Building a bridge over creek…..NEED ADVICE!
Need some bridge advice..
A creek runs through our backyard and I need to replace the rotted bridge.
Current span (of old bridge) is 24′ end to end. I attached pic, but it looks way better here than it actually is. I get nervous when our 3 year old runs across that it might collapse. So, I need to replace for safety concerns. Also, there is a center support here but it is a poor attempt at stability from the previous home owner. I’d like to not have any center support.
I like the arch of this version, but am not as concerned with that asthetic, more with a safe and sturdy replacement. My question is: Can I bolt (and glue if necessary) a series of stagered 2x10s together to make a 3-ply, 24′ beam? I would build two of these, and the bridge will only be 4′ wide and will not carry anything other than people. (no tractors, horses, or any machinery of any kind.) It’s on private property so trffic on bridge is EXTREMELY light so the load will never be more than a few people at any given time.
I live in Portland, OR so will probably use pressure treated due to wet weather.
Thoughts or recomendations? I also considered “I” or square steel beams, but getting those into position might be too challenging…same for log poles due to weight.
Thanks for the advice.
Replies
Steel would be my first choice. I will be heavier that a triple 2x10x24 but i dont see how it would be more complicated to manuver into place.
last project i did (similar to your bridge) a few years ago spand closer to 30', i had the space for some piers to break my span down to 18' and we used power line poles. they were awkward to place but made for a very stable bridge.
problem with the triple beam would be long term exposure to the weather out there and a much faster propensity to rot, and it would not be nearly as sturdy or safe as the other options.
I would consider exterior grade glue-lams.
hire a P.E.
DIY_Dad wrote:
My question is: Can I bolt (and glue if necessary) a series of stagered 2x10s together to make a 3-ply, 24' beam?
That would be a great question for a professional engineer. My recommendation is that you hire one for your project. They may be able to design a method of construction that allows you to handle lighter components that you can build yourself without heavy equipment. This may mean site built glue laminations or trusses.
I would start with your lumber yard. We have been able to have floor trusses made from treated material. 24' is an easy span that way.
Also you could have them incorporate the railing!
Also you could have them incorporate the railing.
If you incorporate triangles in the railing, similar to a truss, you will lighten your bridge quite a bit while giving it much more strength.
Through bolt all the joints with hot dipped galvanized (ACQ/CaC rated) and it should last quite a while
Could you both elaborate...
Thanks for this great advice. Could you let me know what you mean by "incorporate the railings"? Also, I am not sure I know what you mean by the triangles.
THanks
not knowing what is meant by 'incorporate railings' and 'what are triangles' is a little scary, meaning you have zero basic structural knowledge.
however, there is likely something similar to what you want on the net, never to late to learn about trusses.
Search for "simple truss design" and you will get a tutorial on the basics, and if you throw in '24 feet', you will likely get some examples for what yu want.
PS: railings : design the truss so the railing is the top chord of the truss
triangles: they aint parallelograms, meaning if you push on one corner the shape wont change, which allows the 3 sides to be in tension or compressin and form elements of any truss.
calling me out
Thanks for the explaination junkhound. I realized I wrote my previous post to suggest I didn't understand the concept. Quite the contrary. I was just looking for more suggestions as opposed to an intro to it.
I had not considered incorporaating the railing into the structure, so I think that will solve a lot of elements. Wife had an example she wanted me to build, so I was stuck in that mode. HOWEVER, I am going to revise and build the truss design.
THanks for taking the time to help.
Yes, your choices for your main supporting structure are a beam or a truss (or, if you want to get more exotic, a suspension bridge).
A beam is what you have now -- simply a piece of wood (or metal) that is stiff enough to span the distance between supports and carry the weight.
A truss is a series of triangular elements.
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(Pretty much the same as a roof truss.)
What might work for you is a compromise -- a beam that is a hair on the weak side with trusses as the railings to stiffen it. This approach uses the beam as the bottom element of the truss triangles, and the bottom of a truss is the tricky part for a wood structure, since the bottom "chord" is in tension, while the other elements are generally in compression.
I built this trestle for the G gauge train in my living room and it is actually a pretty light structure. The bottom chords are 1x2 and the top chord is 3/4x3/4 red oak.
I weigh 200 pounds and I can hang on it.
http://gfretwell.com/ftp/Trestle.jpg
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First thing you neeed to do is ignore Deadnuts.
A 2x10 spanning 24' is not gonna work. Too long & thin.
Is there much water flow under the bridge? If there is, putting a post in the middle of the creek is a bad idea. Brush that gets washed downstream will get hung up on it. Then more gets hung up on that, etc.
My suggestion would be to pour some concrete abutments on the banks. Make the tops of them a little above ground level. And use them to shorten up the span a little. Then use treated 2x12s for beams to span the distance.
That would give you plenty of room under the bridge for stuff to wash under it and not hang up. Only downside is that you'd need steps (Or a ramp) on each end to get up on the bridge.
Whatever you do, would you come back and post pics? We always like to see how things turn out.
BossHog's post is proof positive (once again) that the small farmer is succumbing to the Darwinian principle of natural selection.
Glad you agree that natural selection is working in following the cognet advice of a small farmer (and professional truss designer) is the way to go,
versus the inane 'hire an engineer' for any trivial job ?
Being honest enough to ask the right questions right off means the OP has enough smarts to educate himself sufficiently to design a 24 ft truss across a creek.
Matt alert - may be a mni flame war brewing ? <G>
In the old days, they could last for weeks, probably flame out in a few hours now?
nice try
"[P]rofessional truss designer"? Wow; that's a great euphemism. Right up there with "sanitation engineer (trash collector) and "maintenance engineer" (janitor). LIke the late, great George Carlin used to say, "people use extra words when they want to sound important". Your referenced job desscription is unimportant.
Bosshog's either a P.E., or he's not. I believe (and have found) that any P.E. worth his salt wouldn't be calling out a specific structural solution in this forum beyond referencing prescriptive code. In the case that Bosshog is not a P.E., then his advice (like all other "professionals" in this forum) is limited to prescriptive code. Hower, if you have an example of truss design in the presciptive code then, by all means, please point it out. Otherwise, you'd be wise to hire a professional engineer.
BTW, Dan, no need to limit one's thinking to the notion that trusses are defined simply as a series of triangular structures. If we do, then it would never occur to us to make use of vierendeel trusses.
Mark 1-22
"The congregation was surprised at his sermon because he spoke as an authority, and didn't try to prove his points by quoting others- quite unlike what they are used to hearing."
Well done! Good form, stay there mark122.
go to another thread where there is black gunk behind a closet wall that smokes when gouged with a chisel.
Maybe the OP there needs some advice on what type of chemist (or would it be a molecular biologist) to hire to analyze the substance to make sure the entire town does not die form a chemical attack?
BTW, the dictionalry definition of "professional' means that one gets paid for doing a certain job well, not only that some gov. agency has 'blessed' them for passing a simple* test and PAYING AN EXHORBITANT YEARLY FEE.
* well, simple for competent folks, probably not for someone who would need to hire somone and cannot do wl^2/8=sigma*b*h^2/6 in their head for the bending components.
BTW2 - didja know that oriville and wilbur never had a PE? Not even a degree?
BTW3 - do you ever get on an airplane? - didja know most of the commercial aircraft wing structural designers do NOT have PE's? ya better quit flying....
just trying to fan the flames, put a little life back into the old place, give Matt some inspiration and resources ?
What got your panties in a bunch?
If you're not mature enough to handle it when someone disagrees with you, maybe you shouldn't be on this forum.
Not gonna frive a truck across it
Ok, why not use a 24 foot 2 x12 vs a 2 x10. Easily done. To beef about deadnuts and 2 x 10s ia a bit silly, no one is driving a truck across that bridge. Get your tables out. look at snow loads for example. (2 x 12s on 16 oc spaning 16 feet are good for 100 pounds snow load! That is 100 pounds on each linear foot. Compare that to one three year old in the middle of that span. Ok, two 200 pound adults.
Then double up 24 foot 2 x 12s, one pair on each side for say a 3 foot wide bridge. Heck double up three pairs. This is not rocket science and no railroad is crossing that bridge. ANd I don't think that creek even needs a 24 foot span.
My goodness. Hire NASA
Comparing bridge beams to rafters isn't much of a comparison.
People don't walk on their roofs every day. They don't drive lawn mowers or ATVs on their roofs.
The rafters are inside and out of the weather. Bridge beams aren't. Home made bridges tend to get left up long after they should be replaced.
So I stand by what I posted earlier.
Of course!
Absolutely! I am sure you feel comfortible hiring an engineer to design screws for you when you last put up the towel rods in your bathroom.
Good Lord. If you don't think 3 doubled up 2 x 12 can handle a lawn mower ... well ... what can I say. Get out in the real world perhaps.
You do realize this is a discussion board, don't you? People are allowed to disagree with you.
Free to disagree/
Sure, I understand that. Do you? I realize you are free to hire an engineeer to design screws for your next towel rod install and it sounds like that is exactly what you will do. I realize that is your right.
But do you understand that BossHog has spent most of his life designing trusses? I'll bet on his understanding of loads before your seat of the pants guess.
But ...
for the love of pete. Have you not stood on a floor constructed with 2 x 10s on 16 OC? And if so you don't think triple 2x 24s DOUBLED up would support a small child? Support a Piano, for that matter? 2x10s spaning 16 feet have no problem with a paino.
There are times and places for Engineers, but the hestaria on these borads .. THE OH MY G!, hire an engineer!!! Just crazy.
Oh, and you are free to disagree as am I. Do you grant me that right ... No? Probably not, you just demand that for yourself.
I stated my case calmly and resonably, and explained why I felt the way I do. You're the one who's having a hissy fit.
This was fun to do - there's a thread on here somewhere - early 2008. Built it in my yard; moved it with a ramp truck.]
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Forrest - used to play here
Great thread
That was a great thread Forrest. Did it dumped during the 'improvements' here?
Have you checked on the bridge lately to see how it's weathering?
Oh wai!. I found it!
http://forums.finehomebuilding.com/breaktime/photo-gallery/rainbow-bridge
All the photos are gone. SMH
truss type bridge
A typical "bridge" like this would be the type that connects the shore to a floating dock at a marina or private dock. You might contact a local dock contractor or supplier in the Portland area...that's a relativley short span for them..it would be light weight and weather resistant.
Your thoughts about using multiple 2x10's is an effort to make sort of a homeade glue lam beam. That works OK as long as the 2x10's are full length...as soon as you start butting shorter pieces together, you cut the stregth significantly. For instance, if you used 3 joists and saggered the butt joints 4-5' apart from each other, it would have the approximate streght of 2 of the joists...In addition, the problem with relatively sort beams is often not strenght, but deflection...it bounces to the point of failure. The taller the beam, the more resistant it is to deflection. 2x12's much better then 2x10's
The truss bridges shown above will not work as easily for a pedestrian bridge when trying to use the top chord for a handrail. To make the truss work effectively, the top chord must be supported laterally - side to side. The train birdges shown have the upper portions of the trusses supporting each other by members conenction them overhead. Often times on pedestrian bridges of this type, there members under the deck extending outward so a diagonal brace can be connected between them and the top chord of the railing.
There is a design using 2x4 to create a 30' arch (see attached picture of basic structure built in driveway to be dis-assembled and reconstructed over stream )- certainly a long span using a minimum amount of material.
If it were me, I would keep an eye out on Criagslist (pretty active in the Portland area) for a couple of longer reclaimed glulams
Can you tell us the load capacity of this particular structure ?
The real question for you is did you contact an automotive engineer and get his stamp of approval to use that truck to haul a bridge?
DW has an engineer do everything around the house <G>