Bridge Footer Construction- Mtbr.com
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  1. #1
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    Bridge Footer Construction

    I'm building (2) 14-16' bridges on a local township trail. I had the local township building inspector look at my plans, he approved them. Double 2x12 stringers with 2x6 decking and a 2x4 curb on each side. Not quite high enough for handrails.

    My questions is about the footers. It only needs footers on each end. I spec'ed out 2x12 pressure treated pine #2 for the sill (the stingers to sit on at each end.

    Is this sufficient? I've gotten different opinions from different people ranging from Gabion baskets (metal basket filled with rocks) to helical pilings.

    FWIW, this is over top of a little more than a trickle of water that feeds a nearby creek. I don't suspect it will get high enough to wash away. Any insight about these footers?

    Bridge Footer Construction-img_20171104_102144787.jpg

  2. #2
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    Lots of ways to go. Couple of cement 3 hole blocks set into the dirt some would work. We usually use 6x6 pressure treated to set the stringers on. Some flat patio pavers would work as well. You going to have a 12-18" high boardwalk off the ground to bike over? End ramp?

  3. #3
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    Thanks. Do you mean you set the 6x6 lengthwise on the ground? If so, do you put any crusher run underneath and or use a stake or similar to keep it from shifting? I also wonder what the life of a PT 6x6 is with full soil contact.

    Hopefully no ramps, I plan on putting the sill beneath grade. Might need to make a dirt ramp or transition.

    Sent from my XT1585 using Tapatalk

  4. #4
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    I build wooden piers out of 6x6 and 4x6 PT lumber. I put a 6x6s slightly longer than the overall width of the bridge under the stringers, and then fill in the gap between the stringers with 4x6's to make a retaining wall I can fill dirt against. I use large 3/8" spike nails to hold it all together, including some through the stringers at an angle in to the pier. This ends up getting buried on one side but the side under the bridge is still exposed to air. The oldest bridge in the park that I steward that used this technique was 10 years old when replaced due to insufficient stringers (PT 4x4 with a 16' span ) but the 6x6 footers were still solid.

    The bridge in the pictures was installed in January 2016. It is a 16' span made with doubled PT 2x10's, bolted together. We did another bridge that was 16' with doubled PT 2x12's and felt the 2x12's were a bit overkill, especially if you have to carry them 1/2 mile into the woods. Most recently we just finished another bridge that is a 20' span using 3 pairs of doubled 2x12's as stringers, it's also a bit wider and has railings because it is nearly 10' high in the center.

    How high off the ground will the top of your bridge be, and how wide do you plan to make it?
    Attached Thumbnails Attached Thumbnails Bridge Footer Construction-cb-bridge-pier.jpg  

    Bridge Footer Construction-cb-bridge-underside.jpg  

    Bridge Footer Construction-cb-bridge-2.jpg  


  5. #5
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    I'm not a fan of wood straight on dirt, so I'd at least excavate out a footer and fill it with rubble. Gabions are pretty neat too.

    We just built a bridge that had higher parameters than yours, we poured concrete footers. It was just off the road though, so it didn't require hauling stuff into the boonies.

    https://www.facebook.com/medicinewhe...0271869465298/

  6. #6
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    Quote Originally Posted by JustMtnB44 View Post
    I build wooden piers out of 6x6 and 4x6 PT lumber. I put a 6x6s slightly longer than the overall width of the bridge under the stringers, and then fill in the gap between the stringers with 4x6's to make a retaining wall I can fill dirt against. I use large 3/8" spike nails to hold it all together, including some through the stringers at an angle in to the pier. This ends up getting buried on one side but the side under the bridge is still exposed to air. The oldest bridge in the park that I steward that used this technique was 10 years old when replaced due to insufficient stringers (PT 4x4 with a 16' span ) but the 6x6 footers were still solid.

    The bridge in the pictures was installed in January 2016. It is a 16' span made with doubled PT 2x10's, bolted together. We did another bridge that was 16' with doubled PT 2x12's and felt the 2x12's were a bit overkill, especially if you have to carry them 1/2 mile into the woods. Most recently we just finished another bridge that is a 20' span using 3 pairs of doubled 2x12's as stringers, it's also a bit wider and has railings because it is nearly 10' high in the center.

    How high off the ground will the top of your bridge be, and how wide do you plan to make it?
    I'm impressed . . . 👍

  7. #7
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    6x6 on the ground perpendicular to the stringers works fine. Cap the ends of the bridge with a 2x10 rim so that the endgrain of your stringers are covered and you have something to backfill against. There's no good reason to use anything bigger than 2x stock for this.

  8. #8
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    Here is a link to a set of bridge plans that were designed by an engineer. They were on the IMBA web site years ago when it actually had helpful information.
    https://www.dropbox.com/s/q8cie82w3p...tails.pdf?dl=0

    Be aware that most 2X lumber is not rated for ground contact. Most 4x4, 4x6, and 6x6 is rated for ground contact. It is a matter of how deep into the wood the preservative penetrates. However, if cut the exposed ends of the ground contact rated wood should be retreated. Wood preservatives are available at most lumber yards, home centers, and on line.

  9. #9
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    Quote Originally Posted by radair View Post
    6x6 on the ground perpendicular to the stringers works fine. Cap the ends of the bridge with a 2x10 rim so that the endgrain of your stringers are covered and you have something to backfill against. There's no good reason to use anything bigger than 2x stock for this.
    I would add: make the bulkhead/dirt dam/end rim as long as the sill (ie, wider than the decking) and cut the ends at a 45 degree angle (make it a trapezoid shape) so that it supports the backfill toe slopes of the approach ramps. If it's only cut as wide as the stringers, the dirt tread in the approach ramp along the edges will eventually collapse, unless you build retaining walls to hold it. Also, pinning the sills to the ground with 3 foot long rebar drift pins may help keep the bridge in place in the event of a flood.

  10. #10
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    Quote Originally Posted by JustMtnB44 View Post
    I build wooden piers out of 6x6 and 4x6 PT lumber. I put a 6x6s slightly longer than the overall width of the bridge under the stringers, and then fill in the gap between the stringers with 4x6's to make a retaining wall I can fill dirt against. I use large 3/8" spike nails to hold it all together, including some through the stringers at an angle in to the pier. This ends up getting buried on one side but the side under the bridge is still exposed to air. The oldest bridge in the park that I steward that used this technique was 10 years old when replaced due to insufficient stringers (PT 4x4 with a 16' span ) but the 6x6 footers were still solid.

    The bridge in the pictures was installed in January 2016. It is a 16' span made with doubled PT 2x10's, bolted together. We did another bridge that was 16' with doubled PT 2x12's and felt the 2x12's were a bit overkill, especially if you have to carry them 1/2 mile into the woods. Most recently we just finished another bridge that is a 20' span using 3 pairs of doubled 2x12's as stringers, it's also a bit wider and has railings because it is nearly 10' high in the center.

    How high off the ground will the top of your bridge be, and how wide do you plan to make it?
    This is a great visual! Very similar to what I'll be building. It will be about 24"H, so I don't plan on handrails and 36"W with a 2x4 curb on each side edge.

    Did you put anything else under your 6x6 sills (gravel, etc)?

  11. #11
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    Quote Originally Posted by Harryman View Post
    I'm not a fan of wood straight on dirt, so I'd at least excavate out a footer and fill it with rubble. Gabions are pretty neat too.

    We just built a bridge that had higher parameters than yours, we poured concrete footers. It was just off the road though, so it didn't require hauling stuff into the boonies.

    https://www.facebook.com/medicinewhe...0271869465298/
    This is my internal debate, wood sill or concrete footers (or stone, .......).

  12. #12
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    Quote Originally Posted by radair View Post
    6x6 on the ground perpendicular to the stringers works fine. Cap the ends of the bridge with a 2x10 rim so that the endgrain of your stringers are covered and you have something to backfill against. There's no good reason to use anything bigger than 2x stock for this.
    Thanks. This is the way I'm leaning, though I'm glad you mentioned the end caps, I didn't plan or envision those. I think that's a good idea and will incorporate that.

  13. #13
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    Quote Originally Posted by old_MTBer View Post
    Here is a link to a set of bridge plans that were designed by an engineer. They were on the IMBA web site years ago when it actually had helpful information.
    https://www.dropbox.com/s/q8cie82w3p...tails.pdf?dl=0

    Be aware that most 2X lumber is not rated for ground contact. Most 4x4, 4x6, and 6x6 is rated for ground contact. It is a matter of how deep into the wood the preservative penetrates. However, if cut the exposed ends of the ground contact rated wood should be retreated. Wood preservatives are available at most lumber yards, home centers, and on line.
    That's where I got my plans originally! This is a township trail but goes through my neighborhood and I had to get it signed off from our building inspector. He said the engineering was fine. This was very helpful. I like your suggestion for treating the cut ends.

  14. #14
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    Quote Originally Posted by Wildfire View Post
    I would add: make the bulkhead/dirt dam/end rim as long as the sill (ie, wider than the decking) and cut the ends at a 45 degree angle (make it a trapezoid shape) so that it supports the backfill toe slopes of the approach ramps. If it's only cut as wide as the stringers, the dirt tread in the approach ramp along the edges will eventually collapse, unless you build retaining walls to hold it. Also, pinning the sills to the ground with 3 foot long rebar drift pins may help keep the bridge in place in the event of a flood.
    Thanks Radair. I can't quite picture what you mean about cutting the bulkhead at a 45. I started planning this about a year ago and have stopped while biking at every bridge I cross to look the construction. I have seen ramps the slope into the ground. Is that what you mean? Like this pic?
    Bridge Footer Construction-img_20171027_145847413.jpgBridge Footer Construction-img_20171027_150009177.jpgBridge Footer Construction-img_20171027_153021556_top.jpg

  15. #15
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    Quote Originally Posted by Perkinscl View Post
    ... I can't quite picture what you mean about cutting the bulkhead at a 45. I started planning this about a year ago and have stopped while biking at every bridge I cross to look the construction. I have seen ramps the slope into the ground...
    What he means is the rim that caps the end of the stringers can be extended wider than the bridge to act as a retaining wall. The 45 degree wing walls (in plan view, i.e. looking straight down) slope back toward the approaches to cut down on the amount of backfill soils you need. We have lots of rock to work with so we tend to build short retaining walls with stone and backfill against it. This one had to accommodate snowmobiles so we made it 5' wide and installed 2x4 curbs on scupper blocks.
    Bridge Footer Construction-hs-loop-bridge-medium-.jpg

    Quote Originally Posted by Perkinscl View Post
    ...Did you put anything else under your 6x6 sills (gravel, etc)?
    When we use 6x6 sills we excavate to mineral soil and put the 6x6s on that. Basically just a trench in the duff.
    Attached Thumbnails Attached Thumbnails Bridge Footer Construction-hs-loop-bridge-medium-.jpg  


  16. #16
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    Quote Originally Posted by Perkinscl View Post
    Thanks Radair. I can't quite picture what you mean about cutting the bulkhead at a 45. I started planning this about a year ago and have stopped while biking at every bridge I cross to look the construction. I have seen ramps the slope into the ground. Is that what you mean? Like this pic?
    Click image for larger version. 

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    Perkinscl,

    This is what I was trying to describe:
    Bridge Footer Construction-elements-sustainable-trails.jpg

  17. #17
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    Quote Originally Posted by Perkinscl View Post
    Did you put anything else under your 6x6 sills (gravel, etc)?
    No, we also dig down to mineral soil (clay here in western PA) and it just sits on that. We don't have a lot of rock in this park, but I could see why having some coarse gravel or small rocks would help keep the sill from sitting in water/wet dirt.

    Quote Originally Posted by Wildfire View Post
    This is what I was trying to describe:
    That graphic helps a lot and is a good idea, I should have done that for the bridge in my photos above. I suppose I can just dig out the ends and add it...

  18. #18
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