Help with simple bridge

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...

Reply to
dpb
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That's kind of what I figured, but tried to plan for worst case.

The premix bagged stuff is pretty easy to work with, but getting water to the site to mix it up might be problematic.

You may end up with some settling over time, but it will probably work if you use a block under each joist (3-4 on each end of the bridge). If I was going that route I would probably dig out an area 6" deep, 16" wide, and the width of the bridge. Then I'd fill it with 5/8- gravel and tamp it down good before placing the deck blocks. It would provide a decent base for the blocks and provide some drainage underneath.

You could probably skip the deck stain if you're in a pinch, and just cover the tops of the joists with that rubberized flashing they use to flash windows. You can find rolls of it at Lowes/Depot in the window and door area.

It probably won't last as long as pressure treated lumber, but the bridge will be up in the air and not contacting the ground (except at the ends), so it should hold up several years at least.

Of course, pressure treated would be the better option if you can get it.

If they don't have it in stock, they may have minimum quantities for special orders. Only way to know is to ask.

I would think PT 2X material would be easier to find in long lengths than

4X beams.

Pressure treated lumber can be quite heavy, as it's still green and pumped full of chemicals. I wouldn't want 20' boards strapped to the roof of my car, but then I drive a tiny VW Rabbit. :)

I have an old carpentry book that has tables that tell the load capacities for various joists and beams for a variety of spans and spacings.

Basically you start with the load you expect to carry, 40 pounds per sq/ft is typical for residential floors, then multiply that by the span. For example: 40 pounds x 18ft span = 720 pounds (assumes 40 pounds on every foot of the beam). Then you find a joist/beam in the chart that can carry that load over that span. It gets a bit more complicated with wood species, deflection limits, joist spacing, etc., but that's the simplistic description.

There are also a variety of calculators online that can give you ballpark figures too.

Anthony Watson

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Reply to
HerHusband

We went for a hike today and saw several small bridges on the trails that reminded me of your situation. They just had pressure treated timbers (6x6 I think) semi-buried in the ground on each end and the bridges rested on those (no concrete piers or anything). They just move up and down with the frosts, no biggy.

I am not completely sure, but it looked like they had just used standard lumber for the bridge itself, not pressure treated.

These are in a city park used by thousands of people every day.

I would opt for the 2x10's. It would only add $28 to your total bill and the extra stiffness would be noticeable for sure.

I've seen 8's, 10's, 12's, 16's, 20's, and 24's.

Can't say I've seen 14's or 18's.

Good luck!

Anthony Watson

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Reply to
HerHusband

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:

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(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.

Reply to
J

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.

Reply to
hrhofmann

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:

formatting link
(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.

Reply to
DerbyDad03

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

Reply to
Ed Pawlowski

Congratulations.

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

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

Reply to
Micky

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