# stringer size question



## Simpleton (May 3, 2004)

We are going to be building 3 bridges: one 12', one 15', and one 20' span.

My question: if we are using pressure treated pine, how large do the stringers need to be?

I know I saw this information in a USFS publication somewhere but I can't seem to locate it now. Any help you can offer is greatly appreciated. Evidently my searching skills are broken today.

Matt


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## Simpleton (May 3, 2004)

Thanks to Fully I found it! http://old.imba.com/resources/trail_building/trailbridgedetails.pdf


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## pascale27 (Aug 26, 2011)

Depending on the bridge length and width, I've used different stuff. For some small logover stuff we used PT 1x4 (which is actually 5/4x4) everything else get PT 2x4 for decking.
This pic is a 10' section with the other side being 7'. We used PT 4x4 on the outside with a 2x4 middle stringer.









We overbuilt this, since I had the PT 4x4s already so we didn't have to buy as much wood and made this thing bombproof.


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## bme107 (Jul 23, 2008)

I noticed that that design guide was from 2004. Recently we've been seeing a reduction in design strengths for new southern pine. Check with a design professional for the most current data for your application.


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## jimithng23 (Feb 11, 2009)

maybe the guy who designed our bridges will chime in here...

we just built 6 bridges on one .86 mile trail extension, with the shortest span being 18'. on the "short" spans, we sandwiched 2 treated 2x12's x2 for stringers. on the 20' spans, we used 3 2x12's x 3. for decking, we used 5/4 rough cut lumber. I'm told the bridges are rated at 100lbs/sq ft.

check this thread for more information on this project.


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## Vibrato (Apr 4, 2007)

Keep in mind that this was a blue trail, and so the width of the bridges has to be at least half the height at the tallest point. Greens and blues have other criteria you should keep in mind.

I chose 100 pounds per sq ft design criteria because I wanted to avoid as much bounce as we could. In the 2012 international code sereis, commercial decks have a 100 lb load and meet the maximum deflection requirements. (Sorry, I'm a code geek- this means they don't bounce much)

The 18 ft bridge was 4 ft wide, the 20 footers were 6 feet wide so as to not need sides 42 inches tall. Going from 18 to 20 feet long and 4 to 6 ft wide adds 48 square feet, that 4800 pounds in additional design load!

If I remember right, the ladder bridge designs oin the IMBA doc (which are fantastic!) didn't go to 20 feet, and had lower loads. Lower loads aren't a problem generally on MTB trails, but to determine what loads you can handle- The American Wood Council has a span calculator for all types of wood located here- Maximum Span Calculator for Joists & Rafters

If using green treat, start with these inputs:

Species: Southern Pine
Size- 2x8
Grade #2
member type: Floor Joist
Deflection limit L/360 (this is the bounce- length divided by 360. 20 foot span will have .66 inch bounce)

Spacing: How far apart you plan to space your runners, measured from center to center. If you don't know, I wouldn't start out with less than 16 inches unless its a REALLY long span. Be aware that 5/4 deck boards cannot be placed over runners more than 16 inches apart, and nominal sized 2 by lumber can only span 24 inches between runners. If using hardwoods you can span much more between runners and over hang quite a bit, too.

Wet service conditions: Yes (subject to wetting and drying from weather)
Incised lumber: no (Southern softwoods are not geneally incised before being treated)

Start with live load of 40 and dead load of 10 to get a 50 lb load per sq ft. This is therating for residential decks and for MOST backwoods mtb trails, its fine.

One more thing- using green treat for decking is a pretty bad idea if you live in an area where it gets wet (dew in the early mornings, trails handle small sprinkles well and people can ride them before the wood dries out, etc. Green treat is AT LEAST as slippery as smooth ice when it is wet, and the small contact patches of the ENDS of the knobs on your tires combine to make this actually pretty dangerous. Rough cut hardwoods are AWSOME. Look for a local saw mill and go buy their scrap its cheap and friggin awesome.

Good luck!!


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## zenolith (Jun 27, 2008)

All the bridges we (USFS) build are rated for 150psf dead load at least and 85psf live load. This is because the stringers, railing and deck, hardware all count as dead load. Live load is any temporary load including 10 feet of snow (common here) and a pack string, or a whole herd of kiwis on bikes posing for a photo.


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## Vibrato (Apr 4, 2007)

zenolith said:


> All the bridges we (USFS) build are rated for 150psf dead load at least and 85psf live load. This is because the stringers, railing and deck, hardware all count as dead load. Live load is any temporary load including 10 feet of snow (common here) and a pack string, or a whole herd of kiwis on bikes posing for a photo.


Excellent points- the inputs I described for the joist calculator are for trails with bikers and hikers only, in an area with a 25 pound ground snow load.

If you build where there's 10 feet of snow and pack animals using your bridges... GO BIG!!! :eekster:


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## Simpleton (May 3, 2004)

*question for Mr. Vibrato*

Thank you very much for your help. What IMBA doc are you referring to? Sounds like I need to get my hands on it!

Our organization is trying to come up with a procedure to design & approve bridges and we need an authoritative source to turn to. USFS seems over-built for our application (MTB'ers and hikers only) but I don't know of other sources. TIA for any additional help you can offer.

Matt


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## Vibrato (Apr 4, 2007)

Simpleton said:


> What IMBA doc are you referring to? Sounds like I need to get my hands on it!


The one I was refering to is this one-

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

I guess it's not technically an IMBA design, I just originally found it on IMBA. Its getting harder to find as it gets older, so you may want to download and save it locally for future reference.

Good Luck!


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## zenolith (Jun 27, 2008)

Google "R6 Trail Bridge Catalog"

Pick a set of drawings from the list (like "Multiple Stringer Log Bridge," use Region 6, RA840-2, drawings are more detailed), open it and scroll down to see the chart. Chose the load for your application to find your stringer size, and go to work.

We "overbuild" on purpose because bridges last longer and can often survive a hit from a big Doug Fir (not the railing and sometimes not the deck), but you can use 9 inch stringers on a 15' bridge with a ped load of 65psf. That is NOT overbuilt.

Have fun!


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