Sizing bridge beams

[Slackdaddy]

I have a CK20HST
I have a smallcreek on my property that I would like to build a bridge over for the CK20.
Clear span of the beams would be ~14' ,, that would get me back 2' or so from the banks.
The terrain is a typ Mid Atlantic "Creek Bottom" ,, pool table flat semi wooded creek bottom (100 yards wide) with 5-6' wide creek.

I plan to go back about 2' from the bank and use post hole diggers and go down about 30" with 2 holes on each side of the creek,
The 2 holes will be the width of the bridge. Put some rebar down the holes and fill them with concrete.
Then build a box form about 12" high and 5' wide , laid on the ground around the post holes,, then fill the form with concrete (reinforced with rebar).
These would be the concrete "blocks" anchored into the ground that the bridge beams would rest on.


I plan to make 2 beams for the bridge, each beam centered under the tires.
Each beam would be made out of P.T. 5 or 6 2x8x14s laminated together to make a 7-1/4" deep by 7-1/2" wide beam
Here is the part I can not find the math on,,, How much point load would said beam carry spanning 14' ?
Truthfully, it would not really be a true point load, it would be spread over the front and rear tires.

Any words of wisdom ?

Below is a picture of a foot bridge I put in about 10 years ago

 

[Hoss710]

Not to derail your line of thinking ( as it sounds like you have a great idea). how about buying a old equipment trailer cheap and use that as the bridge part but still do your columns on either side of the bridge? or even a house trailer frame ??

 

[ROUSTABOUT]

I agree with previous post. An old flat bed trailer from the scrap yard would be better. 40' ones are cheap. Spanning 14' is a lot to ask of wood. Usually here 14' is done with an 18" x 18" solid wood beam, with a banister on each end. We put one on a larger creek here and used 4 big light poles with thick planks to drive on. But I wouldn't drive a tractor across it.

 

[dodge man]

My degree in in civil engineering and I had structural design classes, though I spent most of my career surveying. I know enough that it’s not a simple solution. For a structural engineer it’s easy but they will probably design a $15,000 bridge for you.

Your tractor isn’t real heavy, maybe with implements on it 3000 pounds, but you still want something that won’t dump you in the creek. I’ve seen power poles used with good luck, you could even use four of them.

 

[bandc07]

I also have a creek that's about 20' clear span in my back yard. I used 4 utility poles in pairs and spread the out under the tires. has worked great driven my CK4010 with loader and brush hog on it across many times no issue. Not sure if only 2 poles would work but the 4 I used are very sturdy. I did mound dirt around the ends just didn't have a picture of it totally finished.

 

[LD1]

Wood is tricky to design with because there are too many variables and it isnt constant like steel is. Wood species, Wood grade, whether its in a wet-service location, etc all effect its load capacity. Then add the effect of a moving machine makes it a dynamic load and not just a static load. And most all the span tables are regarding decking and flooring so assuming "joists" and uniform loads in PLF ad maximum spans.....not much for a point load at mid span because thats not typically how lumber is used.

So you have to know the Fb, moment of inertia, and modulus of elasticity of whatever lumber and do some long calculations or plug them into a calculator and know what you are looking at.

Based on all of that.....a typical treated 2x8 spanning 14' should be good for ~300# at midspan.

So a 5-ply should get you somewhere around 1500# per beam. Which IMO is cutting it pretty close to being able to want to drive your tractor over.

As with any type of beam.....strength increases exponentially with depth. For example.....simply going to 2x10's in gonna be ~65% increase in strength. About 500# per 2x10.

But then you have to consider the decking too. How it attaches, and what if you pull onto the bridge and arent directly over your beams. Can your decking support the weight of the machine if a tire is between the beams? OR outside of the beam.

Also consider how high this bridge is gonna be. IF you are spanning a creek that is a foot or two deep....its not really life and death if the bridge fails. But if you are crossing a 15' deep ravine.....whole different story.

Lots of people have done bridges out of trailers. Knock the tongue and axles off and you have a ready made bridge that just needs anchored. And you have piece of mind. Or you could always use steel beams...since steel is alot easier to design for loading and you would know with certainty exactly what it can and cannot carry.

Good luck with the project

 

[2manyrocks]

I found used steel I beams at my local scrap yard for a lot less money than buying laminated beams at the lumberyard when I had to replace the termite eaten support beam in my parent's house. It pays to shop around to see what your options are.

It is also my understanding that most of the strength of wood is in the outer layers of a tree trunk such that a pole made preserving most of the trunk will have more strength than boards cut out of the same trunk. You'd get more strength from using poles than using dimension lumber.

A built up beam isn't going to be as strong as a pole. The strength will depend on how carefully the built up beam is assembled. Will take a lot of nails in a specific pattern. And then water will penetrate between the dimension lumber over time causing rot and a loss of structural strength.

 

[deezler]

I have a similar size creek that I wanted to cross, and also used utility/phone poles like bandc07. Works great for my CK3510, still haven't tried my 6000 lb pickup yet.

My bridge thread

 

[2manyrocks]

I have a similar size creek that I wanted to cross, and also used utility/phone poles like bandc07. Works great for my CK3510, still haven't tried my 6000 lb pickup yet.

My bridge thread

Creek crossing bridges used to be built a lot like that around here using 4 oak (preferably white oak) tree trunks and oak treads obtained from the local sawmill instead of doubled up 2x6s. Bridge treads were something like 3x8 or thicker as I recall that were spiked into the tree trunks.

For those with better budgets, steel I beams and concrete decking have increased durability and one never needs to replace rotted decking ever again.

 

[LouNY]

A used truck bed or over the road trailer would be my first choice.
My second choice would be a used conex box with both ends open.

edit to add,
a used dump truck bed would even give guide rails for the sides.