Building a bridge over a creek (driveway)

[Groo]

The loads on this bridge would actually be pretty small just because the length is so short. While it is true a triaxle truck or fire engine might roll across it, bridge calculations are done on length and weight. In this case, even if a tri-axle truck did roll across the bridge, because the bridge is only 13 feet long, that means the entire weight of the truck is never fully on the bridge. (The front axle is on the bridge, or the rear axles are on the bridge, but never the full weight of the truck.

It still leaves the three rear axles over the bridge granted, but then that load is spread over 12 tires.

The best way to deal with that is, to put down (4) I beams at truck width, angle irons welded x fashion between them so they do not twist under load, then plank the bridge over with 3x8 planking. I would add bumpers on the outside, or railings depending on height. (Just to limit the liability of kids or grand kids falling off if it is high).


I would not over-think this too much.

That is why I discussed highest single axle load.

 

[Renze]

Calculated a bridge with a 10 meter span over a canal for a friend, because the water council needed a calculation for a permit. I calculated four beams HEA400 but he could only buy 9 at a time, used. It can hold a 60 ton dumptruck with a safety margin of two.

As bridgeheads we used inverted T shaped concrete retaining walls used for silage silos, i calculated the lateral pressure as given by the manufacturer, into vertical pressure when used as foundation and it had also a safety margin of 5. Ground pressure was far lower than the lowest rating for the soil in our area, and we buried them deeper than the canal bottom so underwashing of the foundation was impossible. Another friend took the telescopic crane from work to place the bridgeheads so he had a fully licensed 12 meter span, 4 meter wide 60 ton bridge for under 5000 euro.

 

[ch47dpilot]

I've seen people use old flat bed semi trailers. Don't know weight capacity of one but normally are 40' long and 8' wide. Cut in half and you have a 16x20 bridge that probably would hold almost anything you'd need it too.

 

[rScotty]

Nice bridge discussion.
rScotty

 

[Rmart30]

Sometimes you can also find used bridge steel for sale.

Find a demo company and talk to them. They scrap a awful lot of good stuff from building demo jobs.

 

[keegs]

Some counties and states have been using thick micro-lam beams on shorter spans like this. Not sure why... maybe less maintenance using wood versus steel.

 

[RedNeckRacin]

wood is cheaper than steel.

 

[Snobdds]

We had to redo the bridge at our cabin. There are about 20 properties that are only accessible by way of this bridge. THe beams are from a railroad car that crashed in the 1950's and repurposed. The steel has lots of nickle in it and even though we get 20 feet of snow some years, rust in never an issue. I wish we could get steel like that now...

One of the guys up there is a engineer in the oil field and knew about these rig mats used by oil field drillers. He had them made up for our purpose, width and length. These things are strong and durable. It was quite the job to remove the old creosote soaked timbers and shore up the abutments and lay the new decking. It's nice now though...

What ever you do, over build it and make sure your abutments are strong and stable.

 

[johnrdunn1953]

When I bought my farm 20 years ago, I found that I needed a bridge across a creek than ran across the back of my property. Wasn't any way to get to the rest of the acreage to mow without getting on the neighbors land. I bought a used unserviceable 53ft. flatbed semi trailer that had been run under a bridge and the load hit. It took some of the crown out(about 6 inches) and ruined the wood flooring/floor cross supports. I got it for bought and delivered for $1000. I replaced the flooring, removed all the stuff I didn't need and put it over the creek and removed all axles, landing gear etc. welded it to fence posts at the corners and welded fence posts and rails across it. Worked perfectly even when the creek flooded a couple of times. I drive my tractors across it and since it was rated for 80K I figure it will handle anything I will ever need.

 

[chim]

When we built here back in '89 I went to a real engineer and had him design the bridge. We had a temporary OK from the neighbor for the construction traffic to use a corner of his field so we could proceed with the work before the bridge was built. Part of the project included re-routing the creek (removing a large lazy omega-shaped bend). According to the engineer and DEP, straightening the creek offset placing the bridge in the floodplain. Did I mention the approval process took three years?

Anyway, it worked out well because we built the bridge on dry land and then re-channeled the creek to flow under it. I told the engineer I wanted a bridge that would be good for trucks in case we needed triaxle loads of stuff, or something bad happened and we needed a fire truck. I believe the footers are somewhere around 11 feet below grade.

The picture that shows the bridge is looking West, or upstream. The picture with the bridge hiding in the water is looking North during a high water event that happens about 2 or 3 times a year. The driveway curves slightly to the left after getting past the trees and if you look closely you can see the two sides of the bridge. At that level, the deepest water on the driveway is less than a foot. There have only been a few times since we built that we lost access. When that happens, it only lasts for maybe an hour or two. The worst that happens is I get to spend an hour or so gathering old soybean stems, pieces of cornstalks and someone else's firewood.

EDIT TO CLARIFY: Although this is the BIY area, the builder who did the house built the bridge. Since I've had some experience at work, I was thinking about doing it myself. When the builder said he could name that tune for $7K I told him to have at it. I had a price from one of our vendors of over $1K for the rebar alone. I had plenty do do in the house at the time - electrical, plumbing, HVAC, central sucuum, security system, etc. It was a busy 9 months.