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.

 

[deezler]

Agree Lou that those are robust bridge options, but how do you carry and move them into place across an active creek? A crane or telehandler? Lotta logistics there, vs constructing something piece by piece.

 

[LouNY]

A trailer could still be on axles, dump bed would actually slide fairly well, maybe a couple of "ski's".

 

[2manyrocks]

Agree Lou that those are robust bridge options, but how do you carry and move them into place across an active creek? A crane or telehandler? Lotta logistics there, vs constructing something piece by piece.

One option is to temporarily place a couple of logs over the creekbed and then drag the trailer frame over the logs. Then remove the logs.

 

[Rockbadchild]

I went to my local trucking company and bought a retired flat bed for $700 even if you pay double that still going to be cheaper then beams and decking.

see below for my thread,

bridge construction on a budget

good morning TBN members, Id like to share my bridge construction of last summer perhaps it will give ideas to folks with similar project in the future and getting constructive feed back … there is a small creek splitting my property preventing me to access the back end which constitute of...

www.tractorbynet.com

 

[Rockbadchild]

Structural wasn't my strongest class so I am not even going to attempt to calculate it... What is P.T. 5 2x8 ? like tubing? and laminated?? just against each other? regardless the case is I would go with two I beam instead and something like 4'' wide x 6'' tall and that would be as overkill as it can be.

 

[LD1]

Structural wasn't my strongest class so I am not even going to attempt to calculate it... What is P.T. 5 2x8 ? like tubing? and laminated?? just against each other? regardless the case is I would go with two I beam instead and something like 4'' wide x 6'' tall and that would be as overkill as it can be.

Yup. Even a little W6x12 I-beam (matches your 4"w x 6"h specs) would be stronger than the proposed wood.

Just ONE of those 168lb beams could support the WHOLE 3000lb tractor right at midspan.

 

[jonsstihl]

I built a similar bridge 10 foot span, out of two 8x8 hemlock beams and 2x8 hemlock decking about 10 years ago. at the time I had my b7800 and it worked OK but over the years one beam failed due to rot.

I just rebuilt it with steel beams doubled up so i used two steel beams per side so four beams total with the same size 2x8 decking. they are six inch steel beams I think W15 (not sure) I doubled them up to match tire tread widths of all my stuff ranging from ATV to an old manure spreader used as a wood trailer that is wider than the tractor.

I also more than doubled my tractor weight in those ten years. b7800 was approx 3000 with a loader and my L4740 is atleast close to 8000lbs with the loader and filled tires. I get what feels like zero deflection when I cross the steel frame bridge with my L4740.

I hate doing stuff twice. my time is worth something, I vote for steel especially for your longer span.

 

[Rockbadchild]

Ho I see so P.T. 5 or 6 2x8x14 = 5 or 6 pressure treated 2x8 x 14 ... I thought it was P.T. 5 was steel grade.. ok so keep in mind you get most of the strength from the height not the with but the way to make beams is to have plywood (half inch or 3/8) in between each 2x8's and nails (6inch apart in a zipper pattern) and glued together... I am sure you would be fine with what you have as long as its glued and proper nails in with the plywood in between.

 

[LD1]

Ho I see so P.T. 5 or 6 2x8x14 = 5 or 6 pressure treated 2x8 x 14 ... I thought it was P.T. 5 was steel grade.. ok so keep in mind you get most of the strength from the height not the with but the way to make beams is to have plywood (half inch or 3/8) in between each 2x8's and nails (6inch apart in a zipper pattern) and glued together... I am sure you would be fine with what you have as long as its glued and proper nails in with the plywood in between.

The plywood adds nothing more than mass to the beam. Its no stronger or no weaker that equivalent sized dimensional lumber.

In otherwords.....if he builds a beam with 4 2x8's and 3 pcs of 1/2" plywood in between.....making the total beam 7.5x7.5......it would have the same strength as if he just skipped the plywood all together and used 5 2x8's.

And plywood.....even marine grade.....is the last thing I'd want for a wet-location beam.

The reason why plywood became so popular with built-up beams is because nominal sizes. .....

A 2x4 wall is 3.5" thick........a doubly ply header is only 3. So a slice of plywood makes the beam 3.5" wide.
A 2x6 wall is 5.5" thick......a triple ply header is 4.5"......so two slices of plywood make it 5.5.

I am guessing that somewhere along the lines.....someone saw it and mis-understood the purpose of the plywood....and now it seems to be preached as gospel about how much stronger it makes things. But it doesn't add any more than an equivalent size dimensional lumber.

 

[Rockbadchild]

The plywood adds nothing more than mass to the beam. Its no stronger or no weaker that equivalent sized dimensional lumber.

In otherwords.....if he builds a beam with 4 2x8's and 3 pcs of 1/2" plywood in between.....making the total beam 7.5x7.5......it would have the same strength as if he just skipped the plywood all together and used 5 2x8's.

And plywood.....even marine grade.....is the last thing I'd want for a wet-location beam.

The reason why plywood became so popular with built-up beams is because nominal sizes. .....

A 2x4 wall is 3.5" thick........a doubly ply header is only 3. So a slice of plywood makes the beam 3.5" wide.
A 2x6 wall is 5.5" thick......a triple ply header is 4.5"......so two slices of plywood make it 5.5.

I am guessing that somewhere along the lines.....someone saw it and mis-understood the purpose of the plywood....and now it seems to be preached as gospel about how much stronger it makes things. But it doesn't add any more than an equivalent size dimensional lumber.

I disagree, although I agree with the wet location beam, that's a very good point... the wood grains of any planks are longitudinal, the plywood keep the grains bonded uniformly together, so it doesn't separate under stress.