Starting my bridge!!

[Doorman63]

Thank you Doorman for posting. I'll have to look for your barn thread too. I never thought to look at this forum for projects. I'll be looking now. I'd like to see more pix of how the tubes are laid out.
I built a similar size bridge myself. It was one of my most fun projects ever. Hope it is for you too! A couple pix of the finished bridge. I wasn't intending to say so much, but only to encourage others to tackle what might at first seem hard. I built mine of treated wood: mostly 2x6x12's with a few 2x10 x12's. While I wouldn't hesitate to drive a car over it, if I could get it there, my goal was the Ford 8N I'm restoring. I had some free stuff. I salvaged about a 100 bolts which I used to bolt some 2x6's together. I did not bolt all of them, just those used to support the 8N width and my other smaller tractor width. It is sort of like a house floor under the 2x6 planks. I also had two I beams from my garage so those became my support ends on either side of the creek. I sunk 4x4 posts on the inside to hold in the beams which I just laid on a stone foundation. The main span was about 10-11 feet. But I extended both ends to take away the slope. I put one bolt in each of those, not two so that if it needed to rise for the water it would pivot. Only once that I know of did the water go that high. I watched the creek for a few years before this figuring out how high it would go. I used those 4x4 posts too to affix a roof truss to the center of the bridge for added support. I also looked at every bridge I came across while hiking various parks. After all this, I went on-line curious if there was any strength formulas. I guess that should have been my first step. But I was happy that what I did exceeded the expected weight. To relieve my erosion concern, (the bridge is in the middle of a horseshoe) I lined the creek edges with numerous 250 pound blocks that the local recycle shop had for free. But water is powerful. After several washed away, I then pounded rebar stakes through a few fortunate holes in the blocks and then bent over the ends.

Very nice sir!!!

If you poked thru some earlier posts I have pics of my other bridges.

2 are wood and one is a c-channel frame approx 12’ long.

 

[h2opdler]

In my barn thread I mentioned a bridge build and people wanted me to post when I was building it.

Todays the day!!!

Approx 20’ span
Approx 7-8’ wide
2x6 tubes for the stringers
A center 2x6 tube under the stringers
3x3 tubes for cross members

6x6 treated timbers for the “foundations” on each end.

End goal is to drive a vehicle across!

View attachment 786740View attachment 786742View attachment 786743View attachment 786745

Can't wait to see finished project...

 

[dstig1]

Dstig1 had very similar thoughts to mine, he just said it better, mainly in regards to plates at connections and not using rebar. I have seen a lot of rebar handled at construction sites and when guys are carrying long sticks of it it just bows a lot and flops around. In tension that’s fine but in compression it just seems it would buckle easily. Square tubing, round tubing, I beams etc seem to be a better choice. It sounds like you got a lot of scrap a couple of days ago that could be used.

That really isn't so much about the fact that it is rebar, but what the diameter of it is. The modulus of steel really doesn't change much no matter what you do to it. High carbon, lo carbon, alloy, heat treated - doesn't matter - they are all right about 30 Msi (million psi). So you hang out a rod of cold rolled and one of rebar and they are both 1/2" and sticking out the same amount, they will both deflect the same.

But the point is that any thin member in compression is asking for buckling failure, which is why they are not used that way (except in poor designs that are going to fail...). But what the Op was describing was using it a pre-tensioned member to help increase the load carrying ability of the bridge. Per Ning's math above, I also am skeptical if the suggested design of this truss will do much to help.

The problem with steel structures is almost always in the connections which is why I was so concerned with the thin wall tubing tearing or buckling at point loads. Take a look at any steel freeway bridge and where they join the I-beams to each other you will see massive steel plates and about 200,000 bolts or rivets through those plates (yes I exaggerate...). This is why - the connections are always the weak points so they go to extreme efforts to make sure they are overdesigned to a fair degree.

 

[Sodo]

I think adding a tension member on bottom as well as trussing the top of the beam certainly "can" work well. But it can also fail if done improperly

Search google images for "bridge truss design."
For a guy who builds stuff the load paths of the various configurations will make sense.

Trussing the top.... they can buckle if 'bumped' or bent.
But truss offers the benefit of a railing. But maybe you don't want a railing.

Tension rods below is reliable strength, but reduces the under-flow clearance for flood a little.

Round stock for the tension rods will look so much better than visible rebar.
Depends on the price comparison I guess.
Rebar can perhaps be doubled - double rebar will be ~2x as strong.
Regarding welding rebar, 3" or 4" long welds will hold full strength. ---->Good welds. It can be done well.

 

[Streetcar]

That really isn't so much about the fact that it is rebar, but what the diameter of it is. The modulus of steel really doesn't change much no matter what you do to it. High carbon, lo carbon, alloy, heat treated - doesn't matter - they are all right about 30 Msi (million psi). So you hang out a rod of cold rolled and one of rebar and they are both 1/2" and sticking out the same amount, they will both deflect the same.

But the point is that any thin member in compression is asking for buckling failure, which is why they are not used that way (except in poor designs that are going to fail...). But what the Op was describing was using it a pre-tensioned member to help increase the load carrying ability of the bridge. Per Ning's math above, I also am skeptical if the suggested design of this truss will do much to help.

The problem with steel structures is almost always in the connections which is why I was so concerned with the thin wall tubing tearing or buckling at point loads. Take a look at any steel freeway bridge and where they join the I-beams to each other you will see massive steel plates and about 200,000 bolts or rivets through those plates (yes I exaggerate...). This is why - the connections are always the weak points so they go to extreme efforts to make sure they are overdesigned to a fair degree.

Gusset plates are huge because they have a lot of holes in them. They splices are usually placed in areas of zero moment for the dead load, so they are designed primarily for shear

 

[Streetcar]

Search google images for "bridge truss design."
For a guy who builds stuff the load paths of the various configurations will make sense.

Trussing the top.... they can buckle if 'bumped' or bent.
But truss offers the benefit of a railing. But maybe you don't want a railing.

Tension rods below is reliable strength, but reduces the under-flow clearance for flood a little.

Round stock for the tension rods will look so much better than visible rebar.
Depends on the price comparison I guess.
Rebar can perhaps be doubled - double rebar will be ~2x as strong.
Regarding welding rebar, 3" or 4" long welds will hold full strength. ---->Good welds. It can be done well.

Get bigger bar rather than try to double up bars would be easier in most cases

 

[bcp]

If you want side rails and have low clearance below the bridge, you could use a design with the roadway on the lower level of the tension bars.

Also look at photos of inverted king post bridges.

Bruce

 

[ning]

If you want side rails and have low clearance below the bridge, you could use a design with the roadway on the lower level of the tension bars.

Also look at photos of inverted king post bridges.

Bruce

View attachment 789759

The "inverted king post" (what I was descriptively calling "the standoff") only needs to be strong enough to handle the load that's transferred to the tension rods, in compression. In the above bridge pic, there's a substantial beam there because it's probably tying multiple boards together as well as serving as the king post.

Looking at web images, in this example

there's a relatively insubstantial king post, probably because this particular beam is the only load that's being carried.


Found this also, for amusement:

I think that makes a good bridge design. It's nice to have the side rails, and they provide significant strengthening as well without imposing much on the flow of water (& debris) under the bridge.

With a 2x6 under the bridge, and say 6" bridge thickness, an 18" side rail height above the bridge, you have an effective inverted king post height of 30" in the middle of a 240" span. Taking the arctangent of (30/120) gives 14°; the hypotenuse (diagonal) length is about 124. The ratio of the hypotenuse to the height being (30/124), about 4, so for every 1000# you want the tension rods (collective - all of them together) to hold, you need 4000# to be carried by the rods. There being 4 rods, each one has to carry 1000#, if you're relying on all of them.

It's a decent triangle at that height and being supported up higher it seems very workable. If your bridge is happily holding itself right now, and you want to have 7500# travel over it, with this triangle (and no engineering degree) I'd want rods that can hold 7500# in tension each just for overkill (a very non-engineering term). Mild steel as far as I can tell has a 60,000psi tensile strength; 1/2" round is 0.2 sq inch, so 1/2" round should be good for 12,000# tension, so it should be plenty good for this sort of setup (that being the inverted king post like these bridge pics).

Note that in Sherman's patent bridge, the post above the under-bridge beam is really only there to support the rail going across - it doesn't have a significant part to play in the load-transfer-tension setup.

A downside to this arrangement is that the tension rods are attached to the ends of the guard rail, so that rail needs to be strong enough to handle a compressive force without buckling. In the original design where the tension rods are anchored to the ends of the bridge, it's already playing that part well. Maybe if you take one of your 2x6 rectangular, and fill it with concrete, use that as a rail/

 

[dodge man]

I like that king post design. It seems like a perfect way for the OP to strengthen his. It’s been to long since I had structural analysis for me to wrap my mind around how it works exactly but it looks simple.

 

[ovrszd]

I like that king post design. It seems like a perfect way for the OP to strengthen his. It’s been to long since I had structural analysis for me to wrap my mind around how it works exactly but it looks simple.

Works just like a truss, but upside down. The load is straight down on the end pillars.