Load capacity of railroad ties

[RobA]

I found a source of railroad ties and was going to make a bridge over a small creek for ATV's/people/horses. Does anyone know how to figure out how much of a load a railroad tie bridge could support given a certain span? I was going to run 6x8 ties spaced 1 inch apart across railroad tie beams spaced 10 feet apart. I am sure that the bridge would hold an ATV (right?) But what if I were to place the support beams closer together - could it support a lot more? I was going to try to find a spot along the creek to space the beams closer together to carry a bigger load.

 

[EddieWalker]

I'm just thinking here, so don't confuse my thoughts with actual advice or that I know what I'm talking about.
/forums/images/graemlins/grin.gif

If you anchor your two beams across the creak at the width of the wheels of what you plan to cross the creak, they will support the load, not the lumber that you attach to the beams. I've seen many bridges built with two I beams across a span and 2x6's attached to the beams.

To my uneducated eye, it seems the beams support all the weight and placing the load on those beams would be the important part of your design.

As for railroad ties being strong enough, it really depends on the type of wood, condition of the wood and legth of span. My guess is they will be plenty stong.

Sounds like a great project for some pics!!!
Eddie

 

[inveresk]

Rob, here in BC, all I'd have to do is check the Building Code for the maximum point load allowed for a given cross section, span and species of lumber. You'd be best to check out the code in your own locality so you'd be on the right side of your insurance company in the event of a mishap. I'd also post a sign advising of the load capacity of the bridge. At least then you'd have some protection in the event that someone overloads it then tries to claim off you.

Essentially, all the load is going to be taken along the line where the wheels roll, with very liltte load in the middle, so it would make sense to design the bridge accordingly. I don't know what span you need but why not make the whole thing of railway ties laid side by side? You could arrange it so the centre railway ties are laid flat but where you're likely to get load you place them on edge for greater load capacity.

In addition, if you could somehow join them together (Olylog fasteners or the like would work), this would increase their load capability quite considerably. However, bear in mind you'll likely at some time want to remove the odd one and replace it because of rot or damage so you'll want to design in the capacity for some disassembly. You'll also want to ensure they are really solidly supported at either end with a generous end bearing.

 

[KentT]

I can't answer what the limit is -- but on our family farm, growing up, we had two different bridges spanning a small creek about 5' - 6' wide and 3' deep. Both were built with four 12' railroad ties as the beams, and decked with rough sawn 2" oak that we creosoted. They had two RR ties, spiked together, under each of the tire treads, and the decking wasn't really load bearing per se...

We crossed these bridges all the time with loaded pickup trucks, a Ford 8N and later a Massey-Ferguson 135 -- since the bridges connected our two main fields. I know one of them, at least, is still there 30+ years later, but I don't know what condition it is in...

 

[waynokakid]

I drove many a train across wooden trestles. So as to capacity. I would think they would handle just about any type of farm machinery. As longs as they are good solid ones.

 

[RobA]

I didn't do a good job of explaining the positioning of the beams. I was going to use a post hole digger to put 3 ties as posts on each side of the creek. The support beams would be on top of the posts and will be parallel to the creek. Then I was going to run 11 or 12 ties across the creek. These ties will form the deck. The deck will span from beam to beam and be running perpendicular to the creek. Make sense?

 

[have_blue]

Rob,


RR ties used to be made of locust, and were very strong. I have seen large dozers run over them without damaging them.

Condition of the ties and length of the bridge are the main factors. As the length of the span doubles, the load bearing ability roughly quarters. A 6 foot bridge with 3 spanwise ties and a 2x6 deck would hold a mack truck. An 8 footer would start to get iffy on anything heavier than a small tractor, and a 10 footer would be about right for a horse and rider.

Test it out! Block up a couple of cross ties up on a pair 4x4's the width of your tractor tread. Load your tractor up heavy and ramp it up on the ties, and look for sag. The worst thing that can happen is the tractor will plummet 3-5/8 inches to the ground. /forums/images/graemlins/wink.gif Such wooden beams rarely fail catastrophically. They crack and sag long before they actually snap.

Remember, length of span is the enemy, and so are cuts, holes, and gouges in the center of the beam.

 

[RobA]

Bob, that's a really good idea. Really simple but really effective. All I could imagine was building a bridge and then trying to drive a tractor over it - a potential disaster waiting to happen! Doing a test run a few inches off the ground is a great idea. While I said the bridge is for ATV's, horses and people my original goal was for a tractor crossing as well. I was going to make a low water crossing for the tractor and a bridge for the rest. If I can find a way to shorten the span I may be able to use the bridge for everything.

 

[EddieWalker]

How long will the span be? Stretching the RR ties across the creek and fastening them to the posts will maximize the amount of stress you put on the posts. I'd say it's the very worse way to go about it unless it's a very short span.

I don't think I'd be comfortable driving a tractor over anything more than 4 feet max with that design. Walking would be fine, but no weight at all.

Your tires will concentrate the weight of the tractor to the RR ties it's riding on. It will have to absord 100% of the weight, multiplied by the distance from its' mounting position.

This means that when the heaviest part of the tractor is dead center of the span, you are putting your tractor and yourself in danger.

If you put two beams across the creek that can handle the load and the span, then put the RR ties across the beams so the are in perpendicular to the beams, you will be tying everything together.

As for putting the RR ties in the ground, I'd also reconsider that too. I've bought, remodeleld and workd on quite a few homes that have used RR ties for landscaping. In every case, they are rotted out where they touch the dirt.

Don't the railroads put their ties on a bed or rock?

With them planted in the moist soil around the creek, your just a matter of time until they rot out. How long depends on your area, and the individual RR tie.

Save yourself allot of work, money and heartache and buy a culvert large enough for your creek, then cover it with dirt and be done with it. Bridges are very complicated and even the experts get it wrong.

Good luck,
Eddie

 

[jwstewar]

One thing I would be concerned about is the weight of the RR ties themselves on the main beams. I've moved a few of these things around and I remember them being pretty heavy. I would think stretching these things across the 2 or 3 main beams and you would be lucky if they held all of the weight of the ties let alone any thing else. This is just my thought and others can take their shots now. /forums/images/graemlins/laugh.gif