Bridge over ditch

[Mainecoast]

So...without anyone geeking out on modulus of elasticity. 

I'm looking to build a wooden bridge over a small ditch. A tractor could drive through it if needed so it's not some ravine/glacier washout. You know a 3 foot dip to channel water. Looking to span around 12/13 feet to just provide a smooth transition for tractors and stuff like.

I'm thinking it should be around 7 1/2 feet wide to give some wiggle room for the tractor. 

So 7 1/2 feet wide and 13' long.

Toss out some ideas hopefully you've tried your self. 

I have a saw mill and am thinking about getting hemlock or tamarack logs, saw two edges and peel the bark. Lay a 6x6 width wise and set it in the ground on a bed of rocks. Lay 5 of these 10" diameter trees across the ditch. Two in the left and right approximately where the tires go. One down the center. Then saw or buy 2 or 2 1/2 thick material for the decking. Random width or a standard width. And peg these down with wooden pegs cut off with an adze. 3/4 white oak square pegs. 

Thoughts and opinions? 

[Southside]

How big of a tractor?  Maybe put some tire guides running length wise like on a woods bridge to keep your tires from slipping sideways when the wood is wet?  

[Ljohnsaw]

I'd add 2x material lengthwise where the tires go to tie it all together.  Sort of a wear lane that you can replace. Take a look at what @Magicman  has done.

[beenthere]

Look at the crane mats and how they are made. Maybe some ideas will catch for material, size, and construction.  One such video of how a mill does it (sort through the first part for the "how to").
How crane mats are made and some amazing wooden toys must see !! - YouTube

Or just drag in a crane mat if can find one new or slightly used. 

[Magicman]

Use the "Search" above on the left and enter; bridge.  And then by user enter* enter; magicman.  That will take you to several topics where I built and rebuilt bridges on my property.

I have 3 bridges on my property and it is very seldom when all three are cross-able.  At the present time this one LINK has washed out again and the smallest bridge needs rebuilding.....again and the other has a broken board, so all three need work. 

[Mainecoast]

Good thinking in the tires bumpers! Maybe 4x material chamfered.

The tractor would be around 45hp John Deer. 

[Southside]

Hit John's Bridge crossing the Allagash between Big and Little Eagle Lake a few times just a bit fast, was always glad those guides were there.  ;D

[scsmith42]

I'd make it a little bit wider in case you ever need to drive a truck across it.  Maybe 8'6" overall width.

[Magicman]

I started with all 10' bridges and now they are all 12'.

[Ljohnsaw]

I started with all 10' bridges and now they are all 12'.

Wide or long?  If 12' wide, that is pretty comfortable!

[thriceor]

My dad and I built a bridge about 15 years ago.  It spans about 20 feet and is 10 feet wide.  We used  three old utility poles for the beams (fairly good diameter, but not huge).  I decked it with two layers of treated 2x10 horizontally, then two treated 2x10's next to each other, lengthwise, in line with tire spacing for our tractor.  The beams sit on piers/girders of old poles.  It has held up well, the longitudinal 2x is just starting to show it's age.  It will handle approximately 5000 lbs. with minimal deflection.  I, too, recommend bumper boards on the sides- it's kinda exciting when you start to go sideways on a narrow bridge due to frost etc.  Sorry, no pics of it.

[Magicman]

If 12' wide, that is pretty comfortable!

Yes, 12' wide. 

[Al_Smith]

I didn't build a bridge but I did use an underground diesel fuel  tank for a culvert .It was around 5 feet in diameter and approx. 20 feet long covered with blue clay .About 18" of dirt over the top and rip rapped from the remains of a concrete garage floor and rail road ballast stone .It worked just fine .
Every so often though if we got a frog drowner it would back up the water but it never washed out .

[gspren]

Like AL no bridge but 2 culvert pipes. On the extra small creek through my one pasture I have a 24" pipe and on the bigger stream coming out of the woods I have a 48" tube. 36" would have been ideal but I got the 48" used and cheap, just had to dig a bit to get it in. When you can use them culvert pipes are way stronger and longer lasting then bridges.

[woodroe]

My tractor woods bridge span had to be longer at 24' but similar idea to yours
using whats handy for girders and discounted bargain bin treated lumber for decking.
Started with two 17" at the butt Pine trees 14" small end, cut and peeled them in the spring.
These trees were within 100' of the site.
Both trees had identical sweeps in them so placed them crowns up for a arch effect and added strength.
Placed them on some flat rocks that were handy and set them 6' apart.
Layed a strip of rolled roofing across the tops of the logs then placed the 2 x 6 x 8' deck boards on, finished with 2x6x 12' length of span 4 wide each side for tires to run on. 
Figured the rolled roofing would help prevent wood on wood contact rot.
There is an inch or two space between the deck planks but thats what I had to work with from the pallet of 2nds i
found at hammond lumber.
Finished up with some peeled pine log bumpers about 8" dia both sides and
brought in some leftover gravel from driveway project for the approaches. Fun project overall.  
A 6 ton bridge for a 2 ton tractor. Probably have $250 in it. Good luck with yours !

[PatMcD]

Also here in Maine...get friendly with somebody from CMP or the utility pole contractors. Our standard snowmobile bridges have become 5 phone poles: 2 on each side where the tires of a standard tractor would fall, and 1 down the middle. Plank with PT 2x8's. Locally cut hemlock always works, but you never know how long it will last; it'll either be 5 weeks or 5 years. I can take my 55hp tractor with bushog across them up to 32 feet long.

[Al_Smith]

Some time ago the farmers would get old road bridges probably for next to nothing from the county road departments .These usually were riveted steel and plenty strong enough .I only assume they were cheap else the farmers would not have them . 

[WDH]

Whether you can use culvert pipes or need a bridge depends on the area of your watershed.  Magic man would need a pipe large enough to drive a truck through, and how practical is that.   Too much water comes down that stream for any averaged sized pipe to work.   In forestry, we sized pipes by using a topo map to calculate the area of the watershed that the streamcourse drains as an input into a formula to calculate culvert pipe size.  

If the creek floods and if the water gets over the top of the bridge or culvert pipe crossing, you have big trouble.  Debis will build up and dam the up the upstream side and blow your pipe or bridge out.  

[Southside]

Would that be a bridge over troubled water WDH?  :D

[Magicman]

Yes, two of my bridges are over troubled waters since both cross the same stream and the watershed is about 12 square miles.  Many times, such as right now, only one of the two is cross-able.  I have detailed rebuilding both of them several times here on the Forestry Forum. 

[SwampDonkey]

Like Magicman said, you want 12', but no less than 10' wide.

[SwampDonkey]

We have a wooden bridge with steel I beams under it up to the woodlot. Been there over 40 years. Been lots of wood and potatoes hauled over that old girl. But that is overkill for crossing your little ditch. :D ;D

I've seen collapsed or washed culverts replaced by bridges and vise versa to. :D

[WDH]

Chris Jim,

That dates me.  

[Don P]

A bridge too far would date you  :D

[WDH]

 :D :D :D

[Southside]

Chris Jim,

That dates me.  

Sorry to disappoint you Danny, but if you see my post in the Carbon Dating thread you will realize I date Vikings. I like you and all, but not in that way.
:D

[WDH]

I knew that you thought that I was handsome  :D.

[Southside]

You Never Even Called Me By My Name     :D

[WDH]

You were drunk when your Mama got out of prison.  

[Southside]

Dang old train.

[aigheadish]

I just got 2 24"x20' culvert pipes to replace 3 much smaller pipes going under my land bridge. My creek is narrow but a couple times a year we get water that'll wash a lot out. Right now those 3 pipes sit about 2-3 feet under the surface but water will go right over the bridge. The existing pipes only give me about 8' of width and getting the backhoe over it is tense, only a couple inches on each side, and you can watch the dirt crumble while driving over it. My plan is to uninstall the old, widen the bridge, from yonder side to this side, a bit, to accommodate 2 wider pipes. Then I have buried concrete blocks that I'll add back in and likely lots of rock on the uphill side to hopefully keep any of the dirt from washing out. The plan is to then make the bridge about 12' wide, with a fair slope to the edges, rather than the sharp slopes I currently have that I watch crumble away. 

[Splinter]

My small bridge is modified mat style.
2x6 or 8 PT (can't recall without looking) with 2" spacers. All stacked up and put all thread in 4 places right through.
The all thread when tight spreads the load across all the 2x pieces like the timber mats.
I made a trench to hold some PT sills. Just a rot resistant flat and level surface for the deck to sit on at each end.
I  used some 2x wet service load span tables on the internet to come up with the beam depth required for my load, which is an RTV loaded with green firewood and me.
Could drive across without decking, but dogs didn't like it, and ankle twisting was a real possibility.
Was all wiggly until I tightened up the nuts. Really solid now. Been a year and a half. Still haven't had to re-tighten.
I put some 1" hemlock decking on it that was laying around from a previous project. Love the clatter it makes when I drive across.
Holds 5000# vehicle.

Went PT because I didn't want to do it over. Even if the ends get soft, thats not where the stress is in a bridge timber, should last longer than me.
Drill the holes a little oversized for the all thread.  Was a bitch to get it through.
Protect the thread ends when hammering it through.
If interested I can get some pics.
Cost was very reasonable compared to any bridge design I could come up with that had beams. Mostly because beam bridges need stout decking to transfer the load across all the beams. Mat bridges it's all in one.
Anyway it was a great solution for me. Very happy with it.

[richhiway]

a lot of the wooden deck bridges around here use 2x6 and wider on edge for decking. Takes more lumber but very stout.