# Help with simple bridge

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• posted on October 15, 2015, 7:53 pm
A few years back (~5) I built a rustic bridge over our seasonal stream by l aying deck boards over a couple of tree trunks (5"-6" diameter) that I cut down. The stream channel is about 16' from bank to bank and ~4' deep. I c ut the tree trunks to about 20', so about 2' of contact with the ground on either end.
The bridge was a little bouncy, but otherwise seemed structurally sound - t hat is, until now. I had hoped that by placing the ends of the tree trunks on gravel and some pressure-treated 4x4s it would last for many years, but the trunks have rotted in the middle and begun to crack, and the bridge is no longer safe.
The reason I cut down a couple of trees in the first place, was that even t hough it apparently exists, it's very hard to get ahold of 20' pressure tre ated lumber. I had thought that a couple of 4"x6"x20' beams might work wel l to support the bridge - though I never did any actual load calculations.
I'm not feeling the love for the work involved of cutting another couple of trees every 5 years, so I'm looking for a better - and hopefully simple so lution. I don't really have the energy or skill for something fancy. If I could get the 4"x6"x20' pressure treated beams (and a friend to help move them), and if they would work in this application, I would do that. But fa iling that - is there another way to solve this problem? (like maybe nailin g together some shorter overlapping 2"x6" boards?).
Trying to keep it as simple as possible. Thanks in advance for any ideas.
Photos of the current bridge here: https://www.flickr.com/gp/7865232@N08/91 04G5
-J
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• posted on October 15, 2015, 8:00 pm
On 10/15/2015 12:53 PM, J wrote:

What sort of *traffic* are you trying to accommodate? How accessible is this area? What sort of maximum water flow do you have to accommodate (obviously, "0" is your pictured low end! :> )
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• posted on October 15, 2015, 8:41 pm
On 10/15/2015 3:00 PM, Don Y wrote:

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I did a quick beam deflection calculation for SYP 4x6 on 16-ft spacing with a dead center load of 150 lb plus distributed 3.5 lb/ft. Since that's one beam, the actual load for two would be 300 lb which ought to cover the kind of traffic I'd think this would bear given what it looks like would be from the picture...
Anyway, center max deflection for that loading is roughly 0.5" which indicates it would be more than adequate for the purpose.
As for using something other than 20-footers which undoubtedly will be pricey, a couple 10's with a (say) 3/8" flat plate of 3-ft length bolted thru with 1/2" bolts would work and be reasonably easy to do with only a couple of folks and a temporary bracing to hold the ends up while making the connections. Pre-drill the plates and then bore the holes in the beams in place. Could get a little fancy and have somebody fabricate them with a little arch in 'em and have some rise to the center for some aesthetics, too...
Gets as involved from there as may care to be including adding a center riser with a tension rod/cable from each end to stiffen 'er up, etc., etc., etc., ...
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• posted on October 15, 2015, 8:51 pm
On 10/15/2015 1:41 PM, dpb wrote:

Any lumber in that environment is going to rot, mold, splinter (from freezes) etc. over time. (pressure treated lumber?)
I'd, instead, consider placing some culvert sections in the streambed and covering them with rock and soil. Given the apparent amount of debris in the area, something of relatively large diameter may be in order. And, from time to time, it may be necessary to "unclog" portions. But, not prohibitively difficult (any moreso than something coming downstream and taking out a "bridge"
[Hard to get a grasp on the sort of grade that's involved...]
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• posted on October 15, 2015, 9:00 pm
On 10/15/2015 3:51 PM, Don Y wrote:

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Well, I addressed only the sizing question, but yes (or no :) ) I'd not just place the ends on the ground, I'd pour a small pier or similar to set the ends upon...and, if did that, could more than likely with a little extra landscaping make 16's work alleviating the joint or extra-pricey beams.
I'd think it a "feature" as opposed to the maintenance headache of the culvert (and it's at least a possibility he can't fill that contour, anyway, given all the restrictions on waterways and all any more.
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• posted on October 15, 2015, 10:22 pm
On 10/15/2015 2:00 PM, dpb wrote:

Dunno. My uncle had a "brook" (no idea as to the technical definitions of brook, stream, river, etc.) separating his property from neighbor.
Neighbor's property was slightly "uphill" from uncle's. And, brook ran at a slight grade between the two.
He fabricated a bridge spanning the brook (I doubt it was more than 12? ft long -- hard to get an accurate picture of childhood memories where everything always seems so much bigger!)
Brook was not deep -- wade across it with pant legs rolled up. And, only a couple of feet below the bridge (at the "low" side of the bridge).
But, the bridge kept needing work. Soil erosion on the banks, frost heave, "weathering" from the sun/elements, railings getting wobbly, nail pops from the planking, periodic "stain" (wood preservative) etc.
By contrast, the culvert 20 feet "up river" (up brook??) never saw *any* maintenance. I.e., the *easier* task would have been to just drop a length of culvert in place of the bridge (but, it wouldn't look as "quaint")
A similar situation existed a few doors up from my childhood home. Eventually, the "brook" was just routed underground for a few blocks. End of the maintenance issues.
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• posted on October 16, 2015, 1:07 am
On Thu, 15 Oct 2015 13:51:38 -0700, Don Y

Depending on a lot of factors, putting in a culvert in some jurisdictions could create a real legal quagmire. Doing construction in a stream bed, such as for a culvert, can have EPA enforcement implications. Even a bridge could be an issue if the right "authorities" were to get wind of it.
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• posted on October 16, 2015, 1:29 am

The EPA wants to expand its power over waterways. From the Wall Street Journal: http://alturl.com/3z45w
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• posted on October 15, 2015, 11:07 pm

Would some sort of pre made floor truss be at all practical?
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• posted on October 15, 2015, 11:53 pm
On 10/15/2015 6:07 PM, Dean Hoffman wrote:

Standard sizes would likely be quite a bit taller and sorta' show their provenance which might not be desirable. I've no idea re: cost--have never actually used one meself, only bought the one house in TN already in final close-in that used 'em so never had any reason to...
Of course, one could substitute height for thickness on the 4x in a cube power ratio of additional height to reduction in thickness and maintain the same vertical stiffness. That is, I ~ bh^3 so to keep same deflection one needs to raise the height much less than decrease the distance to keep same overall in the vertical direction.
Well, let's just see numerically
4x6 --> 3.5*5.5^3 = 582 2x8 --> 1.5*7.5^3 = 632
so, in fact, the 2x8 would be ~10% stiffer to the vertical load than the 4x6 but more "wobbly" side-to-side w/o any cross bracing.
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• posted on October 15, 2015, 11:17 pm
On 10/15/2015 3:53 PM, J wrote:

Put in a pair of treated 4x4 support posts in the center.
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• posted on October 15, 2015, 11:51 pm

For a span of that length, you really need depth to reduce deflection.
If you can find them, 20 foot pressure-treated 2x12's spaced 12" apart should span your stream nicely. I would add solid blocking every 4 feet or so to add some lateral stability and prevent the joists from twisting.
If you can't find 20' PT 2x12's, you could probably build box beams with PT 2x4's and PT plywood.
Real lumber yards can usually get longer boards than the home centers.
Anthony Watson www.mountainsoftware.com www.watsondiy.com
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• posted on October 16, 2015, 12:40 am
On 10/15/2015 6:51 PM, HerHusband wrote: ...

Yeah, see previous note above--

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Ya' think!!!!??? :)
Presuming 3-ft wide deck, that'd be 4 2x12 -- compared to the calc's above that would support roughly 4500 lb mid-span w/ <1/2" deflection. I don't think there's the intent to be able to drive a car over it... :)
For a pedestrian bridge of minimal loading a couple 2x8 will be sufficient load-wise; some cross bracing likely would be desirable to cut down lateral sway. A third would surely be a major improvement if OP wasn't trying to be terribly penurious.
Those were, as noted in the other response, based on an actual span of 16-ft, not 20 as the pictures made it pretty clear the extra length was simply supporting the ends of the logs on the banks and all needed for a bridge would be some piers at the edges at about 16-ft or perhaps even closer if went down the bank just a little and left a foot or so overhang each end...
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• posted on October 16, 2015, 5:18 am

Hey, you can never be too careful... :)
The original poster mentioned the current bridge was bouncy. I don't think the usage was mentioned. If it's just for one or two people to walk across, yeah, you could downsize the joists and/or spacing. If you need to drive a couple of quads across the bridge, or push a fully loaded wheelbarrow, better to play it safe.
You never know if the bride's entire wedding party is going to pose for pictures on your new bridge. That or a bunch of drunk college kids. :)

I didn't take the time to calculate loads or look up spans in my charts. I just used an online joist calculator to do a quick estimate. I went with 20 foot joists so there could be a foot of bearing on each end, and an 18 foot free span. That would give a little extra on each end so the footings aren't so close the edge of the stream.
Anthony Watson www.mountainsoftware.com www.watsondiy.com
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• posted on October 16, 2015, 1:10 am
On 10/15/2015 3:53 PM, J wrote:

I see that Dean H suggested a truss. I was thinking the same thing, made from PT or other weather resistant wood.
Before you determine the length, you have to do an environmental impact study, get permits from the wildlife commission so a civil engineer can determine the height and span needed.
I bet you can get the bridge done for under \$300,000 if you go with a modest desigh.
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• posted on October 16, 2015, 2:00 am
Well, I think the culvert idea is a non-starter. Right now there are just puddles in the stream (it's been pretty dry lately), but in June we had a 4 " rain event and the water was less than a foot from the bottom of the brid ge, and incredibly intense - jumped the banks at many parts of the stream - though where the bridge is, it's nice and deep. Heck, even the road got p artially washed out and that has a huge culvert running under it. There's generally some pretty amazing water in the spring, moving some pretty big r ocks along with it - which I think would also destroy any mid-span support that I might be able to place.
I'm not sure I'd be able to really get away with a 16' span - at least not without some really major effort to prevent the banks from collapsing where I'd have to put the pier. I don't think I'd be able to pour a pier at all - I think at best, I would just get some pre-cast concrete footers like th ey make for decks.
It sounded like 2x8s might have better vertical support than 4x6s - and the y'd be about 40% lighter. Though, again, getting the 20' length is difficu lt. I'll want to figure out my plan before getting a lumber yard to procur e this for me. Would they really bother helping me with this for just two or 3 boards?
dpb suggested a 3' flat plate to hold two 10' pieces together at the middle . I'm not sure exactly what you mean by that - searching HD and Lowes I on ly saw long narrow metal bars. What do others think of this solution? And if it might work, could someone point me at the appropriate product?
Thanks.
-J
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• posted on October 16, 2015, 5:49 am

Even if you didn't have to worry about environmental rules around a stream, putting any kind of structure "in" the stream seems like a bad idea.

For some reason I was thinking a concrete beam/block on each end, something like 8"x12"x36". That's about five 60# bags of concrete per beam. Not too bad if you can get the bags to the site. I would add a couple pieces of rebar in each block for extra strength before pouring the concrete.

I had no trouble getting 24' 2x12's for the ceiling joists in our garage or the rafters in our house. Of course, those were standard framing lumber, not pressure treated. That might make a difference.
You could always use regular framing lumber and apply a good coat of deck stain on all surfaces. Then add flashing to the top surfaces before installing the decking. Just make sure the ends rest on pressure treated lumber and not directly on the ground/blocks.

The lumber yards I've dealt have been happy to sell me a single board or box of screws.
Getting 20' boards home will be the real challenge. Mine were delivered as part of a much larger order. Maybe you could rent a U-haul truck or something.

I don't know if a 3' plate would be sufficient over that span, especially if the joint was in the middle.
I mentioned a box beam earlier, but you could probably build a laminated beam of sorts. Sister a couple of 2x joists together, staggering the seams, with a good dose of construction adhesive between them. Then drive in a couple rows of lag bolts every 12" to hold everything together.
Having full length joists would be safer, cheaper, and easier.
Anthony Watson www.mountainsoftware.com www.watsondiy.com
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• posted on October 16, 2015, 1:16 pm
For the record, yes, the bridge is for light pedestrian (and XC skier) use. This is just a winding path through the woods. I think I cut the deck boa rds to 30", as 3' just seemed a little too wide and imposing for the settin g. If you looked at the pictures, you can see there's no railing either.

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I've never actually poured concrete before. I probably could get materials to the site. If I remove the mower deck from my riding mower, it can get there. And I can transport stuff (up to a couple of hundred pounds at a ti me) in a little trailer that I can tow behind it. However, I'm hoping I ca n get away with pre-fab 12x12x8 concrete deck blocks they have at Lowes.

or

Great ideas, but too much work for this project at this time.

Oh, I don't doubt they'd be happy to sell me anything they have - just wond ering if it would be worth their while to special order such a small quanti ty of something like this that they don't normally stock. Even the real l umber yard in our area didn't have 20' pressure treated boards on hand (at least not the 4x6 size that I was originally inquiring about)

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I previously transported a 16' 4x6 beam (along with some other lumber) on t he roof rack of my car (subaru outback at the time, though now I have a RAV -4). I was hoping I could do the same for a couple of 20' boards (2x8). T hey should weight less than a 4x6x16' - about 90b each according to a table I found online. I'm sure the weight wouldn't be a problem, especially if I did one at a time, and the length shouldn't be an issue either, as our c anoe is nearly that long. I wouldn't have to drive very fast. About a 10 mile drive, no big roads.
Would a 2x8x20' board be fine for transport if lain flat on two crossbars a bout 4' apart (as in the ascii art below)?
--------+--+--------
Finally, how does one actually calculate the load bearing capacity of diffe rent lumber size options (including the weight of the bridge slats)?
Thanks.
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• posted on October 16, 2015, 3:44 pm

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 www.mountainsoftware.com www.watsondiy.com
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• posted on October 17, 2015, 1:12 am

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 www.mountainsoftware.com www.watsondiy.com