# Need guidlines for bridge width.



## kapusta (Jan 17, 2004)

I've been given the go-ahead to build a bridge to span a small creek at a trail system run by a local city government. 

Here is the issue I am running into: The "manager" looked at some bridge rail guidelines from the Forest Service that recommends 4' 0" clearance between curbs. Including the curbs, this means it will be 4' 7" wide. I was planning on making it 2' 8" wide between the curbs, 3' 4" total.

I want to stick to this narrower width for a number of reasons, and I see that many bridges in legit trail systems are no wider than this (I would like to eliminate the curbs as well, but that is not as important)

So my question is where can I find some legitimate guidelines for bridge widths so I can show the management that this is an accepted bridge width for hiking and biking (if in fact it is)? The bridge is almost 4 miles away from the nearest trail-head. The trails leading to it are rated beginner, though not novice. I don't think the manager in question feels strongly about the width of the bridge, but wants to know that it is being done properly. I don't think that they have any obligation to follow the Forest Service guidelines, it's just the only ones we have right now.

Thanks


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## hado_pv (May 26, 2006)

Bridges don't need to be appreciably wider than the trail they're on.


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## LititzDude (Apr 3, 2004)

IMBA had a great .pdf on their site about pre- engineered bridges. I can't find it listed, but I think I have it at work. If you pm me your e-mail, I will send it to you. 

Our club built two bridges using the designs and they are very good.


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## kapusta (Jan 17, 2004)

hado_pv said:


> Bridges don't need to be appreciably wider than the trail they're on.


Thanks, but where in that document does it mention bridges?


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## smilinsteve (Jul 21, 2009)

If someone rides off your bridge and gets hurt, are you liable?


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## kapusta (Jan 17, 2004)

smilinsteve said:


> If someone rides off your bridge and gets hurt, are you liable?


Me personally? No.

Besides, it's not _*my *_bridge.


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## cjohnson (Jul 14, 2004)

*Vague*

there is a good book, the pdf is online. Wetland trail design and construction, by the USFS.
It describes structures much more narrow than 4' . Puncheons for example. I've been on FS trails with boardwalks less than 3 feet wide and no curbs. there is info on this somewhere, I tried to find it, but no luck. The trail use and location will determine what is required, some things that may help you:

See bridge definition, page 32 of wetland trail design:
On national forests, all
bridges require design
approval from engineering
before being constructed.
Bridges are generally defined
as structures more than 20
feet long and higher than 5
feet off the ground.

page 44: curbs: " curbs are required for "accessible trails." 
Curb and bull rail are two names for the same thing. If the
drop from a boardwalk is about 36 inches or less, a curb
is usually installed. A curb is required for accessible trails.
Curbs help to delineate the trail tread (fi gure 64).

what is an accessible trail? http://www.ncaonline.org/?q=node/659


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## perttime (Aug 26, 2005)

How long will the bridge be? How high?

I have no clue about official guidelines but those things clearly affect the PERCEIVED difficulty of crossing the bridge.


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## kapusta (Jan 17, 2004)

perttime said:


> How long will the bridge be? How high?
> 
> I have no clue about official guidelines but those things clearly affect the PERCEIVED difficulty of crossing the bridge.


FWIW, the crossing is 9' between the tops of the banks, and ~3' 6" at it's highest point. The bridge itself will span 13' and at its highest point the deck is ~4' 4" above the creek bed. The trail is flat and straight on either side.

"Official" guidelines are all that really matter at this point. Everyone involved with this agrees it could be narrower and still be appropriate for the trail, but that is irrelevant if we don't have any official guidelines to demonstrate that.


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## indytrekracer (Feb 13, 2004)

We base our bridges on the IMBA trail difficulty specs

http://www.imba.com/resources/maps/trail-difficulty-ratings

This takes into account that bridges can be narrower for advanced trails. For beginner and intermediate trails we stick to 4' wide as it's easy to cut 8' boards into 2 x 4' boards.


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## Summit Ridge Guy (Aug 16, 2010)

I agree with indytrekracer. I would use the IMBA standards as a guide. Think about who will be using the bridge (if it is mainly MTB what is the difficulty level of the trail?). Length of bridge, elevation, etc, etc need to be taken into consideration. You want to build something fun while minimizing the risk of injuries.

Good luck!


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## mtbikerTi (Jan 15, 2004)

LititzDude said:


> IMBA had a great .pdf on their site about pre- engineered bridges. I can't find it listed, but I think I have it at work. If you pm me your e-mail, I will send it to you. .


For some reason they don't list it on their new website, but it's still up on the old one:

http://old.imba.com/resources/trail_building/index.html

Direct link:

http://old.imba.com/resources/trail_building/trailbridgedetails.pdf


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## kapusta (Jan 17, 2004)

mtbikerTi said:


> For some reason they don't list it on their new website, but it's still up on the old one:
> 
> http://old.imba.com/resources/trail_building/index.html
> 
> ...


Awesome! Thanks!


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## si_ex_pat (Jul 27, 2011)

I noticed you mentioned the bridge would be for hiking and biking. That could be part of the issue. If the trail is for hiking also it may need to be ADA compliant which would mean the bridge would need to be wide enough for a wheelchair and be curbed for safety. If the bridge was for biking only you would be able to get away with a narrower width.


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## slocaus (Jul 21, 2005)

The Forest Service guidelines your land manager references is for multi use trails, specifically horses. That width and the "curbs" as you call them are probably toe rails so that horse hooves don't slide off the bridge if they slip (and they do more than you'd think).

We do most bridges at 24" - 27" wide where they are only on hiking / biking tails if they are straight line. If he bridge has to curve, then the width has to go to 36" - 50" for trailing rear wheel safety. If the highest place on the bride is less than 36" high, then no rail is required. Otherwise, one rail is needed, and over 56" I think requires two rails.

This is in central coastal California.


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## kapusta (Jan 17, 2004)

si_ex_pat said:


> I noticed you mentioned the bridge would be for hiking and biking. That could be part of the issue. If the trail is for hiking also it may need to be ADA compliant which would mean the bridge would need to be wide enough for a wheelchair and be curbed for safety. If the bridge was for biking only you would be able to get away with a narrower width.


It does NOT need to be ADA compliant.


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## kapusta (Jan 17, 2004)

slocaus said:


> The Forest Service guidelines your land manager references is for multi use trails, specifically horses. That width and the "curbs" as you call them are probably toe rails so that horse hooves don't slide off the bridge if they slip (and they do more than you'd think).
> 
> We do most bridges at 24" - 27" wide where they are only on hiking / biking tails if they are straight line. If he bridge has to curve, then the width has to go to 36" - 50" for trailing rear wheel safety. If the highest place on the bride is less than 36" high, then no rail is required. Otherwise, one rail is needed, and over 56" I think requires two rails.
> 
> ]


What I am looking for are the guidelines that state this. This bridge is NOT intended for horses.


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## slocaus (Jul 21, 2005)

kapusta said:


> What I am looking for are the guidelines that state this. This bridge is NOT intended for horses.


I contact the City Ranger and they do not have anything specific in writing. City Building Code for weight bearing, and then the dimensions I stated, based on bridge design. All the photos were from their Open Spaces.

I've emailed the Trails guy at State Parks, but they always have us build for multi-use, even in places that don't have all three non motorized usres, and my guess is that they use the Forest Service Standards.

I Googled "Bridge Standards for Recreational Trails" and got many hits, but all the ones I looked at were the same FS links and urban bike lane standards. You might want to investigate that more thoroughly than I did.

Good luck, I gave you what I could.


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