# IMBA bridge designs question



## RockyMt (Jan 29, 2011)

Does the IMBA publication "Building Singletrack" contain a set of pre-engineered wooden bridge designs for specific widths and spans. I have seen a set of pre-engineered designs referred to as "old IMBA" but I have not yet purchased the current publication.

Thanks


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## BonkedAgain (Aug 23, 2005)

The Singletrack Solutions book (not sure if that is the follow-on to the book you reference) only does a broad overview on most topics, and it dedicates maybe a page to general bridge pros/cons. I have a few trail building books and none of them really have more than a couple basic designs. I think you will do best by going to Trail Bridge Catalog, Trail Bridge Drawings


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## thumpduster (Nov 19, 2008)

There was a set of basic engineer-designed bridge designs that used be available on the IMBA website. This may have been what you were looking for, as it had specific designs and materials for certain spans. After their last website redesign much of the information resources disappeared as they tried to clean things up. Earlier this year I contacted Mark Eller, IMBA's Communications Director, looking for some of these missing resources. He was very helpful and quickly got me what I was looking for. So, Plan A would be to contact him with your request: markellerATimba. com

Otherwise here's Plan B: I know the fellow who authored the bridge design document you're looking for. I'm not going to post his name or contact information without asking him first, but if you'd shoot me PM I can help make the connection and get you what you're looking for.


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## swampboy62 (Feb 10, 2009)

The US Forest Service has some great info, including a Trail Bridge guideline book. This page is worth bookmarking (see the Trail Bridge entry towards the bottom)

Forest Service Publications List - Recreational Trails Publications - FHWA

Good luck.

Steve Z


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## RockyMt (Jan 29, 2011)

Thanks to all who replied.

I plan to follow up on each of your suggestions.


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## TFitz (Jun 21, 2008)

If the spans you have to cross are small (e.g. 25' or less), and if traffic will be bike and foot only (no horses) you can get away with pretending you're building a residential deck. For example, we built boardwalks over seasonal creeks resting on pier blocks using 12-foot-long treated 4X6 (or doubled 2X6) stringers spaced about 21 inches apart, and 32" 2X6 tread boards spaced about an inch apart. You'd need much heavier construction if the tread is high enough that hand rails are required. Otherwise, check out the USFS bridge catalog.


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## Fattirewilly (Dec 10, 2001)

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


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## Vibrato (Apr 4, 2007)

Another resource if you are using rough cut lumber from a local saw mill is from the American Wood Council. Maximum Span Calculator for Joists & Rafters

Enter in the species and diminsions of the limber you want to use.

Enter a grade- if it's solid with few or very small knots I use grade 2, if not I use grade 3 or 4 depending on the condition of the wood.

Member type- floor joists.

Deflection- I usually use Length/360- you don't want a bouncy bridge

I usually hit Yes for wet service areas and No for incised lumber- (incised lumber is different than rough cut and usually green treated)

Next enter the distance between the centers of each joist (Are you building it16 inches on center, 24 inches, etc)

Next use 40 pound live load and 10 pound dead load.

It will calculate what the maximum span distance is for you. Very good for printing and showing the land manager that you didn't just cobble some stuff together. On short bridges you can put the joist 24" wide with 2" nominal lumber for decking, even wider if using rough cut hardwoods- just watch where the knots go, very brittle and subject to breaking unexpectedly.

Good Luck!!


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## DaGoat (Sep 1, 2008)

Thanks for the info folks. Here's some pictures of the Peaks Trail in Breck, they've done some good designs that might work. Looking at other ideas for the Cumbres Pass CDT, so again, thanks! :thumbsup:


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