Bridge

[Donny2guns]

Hope this is the right area for this question if not I appologize. I have a drainage ditch in front of my property that I need to build a bridge over. It is about a 10 foot width. If I took  6 2x10x16's or 2x12x16 and created three beams to span, used 4x4 posts for the ends to sit on in the earth, and drove each beam through the 4x4 into the ground with 1/2 rebar and then planked the beams with 2x4's do you think that this would be strong enough to support the weight of a 4x4 or Toro Dingo like machine? Thanks Russ

[Beavers]

do you think that this would be strong enough to support the weight of a 4x4 or Toro Dingo like machine?

No, I wouldn't want to drive across that bridge.  

There is a LOT of weight that each end of the bridge has to support, I doubt that 4x4's would hold it.  
Most bridges have an abutment at each end, the abutment is usually supported by piling.  The girders then sit on the abutment which acts like a footing to spread out the load supported by the bridge.

Here are couple of links to USFS timber bridge plans.  If you do a little searching on their website you should find more bridge plans.

This one is for a snowmobile bridge.  It has a max load of 7900 lbs. but that is for a tracked vehicle, I'm guessing that you would need a thicker deck to hold a wheeled vehicle of the same weight.

http://www.fs.fed.us/eng/bridges/documents/snowcat/R9_STD_Snowmobile_Bridge.pdf

Here are a few typical plans for different abutments.

http://www.fs.fed.us/eng/bridges/documents/abutment/index.htm


As you can see by looking at the plans, bridges are complicated and expensive to build.  
In highway construction they will usually put in a single pipe, a couple of pipe, or a box culvert.  A bridge is usually built only if one of the other options won't work.  Bridges are expensive...

Have you thought of using either a single large pipe or even twin pipes?  It would be a lot easier and cheaper if you could use a couple of sections of corrugated metal pipe.  With such a short span, I would really try to avoid building a bridge if I could.

Edited to add...

If this is the only access to you property, the culvert would be the only way to go IMO.
Building a bridge strong enough for a concrete truck, propane truck, septic truck, or firetruck, would be big bucks, and possibly out of the DIY realm.  On the other hand corrugated metal pipe with enough fill on top would handle any of the trucks just fine.  Just something else to consider.  

and edited again to fix some spelling errors. 

[eddiescabin]

I'm with Beavers, only because here in Cali the gubmint is enforcing a code in which only bridges capable of supporting a fire truck are legal, they are ripping out many that span the little creek here.

[ScottA]

We have similar problem. I'm looking at a 4' x20 pipe with claygravel fill over it to solve the problem. Seems easier than a bridge. For a bridge I think 4x6" decking at at a minimum and pilings would need to be 6x6".

[Donny2guns]

I have looked into the culvert. In the county im in you need to lay down $300 just to apply for a permit to build one then whatever the cost is on top of that (have heard around $600) Im trying to avoid this because Im trying to save the $ for the cabin Im going to start building here in the next few weeks. Last fall there was no water in the ditch all summer through winter. Im guessing this is spring melt water. Im also thinking that this water must be here because there is a blockage somewhere down the line. Ill have to do some investigating and see if I can find a blockage. Im not looking to build a bridge for a car and a wheeler wouldnt have to drive over it. Really curious about the dingo (weight about 2000 lbs).

[Beavers]

That is pretty high dollar for a permit.  You don't need one for a bridge though?

I'm in the same boat as you...need to save all the cash I can for the house. 
Aren't you building in northern Minnesota? 

How about going the ultimate cheep route and build a bridge out of local materials.  There shouldn't be a shortage of rocks or timber in your area. 
Have you looked at the USFS plans for the Gabion Geocell abutment?  Just need some welded wire fencing, threaded rod, a couple of small steel plates, and rocks.
http://www.fs.fed.us/eng/bridges/documents/abutment/R1_Abutment_Gabion_Geocelll_R1933.pdf

If you look at the bridge plans for the snowmobile bridge it only calls for an 8x8 sill, gotta tree that you could drop on your property to use for that?  For the 10' span looks like the 2x12 beams you suggested in your first post would be good too. 

If you do a search for "beam tables" or "span tables" on this site you should find what the load rating of a 2x12 is.  I'm pretty sure that Don_P has posted a span table for logs before.  You could then plug the numbers from the 2x12's into that table and figure out what size logs you would need for stringers/girders for the bridge.

If you don't have trees on your property that you want to cut for the bridge, you can get a firewood permit for Superior National Forest that lets you cut up to 4 cords of wood per year for $20, nothing says that you have to cut the logs into 16" pieces, cut them long enough for your girders. 

If you get all your material locally the only cost would be the fencing and rod for the geocell abutment and some 2x for the decking.  Plus you'd have a bridge that the USFS says is strong enough to carry 7900 lbs. 

BTW-I really wish I was building a cabin in Northern Minnesota, it's my favorite place to be.  I'm usually up there a couple times a year in the BWCA.  Will be heading up the Sawbill trail in a little over a month.

[glenn kangiser]

Hey Russ, I would check the beam table for load in a 10 foot span, for safety, but from my experience 2 4x10's 18 feet long span each side will hold about a 4000 lb car so I think one 4x10 or better 6x10 built up on a 10 foot span would support the dingo - provided it is not in a place that would be a safety violation for a fire dept vehicle, and you realize I am not an engineer and ultimately you are responsible for your own safety.

I would use a railroad tie at each end flat on the ground ideally on gravel for drainage if it was me.  I would put one beam under where each of the wheels run - two total and extend the boards a bit to each side.  Consider the depth of the ditch also - put up guard rails if necessary for guidance.  I have a Bush Hog and would drive it over it - similar weight I think.



Looks like about 16 foot span here unsupported -I know we ran some numbers on the beams before running the Mercedes onto them.  There is no center support in the pix - center post is outside only.  There are 3 boat docks there and each side of the Mercedes has two 4x10s under the wheels.  Six total beams but the outside ones in the pix are not loaded.

Just my experience - no guarantees.  Your results may vary.  

[rwanders]

A critical point to remember is the bearing area at each end of a bridge-----think of it in the same vein as a house's foundation. The more area, the less loading at the banks and better stability for the bridge. I would like at least a bearing width of 16" at each end (two railroad ties should work well on a well compacted and drained base material)

I am not an engineer but, did design and build an 80' span across a creek which supported an 80,000 lb load using round logs and rough cut 3x12 timbers. It didn't deflect or even groan when loaded with the rolling load of 80,000 lbs.

[Donny2guns]

Glenn, are those 4x10's built up 2x10's? Im going to go out there this friday and see if I can find blockage. If not Im going to see if I can find a more narrow area to build a "bridge". Thanks for all your help guys. I need to figure this out because i am almost ready to start digging holes for posts and am at a standstill until I get this worked out. This county is rediculous. $95 for a privy permit I will never build (going to use a honey pot or compost) You have to have a privy permit to get a building/land use permit. That is another $120. In order to get a culvert it is $300 for the permit and then around $600 to get it built. I work for the county and I sure as hell am not seeing any of this cash lol.

[Ndrmyr]

Ultimately I too am going to build a bridge, but it may not be this year.  As I scoured the building material auctions, i would hate to say how many glue-lams I passed up, some as long a 40' that went for a pittance. I would have water sealed and perhaps even wrapped the top with aluminum for water protection. They would have been perfect for a bridge, and boy were they cheap. Sadly, at least for the time being, the auctions have virtually dried up. Still, check the paper and craigslist for barn beams and even steel i-beams.  Some nails in used beams will be no bother for a bridge once it's decked. I bought 20 old growth doug fir 3" x 12" x 20's for $10 ea. off of craigslist. Pretty cheap wood if you ask me. Good Luck.

[cbc58]

i was just looking at the government auctions website and they have 200,000 lb. bridge sections for sale at auction....   amazing what you can buy from uncle sam.

[Dallas2build]

We used an old flat rail car to build a bridge, works very well.

[Whitlock]

We used an old flat rail car to build a bridge, works very well.

That is a good idea. I have used large truck trailers the same way.

[glenn kangiser]

Glenn, are those 4x10's built up 2x10's? Im going to go out there this friday and see if I can find blockage. If not Im going to see if I can find a more narrow area to build a "bridge". Thanks for all your help guys. I need to figure this out because i am almost ready to start digging holes for posts and am at a standstill until I get this worked out. This county is rediculous. $95 for a privy permit I will never build (going to use a honey pot or compost) You have to have a privy permit to get a building/land use permit. That is another $120. In order to get a culvert it is $300 for the permit and then around $600 to get it built. I work for the county and I sure as hell am not seeing any of this cash lol.

Those are treated 4x10s  in 4 foot wide boat docks. I put 3 docks together for a 12' wide floor.  Treated is not as strong as untreated plus these docks had been in use for years so I think the built ups will work fine -stagger nailed as PEG explained in a built up beam description here elsewhere.

[glenn kangiser]

A critical point to remember is the bearing area at each end of a bridge-----think of it in the same vein as a house's foundation. The more area, the less loading at the banks and better stability for the bridge. I would like at least a bearing width of 16" at each end (two railroad ties should work well on a well compacted and drained base material)

I am not an engineer but, did design and build an 80' span across a creek which supported an 80,000 lb load using round logs and rough cut 3x12 timbers. It didn't deflect or even groan when loaded with the rolling load of 80,000 lbs.

That is exactly how my uncles and family used to build bridges in Oregon - logging country.  They were mostly loggers.

[rwanders]

The big bridge was built with 8' deep by 10' wide  benches cut into each bank. I then laid 12" diameter logs side by side to serve as the abutment foundations and bound them together with 3/4" steel cables. The span was was formed by 80' long 36" diameter round logs side by side across the full 10' width also tightly bound together with 3/4" cables. I then spiked 3x12 timbers to form the bridge deck and added 6x6 timbers as curbs. Spread gravel for the approaches. I admit I did chicken out and added a center support of 12x12 timbers in case I had not built stout enough----but the span didn't seem to notice it when I drove the 80,000 lb drill rig across. That rig was brand new so the trip was definitely a little nervous. I had looked everywhere for engineering data for round log bridges but never did find any. The logs were a mix of Sitka Spruce and Hemlock. The bridge was the longest of three I built in Alaska about 60 miles south of Cordova Alaska in 1985 in conjunction with some oil exploration.