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Criticism of small bridge design welcome.

  1. Jan 8, 2013 #1
    Criticism of small "bridge" design welcome.

    I live near a park that has two large hiking areas divided by a stream. If those areas were linked by a bridge the number of possible hikes that could be taken would be increased. The stream at one "crossing" is about 30 feet wide and normally has a small flow of but always enough water so you can't cross without getting wet (sometimes stones get lined up but higher water moves them. I'm guessing that at times water up to two to three feet high with flows of say 10 miles an hour can occur. Below is a scan of what I think is a simple and relatively inexpensive bridge design. How might you improve this design or is it fundamentally flawed?

    Thanks for any help!
     

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  2. jcsd
  3. Jan 8, 2013 #2

    Simon Bridge

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    Re: Criticism of small "bridge" design welcome.

    I suspect that getting a bridge approved will be more difficult than building one.

    For your design - numbering the planks left-to-right - I'd put planks 2 and 4 on top of plank 3 to keep all planks level. If the high-water flow can move stones around you want to make sure that peak flow won't move the tire+concrete+pole - remembering that debris will pile against them.

    Some people may have concerns about the tires breaking down in the water over a long time ... I wouldn't know how important those concerns would be compared with general levels of pollution, in the area, from other sources.

    My personal thoughts are that, in many places, when you build something for general use, and someone gets injured using it, you find yourself under criticism. Look through your design for avoidable ways people could get hurt ... like: you'll want to cut the pipe so the ends sit below the hole - paint the ends of the raised planks (1 and 3) white (or with danger stripes), stuff like that.

    Lastly - have you compared with other building methods?

    Aside:
    NZ has a lot of bridges or raised walkways in the context you are looking at - the usual approach is to use treated pine or macrocarpa. Poles are set in concrete and lowered into holes bored in the stream-bed... two for each span. I've only seen the tire+steel-pole approach for holding up volley-ball nets.
     
  4. Jan 9, 2013 #3

    AlephZero

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    Re: Criticism of small "bridge" design welcome.

    What Simon Bridge said. You would probably have a lot less bother if a 50 foot tall tree "accidentally" fell across the stream, than getting permission to build your desigm.

    If your design isn't securely fixed to the stream bed (not just resting on it), there is no guarantee it will still be there after the first flood of water. A tire full of concrete only has the same density as a rock, so it's just as easy for the water to move it.

    The current could also wash away the stream bed from underneath your tires (unless it is solid rock) which would soon make the bridge in unstable.
     
  5. Jan 10, 2013 #4
    Re: Criticism of small "bridge" design welcome.

    As this is a stream were dealing with you will probably also have to have a minimal depth to sink your uprights into. Tires filled with concrete by itself will not prevent stream flow from undercutting under the tires
     
  6. Jan 11, 2013 #5

    Bobbywhy

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    Re: Criticism of small "bridge" design welcome.

    Spinnor,
    steel pipes rust
    bicycles (and people) slip off a narrow plank and fall into the stream
    wooden planks become more slippery when wet
    And, the liability issue already raised by Simon Bridge above
    Rather than trying to "reinvent the wheel" why not research how others have solved this?
    Cheers,
    Bobbywhy
     
  7. Jan 11, 2013 #6

    Simon Bridge

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    Re: Criticism of small "bridge" design welcome.

    @Spinnor: a little taste of a scientific working environment... you comes up with an idea that seems quite all right and then you talk about it and everyone, everyone, dumps on it.

    But I think you can also see the benefit of this process - we should have given you something to chew on here and a more robust way forward should present itself.
     
  8. Jan 11, 2013 #7

    Bobbywhy

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    Re: Criticism of small "bridge" design welcome.

    @Spinnor: OK, so Simon Bridge (pun intended) recommends members here give “something to chew on here and a more robust way forward” to solve your request “If those areas were linked by a bridge”.

    A few moments using Google search terms “simple bridges to build” about 22,400,000 results appeared. Here are a few:

    http://www.outdoorlife.com/saveastream/project-1
    http://pghbridges.com/basics.htm
    http://www.instructables.com/id/Popsicle-Stick-Bridge/
    https://www.google.com/search?q=sim...sH5GO9AT8pID4Ag&ved=0CGUQsAQ&biw=1366&bih=660

    Let us know if and how you resolve this.
    Cheers,
    Bobbywhy
     
  9. Jan 12, 2013 #8

    Simon Bridge

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    Re: Criticism of small "bridge" design welcome.

    ... hmmm, could be taken two ways couldn't it?

    @Spinnor: so... how are you getting on with this?
     
  10. Jan 13, 2013 #9
    Re: Criticism of small "bridge" design welcome.

    Simon it is all good! Will respond latter today, thanks again to all!
     
  11. Jan 13, 2013 #10
    Re: Criticism of small "bridge" design welcome.

    Went for a walk and "studied" the site. It appears the crossing is greater then 40 feet wide. A couple of inches of loose gravel covers bedrock across the stream bed. Even after a good rain two or so days ago the average depth of the water was about 3 inches. Various sized rocks made a foot path across the stream, with most rocks out of water. A hiking stick allowed me to cross without my boots getting soaking wet. Bikes are not allowed on the hiking trails. Rocks of size about 12x10x42 inches (about 270lbs.) have been at this crossing for years as well as smaller sized rocks.

    A design factor that should be considered is the fact that if there were an easy way to cross the stream said crossing would probable not get a lot of use and so probably does not warrant any significant expenditure.

    While I'm not convinced my design (with refinements from previous comments) is fundamentally flawed for this crossing I now think there is a better way forward. I see two options now, see scan below. The crossing is located at the south-eastern most trail in the park, of the Upperwoods trail and the crossing is at the south-eastern most part of the trail. The park owns land on both sides of the stream and trails could easily be made to the north-east linking state highway 313. There one could cross the state bridge over the stream, that is option one.

    The other option involves cast concrete stepping stones, of starting size about 2 foot base, 1 foot "flat" on the top and a height of about 3/4 foot. Such a cone made of concrete would weigh about a 190 lbs., would cost less then 12 dollars in concrete and could be moved by wheel-barrow. The shape could be optimized against "tipping" and moving by water currents (care to optimize shape, and spacing, straight line or zig-zag?). See a sketch of a possible "ideal" shape. For the entire crossing I would need about 20 such cones. That is still a lot of work for a seldom used crossing.

    Thanks for any suggestions!
     

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  12. Jan 13, 2013 #11
    Re: Criticism of small "bridge" design welcome.

    Making the form-work for a "rectangular" truncated cone would be far easier but the "round" stepping stone has less "drag" for a given cross-sectional area but the "rectangular" cone weighs more. Which is best?
     

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  13. Jan 13, 2013 #12
  14. Jan 14, 2013 #13

    Simon Bridge

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    Re: Criticism of small "bridge" design welcome.

    The first link highlights the liability issue doesn't it?
    I'm not sure that a bunch of pics of stepping stones constitutes "finding out how other people have solved the same problem".

    Coming from NZ it is difficult to understand why such a slight stream in a hiking trail should present such a difficulty as to benefit from a bridge ... wading rivers is common over here: if the water is under your chest you don't need one...

    ... though I realize some people don't like to get wet. You may, from the description, be best advised to search up and down stream for large rocks and lift them into position at the ford.
     
  15. Jan 15, 2013 #14
  16. Jan 17, 2013 #15
    Re: Criticism of small "bridge" design welcome.

    So it seems the most cost effective solution is local materials, large rocks. I like that,

    thanks!
     
  17. Jan 17, 2013 #16

    berkeman

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    Re: Criticism of small "bridge" design welcome.

    You have not addressed two important issues in most of your thread (while others keep pointing them out to you). First, there is liability involved in creating something like a bridge or stepping stone path for people to use. You must address that in some way.

    Second, since the Parks own the land on both sides of the stream, you will need to get permission from them before you do anything with the stream and banks.

    So, the option you outline above seems like the best practical option by far. You need to work with the Parks anyway, and they will almost certainly say "no" to any bridge or stepping stone option for cost and liability reasons, respectively.

    You should find out who does the planning for Park improvements, and present your trail extension ideas to them.
     
  18. Jan 27, 2013 #17
    Re: Criticism of small "bridge" design welcome.

    Went for a walk today in a nearby amenity area and took these pics of one of several amenity bridges for walkers and cyclists. It is over a small side stream like yours and formed by stanking around a flat row of large bore pipes with sandbags and infilling with rock chippings.
     

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  19. Jan 27, 2013 #18
    Re: Criticism of small "bridge" design welcome.

    The grass is greener where you are! The ground here is frozen, though the grass was green a couple of weeks ago, S.E. PA.

    I should have taken pictures also, will do so this week. The "bridge" at my site won't need to carry bikers as they are prohibited. I scouted near the site and found many big stones that would likely not go very far in flood conditions and would make nice stepping stones, easily moved with a wheel-barrow.

    Have a good week!
     
  20. Jan 27, 2013 #19
    Re: Criticism of small "bridge" design welcome.

    There is another photo from today here

    https://www.physicsforums.com/showthread.php?t=667304

    As you can see there is some flooding, due to rain and meltwater at the moment since the park is in the valley. There is still snow up on Exmoor (SW UK) nearby.

    Go well
     
  21. Jan 27, 2013 #20

    berkeman

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    Re: Criticism of small "bridge" design welcome.

    WTH is that thing? An ancient wooden Stonehenge? Or an artist's rendition...
     
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