Building a bridge: PT or rough cut??

[ovrszd]

:laughing: I bet if you started a thread here asking for ideas, you'd be amazed what folks would come up with!

My BIL built this for me 30 years ago. I use it for a small portable anvil. You can beat the snot out of things on it. Survived my shop fire ten years ago.

 

[Diggin It]

John Deere X748 weighs around 1,200 pounds. Add another couple hundred for a 54 inch mower deck

So, with the operator, fuel, etc., you're talking close to 2,000 pounds, which means the design needs to be for a minimum of 3,000

For that span, with those rails you will most certainly get some deflection.
All things considered, it will probably work for only 1200 lbs. (plus the decking material).

8x8 timbers are heavy, massive for posts, but equal dimension timber for a span is not the best idea.
The deeper the timber cross section, the stronger the span will be.
For a tractor potentially much heavier than the aforementioned 1200 lbs.,
Beams made up of 3-2x12s laminated (glued and spiked), would be a much better choice for bridge carrying beams.

1,200 lbs for the machine + 'a few hundred' for the mowing deck + 200 or so for the operator + fuel and whatever else they might be carrying = 1,800-2,000 lbs + 50% safety margin = 3,000 lb target.


Laminating beams might work if they're done right.

 

[Diggin It]

Anyway... here, about two feet on each side will be on large flat rocks so that leaves an actual span of sixteen feet or so. I'm more than willing to listen to anyone might have information as what kind of risk this might involve for the intended purpose.

How deep is the creek (and do you have a paddle ... err, nevermind that part)? More to the point, how far will the deck be above the creek bed? And does the creek rise, flood, go to swift water that could wash things out?

 

[newbury]

I've a lot in Vermont that has a similar sized stream and had a 2 rail bridge across it until it washed away tearing one rail off the bank.
Any chance you (OP) can get the third rail?
Three should definitely be enough.

 

[Industrial Toys]

A third rail would give you a convenient place to pee. :laughing:

 

[jymbee]

I've a lot in Vermont that has a similar sized stream and had a 2 rail bridge across it until it washed away tearing one rail off the bank.
Any chance you (OP) can get the third rail?
Three should definitely be enough.

I believe there still may be a 3rd rail at the original site but may be in the stream under some rocks. I'll have to take a look when I get a chance.

I guess I could always cut down some hard maple logs around 8-10 inches in width and long enough to span the creek for the support, but I expect there may be multiple issues involved there as well. At one point my no-help-at-all wife suggested I build a stone arch bridge. No shortage of rocks around here but-- talk about overkill!

 

[BrokenTrack]

I have a timber frame home, and I did the weight bearing beam 8 inches by 8 inches, and 12 feet long, and just one would support 4500 pounds.

We use Log Mats, also called Enviro Mats, or Crane Mats that are 16 feet long, 4 feet wide, and made out of 8x8 beams bolted together, and use them for bridges for skidders for crossing streams. My skidder weighs in at 16,000 pounds. If my skidder can cross a 16 foot 8x8 laden bridge, a 3000 pound tractor could.

You can buy used Crane Mats for a couple hundred bucks....so lay (2) side by side for a 8x 16 foot bridge. Or buy just one and cut it in half so that you have (2) 2 foot by 16 foot bridges to cross over on by straddling. Or, buy 8x8 rough sawn beams and make your own Crane Mats.

 

[newbury]

I have a timber frame home, and I did the weight bearing beam 8 inches by 8 inches, and 12 feet long, and just one would support 4500 pounds.

We use Log Mats, also called Enviro Mats, or Crane Mats that are 16 feet long, 4 feet wide, and made out of 8x8 beams bolted together, and use them for bridges for skidders for crossing streams. My skidder weighs in at 16,000 pounds. If my skidder can cross a 16 foot 8x8 laden bridge, a 3000 pound tractor could.

You can buy used Crane Mats for a couple hundred bucks....so lay (2) side by side for a 8x 16 foot bridge. Or buy just one and cut it in half so that you have (2) 2 foot by 16 foot bridges to cross over on by straddling. Or, buy 8x8 rough sawn beams and make your own Crane Mats.

Where does one look for used Crane Mats?

 

[BrokenTrack]

Where does one look for used Crane Mats?

They are all over the place. We have a local swap/sell magazine where they are listed, and I have seen them listed in the newspaper for sale as well. I have also seen hand painted signs where there is a big pile of them saying they are for sale.

You could also ask any logger where you might get some, or a log yard, or a construction company, electrical company: really anyone who does work outside with equipment would know where to find them.

They are dinged up of course, but still have a lot of life left in them as a bridge.

They are made out of any hardwood, except Ash, Basswood and Popil, so they will not rot anytime soon.

 

[fried1765]

I have a timber frame home, and I did the weight bearing beam 8 inches by 8 inches, and 12 feet long, and just one would support 4500 pounds.

We use Log Mats, also called Enviro Mats, or Crane Mats that are 16 feet long, 4 feet wide, and made out of 8x8 beams bolted together, and use them for bridges for skidders for crossing streams. My skidder weighs in at 16,000 pounds. If my skidder can cross a 16 foot 8x8 laden bridge, a 3000 pound tractor could.

You can buy used Crane Mats for a couple hundred bucks....so lay (2) side by side for a 8x 16 foot bridge. Or buy just one and cut it in half so that you have (2) 2 foot by 16 foot bridges to cross over on by straddling. Or, buy 8x8 rough sawn beams and make your own Crane Mats.

The load carrying ability of any beam has a direct relationship to the depth of the beam.
An 8 x 8 beam may carry 4500 pounds, but a deeper beam, with equal cross sectional area will carry more weight.