Wooden Bridge Question

[jkent]

Hi! I recently built two wooden bridges to span a creek in the woods. The bridges are 52' and 42'. The 52' has 4" x 6" posts spaced 6' on center lengthwise and 5' on center widthwise. The 42' has 2 7/8" oilfield tubing for posts, spaced 6' on center lengthwise and 52" on center widthwise. Both bridges have beams 2" x 8" on both sides of the posts. There are six runs of 2" x 6" joists spread on top of the beams. Both bridges have 1" x 6" x 6' decking. All posts were set 24" deep in concrete. Can anyone help with calculating the load bearing capacity of each bridge? Would like to use small IH row tractor, zero turn and utv. Any help would be appreciated.
Thanks!

 

[Tinhack]

Interesting that you would be asking farmers about an engineering problem. :confused3:

Are you going to grow potatoes on these bridges? :confused2:

 

[KennyG]

If we assume the posts are well braced and primarily in vertical compression, the limiting members will be the two 2x8 beams spanning 6 feet. The joists should distribute the load more or less evenly, so we can just say we have two 2x8s, 6 feet long evenly loaded. The strength will vary by species of wood and condition, but I would estimate a capacity of about 3500 lbs. This would probably be a conservative safe load if everything is in good condition. I'm using the beam tables from an old copy of Marks Handbook for Mechanical Engineers.

By the way, I would guess there are more degreed engineers on this site than full time farmers. :cool2:

 

[Tinhack]

And how many of them are willing to accept the liability for free? :laughing:

Well, at least one.

 

[fried1765]

If we assume the posts are well braced and primarily in vertical compression, the limiting members will be the two 2x8 beams spanning 6 feet. The joists should distribute the load more or less evenly, so we can just say we have two 2x8s, 6 feet long evenly loaded. The strength will vary by species of wood and condition, but I would estimate a capacity of about 3500 lbs. This would probably be a conservative safe load if everything is in good condition. I'm using the beam tables from an old copy of Marks Handbook for Mechanical Engineers.

By the way, I would guess there are more degreed engineers on this site than full time farmers. :cool2:

Yup.....probably true!
Farmers USE their tractors to make a living, and likely spend very little time on TBN.
Engineers simply PLAY with their tractors...for FUN!

 

[fried1765]

And how many of them are willing to accept the liability for free? :laughing:

Well, at least one.

The safe answer to that bridge question, is that it MIGHT be an acceptable bicycle bridge!

 

[EddieWalker]

... wooden bridges...... 52' and 42'. ...Both bridges have beams 2" x 8" on both sides of the posts.

I eliminated what wasn't important in your question, and left what matters. How far is the span of your 2x8 beams? From memory, you can go 16 feet with a 2x8 for a roof, but not a floor.

from what you have given, I would guess that your max load will be in the one hundred to two hundred pound range. I would be nervous walking across it. I doubt that I would carry a hammer or anything heavy if I walked across it. I would not let livestock walk across it. Maybe my dog, but only one at a time.

 

[jkent]

Thanks for the input. All wood is new pressure treated.

 

[deezler]

Hi! I recently built two wooden bridges to span a creek in the woods. The bridges are 52' and 42'. The 52' has 4" x 6" posts spaced 6' on center lengthwise and 5' on center widthwise. The 42' has 2 7/8" oilfield tubing for posts, spaced 6' on center lengthwise and 52" on center widthwise. Both bridges have beams 2" x 8" on both sides of the posts. There are six runs of 2" x 6" joists spread on top of the beams. Both bridges have 1" x 6" x 6' decking. All posts were set 24" deep in concrete. Can anyone help with calculating the load bearing capacity of each bridge? Would like to use small IH row tractor, zero turn and utv. Any help would be appreciated.
Thanks!

Holy crap thats a lot of wood. 52' feet long? So you're saying you have 27 4x6 posts in the ground supporting this bridge? How long/tall are the 4x6 posts? If the bridge is only high enough that you could easily support it with 4x6 posts every 6 feet along it...., why not just drive down into the depression and back up? Does it cross water?

I'm struggling to understand how you could invest thousands of dollars and dozens of hours into a colossal wooden bridge (sounds more like an elevated causeway?) without knowing if it would support your vehicles that you want to use on it.

PICS! Post lots of pics. Then we can advise you.

 

[Tinhack]

Just a question--Has anyone here seen these bridges?

They could be held together with 80mph Tape and Bailing Twine. This is Tractor By Net; Remember? :laughing:

 

[fried1765]

I eliminated what wasn't important in your question, and left what matters. How far is the span of your 2x8 beams? From memory, you can go 16 feet with a 2x8 for a roof, but not a floor.

from what you have given, I would guess that your max load will be in the one hundred to two hundred pound range. I would be nervous walking across it. I doubt that I would carry a hammer or anything heavy if I walked across it. I would not let livestock walk across it. Maybe my dog, but only one at a time.

Max span for SPF 2x8 floor joists @16" OC, is 12'

 

[EddieWalker]

Max span for SPF 2x8 floor joists @16" OC, is 12'

So that would mean that the bridge would have to have support for the bridge every 12 feet for the 2x8's to rest on. Then there would have to be 2x8 joists every 16 inches apart for the width of the bridge. 8 foot wide bridge would have to have 7 2x8's for every 12 feet of length. 8 feet isn't wide enough to drive across in my opinion, but it's easy math for this situation.

You didn't mention what the rating is for that span, but if it's for a house, it should be strong enough to walk across and maybe handle a riding mower. I wouldn't trust a tractor or even a side by side ATV on it. I have a bridge that I'm going to make for my overflow dam at my pond and for 12 feet of span, I am going to use treated 2x10 every 12 inches so I can drive my zero turn and Kawasaki Mule over it. I will not drive my 35 hp tractor over it.

This also relies on the posts in the ground being strong enough to support the load of the bridge and what's on it, every 12 feet, for the length of the bridge.

 

[deezler]

^ eddie, he said he has a full grid of 4x6 posts - every 6' along the bridge length, and every 5' across the width. So I presume this means he has a 10' wide bridge with rows of 3 posts every 6 feet. The 2x8s probably go sideways to connect the posts, and then he has six 2x6s going 6' between every row of posts.

I'm most concerned with the 1x6 decking. I would never drive a heavy vehicle on 1x6s spanning over a foot between joists.

 

[fried1765]

^ eddie, he said he has a full grid of 4x6 posts - every 6' along the bridge length, and every 5' across the width. So I presume this means he has a 10' wide bridge with rows of 3 posts every 6 feet. The 2x8s probably go sideways to connect the posts, and then he has six 2x6s going 6' between every row of posts.

I'm most concerned with the 1x6 decking. I would never drive a heavy vehicle on 1x6s spanning over a foot between joists.

If the 1x6 decking should fail, it would only be ugly, and messy.
If the supporting structure failed it would be DANGEROUS!

 

[EddieWalker]

^ eddie, he said he has a full grid of 4x6 posts - every 6' along the bridge length, and every 5' across the width. So I presume this means he has a 10' wide bridge with rows of 3 posts every 6 feet. The 2x8s probably go sideways to connect the posts, and then he has six 2x6s going 6' between every row of posts.

I'm most concerned with the 1x6 decking. I would never drive a heavy vehicle on 1x6s spanning over a foot between joists.

Thank you, that means I got it all wrong.

 

[fried1765]

Thank you, that means I got it all wrong.

Nope!
You sure as He11 did not!

 

[deezler]

Thank you, that means I got it all wrong.

Nah, it's a pretty confusing description, I'm not sure I got it right either.

What I am wondering is why the bridge needs to be 52' long if you have solid ground to place posts in every 6'.... I guess it's more like an elevated causeway, or trestle bridge.

 

[BoylermanCT]

Do you have pics of the bridges? Maybe a drawing showing the posts and any support beams connecting the posts?

 

[airbiscuit]

I don't have a clue about load bearing capacity, but 1" decking is a very limiting factor. 52' seems quite long for a creek crossing. Could you please post pics? I have seen similar posts recommend using a scrapped semi trailer for the bridge underpinnings. Was that something you considered?

 

[KennyG]

It would be nice if the member who started this would come back with some clarification, that is, if we haven't scared him to death already. If I understand this correctly, the structure has a bunch of 4x6 posts, connected by 2x8s, with 2x6 joists supported by the 2x8s. Then it has 1 x 6 decking on the 2x6s. If the 1x6s have a maximum span of 1 foot across joists, a rough calculation indicates they should take about 1000 lb load. With the 2x8s good for about 3500 lbs, as I calculated above, the 1x6s should handle wheel loads.

I would have used 2x6s but this is probably OK for the zero turn. Of course, if the connections are not well made for any of this, it all goes out the window (or into the creek).