king post under bridge is a much simpler design for stress and loading. your deck doubles as bracing for lateral loads in this design.

It's not a re-design; he brought up using a cable for tension half the thread ago; it was refined and calculated out shortly thereafter. Turning the bridge into a tressle is a redesign he doesn't need.

We're not talking about a truss design. The end posts for tension rods are similarly in compression; since they don't have equal forces on each side, that's why there's the rail between them.

Now, if you go making that rail do more work than just compression by adding diagonal trusses between it and the bridge, *that* is some serious complication and I'm not going to touch that with my math. I'd stick with the end posts, a simple beam between them that doubles as a hand-rail or just visual rail, and a couple tension rods. Very simple math there and a tenth the cutting and welding.

ning
your design adds a lot of new forces that must be accounted for to safely cross the bridge. It is not as simple as you think it is