Finally got the bridge started.

[Woody65]

So, I’ve been wanting to build a bridge over a small creek for sometime. We were able to cross it with a sxs, quad, tractor and excavator but it was slightly difficult. Also, thought it would look neat.
I got a hold of (4) W12x26 steel beams 24’ long. I dug out an abutment on either side to accept 3 layers of 80lb bags of concrete 8’ wide. I let the bags sit for a couple of weeks to harden and lock in place.
I set the beams on top of the concrete with a 4’ span between the center beams web to web. And a 7’ span between the outer beams web to web. I plan on making the bridge 9’ wide. If any of you guys are engineers or play one online, feel free to quote me the capabilities of this setup. The span unsupported is 22’. The 4 beams are also tied together with 3x3x1/4 angle iron. Thanks.

 

[dodge man]

My degree is in civil engineering but I was mostly a land surveyor, no license in engineering.

My calculation is as follows. The modulus of elasticity for steal times the aspect ratio of the sectional density of that steel beam times 4(4 beams) divided by the integral of X over Y squared is equal to 3. Yep, that’ll work!

Actually that is just a bunch of random math terms that make no sense, I haven’t done a structural calculation since I graduated in 1984. My seat of the pants is it’s plenty strong for a tractor.

My opinion also is the abutments are the weak point. The bags directly under the beams are the only ones doing any good, the others aren’t helping because there are no rebar. They will help with erosion though. If you took something really heavy across I could see the Sackrete bags crushing some but I don’t think you would get a sudden failure.

 

[Streetcar]

As dodge man said abutment is weak point, what are you using as deck

 

[KennyG]

Those are impressive beams. You should check my work, but I find the elastic modulus for a 12x26 to be 33.4 IN*3. Using W=8fS/L for allowable load and 20,000 psi for stress limit, I get about 20,000 lbs per beam. I would have formed up and poured reinforced foundations to match the beams. Your concrete will give way long before the beams.

 

[newbury]

Now you need one of those road signs stating weight limit 80,000 lbs!

 

[Rockbadchild]

I have a course in structural engineering from my mining and civil technologies degree but that was the only course I almost failed lol any how you don't need to be a genius to see that this bridge is bomb proof, the only thing like other have point out are the abutments, although it will most likely be fine I don't understand the reasoning behind placing cement bags as base, at this point why not mixing it? It is a waste of money and effort in my opignion, gravel would've do the same and been more cost effective...

 

[dodge man]

I actually think the bags of concrete help. Not being mixed up they will harden with rain but I can see them gradually crumbling. Unless this bridge gets a lot of heavy use I think it will be fine for several years.

 

[Woody65]

Thanks for all the thoughts and analysis. The heaviest thing going across will be my excavator that weighs 19k. Other than that, just quads and Utv’s. The concrete is on virgin soil and buried in the bank. The only part visible is the very top where the beams sit. I don’t see them moving at all. I have milled up some 3.5” x 12” lumber for the decking. I have it stickered at the moment. I will be coating them with a 50/50 mix of old motor oil and diesel.

 

[JJT]

Is there a reason you didn't go with a large culvert? I have a much more substantial creek and used a 4' box culvert, built on site. 20 years and it's only been overrun once in the spring, when it plugged up with a fallen tree.

 

[2manyrocks]

Forgot the actual term for it, but it looks like the creek bank sometimes overflows in the area where the beams land up to the area of the dashed red line and the trail finally climbs up past this area where the solid red line is shown.

My concern would be debris collecting. I would want the bridge higher off the creek bed.