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: