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

tennesseejb · Dec 21, 2015

Luke, I have pulled a couple trailers back there behind my quad, and I did take my truck back there a few times early on, but may need to clear a little on the path to get it back there again. I would plan on pulling the lumber on a trailer, either behind the tractor or my truck, even if I had to make a few runs. Is that what you are talking about, or do you have another idea?

bigdeano · Dec 21, 2015

You should be able to find info online how to make concrete beams. I built a house with tip up concrete panels I cast in the yard. I found the info on how much rebar, thickness, etc in the building code book. Was an interesting project.

Could you cast them on level ground where a cement truck can reach and then drag them with a vehicle?

tennesseejb · Dec 21, 2015

I'd be hesitant to do that. The trail to the creek is about 1/2 mile and includes some muddy and some rocky areas....

KennyG · Dec 21, 2015

The beams could be evenly spaced. The 2x decking will distribute the load. An alternative would be to double or closer space under the wheel path. That would add a little strength margin. There is no real need to double the beams, as long as you block them, as I said earlier.

As an engineer, I would hesitate to use cast concrete beams, unless you had assistance from someone experienced. I wouldn't do it on my own. The problem is that beams by nature get strength partly from tensile stress and concrete has poor tensile strength and is subject to sudden fracture. The reinforcing adds the tensile stress but it's placement is critical. No offense to bigdeano but the reinforcement placement and concrete cure process seems to be much more critical for a beam than a wall panel. I like wood because it's so forgiving and balanced in compressive and tensile strength. Just my personal thoughts.

ovrszd · Dec 21, 2015

For the kind of money you seem willing to spend, why don't you look for a flatbed off a truck? Pour your piers, drag it on, push dirt up against each end, done, bridge.....

tungularafishcamp · Dec 21, 2015

I have one of those 500$ cement mixers that are 3 pt mounted. Not the best built unit and only does half a yard at a time but it is a whole lot easier than by hand, just back up to your sand gravel pile throw in a few 5 gal buckets a bucket of cement squirt a little water mixing it while you drive to the dump site back right up n dump. It has held up for the foundation of a 28x50' building and lotsa other small jobs over the last 4 years or so. I have the blessing of all the free sand and gravel I could ever want and no chance of ever getting a cement truck here so it was an ez fix for me.

teg · Dec 21, 2015

tennesseejb said:
I have pulled a couple trailers back there... ...or do you have another idea?

He might be thinking of using a 16 foot trailer as a bridge... (just guessing).

Trials · Dec 21, 2015

You really should price out 2 short lengths of large diameter culvert before you invest a whole bunch of money in P.T. wood. Culverts have the arch shape needed to transfer heavy weight more effectively then a straight span, and they will outlive you. For backfill you can use natural available rock material. They also roll real easy, so you can get them back into the woods with no problem. Plastic and steel are almost the same price, the plastic is a little nicer to work with imho. I think you might be pleasantly pleased with the cost once you have priced out the total construction cost using P.T. wood, fasteners and concrete.

RedNeckRacin · Dec 21, 2015

tennesseejb said:
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?

I think you can buy culvert pipe for alot cheaper than your cost estimate. I just bought some 15" hdpe pipe thus summer for about 140 for a 20' stick.

newbury · Dec 21, 2015

tennesseejb said:
Luke, I have pulled a couple trailers back there behind my quad, and I did take my truck back there a few times early on, but may need to clear a little on the path to get it back there again. I would plan on pulling the lumber on a trailer, either behind the tractor or my truck, even if I had to make a few runs. Is that what you are talking about, or do you have another idea?

If you can drag a trailer behind your truck just use a trailer.
If the span of the creek is only 10' you should be able to buy an old equipment trailer with bald tires for well less than $1500 and drag it back there. I frequently see CL listings for equipment trailers that need tires going for around $1,200 in NE Mississippi and Memphis.