Another Bridge Thread

tennesseejb · Dec 21, 2015

I've found a number of great threads on bridges. I've been thinking of a way to build one that just seems very simple, so I'm wondering of there is an aspect or risk I'm not considering. We have a creek near the back of our property that I'd like to be able to get an ATV and tractor across. The span of the creek is about 10 feet and the creek does not rise above the bank (in this spot) when it rains.

It would be difficult at this point to drag a trailer back there to span the creek and single pieces of culvert that wide are pretty pricy. That's not mentioning that I really just like the idea of wood.

What I'm considering is this: Pour concrete pads on both sides of the creek. Then lamniate 2x lumber across the entire width of the bridge. I realize that I could just laminate some beams and then lay a decking across them, so part of what I'm asking is whether there is a structural reason that is better than just running 14 or 16 foot lumber across and gluing/nailing/bolting it all together until it's 8 ft wide. At 1.5" each, that would mean 64 boards... at roughly $15/ea for 2x8x16 or $19/ea for 2x10x16, I'd essentially have a 8 or 10" x 8' slab of wood across the creek, which has me in it for lumber at $1000 - $1250. Seems it would be very simple to construct.

What say the group?

KennyG · Dec 21, 2015

Not sure how much weight you are talking about, but you might not need that much structure. You'll get the cheapest build by using the deepest section possible and limit the number of timbers in the span. In other words think about how many 2x12s you need to support the weight by using beam tables.

Here is my slightly smaller bridge that seems to follow the pattern you are considering. My tractor with attachments maxs out at about 2500 lbs and there is lots of margin with this design.

http://www.tractorbynet.com/forums/projects/289644-my-bridge.html

tennesseejb · Dec 21, 2015

The tractor I have access to right now is a Kubota MX5200... specs show it weighing 3716lbs. I'm guessing that's without implements (or a driver), so we'll have to go up from there. I don't imagine myself getting anything larger than that for this property. In my example, I would use 64 boards all on their side. If I didn't do that, I would need decking, so let's say we used 2x6x16 and cut them in half. Then by my calc I'd need 18 of them at $11/ea or roughly $200. But how many 2xX's would I need in each beam to carry that weight? It looks like in your picture you used pairs of 2x lumber. Is it better to have more beams that are narrower? Or a couple really beefy beams?

Thanks for your quick response!

JB

KennyG · Dec 21, 2015

I doubled mine because I had to clear the existing retaining walls and I wanted to minimize the overall height. I originally wanted to use 2x8s to keep the height down but 2x12s not doubled would have been the best structural approach. So it's a compromise. Doubling doesn't matter as long as you block between the timbers to prevent rotation.

With 5K+ tractor and attachments, I would say design for 8K point load (16K distributed load) and you would be very safe. If you actual span between foundations of 12 feet, with a bending stress allowable of 1000 psi, you get about 1750 lbs distributed capacity from each 2x12. I would say about 9 2x12s would be OK. To get the same capacity with 2x6 you would need about 38 of them. 2x4s would not give you adequate capacity. (You probably want to check my numbers. I did this quickly.)

When I built mine I bought 2x6-8 ft at significantly less than half the cost of 2x6x16. And no cutting. Check the numbers, but I think buying long timber and cutting in half always costs more in the long run.

Luke'sScreenName · Dec 21, 2015

Wouldn't water get between and rot faster?

Egon · Dec 21, 2015

Think about point loads and then do more planning.

Just remember the section modulus figure is a design maker/breaker.

bigdeano · Dec 21, 2015

Here in the pacific northwest I don't think a bridge like that would last very long. Maybe it's different there. Maybe you could cast a couple of concrete beams with rebar near the bottom for strength, then use pressure treated decking. Maybe cheaper than all that wood and wouldn't rot.

tennesseejb · Dec 21, 2015

KennyG - Thanks for the additional input. Just looking online, it seems the 8 footers are about half the 16, but less cutting is indeed better. In your example, if I went with 9 - 2x12's, would you then evenly space them on 12" centers? Or would you double them up and space them on 24"s? Does it matter when considering "point load" which I presume accounts for most of the weight being on one or a few specific spots or beams? In other words, is it better to try and localize the beams under the general path of the tires? Or does the bridge perform the same if the beams are spread evenly?

Luke, you bring up a good point. I have had PT deck boards at the house rot faster than I wanted, and I presume it was because there was no space between them, so more likely that water, and other organic material like leaves, etc gets trapped. If that's true, then it would seem to lend itself to spreading out the beams and also leaving some space between the decking to allow for easy drying of the entire structure. Thoughts?

Egon, I don't know what section modulus figures are, but will go google them. I'm by no means an engineer of any sorts, which is part of the reason I'm here. I can generally teach myself things through research, but when faced with the alternative of having to pull my friends tractor out of a creek, I'll take as much advise as I can get.

Design breaker seems important, so I'm off to google.

Thanks again to everyone!

JB

Luke'sScreenName · Dec 21, 2015

Could you get a smaller equipment trailer in there like a tag along or even a 10ton bumper pull?

tennesseejb · Dec 21, 2015

bigdeano - Longetivity is important... though we plan to build in the back of the property (across the creek) in about 12 years, at which point I'll be looking to put a more permanent/durable bridge in place for the driveway. In that case, I imagine it will be steel and/or concrete, but will also be rolled into the cost of the home.

Casting concrete isn't out of the question, except that I don't have a way to get a concrete truck back to this location, so I would be mixing it all by hand (or technically by generator / small mixer). You bring up an interesting point though... I hadn't considered forming just the beams. I did pour a sidewalk once by hand, so the labor is not tremendously overwhelming, but I don't know how you factor in all the variables with regards to how well I do with the concrete. Is it such that concrete would be so overbuilt for my needs that I couldn't screw it up? I could possibly frame it up in place, and not even have to move them once set. Interesting thought.

Thanks,
JB