Building a bridge to cross stream

spo307 · Jan 14, 2010

I need to build a bridge to span a stream 34', tractor with back hoe weights #7500, so I figure 10k lb bridge, the deck i would probably use 3' x 10' subway grating either steel or galvanized. the easiest thing to do would be get a 40' flat bed trailer ( full rails front to back ) take the axles off and 5th wheel, drop into place with the 200 excavator and be done. i have seen flat beds for 3K, the i thought 2-12" I beams, with the decking on it, or 3 8" beams, anyone know span and loads for I beams, i am a welder just don't know the tech stuff, I know there's a table for calculating it on the web but again, i took a small bus to school. thanks Dennis

Cami · Jan 14, 2010

Engineers are cheaper than failed bridges.

It doesn't matter what bus you took to school, or what you know, because specialist help is just a phone call away. When I run into something I'm not sure I can do properly, you can bet that I call somebody who can; whether it's for advice or for them to actually do it for me.

Is a bridge the only option? Can you make a ford or install a culvert? Is this permanant or just for the duration of a job? Can you rent/buy a Bailey Bridge?

I've made a couple bridges, if you can call them bridges, by making a pair of ramps from two logs lashed together with wire rope. ....but if you're not sure, you need an engineer to tell you how big/what species of a log to use! It's not for the faint of heart but can work in a pinch.

LD1 · Jan 14, 2010

There is more to it than just a 8 or 12 inch beam. There are many 8" beams and many 12's and there is a big strength difference.

The beams are designated like W12 x 35 whereas W means wide flange, 12 is the depth and 35 is 35lbs per ft.

A W12 x 22 would not be nearly as stout as a W12 x 35 but they are both 12" beams.

A good program to use is Beamboy. You can google it and it's a free download. I cannot get it to work on my computer for some reason but you may have more luck. All you have to do is choose the beam size from the list, and it knows the modulus of elasticity, section modulus, and all that other engineering stuff.

You select the beam, select the length, and select the load. Then hit calculate and it will tell you the how much the beam will deflect and the how much it is being stressed in ksi and also the maximum bending moment.

When I did the beams for the loft in my shop, @ a 30' span with 100lbs per ft distributed load and 1500lb point load in the center (for occasional motor yank) the minimum I could use using L/360 deflection limits was a W10 x 19. I ended up going w12 x 26 cause I found them at the right price.

Given your 34' span, using two beams with a point load of 5000lbs centered @ 17', just start picking beams in that program that will give you about 1.13" or less of deflection.

inmaestro · Jan 14, 2010

All good suggestions. I suppose if you know the rating of the 40' trailer, and build adequate concrete abutments at each end, your idea could work. Be advised, as others mention, once in place, you own it, and if somebody comes along with a 10 ton load...well, you get the picture.

Best bet, sketch something up and have a local Civil Engineer size the beams for you. A 34' span is pretty long, so you may end up with some pretty deep beams. Like the others state, depth is only one consideration. Weight is just as critical. Don't forget the design of the abutment. Even the best steel W beam turns to spaghetti when its on its side. I suspect you might need a little X bracing, too.

Let us know how it turns out. I got a 7500lb tractor and a little stream I want to get over, too. Best of luck.

CurlyDave · Jan 14, 2010

While I have seen trailer bridges and I know they work, the maximum width is only about 8'. This seems very narrow, especially for a relatively long bridge.

I am old enough to remember Teddy Kennedy and Mary Jo...

ray66v · Jan 14, 2010

Dennis, if your engineering skills have not let you down so far, I say build the bridge yourself.

If you have not done well in the engineering department, or have limited skills, that would be different.

A bridge is the last thing on my, "Things I have built" bucket list. I sure would love to do one.

rambler · Jan 14, 2010

Dad built 2 bridges, they had to be 12 feet wide, 8 feet high for the ditch flow. He made them 12 feet wide, he always regreted that, said it shoulda been 16.

Anyhow, he started with 7 I beams and a couple truckloads of concrete. Put 3x8 home sawn white oak lumber across it.

He built it for his 4500 lb tractors and 2 row pull type combine, and 110 bu barge wagons.

Fortunately he overdesigns such stuff.

Now I use it with big wagons, bigger combine, and a couple years ago the fertilizer applicator misunderstood me & took his big loaded applicator over one. Well, I guess if it holds that.... I think we are talking 12 ton on an axle there....

Anyhow, 30+ feet is a good span. And you are talking a good load. A trailer has wheels not quite to the ends - be sure the span matches the loads you want to put on it. Sounds like a good plan, but - be sure. Overbuild. What happens when you find a good deal on the next size up machine, too good to pass up? Be sure you overbuild.

--->Paul

spo307 · Jan 15, 2010

thanks to all for the info.

jayste · Jan 15, 2010

If you do a search on here for bridges I'm sure you'll get more than enough information on builds and engineering technical information. Be prepared for a BUNCH of technical information. I've been impressed by the builds as well as the engineering (and I don't mean train driving

) information.