# Bridge to Cross Wetlands



## sick4surf (Feb 4, 2004)

We have a proposal to build a trail through a seasonal wetlands area, the water rises from 0 feet to 2.5 feet. The special regulations for building a bridge in wetlands are as follows:

1. No digging or disturbing of soil allowed

2. Sunlight must be able to penetrate below so the vegetation doesn't die

The span is 52 feet.

Ideas? I need to develop drawings for a proposal next week. Thanks!


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## Harold (Dec 23, 2003)

For one, sounds like a floating bridge needs to happen.

The sunlight thing is trickier. Might have to use something like a 2x2 for the decking, with more space between the wood. So maybe 3-4" of gap between each 2" piece of decking. Or, possibly make use of a heavy duty steel mesh material.


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## sick4surf (Feb 4, 2004)

Yea I was thinking at least 1.5" spacing between planks. The support might be cribbing. They didn't provide details.


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## TFitz (Jun 21, 2008)

This is a tough situation. Several thoughts: First, there are some foundation systems out there (sorry, no links) targeting building in wetlands. The one I thought simplest and cheapest uses pier blocks with crossed conduit (or something else that won't corrode too much in moist soil) underneath to really spread out the load. Second, 2X2 decking will be too fragile. Is this trail bike-only or mulitple use? The big load for biker-hiker trails is not people on bikes but people on foot. Imagine a 50-ft bridge with a bunch of bikers crossing in tandem vs the local high school track team out for a practice...
Tread boards should probably be 2X8 minimum; 1.5" spacing would be ok, but dog walkers would like .5"; you can't please everyone. Third, how much two-way traffic will there be? One of my local fave trail systems has a long curvy boardwalk through a wetland that is only about 18" wide, and you can't see the other end when you get on the boardwalk. This is on a 2-way trail which is not ideal. Standard width for a residential sidewalk is 5 feet, just to give an idea of what non-motorized 2-way traffic generally expects. Good luck!


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## sick4surf (Feb 4, 2004)

Thanks, this will be multiple use. Good point about the high school track team. It will also be one direction only but I am wary about curves as it could be slippery with morning dew, snow, rain etc. I was thinking 2 - 2x12s (not doubled up) spanning about 12 feet on 24" centers with 2x6" planks for a 36" finished width.

Thanks for the suggestion for the pier blocks, I've seen them used before anchored by re-bar on an angle. I'm going to provide them with three choices in case they don't like one they can choose another.

The other choice will be 6x6" cribbing, which will also allow light to penetrate.


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## Cotharyus (Jun 21, 2012)

Use expanded steel for the decking. Good traction, and it lets light through.


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## TFitz (Jun 21, 2008)

Your framing plan sounds totally up to the job. If you're using pier blocks with brackets, treated 4X8s also will span 12 feet OK. Here in the PNW I roll out a strip of tar paper on top of the stringers to extend service life. For tread, if letting light through is a high priority, maybe try 2X4s on edge 1.5" apart. You'd want 4-6" long 2X4s for spacers down the middle and 2X4s along each side of the tread for stability as well. That's a lot of labor due to all the small pieces of lumber involved but would certainly let lots of light through when the sun is more or less directly overhead.

The boardwalk I described above just uses pier blocks without any rebar or conduit underneath to spread the load and everything seems to be holding up well after well over 10 years since construction, but this is a wetland that is very dry through the summer so the soil isn't too saturated.


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## sick4surf (Feb 4, 2004)

Cotharyus said:


> Use expanded steel for the decking. Good traction, and it lets light through.


 Where would we purchase that and how would it be attached to wood framing?


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## sick4surf (Feb 4, 2004)

Floating piers, no digging: https://www.diamondpiers.com


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## Cotharyus (Jun 21, 2012)

sick4surf said:


> Where would we purchase that and how would it be attached to wood framing?


You'll usually find it available at metal suppliers. The sort of place a fabricator would shop for supplies. A number of ways come to mind to attach it to wooden frames - but I'd got for a good old U-nail like you'd use to attach fence to a post.


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## CycleKrieg (Dec 19, 2013)

A few engineering thoughts here:

*Floating vs. pier*: Floating might be good if you have large changes in the relative height of water. Also, if you get freezing in the winter. However, floating boardwalks are not for the faint of heart. They need rounded bottom floats to avoid killing large number of plants and getting on top the ice, assuming you have any. Because of the mechanics of a floating boardwalk, you have to have the float locations in such a way that they don't twist the frame. The usually means the floats are separate subsections the decking frames hinge onto/from. Also, narrow boardwalks need long float locations to stabilize the deck. You want a wide triangle on the ground so the boardwalk sections don't roll. Remember, when its off the ground, its really a pontoon bridge. https://en.wikipedia.org/wiki/Pontoon_bridge

*Pier types:* The diamond piers are nice, but watch the use of high PSI footings in wetland soils. Wetland soils tend to be almost entirely silt and organics and compress easily. If you were going with smaller posts, I would suggest the OZPOST footing as it relays on vertical friction and not the soils horizontal stability.

*Decking material:* If you want grip and what lots of sunlight through it, expanded metal mesh is the ticket. A few warnings though: You need stringers every 12" to 16" to give it some strength. You can not go the whole 48" width of your boardwalk with a panel, the weight will bend the panel at the center, so you will need those stringers. Also, if you are going with wood with metal setting on the stringers, I would suggest some sacrificial wood on top of the stringer, furring strips or something. DO NOT use tar paper for reasons mentioned below.

*Grip enhancement; tar paper/shingles:* If you go with wood, its likely you will need some kind of grip enhancement. The best option is welded wire fencing. A lot of groups try chicken wire or rabbit fencing and come away disappointed. Those types of wires and fencing aren't thick enough to take the stress of tires and boots. Good welded wire fencing strong enough for small livestock, it can take stress really well. It comes in 3'x50' rolls and laid down length ways along a boardwalk. A roll is $20 at Menards. Why not shingles and tar paper? You should never, never ever use tar paper and shingles in any setting that is natural. I know they do in the PNW, but its a bad idea. Almost all forms of tar paper or shingles contain bituminous (crude oil) between 10% & 35%, and sometimes, dioxins in some form. As they degrade, and they will degrade quite quickly, they release a whole host of nastiness into the ground and water around where they are used. Because they don't last long, over long periods, new shingles and new pollution vectors are added constantly. Plus, they look like crap.


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## karthur (Apr 20, 2018)

It sounds like you are in over your head on this one. Steel is the way to go for being strong enough and allowing enough light for plants to grow underneath. Is there any current when the water rises?


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## Schulze (Feb 21, 2007)

What is your budget? It seems aluminum would be superior to steel for its corrosion resistance.


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## TFitz (Jun 21, 2008)

I'd never advise using tar paper or shingles on top of the tread for traction; they wear away really fast, apart from the pollution aspect (saw this application on a heavily used trail in TN). Tar paper between the stringers and tread boards is pretty stable and helps the stringers last longer. Even though some stuff will leach out over time I recommend treated lumber for the stringers, if the permitting authority will let you. You could use a non-treated 4X4 as a kick rail on top of the tread (metal or wood) but it will degrade a lot faster than treated.


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## sick4surf (Feb 4, 2004)

Thanks for all the advise. There is no current to deal with. The water rises from the ground water. The area is an old sand mind that was dug below sea level so this section of the trail will have varying water levels. It is about 1.5 miles from the nearest road.

We can't use tar paper or obviously shingles in a sensitive freshwater wetlands. I'll look into the livestock fencing for traction though. OZPOST looks like an option too. Thanks!

We didn't create a budget yet but I would prefer pressure treated wood because we will be hand carrying it through a tight forest for about 1.5 miles.

The water level rises about 2 feet and I'm figuring the bridge deck to be 3 feet tall. At what height would you consider adding a railing for safety?


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## TFitz (Jun 21, 2008)

I could be wrong (check your local building code) but at 3 feet above grade I think you're well above the threshold for needing railings. This will entail much heavier construction than would be needed just for spanning 12 feet. You need substantial (i.e. at least doubled) 2X construction for side stringers at or near the edges of the tread, plus lots of blocking between stringers, plus carriage bolts fastening posts to the stringers to hold the railing boards (2X6's every 12" to a height of 36" above tread).


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## sick4surf (Feb 4, 2004)

Thanks for that railing advise. I found a dock building supplier nearby on Long Island that sells 3x and 4x stringers. They also carry this flow thru fiberglass decking that allows light thru: https://thruflow.com/products/surge/


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## CycleKrieg (Dec 19, 2013)

TFitz said:


> I could be wrong (check your local building code) but at 3 feet above grade I think you're well above the threshold for needing railings.


The standard on distance from surface of elevated section to ground is 30". OSHA also requires a 42" railing if you are above 48".


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## CycleKrieg (Dec 19, 2013)

sick4surf said:


> They also carry this flow thru fiberglass decking that allows light thru: https://thruflow.com/products/surge/


Those are amazing! I looked online for a price and didn't find those panels with a listed price. If you don't mind me asking, what was the cost per a panel?


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## sick4surf (Feb 4, 2004)

I'm going to give them a call this week and will let you know.

BTW this is a black diamond mountain bike trail though also multi use, I guess I still have to put a railing just in case toddlers are with inattentive parents. We don't want any drownings even though the water is only 2 feet deep at times.


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## ki5ka (Dec 17, 2006)

sick4surf said:


> ...I guess I still have to put a railing....


Let's make sure we're not putting an onus on sick4surf that isn't required. I've seen it happen time and again where well-intentioned people put out misinformation and others take it to heart.

OSHA regulates workplace safety; do OSHA standards apply to trails?

These features don't have rails and they look and read like legitimate projects that have met the required standards.

























I realize this is a multi-purpose trail. Does someone in this thread know from first hand experience what standards apply for railings on trails?


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## CycleKrieg (Dec 19, 2013)

ki5ka said:


> Does someone in this thread know from first hand experience what standards apply for railings on trails?


https://www.aswm.org/pdf_lib/2_boardwalk_6_26_06.pdf - Page 6, bottom of the page.

OSHA's requirements are the same as the IRC (International Residential Code).

Ask the landowner. Sometimes they be OK without rails.


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## ki5ka (Dec 17, 2006)

CycleKrieg said:


> https://www.aswm.org/pdf_lib/2_boardwalk_6_26_06.pdf - Page 6, bottom of the page.
> 
> OSHA's requirements are the same as the IRC (International Residential Code).
> 
> Ask the landowner. Sometimes they be OK without rails.


This is not occupational and not residential. Why are you suggesting either of these standards apply?


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## mbmtb (Nov 28, 2013)

A quick google search turns up some usfs guidelines... which are pretty reasonable. About usage area, types, height off the ground, etc.
The photos above of course are more trail features than bridges. And on labeled advanced trails.

I'd be calling to find out how much a prefab aluminum bridge costs. Probably a lot.... your marine dealer may be able to help since these people look to be the same as the decking system you found: Trail Bridge - GatorBridge

But if you could get a single span, partially assembled on site, and can just put in some footings.... Overkill city but nice and durable?


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## CycleKrieg (Dec 19, 2013)

ki5ka said:


> This is not occupational and not residential. Why are you suggesting either of these standards apply?


Well, I'm just answering a previous question about the possible need for railings. OSHA, ADA and AASHTO are the standards I deal with all the time at work and they all reference the IBC. In fact, nearly state and federal government agency uses IBC numbers for things regardless of the agencies focus.

Additionally, I've worked on boardwalk projects in the past ( https://crowwing.us/294/Milford-Mine-Memorial-Park ) and I recently ran into this with my club's backcountry trail loop; luckily we had a Type 7 wetland with a large de minimus fill limit, so we could use puncheons.

Even the USDA Forest Service trail guidelines mention the IBC code in referencing the need for guardrails for non-remote locations ( https://www.fs.fed.us/t-d/pubs/pdfpubs/pdf07232329/pdf07232329dpi72.pdf ). The outlier here is USFS's remote guidelines, that is on trails not covered via state, ADA, AASHTO or other requirements, and they put the fall height of 4ft instead of 2.5' without a guardrail, 8ft if fall zones are in very remote areas. Seeing as the OP's boardwalk is on Long Island, NY it seems unlikely one could claim its remote. Additionally, with a water height of 2.5', you would want the base of the stringers at about 3' min., 3.5' to be safe. With stringers (2"x8" min.) and decking, you would at or over the required height for the most liberal standard.

This is a multi-use trail and, rating aside, it likely would likely tick some box in the agency noodle pile above or some state requirement I can't know in NY state. However, as I mentioned in my previous comment, ask the land owner. They might have different thoughts.


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## Schulze (Feb 21, 2007)

There's a wetlands bridge in San Antonio on the greenway south of Mcallister Park that doesn't have railings. It has a 6" high board on each side that looks very well designed to catch a wheel or foot and flip you off the bridge into the water. The city built this and you know they have an army of lawyers.

I found a picture of it. Also notice the angular turns and the fact they built a part of it to go around a Huisache tree. A frickin Huisache tree, basically a large weed!


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## sick4surf (Feb 4, 2004)

Thanks for all the additional info. As much as I would love to have a narrow bridge with no railing, we already got sued once and lost our insurance then when we were able to get insurance again it was 3xs the cost so we have to do the responsible thing and add a railing. The riding height will be higher than 3 feet.

That aluminum span bridge looks cool, gonna check the pricing. Might have to go with wood but I bet the aluminum would be easier to carry 1.5 miles into the woods. The 60 foot span would not need footings because we will be descending down and up two hummocks on either side of the water.


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