Bridge questions

Keith Jul 22, 2015

  1. Keith

    Keith TrainBoard Supporter

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    Have a few questions on bridges. And wasn't sure which forum to
    post to! But, went here, since my questions regard G Scale girder bridges.

    I have 4 Garden Metal Models 12" girder bridges for use outdoors eventually!
    Before I attempt to install them, somehow, I'm wondering about clearances.
    As trying to figure out HOW to build the needed abutments!

    That is, a minimum height, maximum height, or average height above a road, water etc...
    A couple of mine will be crossing over a small, DRY river bed. Used for water drainage from
    roof downspouts. Plus, it adds a little variety to the railroad.

    Anyway,
    Information appreciated.
    Thanks.
     
  2. BoxcabE50

    BoxcabE50 Staff Member TrainBoard Supporter

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    You are talking about clearances such as for a train to pass underneath? If you have enough space, why not give at least two or three inches all around?

    Can you cast your own abutments from cement?
     
  3. Keith

    Keith TrainBoard Supporter

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    In my case, no, tracks will not be crossing over one another.
    I've thought about casting concrete abutments. I'm still temporarily
    stumped as to how to create a mold for abutments, and out of what?
    I'll get it figured out some day!
     
  4. ScaleCraft

    ScaleCraft TrainBoard Member

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    Um....bare minimum, depending on scale, is 9" above railhead to bottom of bridge.
    (1:32 is physically smaller in height than 1:20.3, so what scale are you using?)
    More in some cases (like Pardee-Curtin Botch Shays with HUGE stacks).
    Dry streams, anything you want, whatever fits with your surveyed roadbed right of way.
    Abutments...DON'T use concrete with re-bar...re-bar will rust and split it all over.
    I use cinder blocks, 1/2 ones, 8X8X8, and even over a 4X4 pressure treated post.
    Dave
     
  5. ScaleCraft

    ScaleCraft TrainBoard Member

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    ah..you posted that as I was posting. No tracks crossing tracks, then whatever you want, thinking "high water mark".
     
  6. BoxcabE50

    BoxcabE50 Staff Member TrainBoard Supporter

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    Scrap wood? Wood will stick, so you might check with the folks who pour concrete floors and foundations, how to treat that wood. I'd not be surprised if you can just spray it with WD40.

    Reinforce with something such as fiberglass window screening.
     
  7. John Easterwood

    John Easterwood TrainBoard Member

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    The National Model Railroad Association has standards and recommended practices for just about every aspect of the hobby, including clearances. The table below is based upon their recommendations. For more detailed information, see theirStandard S-7. Remember, these are only recommendations and only the minimums...you may desire greater vertical clearances in some areas for scenic effect or for access to the trains. (Our fingers don't scale down.) Also, these figures do not take into account any additional height required for the roadbed and benchwork used to support your tracks. You can make a clearance gage using NMRA S-7 figures for your scale; the gage will give you the horizontal and vertical minimum clearances. The prototypes (real railroaders) use the 100 year flood data to determine minimum clearances above the water feature being crossed; what's the maximum water height at the bridge crossing that you've seen from previous rains? Add a couple inches to be safe. Wood can be used as forms for your abutments; use used motor oil as a release. I use pipe straps and driven rebar to hold the forms in place. I would also use driven rebar (depth determined by soil conditions) wired together to make a tee (rust requires oxidation; rebar covered by concrete can't oxidize, hence it is used in the slab of your house) to hold the concrete abutments in place. I use a plasticized concrete because it forms and finishes better; be sure to gently tap your forms while the concrete is newly placed to get rid of voids.
    Minimum Vertical Clearances
    • Z Scale: 1 3/16 - 1 1/4" 30 - 32mm
    • N Scale: 1 9/32 - 1 23/32" 32 - 44mm
    • TT Scale: 1 11/16 - 2 5/16" 43 - 58mm
    • HO Scale: 2 11/32 - 3 5/32" 59 - 80 mm
    • S Scale: 3 3/16 - 4 5/16" 81 - 110mm
    • O Scale: 4 1/4 - 5 3/4" 108 - 146mm
    • Large Scales: 6 3/8 - 9 17/32" 162 - 242mm*
    • 1/1 Scale (Prototype): 17 - 23' 5.181 - 7.01m
     
  8. Keith

    Keith TrainBoard Supporter

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    The one bridge installed currently, is going to be removed
    in the near future. Going to require TOO much work to do right.
    Just a not worth the effort. Figure I'd have to lift the entire railroad
    by at least 4" to maybe get close.
    Gonna put the 4" PVC pipe back in, and see if I can find a culvert
    facing for two of them. With 4 girder bridges, those are now gonna become
    a $400+ display instead.
    Mainly since I never got the information and/or the help I needed.
     
  9. Rocket Jones

    Rocket Jones TrainBoard Member

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    I know a guy who does concrete sculpture. He builds his forms out of wood - mostly two-by- - and then coats the insides with cheap petroleum jelly he gets at the dollar store.
     
  10. John Easterwood

    John Easterwood TrainBoard Member

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    Sorry we didn't help you.
     
  11. ScaleCraft

    ScaleCraft TrainBoard Member

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    Yes, it does, if you count rust as oxidation.
    My son is a rebar iron worker, now foreman. It can be such an issue they now use coated rebar in some places.
    Marc Horowitz of Garden Railways had a series on it once. All his bridge abutments split out.
    BIG difference between a six in square bridge support and a foot thick, 3-8 foot high, 30-50 foot long foundation with footing.
    But go ahead, use straight rebar in a small bridge abutment. I gave up arguing years ago.[​IMG]
    That is a foundation.
    http://www.andersal.com.au/services/concrete-cancer-spalling-repair/guide-to-building-defects/
    Look at the photo of the side of the bridge where you can see all the vertical rebar:
    "Water damage can cause the concrete reinforcement to rust and expand, which adds pressure on the surrounding concrete."
    I am waiting to get permission to post the photos from Garden Railways Magazine on a small scale concrete structure.
     
  12. ScaleCraft

    ScaleCraft TrainBoard Member

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    [​IMG]
    [​IMG]
    Used with permission, Marc Horowitz/Garden Railways Magazine/ Kalmbach Publishing

    Rebar. Concrete. Failure. Small items. Outdoors.
     
  13. BoxcabE50

    BoxcabE50 Staff Member TrainBoard Supporter

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    Even when in theory concrete is "cured", it contains a certain amount of moisture and always does. Bare steel trapped inside that moisture corrodes and actually expands. Crack, pop and spall. This is why more and more is done with coated rebar today.

    I was thinking that some fencing is galvanized, so that might work. Such as 2 by 2 or 2 by 4 mesh.
     
  14. ScaleCraft

    ScaleCraft TrainBoard Member

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