# bike / foot bridge



## endo_alley (May 28, 2013)

We have a small bridge to design and build over a very small wash. About a 7' - 8' span. Maybe 4' drop to bottom of the wash. I was thinking about precutting pieces of ground contact grade pressure treated wood, and assembling the bridge in place. About a 1/4 mile hike for the pieces. Landscape timbers for beams, and 2" x 6" material for the bench. Does anybody else have experience in bridge building? Any engineering ideas?


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## krel (May 9, 2017)

There's tons of plans online for stuff like that. Easy to cut and drill everything ahead and just lego it on site, I'd think? Sounds like a fun project. 


Sent from my iPhone using Tapatalk


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## old_MTBer (Feb 16, 2014)

Our club has built around a dozen small bridges by precutting the materials and assembling on site. For these small bridges we use a set of drawings designed by an engineer that were on the IMBA web site at one time. That way if there are any issues in the future you have plans that carry a name with P.E. (Professional Engineer) after it. Dropbox link to plans.
https://www.dropbox.com/s/q8cie82w3pshqxm/trailbridgedetails.pdf?dl=0


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## leeboh (Aug 5, 2011)

How wide? Dog and hiker use? 30-36" wide is usually good. 3 stringers of 2x8 pt with pt 5/4ths decking works well. Galvanized ring nails or deck screws to fasten. 4x6 or 8x8 pt blocking for the end ground contact. Just be sure it has a solid base/foundation. End boards to slope down? Sometimes on each end we will use some big flatish rocks for transition.


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## JustMtnB44 (Nov 8, 2004)

I've designed and built several bridges for the local park I steward. I used the plans old_MTBer linked to as a guide, along with a few other guides to create my bridge designs. My general guidelines are:

*Width:*
30"-36" depending on type of trail and height off the ground, but this range is good for bikes, hikers, runners, etc. 24" for a very low or narrower technical / feature type bridge.
*Stringers:*
6' or less I have used PT 4x4 stringers
6'-12' I use PT 4x6 stringers (>32" wide use 3 stringers, one in middle)
12'-14' could be doubled up PT 2x8 stringers, bolted together
14'-16' I've used doubled up PT 2x10 stringers, bolted together
>16' I've put a support of some type in the middle
*Decking:*
Always PT 2x6. Other people in my area have used PT 5/4 decking boards in the past and they don't last.
*Footers/Piers for ground contact:*
Usually PT 4x6 sitting on ground, sometimes vertical 4x4 in post hole if needed
*Hardware:*
Deckmate coated 3" screws for decking with Torx drive
3/8" galv spike nails to hold stringers to footers or any 4x material together
3/8" galv carriage bolts for doubled 2x stringers

Some examples:


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## leeboh (Aug 5, 2011)

^^^ Seems some really beefy build specs, should hold up very well. Should space the decking boards 1/4" apart or so for water and dirt to get though. Watch for really wide spacing though, dogs feet might have some issues.


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## JustMtnB44 (Nov 8, 2004)

leeboh said:


> ^^^ Seems some really beefy build specs, should hold up very well. Should space the decking boards 1/4" apart or so for water and dirt to get though. Watch for really wide spacing though, dogs feet might have some issues.


I am firmly of the opinion that if we are going to spend the time and effort to haul materials deep into the woods to build this stuff it needs to last as long as possible with minimal maintenance. The small added cost of doubling up stringers or using bigger lumber is worth it if it lasts 15+ years.

I realized I forgot to mention deck board spacing last night while out working on trails. I usually do 3/4" spacing between deck boards; I cut a couple scraps to use as spacers when installing decking. I agree more than 1" has issues for dog feet, and less than 1/4" makes it hard for muck/dirt/debris to fall through.

If the bridge is in a turn (like my first picture above) or a steep feature (like the last picture) I screw down steel lath for traction using 1/4" fender washers and short deck screws.


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## jonas_24112 (May 16, 2016)

endo_alley said:


> We have a small bridge to design and build over a very small wash. About a 7' - 8' span. Maybe 4' drop to bottom of the wash. I was thinking about precutting pieces of ground contact grade pressure treated wood, and assembling the bridge in place. About a 1/4 mile hike for the pieces. Landscape timbers for beams, and 2" x 6" material for the bench. Does anybody else have experience in bridge building? Any engineering ideas?


Don't use landscape timbers for beams, use pressure treated 4x4's at least. For that span, (2) 4x4x8' will do for beams and pressure treated 2x6's are good for decking. Pre-cut the 2x6's and haul it all in a wheel barrow. I suggest using screws for attaching decking to the beams. And use rocks you scrounge from the area or creek for footers. Should last you at least 10 years.


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## aero901 (Apr 11, 2012)

Expanding on treated wood: treated lumber comes in different grades which have different levels of preservative chemical retention and longevity. Most commonly found treated lumber will be rated for either "ground contact" or "above ground" use.

Ground contact (GC) grade should be used for anything that will be in contact with soil or water for long periods of time. Most posts (4x4, 4x6, 6x6) come exclusively in this grade, but this isn't the case with 2x material. In this product, treatment chemicals have, typically, penetrated the entire piece of lumber.

Above ground (AG) treated lumber is cheaper and the chemicals, in my experience, only penetrate 1/4"-1/2" into the wood. Meaning, if it is cut/damaged after treatment, there will be untreated wood exposed and it will rot quickly. This is especially a problem with exposed endgrain since water will wick into the core of the board.

Example of the preservative penetration in a re-sawn AG treated 2x4 (not much):


I prefer GC lumber for trail structures for added longevity. It costs more up front but lifespan costs are actually less.

To tell the difference between types there is usually a plastic tag stapled to the end of the piece of lumber (see photo below). Trust the tag and not the person selling the lumber.



In our area we use rough sawn white oak for improved traction on bridge decks. Treated wood can be like ice when wet and I've seen riders go down on damp treated decking during light braking. 1" deck board thickness is typical unless wider joist spacing is needed; 1" oak can span 18" without feeling too flexy to hikers/runners. I use a spare piece of decking or 1x material set on edge for plank spacing. White oak decking will last 8-12 years if kept out of the dirt in a temperate forest climate.

I'd recommend 2x material for stringers/beams over 4x4s or other post specific cuts since they are a more efficient use of material and cheaper/lighter. Designing bridges to withstand a 50psf load (10psf dead load & 40psf live load), which is common for most residential decks, is good practice. Dead load is the structure's weight, live load is the load that is being applied to the structure, and psf = pounds per square foot. No. 2 2x6s on 16" centers are good for ~9' of span at a 50psf load with minimal deflection.

American Wood Council - Maximum Span Calculator for Wood Joists and Rafters

Typical values if using this calculator for _simple_ trail bridges: 

_Species_: Southern Pine (typical for treated wood products but species availability can be regional)
_Grade_: No. 2
_Member Type_: Floor Joist
_Deflection Limit_: L/240 or L/360
_Wet Service Condition_: Yes
_Dead Load_: 10psf
_Live Load_: 40psf
Galvanized or coated fasteners are also recommended with treated wood to prevent corrosion related issues. Hot dipped galvanizing is usually favored over electrogalvanized fasteners since the coating is much thicker and resistant to attack.


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## Walt Dizzy (Aug 18, 2003)

Thanks to all the contributors to this thread.

I need to repair an existing bridge. The repair will center involve adding support to the middle of the structure. (The stream is in the process of moving by carving away one bank and dumping gravel against the opposite side.)

Long story short, I plan to fix the bridge by building a support under the middle. I plan to dig a trench to seat a pressure treated 8"x8" timber for the new footing. I'd like to stake the footing timber into the gravel in case the creek rises onto the new bank, but I don't know what to use for stakes. Steel rod of some kind, driven in by an impact driver. It would be nice to have something hard enough to split any rocks I will inevitably run into.

Ideas?


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## endo_alley (May 28, 2013)

First, maybe armor the bank with some large boulders to keep it from eroding. That is not always possible, and may require a permit. You can create a stone or cinder block footer beneath the 8"x8" baseplate set in concrete. And set heave rebar in the concrete. Drill holes into the base plate to accept the rebar studs.


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## Walt Dizzy (Aug 18, 2003)

endo_alley said:


> First, maybe armor the bank with some large boulders to keep it from eroding. That is not always possible, and may require a permit. You can create a stone or cinder block footer beneath the 8"x8" baseplate set in concrete. And set heave rebar in the concrete. Drill holes into the base plate to accept the rebar studs.


Thanks for the ideas, endo.

I'd enjoy armoring the eroding bank and do it if I thought it would stay put. We originally had an armored crossing at this location. It was constructed of ~400 lb rocks. The stream runs fast in the spring melt. It tossed those stones downstream. Hence the bridge.

I like your idea of making a concrete or stone base under the footing and studding them together. Not sure how the digging is going to proceed. I may be being optimistic to believe I can bury the footer. I may have to lay it on the gravel and hope that my iron stakes will secure it in high water.


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## endo_alley (May 28, 2013)

Picks, rock bars, sledge hammers and shovels; and you can get through just about anything. Did you set those boulders battered into the banks properly? My property goes midway into the Eagle river of western Colorado. And the flows are from 120 cfs to sometimes 8,000 cfs along a class three whitewater rapid. And my banks are not eroding noticeably. When armoring a bank, make sure you do not impede the flow at all or pinch the stream bed in any way. Not only is that bad hydrology, it is usually quite illegal. They usually only allow the federal (or maybe state) governments to legally really screw things up really badly in our waterways. With everyone else, even a simple common sense fix on your property and you open yourself up to a fine. So be careful doing improvements along a moving stream.


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## Maday (Aug 21, 2008)

I've used diamond piers on a few bridge supports...
Diamond Pier


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## radair (Dec 19, 2002)

Maday said:


> I've used diamond piers on a few bridge supports...
> Diamond Pier


We have also used 4x4s driven like a pile for mid-span support. Cut the bottom end at a point and drive them to refusal, then cut off the excess. The problem with trying to put in a footing is scour will occur at high flows and undermine it.

As others pointed out, a wetlands permit is usually required to armor stream banks and the regulators can get pretty excited about that stuff.

85 psf is standard pedestrian live load, which is a bit overkill. When designing long spans, I will typically use 85 psf but ignore deflection as this will further increase stringer sizes.


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## Legbacon (Jan 20, 2004)

These guys know how to build a bridge. Sort through their FB. Curved, roller coaster, big, and small. https://www.facebook.com/RiverRatz


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## jonas_24112 (May 16, 2016)

Also check local junk yards for box trucks with pull out aluminum ramps. Our local trail guy picked two up for $100 each. A 12 and 16 footer. Gonna paint em brown and set em.


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## bamwa (Mar 15, 2010)

^ brilliant!


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## JustMtnB44 (Nov 8, 2004)

jonas_24112 said:


> Also check local junk yards for box trucks with pull out aluminum ramps. Our local trail guy picked two up for $100 each. A 12 and 16 footer. Gonna paint em brown and set em.


I did this at my parent's house a long time ago when I found a moving van ramp in the ravine next to their property. It does work well, although in some urban situations I would be worried about them getting stolen (for scrap).


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