Help with simple bridge

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On 10/15/2015 9:00 PM, J wrote: ...

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It's flat plate, obtained from your local ironmonger...make each leg 3 ft was what was intended altho I see as I wrote it, it does sound like meant overall.
Or, of course, you can sister another tubaX the same way or use exterior ply for the splice with a cap to hide the exposed edge.
The metal solution would, with only a little extra effort/expense, let you have the pieces fabricated with an angle so the center could be higher than the ends that could lend a nice aesthetic touch. Need a buddy who can weld or have the metal shop do it for you.
I don't recall now precisely what it ran, but we put a 12-ft 1/2" x 8" plate across the 10-ft span of the barn driveway after we jacked the sag out of the middle of the span when reroofed and repainted/repaired the old barn here a few years ago. That piece was under $200 as I recall new; if the scrap dealer in town had happened to have a piece at the moment I called it could've been much less. He's (the scrap man, that is) is really complaining these days that steel prices are way off, so while I've not priced anything recently, it shouldn't be too bad. Again, go to an iron/metals monger, not the box stores.
And, two 3/8" are probably overkill; two 1/4 or 5/16" would likely suffice as they're constrained from flexing sideways by being bolted to the beams it's the vertical height that is key dimension; same effect as the 2x8 vis a vis the 4x6 for stiffness in the vertical direction; the section is still rectangular x-section so same formula for I.
Actually, of course, you could make the same arch construct with the ply gussets as well. I've never tried it; don't know how well the pressure treated would bond with resorcinol to do a glue laminated joint; you might try to do some research and see if anybody has any suggestions on that. The polyurethanes (Gorilla glue) are waterproof and one part; again I don't know about the durability with PT material. Be interesting to look on the PT manufacturers' trade association web site and see if they have any info on laminating them--I've only done untreated.
And, of course, as somebody (or bodies) have mentioned there's always the manufactured/engineered beams but you may start talking more monies quickly for exterior application...
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On 10/15/2015 01:53 PM, J wrote:

Have you considered 20' I beams? You'd probably get away for under $300 and they wouldn't rot any time soon?
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On 10/15/2015 9:24 PM, rbowman wrote:

From somewhere else, "I have seen some folks use old mobile home frames for bridges. Seems to me it would be cheaper and easier to go that route and be plenty strong." My idea, a shipping container would make a nice covered bridge, or make some reinforced concrete beams, sometimes the county here gives the old bridge beams away when they repair a county bridge. I got one to take the bounce out of a 2nd story floor.
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I just wanted to thank everyone again who responded and post an update. Th anks to the mild weather through December, I found some time to do this pro ject.
Even though the 16' timbers that I ended up using seem marginal in length, that is what I went with for simplicity (easier to acquire and transport - I went with 2x10s). The bridge rests on either end on pressure treated 4x4 s embedded in the ground. They 2x10x16s are spaced ~20" apart (on center) a nd there are five 2x10 cross-members (roughly every 3') for lateral stiffne ss. I reused the deck boards from the original rustic bridge, which were co nveniently held together in 4' lengths by the 2x4 edge railings. I split t he extra 4' length of deck boards into two and used it for ramps on either side of the bridge (supported by 2x4s bolted to the main bridge) so that we can easily ski across (when/if we get enough snow). The new bridge can be viewed here: https://flic.kr/s/aHskriAbFc (some pics are before the additi on of the ramps at each end)
I'm not too worried about the bridge rotting where it comes into contact wi th the ground. There is some gravel there to help out with drainage, and t he 4x4s that were there supporting the original beams (which were just fell ed trees) seem practically as good as new after 5 years. I suspect erosion and undercutting of the banks will cause the demise of this bridge long be fore rot. But I'm hoping that won't happen too soon.
Among the benefits of the new bridge are that it is much more sturdy (much less deflection than the original rustic bridge - in fact, hardly noticeabl e) and that I was able to level it.
I was able to construct the new bridge almost entirely by myself - I had a little help from my 9yo daughter. Though afterwards, my next door neighbor complained about not being asked (invited?) to help. He also said, that h e preferred the "Indiana Jones" feel of the old bridge :-). However, it's a good thing no one got hurt on the old bridge, as it actually fell into th e creek when I was removing the deck boards. I think the pinching together of the 2x4 railings as the old bridge sagged, was actually what was holdin g it up! This past spring before I realized how unsafe the bridge was beco ming, we actually crossed it during a local flood event. The creek was a torrent, with water level about one foot or less below the bottom of the br idge. You could hear the rocks being moved along the bottom of the creek. Had someone fallen in then, they would probably not have survived. I thin k next time, I won't be stupid enough to cross the bridge under those condi tions, even if I think the bridge is perfectly sound.
Cheers everyone, and happy new year.
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On Tuesday, January 5, 2016 at 3:13:05 PM UTC-5, J wrote:

Thanks to the mild weather through December, I found some time to do this p roject.

, that is what I went with for simplicity (easier to acquire and transport - I went with 2x10s). The bridge rests on either end on pressure treated 4 x4s embedded in the ground. They 2x10x16s are spaced ~20" apart (on center) and there are five 2x10 cross-members (roughly every 3') for lateral stiff ness. I reused the deck boards from the original rustic bridge, which were conveniently held together in 4' lengths by the 2x4 edge railings. I split the extra 4' length of deck boards into two and used it for ramps on eithe r side of the bridge (supported by 2x4s bolted to the main bridge) so that we can easily ski across (when/if we get enough snow). The new bridge can be viewed here: https://flic.kr/s/aHskriAbFc (some pics are before the addi tion of the ramps at each end)

with the ground. There is some gravel there to help out with drainage, and the 4x4s that were there supporting the original beams (which were just fe lled trees) seem practically as good as new after 5 years. I suspect erosi on and undercutting of the banks will cause the demise of this bridge long before rot. But I'm hoping that won't happen too soon.
Nice job but I'd like to suggest a minor improvement, if for nothing else b ut longevity.
In the spring, why not extend the beams farther up on to the banks? You cou ld sister some 2 x 10's, maybe 6' long, onto the beams for 2 to 3 feet. That would extend the bridge 3 to 4 feet onto the solid ground on each side. If the bank eroded, the bridge wou ld still be supported.
If nothing else, that would give you time to get some big boulders or somet hing else to rebuild the bank while still having use of the bridge.
That looks like it was a fun project.

h less deflection than the original rustic bridge - in fact, hardly noticea ble) and that I was able to level it.

a little help from my 9yo daughter. Though afterwards, my next door neighb or complained about not being asked (invited?) to help. He also said, that he preferred the "Indiana Jones" feel of the old bridge :-). However, it' s a good thing no one got hurt on the old bridge, as it actually fell into the creek when I was removing the deck boards. I think the pinching togeth er of the 2x4 railings as the old bridge sagged, was actually what was hold ing it up! This past spring before I realized how unsafe the bridge was be coming, we actually crossed it during a local flood event. The creek was a torrent, with water level about one foot or less below the bottom of the bridge. You could hear the rocks being moved along the bottom of the creek . Had someone fallen in then, they would probably not have survived. I th ink next time, I won't be stupid enough to cross the bridge under those con ditions, even if I think the bridge is perfectly sound.

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Looks nice. Is the snow from the old bridge deck too?
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Congratulations.

I can see why you didnt' invite him, or ask him.

What's the difference between a flood event and a flood?

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Looks like it should be good for 10+ years unless the supports wash out. You could put some large stones/boulders along the banks up-river of the support beams to slow down any washout of the banks.
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