Concrete Foundations for Wooden Bridge

[glennmac]

In a few weeks, when my son is home, I am going to build another bridge over my creek so I can get my tractor to my back 8 acres. The double-culvert bridge I built last fall has washed out several times, and I consider that approach to be a failure.

I need some advice on how, in what shape, and where to pour the concrete foundations for a wooden bridge. I have never done anything like this before.

Here is the basic construction idea. The bed of the creek has fairly vertical walls, almost U-shaped. It is about 3-feet deep. At the location where I will span, the gap between banks is about 8' maximum. I am going to use four 20' long 4'' x 12" beams (long side vertical) to span the creek. So, I will have at least 6' of beam on the ground on each side of the creek. Across the span beams, I will nail 12' long 2" x 6" boards. I want to put concrete foundations under the beams, and then somehow secure the beams to the concrete to prevent the bridge from washing away during overflows.

Different ways to pour the foundation have been suggested to me.

1. Use 12" diameter sono-tubes at each end of each beam, each sono-tube buried at least 3' (4', 5', ????). I dont know whether to put the sono-tubes at the very end of each beam, or to put it maybe 3' from the end of each beam, which would still leave it 3' from the edge of the creek. In other words. should the cross spans be end-supported or cantilevered on the sono-tube foundations. Also, should I elevate the sono-tube above ground level, so that the span beams are elevated and contacting only the sono-tube. Or should the sono-tube be basically flush with the ground so the beam is also resting on the ground in addition to the sono-tube.

2. Dig a big rectangular hole in the ground, maybe a foot deep, 12' long and 2' (3', 4', ???) wide, and pour concrete in the hole. This would give me one rectangular foundation on each side of the creek, flush with the ground. This method also raises the end support vs. cantilever issue.

3. Build a rectangular wooden form out of 2" x 12"s, in whatever dimensions I would do for method 2 above. Put this form on the ground (leveled), and pour the concrete into the form. This will give me one rectangular foundation on each side of the creek, but raised 1' above ground level. Same end-support vs. cantilever issue.

4. A combination of 2 and 3. Build the form and partially submerge it in the ground with a little bit protruding above the ground. Cantilever issue again.

Finally, however I pour the foundation, I have two other issues. First, do I need to use rebar if I use rectangular foundations, and if so how, or is the concrete itself enough. Second, how do I secure the beams to the foundation. Bolts set in the concrete have been suggested to me, but it is not clear to me how this is done.

I would appreciate any thoughts, experience or comments, as this contruction stuff is all completely new territory to me.

 

[kerr]

You can use fibermesh in the concrete it works very good,with or without rebar.I would stay back from the creekbank as far as pos. with my foundations.Im thinkin steel I beams instead of wood myself.Ever thought of cutting the bank down and just put in a low water crossing?

 

[Alan L.]

Glen - I have an almost identical situation - I've got about 12 acres I haven't seen in over a year except what I can see across the heavily wooded creek. I can't offer experience, but after using the I-beams to build my metal building the idea struck me to use these for spans. Seems like they aren't all that expensive, but I forgot exactly how much they are. They would seem to have more resistance against decay and also might handle flooding situations better. My whole creek bed is about 15' lower than the surrounding area, and then the creek is maybe 5' deep with vertical banks, and about 8' to 15' wide. Whatever bridge I put in will be under several feet of water at least once a year.

The rectangular concrete structure sounds like a pretty good design against washouts, but a 2 X 4 X 12 beam will take about 3 yards of concrete - times 2 is 6 yards which means you will have to have cement truck access. I dont' think I could get a truck near enough to it to get it done so I might have to go with the piers.

As for how to attach the beams to the concrete support I would bolt a 12' piece treated lumber cross ways, and put your spans on top of that. I may attempt to bolt my I-beams directly to my piers through the lip on the bottom.

I'd like to get my hands on some old bridge planks to deck it with. The county is taking down old bridges all the time - surely they aren't burning that good lumber.

Alan L., TX

 

[TWINKLE_TOES]

Glenn,

A couple of links that may help with your placement of the bridge supports. For maximum load support you would like the bridge abutments as close together as possible. For protection from erosion, as far back from the creek as possible. You need to settle on somewhere in between.

Both links provide shareware for load and span calculations. also note that the classification of hardwood Vs softwood is not based on the hardness of the wood. (Cottonwood is a hardwood). For your application and location, Douglas Fir (select structural) is a good choice. You also might look at some of the used structural beam yards. Up here big timbers heavily treated with cresote are abundant. I have access to telephone poles which the power company has removed from service, they would make great bridge supports.

http://woodbin.com/ref/wood/strength_defs.htm

This link will allow you to download an evaluation copy of beam analysis software. I think you can run it about 30 times and then "Phoof" it dissapears

http://www.orandsystems.com/

 

[HR_Mutt]

glennmac
Wishing I had a river to bridge on my property...I'd go with I-beams previously mentioned, tie them together with angle iron which would also provide a framework of steel construction and space the angle iron to accept the size timber being used.For the piers (depending on what you have there) I'd want some mass down in solid ground to frostline+.Then possibly build some forms from plywood to accept the I-beams when cement is cured.Hope I don't get laughed at too bad by engineers out there who know how to really build it/w3tcompact/icons/wink.gif.It sounds like a fun project.
regards
Mutt

 

[Bird]

A little out of my area of expertise (as most things are), but I'd probably go with #4, the combination, but I'd drill at least two piers, or footings, at least two feet deep, under that slab foundation.

Bird

 

[cowboydoc]

Glenn,
I've got the exact same deal as you except my creek is deeper and wider. I just went with three telephone poles and nailed in bridge plank for the surface on and used an old truck bed and put the plank on top for the other one. I take everything across it. Even had one of the big spray rigs across it a couple of times. I had tried cutting down the bank and making a low crossing, didn't work. Put in a culvert and washed out. So I went to this. All I did was cut down the bank so it was level with the truck bed and the pole bridge and that was it. I didn't have anything else anchoring them at all. I wanted to drive telephone poles into the creek bed and support the bridge that way. The old farmer that was helping me said forget it it'll hold just fine. I figured after the first big rain it would be gone but wasn't. When we get big rains, which is often, the creek turns into a river and the bridges are completely covered. They have never moved one inch. Before I went to all the expense of concrete piers, etc. I'd sure try it that way.

 

[RPM]

I've worked on a couple of bridges over a creek that was around 30 feet wide. Would go from 2' deep to 8' deep between steep banks after a storm.

First something no-one has mentioned -- first law of home-bridge-building is not to put the bridge in over a bend for obvious reasons. Find the straight run.

With regard to concrete foundations, I doubt you'll need much. The guys I worked with didn't use anything other than stakes made out of angle-iron. The weight of the lumber in the bridges held them down.

The only precaution we took was because this creek is a known bridge-destroyer over-winter. Both ends were chained to trees (we were in a wooded area) about 30 feet back from the banks. They said it was much safer than putting in heavy concrete -- if the banks are going to wash out, they're going to wash out. If there's nothing under the concrete it's just going to become part of the new creek bed.

We lost the first bridge to a downed tree (fell on the bridge). After that we took out the snags and leaning trees 40-50ft up and downstream. We almost lost the second bridge to a tree that was carried downstream after a big storm -- the chains saved it. We just went out after the storm and trimmed back the big branches then readjusted the bridge with the tractor - good as new. Next storm floated the trunk further down stream.

Patrick

 

[RPM]

Should have pointed out on that last post that we had a big boulder mid-stream that we used for a middle-support (with a bit of joinery). Didn't bolt to it - just used it for mid-span support. 30ft otherwise would need some heavy I-beams.

Patrick

 

[gordon]

Guess you want to end up with something like this huh?
See attachment above.
Gordon