# Planning a bridge



## Maria (Apr 24, 2003)

I was wondering if anyone here has experience building bridges, or knows of any reference book that would tell me how to calculate how much weight a given type of bridge can hold.

We are going to build a timber bridge across a small creek that runs across our property in order to get our tractor and hay equipment to the other side and harvest hay from the *other* side of the creek as well. 

We currently have a small bridge we built 14 years ago, but it was only good enough to for our small tractor with no other load and is ready to fall apart now and is only safe for foot traffic. 

The creek is 5 feet deep and 10 feet wide. When in flood, it fills that whole space, so we can't really do a couple of culverts with fill on top. (We talked to a culvert salesman and anything they can do would reduce the peak water flow too much) edit: The space the creek takes from bank to bank is 5' deep and 10' wide. Most of the time the water is just a trickle in the bottom.

We are thinking of doing another timber bridge, but much bigger and stronger than what we did before. 

We'd have a 16 ft. span and the stringers (?) would be 2 x 12 x 16 pine- common enough lumber that will be comparatively cheap to purchase. I'm thinking of bolting 2 such boards together and doing a set of those every two feet for the entire width of the bridge. On top of that would be 2 x 6 boards and on top of that would be another layer of boards going at an angle to the 2 x 6s. 

Properly supported on piles with a cap - would such a structure hold up to a couple of tons of tractor & loaded haywagon over a 16 foot span? I'm just guessing on the weights. It seems like it would hold up, but I can't be sure. If any of you have any idea whether this would work or not, I'd love to hear your opnion. 

I suspect we are designing it much stronger than it has to be- but I have no way to know. I'm hoping someone will tell me we could drive a semi across such a bridge and it'd be fine!

I'd especially like to hear how any of you have bridged a small creek like this.


----------



## PhilJohnson (Dec 24, 2006)

I have seen some folks use old mobile home frames for bridges. Seems to me it would be cheaper and easier to go that route and be plenty strong. The neighbor behind me built his own bridge which was right around 20 feet if I remember right. He bought an old steel bridge that was going to be scrapped. The bridge was plenty sturdy but they were widening the road. Could be a possibility if you can find anything like that. I myself am not a big fan of wooden bridges but most of the ones built around here use much bigger lumber than 2x12s. I could take a picture of one of the wooden bridges that are in use around here if you want to get some ideas what one that has lasted for the past 50 years looks like. Otherwise most of the other bridges I see are ones where the wood planks are covering two steel I-beams. I myself wouldn't trust a bunch of 2x12s to hold up some heavy tractors plus hay. Another option is to get some huge steel culverts and put gravel over it. For a 16 foot span probably looking at two or three of them.


----------



## ericjeeper (Feb 25, 2006)

Biggest problems most folks face when building a bridge is.. Not a proper foundation on the end bents.You need some serious footers in order to keep it all from washing away.(floating)
If you are going to insist on using wood for the main beams.. Find some oak or Elm preferably.
One inexpensive option for you to think over.Bring in a dozer and make a ford.Place a 3 foot oval culvert in the bottom. Then pour massive amounts of concrete over it.. When the floods come it simply flows over the ford.
If I was going to build a bridge it would have some serious I beams sitting on concrete piers.
This is not going to be a cheap endeavor if you choose to do it right, and only once. This is all just my opinion.


----------



## PhilJohnson (Dec 24, 2006)

ericjeeper said:


> Biggest problems most folks face when building a bridge is.. Not a proper foundation on the end bents.You need some serious footers in order to keep it all from washing away.(floating)
> If you are going to insist on using wood for the main beams.. Find some oak or Elm preferably.
> One inexpensive option for you to think over.Bring in a dozer and make a ford.Place a 3 foot oval culvert in the bottom. Then pour massive amounts of concrete over it.. When the floods come it simply flows over the ford.
> If I was going to build a bridge it would have some serious I beams sitting on concrete piers.
> This is not going to be a cheap endeavor if you choose to do it right, and only once. This is all just my opinion.


Didn't think of the ford idea. There are a few roads around here that have them. Two come to mind right away. I seem to remember the one had cement poured down and the other one just had a bunch of good sized rocks piled underneath. I suspect they have to dump a load of rock every once in a while because it would wash out. All the wooden bridges around here have cement footers at the end. Probably would be the cheapest to do a ford.


----------



## Wis Bang (Feb 20, 2009)

Local township has a few old bridges on back roads that are 2 I-beams spanning the stream from concrete abutments. The bridge deck is simply 2x6's on edge and they drive trash collection trucks across them though they are being replaced, budget permitting. One did fail when a redi-mix truck took a wrong turn... 

Our motorcycle club took the running gear off an old flatbed trailer to span their small creek. They drive the tractor/mower over it.

I'd check w/ the local environmentals b/4 I built a ford though that would work too...if they allow it; alot of states are getting protective os streams ans wetlands.


----------



## Harry Chickpea (Dec 19, 2008)

What you are essentially making is a deck. There are a number of concerns and issues.

The first issue is the initial strength and loading. With a tractor and hay wagon, your load will be concentrated to the area under the wheels rather than spread out along the span. When the wheels reach the center of the span, there is maximum stress. The actual load could easily be heavier than a couple of tons, and safety margins multiply that figure.

The second issue is the inevitable rot and insect damage, which can be hidden until it is too late if the support structure is entirely beneath the bridge. An area like this is almost always going to be high humidity, which makes the problem worse.

Doubled 2 x 12s 24 O.C. isn't enough for a 16' span. However, you can modify your design to easily have a strong enough bridge for light truck traffic. I'll give you the concepts first, and you can go from there.

When you halve the length of a span, the strength goes up, for all intents, by the square. In other words, if you can somehow break your unsupported 16' span into two smaller 8' spans, you bridge will carry a manure speaderfull more load instead of a kiddie tractor.

How can you effectively do that safely, if you can't drop a support in the middle of the creek? You could use posts or trees at either end, and suspension cables of the same type of cable used with winches to make a suspension bridge. More elegantly, you can use the compression strength of wood and short lengths of steel rod to do the same job. This is called a kingpost or kingrod truss bridge. There is a diagram on the link. http://www.vermontbridges.com/pinebrk.htm

Because you aren't doing a full covered bridge, you need to keep the strain from pulling the top part of the truss triangle down to the side of the bridge or across it, causing a collapse. That is the purpose of the outrigger and brace on the bridge in this link http://en.wikipedia.org/wiki/File:King_post_truss.jpg

You have to make a brace that will keep the top of the triangle forming the truss from swaying either to the right or the left. As long as the truss is close to vertical, it takes relatively little strength. Once the truss is more than a few degrees off vertical, and is under load, it takes a LOT of strength. So you want a VERY rigid brace. 3/4" marine or exterior grade plywood in a triangle, bolted in place and properly sealed, would work nicely.

So, do the basic deck the way you designed it, but add a truss on either side, along with the sway braces. I would have the king rods pulling up on a small steel beam under the center of the deck, and I would whitewash or paint the truss and steel to minimize moisture damage. The cost of adding the trusses would be minimal, the bridge would be far stronger, and it would be cool to look at.

I might also use 8' or 10' joists under the deck rather than a single span of 16' (except for the outer joists, which have to form the rigid bottom sides of the truss triangles and have the upper sides bolted to them). That way if some had to be replaced, it would be easier. Remember that you need to crossbrace between the joists. If it were me, I would measure the distance between the wagon or tractor wheels, and put two sets of joists one foot apart, directly under where each of the wheel tracks would be in crossing the bridge. The center and sides would need less strength than those two tracks.


----------



## Wis Bang (Feb 20, 2009)

The Delaware Canal had 'Camelback' truss bridges where the center portion was level and there were two sloped sections transfering the load to the abutments. There reason was to make clearance for the canal boats. Six of the originals remain and the 'Freinds of the Delaware Canal' are trying to replace/reconstruct them because of the unique design and they want to replace other, newer, bridges w/ the original truss design. The abutments could be slightly below the grade at the top of the bank w/ the main deck several feet higher. The canal has several covered bridges over it too as Bucks County, PA has a bunch of them left too!

Looking at the 'freinds' web site I don't see any pictures of an actual camelback truss only the flat floor replacements w/ siderails made to resemble them...


----------



## Maria (Apr 24, 2003)

Thanks, you've given me a lot to think about.

We aren't dead set on either the timber aspect of the bridge, or the 16 ft span, really. We are just trying to keep that whole creek bed, 5' x 10', free of culverts or pilings that would reduce or change the water flow when it's flooding. It seemed like a timber bridge would accomplish that.

I'd forgotten about insect damage, though. Treated wood would only buy time, not make it impervious.

We do have a place we use as a ford, but it would take some serious earthmoving to smooth out the approach on both sides to allow a trailer across. We liked the other site because it's practically level on both sides and would require only cutting a couple of trees. I suppose, though, if we rented a bulldozer or backhoe and modified the existing ford, and put in some culverts and cemented them in that would be a more permanent solution than a wooden bridge.


----------



## Harry Chickpea (Dec 19, 2008)

Another thought is to skip the dimension lumber and buy new railroad ties. A treated railroad tie will last longer than you or me.


----------



## Maria (Apr 24, 2003)

The decking of the small bridge we built 14 years ago is supported on railroad ties. They look like they did the day we put them in.


----------



## The Paw (May 19, 2006)

Maria said:


> The decking of the small bridge we built 14 years ago is supported on railroad ties. They look like they did the day we put them in.


Railway ties will last, but they will still deflect on a 16 ft. span, even though they are fairly thick. And most of the railway ties (at least around here) tend to come with large naturally occuring cracks. I think they are fine as sleepers or for foot bridges, but I would be nervous about spanning that distance with them.


----------



## Drew Cutter (Dec 30, 2008)

With rail road ties their are cheap quality ones and their are quality ones . My neighbor cheap rail road ties are rotten after only 7 years. What about building permit ? I suppose in rural area you can forgo this ?


----------



## Chixarecute (Nov 19, 2004)

Not exactly an answer to your question, more of a wet blanket thing. Here in WI, my nephew was injured riding a 4 wheeler on his dad's 40. Somehow, DNR got involved, & asked how he got onto the 40. It is adjacent to the north edge of ours, and down a steep bank, across a VERY shallow creek, & up a steep bank twice as high. 

Well, nephew & his brother had build a log bridge & decked it over so they could cross. Only problem was that 12" deep creek in years of extreme flooding is considered "navigable." So the bridge interfered with navigation, and the family was threatened with considerable fines from the DNR. I'm not sure if the fines were dropped or not...it was probably 15 years ago. Stupid thing. 

Just be sure (quietly) that what you want to do is legal. Even if it's in the middle of nowhere, on your land, it may not be legal. And if an illegality is discovered... you could be facing some stiff fines.


----------



## Bearfootfarm (Jul 13, 2006)

I'd use 3 or 4 culverts as large as possible, to give the most flow, cover them with rock and dirt, and then lay telephone poles across for added support. Cover the poles with some tarps and a layer of dirt and plant grass to hold it together.

We've done that here and driven huge tractors and combines over them for years.


----------



## stickinthemud (Sep 10, 2003)

Soil Conservation Service encourages bridges & may be able to help you. Meanwhile, Google "soil conservation bridge stream" "riparian rules" etc and find lots of ideas:
http://www.dnr.cornell.edu/ext/bmp/contents/diy/diy_streamcross.htm
http://lincolncd.org/streamcrossing.html
http://www.vacd.org/rcd/documents/SkidderBridgeBrochure.pdf
Missouri:
http://olp09.wordpress.com/law/missouri-water-law-primer-streams/
http://www.dnr.missouri.gov/env/wpp/wpcp-about.htm
Railcar bridges:
http://www.railcarbridge.com/?gclid=CIPkndy_rpoCFQObFQodnnv5bg
Historic bridges, some of which might give you ideas:
http://bridgehunter.com/


----------



## Evons hubby (Oct 3, 2005)

I built this one a few years ago. I used doubled 18" Steel I beams for the stringers and 3 inch white oak decking. Its overall length was 60' with three 20' spans. "on paper" its supposed to support 93,000 lbs. We have had trucks in excess of 50,000 lb on it.


----------



## Evons hubby (Oct 3, 2005)




----------



## Evons hubby (Oct 3, 2005)




----------



## agmantoo (May 23, 2003)

Here is a bridge I built. This bridge is made for the water to go over the top in major flood situations. In the pic with the assistant and the truck that entire area at the truck is also under water at times. The supports on the ends of the bridge are anchored to bedrock using 6 inch well casing drilled into the bedrock and filled with major diameter rebar and concrete. In less than 10 years the water has been over the top of the bridge not less than 3 times. The working load calculates at over 250,000 lbs.


----------



## ronbre (Apr 26, 2009)

i was also wondering if telephone poles might be your answer for the "beams" ..i wouldn't drive a cement truck over it..but it might work


----------



## cbdinc (Jul 20, 2009)

If it were me. I'd go with one of two options.
A simple timber bridge or a transverse laminated deck.
Simple timber bridge would be made up of 12" butt timber piling driver to the point of refusal. Use three on each end. You can then use a single cap. 12x12 or a double cap bolted to the sides. Say three 2x12's nailed together then through bolt with a length of all thread rod. I'd go 3/4 to 1". 
now for a simple timber bridge I'd use three 2x12x16 nailed together every 16". Then use a double 2x8 deck.
For the transverse deck. Use 2x12x16 set on edge across the entire width of the bridge. Nail together and insert 5/8 rod every 2'. Use an og washer and nut on rod. Shoot me an email and I'll send drawings. [email protected]


----------



## Maria (Apr 24, 2003)

We had to abandon the project due to my husband developing severe back problems. Thanks anyway.


----------



## CIW (Oct 2, 2007)

Have you thought about using an old flatbed railroad car.
I helped a friend pour the pilings on each side of the creek and then we used a trackhoe to hang the car across the span.
Over the years it has endured many 13 yd. loads of concrete crossing it.


----------

