I've built a fair number of projects like this. Here are my opinions if you care to use them at your own risk, etc.

1) Presuming a 12 foot span, 4x4's are NOT strong enough. 6x6's might work, but I would strongly recommend using 2x12's on 12 inch centers evenly spaced across the entire width. The result will be much stronger. Don't forget, the bridge must hold up not only you and your ATV but also its own weight including the decking material. The bridge alone will likely weigh several hundred pounds before you're done.

2) At each end, use an appropriate length of 2x12 as an end cap.

3) Every couple of feet along the length, insert braces to help keep the 2x12's straight. Twelve feet is a big span and those 2x12's will want to twist over time.

4) Deck the result with appropriate decking material. I prefer composite like Trex because it will never need maintenance.

5) Do put 4x4's along each side on the top to act as guard rails.

6) Where the ends will touch the ground, install some form of pressure treated lumber. Do not let the 2x12's touch the ground.

7) Yes, you should stake the ends to the ground. Get some pieces of 3/8 or 1/2 rebar, 3-4 feet long each, drill appropriate holes in each end (in the PT lumber), and drive the rebar through the PT lumber straight into the ground with a sledgehammer. Two or three such stakes, going down 3-4 feet, will keep that end from ever moving.

8) Use screws everywhere. Do not use nails. Use galvanized, stainless, or decking screws so they don't corrode. Drill pilot holes so the wood doesn't split.

These steps will require a bit more time and money, but when you're done you'll have a bridge that will last for 10-20 years, won't sag and give you the creeps when you cross it, and remain safe for its entire useful life. Anything less and you're gambling with your life and your equipment - and perhaps your liability insurance if someone else gets hurt with or without your permission.

Which reminds me - post "No Trespassing" signs at each end, too.

 

Might as well throw my 2 cents in here. [img]i/expressions/face-icon-small-happy.gif[/img]

I'm digging a pond with an island and have been thinking about the kind of bridge I want. I think the best look in my case is to have a curved bridge. I plan to use a hydraulic jack, chain, and "time" to bend some 4x4's to a decent curve. Then set concrete piers in the ground on each end and fasten the 4x4's. It should hold ungodly amounts of weight! Once the wood is bent, it won't break before the piers sink in the ground.... That's for sure! But I'm only planning on walking across mine.

 

I'm with WAATV. I'm a General Contractor and have built lots of bridges. 4x4's = no way will it work. 6x6 is possible but you might as well use 4x6.

Go with 2x12x 16' spaced 12" OC. All pressure treated is better but it will work as long as the ground contace points are PT lumber. I don't think that composite decking is the best choice - aside from being more expensive, composite will bend more under load and will be more slippery. I'd just go with 5/4 PT decking.

Don't be shocked at the price of lumber. Unless you have some used lumber around, you're not going to build a 16' bridge for $120. More like three times that figure.

Jaybee

 

<div class="FTQUOTE"><begin quote>Originally posted by: jaybeecon55

Don't be shocked at the price of lumber. Unless you have some used lumber around, you're not going to build a 16' bridge for $120. More like three times that figure.



Jaybee</end quote></div>

I've also been told that the power company is charging for poles now. It use to be that if you called them they'd drop off any old or broken poles when they were in your area. I've had dozens dropped off over the years. I use them for corner posts and for bridging creeks and ditches. I've heard that they're not doing that anymore and that they're selling them for fuel purposes to cement and power plants.

I know that I use to be able to get free discarded railroad ties from the railroad. All I cared to haul. I used them as bridge decking over telephone poles when I built bridges, for fence posts and for various retaining walls and such. Now I understand that now the railroads are also selling them for fuel purposes. They chip and shred them to burn at power generating stations and cement factories too.

It's a shame because I could build a bridge strong enough to drive a tractor and a loaded grain wagon across with telephone poles and railroad ties. The only costs were for bolts to hold everything together.

 

Around here they charge by the foot for poles, not sure how much but it's cheaper (and stronger) than buying dimensional lumber. The government dismantled the railway in Newfoundland back in the late 80s/early 90s, there were lots of ties to be had back then but they're all used up now.

 

Just an update, the banks seemed fine, but after a good rain and re-examining the situation, I thought better of building a bridge. The banks are to sketchy. One day it would be fine, the next would be a death trap. I don't want that on my head so I am just gonna continue riding down the road to the trails. It hasn't been a problem yet, so. Have lots of lumber now though, LOL. I am sure I will find a way to use it. Your right, cost me $298.00 for the lumber, I have a freind who works at lowes, and that was with his discount. Would have been well worth the money if the ground was permanately sound.

 

Ahhh, don't bother with the bridge. Just make a ramp on either side and jump it!!!!! [img]i/expressions/face-icon-small-happy.gif[/img]

Jaybee

 

You can still build it, you just need to have the bridge extend over the bank 3 or 4 feet, the bank isn't going to cave that much over night. if the water isn't too deep and the water level doesn't fluctuate much you could even dig the bank down on either side so that the span isn't as wide and the bridge isn't as high above the water, just don't shovel the dirt into the ditch.

Don't give up now man, there is some way you can do it.