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I am in the planning stages of a bridge that I intend to build over a creek at my hunting property, actually 1 of 2. This bridge will span 20' and be 10' wide. The use for this bridge will be ATV/UTV and tractor use. My tractor will be the heaviest item to use the bridge with a weight including tractor and implements approximately 11k pounds. I have placed a pipe in the creek and it has held up fairly well but the limbs, leaves and other items coming down the creek are clogging up the inlet side and I am looking for a better long term solution as I do not live on the property and do not get to visit as often as I would like.

Here is what I am thinking...I plan to use concrete on both sides where the beams will rest. The foundation for the beams will be poured into the banks on either side measuring approx. 18"x18"x12'. The beams will be anchored to these and will span the 20' across the creek. I plan to use 3 pieces of plate steel in between the beams to anchor them all together to prevent rolling or shifting. I plan to use approx. 35 4"x6"x12' pressure treated timbers laid horizontally spaced approx. 2" across the I beams. I will then place 3 2"x8"x20' PT boards across the bridge at the location of the tractor tires. All fasteners will be hot dipped galvanized.

The only item that I am unsure of is what size I beam do I use. I am considering 4" or 6". While I am no engineer, I would think that a bridge designed and built like this would hold more weight than what I need. It would also open up the creek under the bridge to prevent obstructions from stopping the flow of water. Any input or suggestions would be welcomed. I have attached a drawing of what this bridge would look like. bridge model.jpg
 

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This looks like a very interesting project.
You have certainly thought this through well so far.

Timber bridges are an incredibly fun engineering exercise.

Well I am unable to lend an opinion on your question of beam size, if you have somewhat of an engineering and mathematical mind, I can point you to a great document

http://www.woodcenter.org/docs/em7700_8--entire-publication.pdf

It is very technical but, if you understand it... you can figure out your beam requirements from the formulas and data in there.

If you aren't so inclined the best piece of advice I can give is that overbuilding is often cheaper for the layman, and keep in mind that a moving point load (tires on a tractor) requires substantially more strength than a uniform static load.

Good luck with this project and I hope to see pictures of your progress :thumbup1gif:
 

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Discussion Starter #3
This looks like a very interesting project.
You have certainly thought this through well so far.

Timber bridges are an incredibly fun engineering exercise.

Well I am unable to lend an opinion on your question of beam size, if you have somewhat of an engineering and mathematical mind, I can point you to a great document

http://www.woodcenter.org/docs/em7700_8--entire-publication.pdf

It is very technical but, if you understand it... you can figure out your beam requirements from the formulas and data in there.

If you aren't so inclined the best piece of advice I can give is that overbuilding is often cheaper for the layman, and keep in mind that a moving point load (tires on a tractor) requires substantially more strength than a uniform static load.

Good luck with this project and I hope to see pictures of your progress :thumbup1gif:

Thanks for the link. I would not consider myself a math wiz but I can usually fumble around and get the answer I need. I intend to take several pictures of this project as it goes along and will share with everyone here.:thumbup1gif:
 

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Good project

This looks like a very interesting project.
You have certainly thought this through well so far.

Timber bridges are an incredibly fun engineering exercise.

Well I am unable to lend an opinion on your question of beam size, if you have somewhat of an engineering and mathematical mind, I can point you to a great document

http://www.woodcenter.org/docs/em7700_8--entire-publication.pdf

It is very technical but, if you understand it... you can figure out your beam requirements from the formulas and data in there.

If you aren't so inclined the best piece of advice I can give is that overbuilding is often cheaper for the layman, and keep in mind that a moving point load (tires on a tractor) requires substantially more strength than a uniform static load.

Good luck with this project and I hope to see pictures of your progress :thumbup1gif:
You will also have substantial weight from the decking above the beams. That needs to be included in the loading. I would use "wet" weight for the lumber as the bridge is out in the open and wood absorbs water.

The point about a dynamic weight vs. static weight is important, particularly with implements that could be bouncing on the back of the three point hitch.

Treefarmer
 

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Does the creek flood? Hard to tell with out knowing the creek banks but I think you may want more depth in you cement footings. Even a small flood can wash out a lot of material real quick and may take your footings with it.

We have a wood over steel bridge that our neighbors use. Tonight or tomorrow I'll go and take some measurements of the beams and some photos for you. They did use to move a house trailer across so that is probably over kill for you.

Sent from my iPhone using Tapatalk
 

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Discussion Starter #6
You will also have substantial weight from the decking above the beams. That needs to be included in the loading. I would use "wet" weight for the lumber as the bridge is out in the open and wood absorbs water.

The point about a dynamic weight vs. static weight is important, particularly with implements that could be bouncing on the back of the three point hitch.

Treefarmer
Thanks for the input TF. I will take that into account
 

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Discussion Starter #7
Does the creek flood? Hard to tell with out knowing the creek banks but I think you may want more depth in you cement footings. Even a small flood can wash out a lot of material real quick and may take your footings with it.

We have a wood over steel bridge that our neighbors use. Tonight or tomorrow I'll go and take some measurements of the beams and some photos for you. They did use to move a house trailer across so that is probably over kill for you.

Sent from my iPhone using Tapatalk

Thanks Felix, that would be great. The creek has never flooded that I am aware of. This creek is actually a feeder creek for a larger creek that is as deep as 10' or more in places. I have seen the water reach to within a foot or so of the top but never come out of the banks, at least not within the last 25 years. I will be at my place this Saturday and will be able to get a better idea of where the footings will go and how much water contact the banks will get.:thumbup1gif:
 

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Is frost heaving even a worry in AL? Up the cold state of MI the frost depth in my area is 42"
 

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Your beams need to be 10". Rule of thumb for beams based on deflection, Span in feet/2= beam depth in inches.

The number of beams you need will be determined by the load and how the load is distributed. I'm guessing you will need 4.
 

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Discussion Starter #11
Is frost heaving even a worry in AL? Up the cold state of MI the frost depth in my area is 42"
Frost heaving is not much of a concern here. My thinking was that the weight of the concrete and other materials would put the total weight of the bridge at somewhere close to 4.5 tons and any freezing temps would not cause a problem. I received another suggestion to make the footings larger so I am thinking that I could make them 24" deep with the top being at grade.
 

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Discussion Starter #12
Your beams need to be 10". Rule of thumb for beams based on deflection, Span in feet/2= beam depth in inches.

The number of beams you need will be determined by the load and how the load is distributed. I'm guessing you will need 4.
Thanks Fred. I checked with another local steel supplier today and learned that I can buy 6" beams for the price of 4" beams from the other supplier. I was thinking that going with the smaller 6" beam would make for easier handling and by adding one more beam would give me the strength that I am looking for. Am I wrong to think this way?
 

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I'm no engineer either; but I think you need a couple of poured concrete 'dead men' tied to your concrete piers. The 'dead men' would extend perpendicular from your piers say 10' or more into the banks. Think of the math symbol for PI in plan view. This would help tie the bridge to the banks in case of higher than normal flow and erosion that may get behind your piers.

Never underestimate the power of running and freezing water to move stuff.

I also like the previously mentioned idea of the pre-cast culverts as they would be quick and easy to install assuming you have a way to lift them in place. I've seen them used around here.

If in doubt, consult an engineer.
 

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Thanks Fred. I checked with another local steel supplier today and learned that I can buy 6" beams for the price of 4" beams from the other supplier. I was thinking that going with the smaller 6" beam would make for easier handling and by adding one more beam would give me the strength that I am looking for. Am I wrong to think this way?
A 6" beam will bend under it's own length at 20-24'. You really do need a bigger beam set. Get on craigslist and find a moonlighting structural engineer. Get the piers, abutement, and beams correct and you can do anything you want on top. This is a safety issue of huge magnitude. There are large dynamic loads to consider as you enter onto the bridge and residual "bouncing" once at the midpoint. There are lateral bracing requirements with a deeper cross section and the large span.

Spend a couple hundred bucks, hunting or fishing rights, etc and get the foundation of the bridge done properly. 20', tens of thousands of dollars in equipment, and you and yours health and safety is worth it.
 

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Discussion Starter #16
A 6" beam will bend under it's own length at 20-24'. You really do need a bigger beam set. Get on craigslist and find a moonlighting structural engineer. Get the piers, abutement, and beams correct and you can do anything you want on top. This is a safety issue of huge magnitude. There are large dynamic loads to consider as you enter onto the bridge and residual "bouncing" once at the midpoint. There are lateral bracing requirements with a deeper cross section and the large span.

Spend a couple hundred bucks, hunting or fishing rights, etc and get the foundation of the bridge done properly. 20', tens of thousands of dollars in equipment, and you and yours health and safety is worth it.
Thanks Fred, I am still in the planning stage and I do see what you mean about the center portion of the bridge. I have looked at the possibility of adding 2 vertical supports in the center of the bridge to give additional support in that area. One advantage I have is that this creek will dry up for most of the summer.
 

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Discussion Starter #17
I'm no engineer either; but I think you need a couple of poured concrete 'dead men' tied to your concrete piers. The 'dead men' would extend perpendicular from your piers say 10' or more into the banks. Think of the math symbol for PI in plan view. This would help tie the bridge to the banks in case of higher than normal flow and erosion that may get behind your piers.

Never underestimate the power of running and freezing water to move stuff.

I also like the previously mentioned idea of the pre-cast culverts as they would be quick and easy to install assuming you have a way to lift them in place. I've seen them used around here.

If in doubt, consult an engineer.
I did look at the pre cast culverts as we have a supplier here. The only problem I have is getting them into place. The creek is back in the woods and the only way to get there is by atv/utv or tractor.
 

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So this morning I was able to measure up the wood over steel bridge for you and I also did the cement over steel bridge that I use to cross our creek. Well hope this helps. Let me know if you want any other measurements or larger pictures.

Wood over steel

Beam hight24"
Beam with6 1/2"
Material thickness1/2" (tapering up to maybe 3/4")
Span32'
Number of beams3
Beam Spacing53"

IMG_2320.JPG IMG_2321.JPG IMG_2325.JPG IMG_2324.JPG

Cement over steel

Beam hight18"
Beam with12"
Material thickness3/4"
Span24'
Number of beams2
Beam Spacing44 1/2"
Width of driving area12'
Cement thickness9"

IMG_2330.JPG IMG_2326.JPG IMG_2327.JPG IMG_2329.JPG


IMG_2328.JPG IMG_2331.JPG
 

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One thing you might want to consider is installing a trash fence upstream. I have seen many of them here in Kentucky. Took me a while to figure out what they were doing.

Many use old fence gates upstream about 50 or so feet. Trash collects on the fence, then rolls around the outside supports. Eats the bank up a bit, but not their low-water bridges and abutments. Clean the debris out at your leisure.

Just a thought..
 

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Discussion Starter #20
So this morning I was able to measure up the wood over steel bridge for you and I also did the cement over steel bridge that I use to cross our creek. Well hope this helps. Let me know if you want any other measurements or larger pictures.

Wood over steel

Beam hight24"
Beam with6 1/2"
Material thickness1/2" (tapering up to maybe 3/4")
Span32'
Number of beams3
Beam Spacing53"

View attachment 46571 View attachment 46572 View attachment 46573 View attachment 46574

Cement over steel

Beam hight18"
Beam with12"
Material thickness3/4"
Span24'
Number of beams2
Beam Spacing44 1/2"
Width of driving area12'
Cement thickness9"

View attachment 46580 View attachment 46575 View attachment 46576 View attachment 46578


View attachment 46577 View attachment 46579
Thank you Felix, those are some serious bridges. The wood top steel underneath is what I have pictured in my mind.:thumbup1gif:
 
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