My cousin built a bridge on his property crossing a creek. It has a 35' span and uses 2 I beams under a wood deck. The beams are spaced to match the wheels of his tractors. Each beam is 18" tall and 7.5" wide. The steel is 1/2" thick on the top and bottom and 3/8" thick in the center. He wants to drive his loaded tractor and trailer across. His estimate is that the tractor is 6,000 lbs. the trailer is 2,500 lbs and the wood decking on the bridge is about 4,500 lbs. So about 13,000 lbs total load with 8,500 moving (tractor and trailer). Anybody got a way to estimate the strength?

4/29/2015 9:19:25 PM

Do your own webassign

4/29/2015 10:37:26 PM

guys pls he has a maths tonight

4/30/2015 10:57:30 AM

Got a picture of this cousin's bridge?

4/30/2015 11:18:10 AM

Quote :

"Do your own webassign"

LOL, gg

4/30/2015 11:24:32 AM

^ indeed..

winner post #2

4/30/2015 11:28:53 AM

what is supporting the I beams? Is there any cross bracing to keep the I-beams in position and square so this "I" doesn't turn into this "I"

4/30/2015 11:33:38 AM

Pretty much what you see is what you get. He doesn't have any additional cross bracing except the decking itself.

4/30/2015 5:22:21 PM

Yea... That doesnt look safe for that load/span

4/30/2015 5:34:25 PM

is that untreated 5/4" lumber he's using for a bridge deck?

How's he planning on securing the bridge on each side without crushing in the creek embankment?

4/30/2015 5:36:28 PM

He's got the I bolted to the RR tie you see. He said its pretty solid ground under the tie. Its 2"x8" rough cut oak deck.

4/30/2015 7:46:36 PM

in a situation where there is no side or torsional load, it would be WAY more than strong enough, I'm sure. An 18" I-beam would be hellaciously strong; 2 of them even more so...in the bending mode that they're intended to support. However, they don't normally fail in bending in the web. It would fail by twisting or by a flange bending sideways/rolling over (and then probably the whole thing twisting).

The planking, if they're all fastened down on top of both beams, gives it some resistance to that sort of failure. It's not like it's fully boxed in and cross-braced, though.

My concern is that isn't exactly straight/level due to the irregular environment it's built in, or that it won't be if you drive a big tractor on it and shit sinks into the ground some. That will generate those irregular loads that the I-beams aren't designed to support. I still kinda suspect it'll be fine, but that's a hell of an assessment to make from the other end of a computer connection without seeing anything.

4/30/2015 8:43:48 PM

he has some oak runners on top of the I bolted down then the deck is nailed down to the runner. He placed the I the exact measurement of his tractor which he plans on using to cross it.

4/30/2015 9:02:00 PM

yolo

he should just drive his tractor across the bridge and see what happens.

4/30/2015 10:00:15 PM

right, but i'm saying that it might not be exactly flat and straight, and it's certainly not on a rock-solid foundation that's sure to keep it that way.

i think it'll work, but there are several ways it could go wrong.

4/30/2015 10:04:36 PM

I'm not an engineer but I think if you throw in a third beam it would hold a tank pretty much.

[Edited on April 30, 2015 at 11:32 PM. Reason : that bank may collapse without a more even weight distribution ]

4/30/2015 11:30:57 PM

He doesn't need a 3rd I beam, he needs cross bracing, better decking, and ideally a better landing foundation.

5/1/2015 7:20:27 AM

still better built than a Top Gear bridge

5/1/2015 8:26:13 AM

he needs something to stop it from flattening i. 115e if he isn't 100lined up. Also needs more than decking to stop it from say splitting like a zipper down the middle

5/1/2015 8:29:48 AM

Quote :

"he should just drive his tractor across the bridge and see what happens."

film it too, plz

5/1/2015 8:40:15 AM

^^^^ I think he'd get away with the decking. It's the cross bracing and foundation that might possibly be a problem.

5/1/2015 9:39:11 AM

I really wished he'd placed a concrete foundation/footer and had cros braced. Since its already up he may be able to add some angle iron on the bottom to connect the I's together.

5/1/2015 9:53:32 AM

IMO what he should do:

1. throw a few cross braces in

2. excavate to the side of the current landing points, down maybe a couple of feet. stack a couple more railroad ties to build the landing point back up about a foot. assuming his tractor is capable, slide the existing bridge over onto the new ties. I think this will achieve three things:
a) lower the elevation of the deck, so he won't have to drive up a ramp to get onto the bridge
b) the bridge will be supported by better bearing soils. i.e. not surficial topsoil
c) move the landing point back away from the slope

I don't think this would be that difficult of a solution, and he'll have a much better bridge without a lot more work

5/1/2015 10:38:04 AM

^ he may be able to raise the bridge in the air, excavate, and then fill with concrete. What do ya think?

[Edited on May 1, 2015 at 12:06 PM. Reason : ^]

5/1/2015 12:06:28 PM

he's still going to need cross bracing

5/1/2015 12:33:08 PM

This model has 2" x 2" x 3/8" angle iron bolted/welded in.

5/1/2015 1:39:14 PM

don't make your decking a structural member, use x cross bracing

5/1/2015 2:03:02 PM

Quote :

"excavate to the side of the current landing points, down maybe a couple of feet. stack a couple more railroad ties to build the landing point back up about a foot. "

that's how we did the bridge on our farm - a bed of railroad crossties buried deep enough that the bridge deck could sit flat on either side. We also didn't anchor the bridge to the foundation, with the thinking being that we'd rather the bridge wash away during a 100 year flood and we'd just retrieve it instead of risking the embankment being torn apart at our best crossing location.

5/1/2015 3:11:08 PM

that's a hell of a lot better constructed than the tractor bridges I've considered ... at least material-wise.

my issue is with the ends of the bridge. there will be a lot of weird forces as he enters/exits the bridge. presumably he's got some kind of ramp in mind.

the beams are plenty strong. i'd just brace the ends of them to prevent them from rolling over. they're not wide enough to really prevent that on their own, and the points at which they're fastened aren't wide enough to provide much leverage for resistance

weld/affix something like this:

     -------
        |
        |
        |
        |
        |
        |
     -------
     |     |

     -------
      / | \
     /  |  \
    /   |   \
   /    |    \
  /     |     \
 /      |      \
-----------------
|               |

5/1/2015 4:28:34 PM

IXI

5/1/2015 5:19:08 PM

redneck, that angle iron will not improve the rigidity of the bridge.

look at this box.



imagine putting your ass on the edge of that box and pushing with your feet to try and slide the box. the box will want to slide across the floor because the X is not allowing it to collapse.

now imagine sitting on the same box, but without the X bracing in the middle. if you tried the same thing, the box would collapse rather than slide.

Quote :

"^ he may be able to raise the bridge in the air, excavate, and then fill with concrete. What do ya think?"

concrete would be great, and i considered that. but a couple of things.
1) i assumed that he only had one backhoe. maybe he has two. but one would have to hold the bridge up for quite some time while the other backhoe excavates. it would have to be elevated long enough for the concrete to cure.
2) i assumed this creek crossing is in an area that is somewhat inaccessible to regular vehicles (hence it being a tractor bridge). you will need a moderate amount of concrete, best bought from and delivered by a ready mixed company. you'll need more than a few sacks of quickrete. plus that shit's expensive for a random tractor bridge.

to me, my solution is quicker and cheaper, to me

[Edited on May 2, 2015 at 12:12 AM. Reason : is it too early to feel like i'm being trolled]

5/2/2015 12:05:47 AM

I'm going to let him know the info. All good ideas.

I think his plan for a ramp is just to add a lot of soil on the sides to build up to the decking.

5/2/2015 12:33:18 AM

Quote :

"1) i assumed that he only had one backhoe. maybe he has two. but one would have to hold the bridge up for quite some time while the other backhoe excavates. it would have to be elevated long enough for the concrete to cure. "

no backhoe is needed, he can use cribbing and jacks to raise the bridge and excavate out at either end.

5/2/2015 1:54:59 PM

if both ends are in concrete blocks that will help.

right now there is nothing besides the decking to stop the bridge from splitting into 2 peices (them moving away from eachother, or falling over so modes of failure I see right now


______________
I I

turning into

_______ ________
I I


then

_______ ________
/ \

falling in


or


______________
I I

5/2/2015 7:37:20 PM

crazy web needs an ascii art plugin

5/2/2015 7:37:48 PM

^Use the "code" tags and it will appear in a monospaced font, like so:

Quote :

" ______________ I I turning into _______ ________ I I then _______ ________ / \ falling in or ______________ I I "

[Edited on May 3, 2015 at 2:24 PM. Reason : Well, you have to also lay it out in a monospaced font. . . I leave that to you, but it works.]

5/3/2015 2:23:14 PM

Her Birthday is june 16th

5/3/2015 3:24:26 PM

5/3/2015 9:58:04 PM

so has the bridge collapsed yet?

12/1/2015 12:34:15 PM

This won't pass code.

12/1/2015 12:39:08 PM

Would be a good statics problem on an engineering exam.

I know we're months later, but I agree with everything Duke866 said. The I beams should do awesome although to be sure I would need to do the math, but saying "it's on solid enough ground" isn't good enough long-term. Some massive rainstorm can come in a year or two and change things a lot.

[Edited on December 2, 2015 at 12:18 PM. Reason : /]

12/2/2015 12:12:25 PM

Any lateral force, which there will be, is going to cause those I-beams to rotate. That's if the bank doesn't give way first.

12/2/2015 12:38:49 PM

just click http://imgur.com/gallery/LUtnHJm

[Edited on January 13, 2016 at 2:11 PM. Reason : clicky]

1/13/2016 2:10:56 PM

^FTS



so did this dude and his cousin die or what?

1/13/2016 2:12:45 PM

LATERAL
TORSIONAL
BUCKLING

1/13/2016 4:26:31 PM

just designed a bridge just like this except with 4 girders vs 2. We had W-section cross bracing welded/bolted to the girders at five foot intervals along the length of the bridge and full concrete retaining walls as the abutments on each side.

Soooo, good luck with this. I've always wanted to know just how over-engineered typical bridges are, I guess we'll find out

1/14/2016 9:07:56 AM

its gonna go like dis

1/15/2016 7:59:24 PM