# MTB bridges using steel bar joists



## willworkforbeer (Jan 14, 2007)

Anybody done a bridge using steel bar joists?

We are looking at a 24ft span, and the specs we are using for a wood stringer mtb bridge says we can't do it (based on 100 lb/sq ft design load). Got a guy local that has some surplus 24 ft bar joists, and I am thinking that 2 of those with some treated 2x6 on the top, plus some cross bracing to prevent tilting might do the job. 
Chased some basic bar joist info, and the size we are looking is rated at well over 200 lb/sq ft. 
2 of these would cost less than trying to build a shorter bridge from 2x12s.
Probably looking at a 3 ft wide bridge (not wide enough for a 4 wheeler)

Anything I am missing? This sounds too easy.


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## Joules (Oct 12, 2005)

weatherproofing/rust prevention? Weight of the joists and how you're going to transport them to the site. 

With a span that long, metal beams seem to me the best way to do it.


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## willworkforbeer (Jan 14, 2007)

Not sure on rustproofing, but I think even bare they will last quite a while once the initial rust forms, and as long as no water is trapped on them. (yes, I know rust never sleeps....) I am thinking a layer of tar paper between the steel stringers and the wood decking to shed most of the water. Would not be that big of a deal to slop on a bunch of paint before the install though.
Weight - don't have it in front of me, but I think it was 6-7 lbs per foot - a lot less than wood.
2-3 guys could easily carry one in, and we don't have far to go.


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## Trail Ninja (Sep 25, 2008)

I think you have it covered. Bring lots of batteries for your drills. Take the time to paint the joists. I wouldn't bother with the tar paper. A couple of coats of paint will last WAY longer.


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## Dave_schuldt (May 10, 2004)

Sounds cool. How will you connect to abutments? Do you have snow load? If cambered you may need to have one end slide. Bridging is critical.
Post pictures.


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## Trail Ninja (Sep 25, 2008)

Dave_schuldt said:


> Sounds cool. How will you connect to abutments? Do you have snow load? If cambered you may need to have one end slide. Bridging is critical.
> Post pictures.


OP's in Memphis. Snow shouldn't be a problem. 
In Canada here we would space the decking over 1" and not even worry about snow on a 3 foot wide bridge.


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## willworkforbeer (Jan 14, 2007)

yeah, snow is not a real issue in Memphis, but gonna space the decking about 1" apart to clear mud. Am planning on this being relatively level construction, both end to end and side to side.
Still thinking about the abutments. Probably going to use 2 4x4 posts sunk into the ground with one 4x4across the top on the end that has to be elevated to get level. The other end, either a section of railroad tie or treated 6x6, secured to the ground by bolting to a couple of 4x4 stub posts. Kinda depends on the configuration of the end of the joist; will see once we get them. One side of the ditch is pretty steep, the other, not so much, so I think the methods will have to vary from side to side to maintain a level bridge.
This area floods sometimes, so it will have to be attached to the ground.
Will take plenty of before and after pics.


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## RidingMyTrail (Oct 7, 2012)

Are you on MSTA?

Where are you building the bridge and do you need help?

I've considered building a couple of bridges out of bar joists to span about 40'.


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## mtbty (Jun 15, 2012)

maybe try using more natural rot resistant decking, so bad stuff doesnt lech in the the ground water. 
A trail that i'm over seeing in the northwest has 2 bridge that spans fish steams with 2 75' steel I-beams spanning to concrete abutments on ether side decked with 3" ceder decking and bummers.

go for your most natural rout resistant lumber (3"+ thick) and concrete formed abutments! don't forget the rebar. 

plus being able to say you avoided chemical leching products for more natural stuff sounds good to land mangers and enviros.


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## Ridnparadise (Dec 14, 2007)

These pics come from a trip to Jindabyne and Lake Crackenback Resort and Spa up the valley.

The decking is clipped to the frame which is square tube steel. The ends are secured to similar steel tubes protruding from the intended base with simple square brackets - peg in hole style. The brackets are simply brazed to the frame and finally bolted to the posts. Simple. Some of the bridges are grand, some are simple. The surface is very grippy like sand bonded on.

http://forums.mtbr.com/attachment.php?attachmentid=737183&stc=1&d=1352798251

http://forums.mtbr.com/attachment.php?attachmentid=737181&stc=1&d=1352798251

http://forums.mtbr.com/attachment.php?attachmentid=737184&stc=1&d=1352798251

http://forums.mtbr.com/attachment.php?attachmentid=737185&stc=1&d=1352798251

http://forums.mtbr.com/attachment.php?attachmentid=737182&stc=1&d=1352798251


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## willworkforbeer (Jan 14, 2007)

Yep, MSTA member, have discussed this one with Brad a little. He gave me the local specs for mtb bridges, but they don't address this type of construction.
Bridge is going to be just east of I-40 near the lake, first ditch crossing. Right now there is a section of pallet rack upright laid down with some 1x6 screwed to it. Very flimsy, we never ride over it, and don't really like walking over it. To call it a bridge is an insult to all bridges elsewhere.
May need a couple of people when we get it all worked out and get all the materials on hand. Right now I am thinking building the abutments one weekend, and setting the stringers the next so the concrete gets a chance to dry. If it gets too cold to fast, may have to wait until spring, don't want the concrete to freeze before it sets properly.


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## RidingMyTrail (Oct 7, 2012)

I suspected that might be the area after reading the reroute post that percolated to the top yesterday. My basic thought was to hang the bar joists from the abutments like they attach them for a roof. Look up when you are in a Lowes to see what I mean. The options I had are to pour concrete and bolt the joist to that or place some I-beam and weld the joists to that. I do like the above pictures though and that gives me some ideas tool.


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## RidingMyTrail (Oct 7, 2012)

Ridnparadise said:


> These pics come from a trip to Jindabyne and Lake Crackenback Resort and Spa up the valley.
> 
> The decking is clipped to the frame which is square tube steel. The ends are secured to similar steel tubes protruding from the intended base with simple square brackets - peg in hole style. The brackets are simply brazed to the frame and finally bolted to the posts. Simple. Some of the bridges are grand, some are simple. The surface is very grippy like sand bonded on.


I like that decking. I would like to know more about it and get an idea on where to source it. Does anyone recognize this material and have information about it?

Added: Went on a search and it appears that the deck is fiberglass reinforced plastic (FRP). Fairly expensive though. More info: FRP Floor Grate Fabrication - Cutting and Installation of Grates


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## Bill in Houston (Nov 26, 2011)

that looks like fiberglass grate with carbide or aluminum oxide grit embedded.
Fiberglass Grating - Direct Metals


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## swampboy62 (Feb 10, 2009)

The first thing I'd wonder about is the bearing condition - are they top chord bearing joists? Do they have the extended bottom chord to tie in?

How deep are they? Having flood-borne debris come downstream against the joists would be a bad way to load them. 

Usually bar joists require some pretty solid connections, and would need a fairly substantial fixed footing at each end to keep it from moving.

And I would think that you'd want make sure that the bridging is adequate to keep the joists fixed to each other - you wouldn't want one of them to start creeping or twisting.

I may be overthinking this, and it certainly could result in a strong and long lasting structure, but I'd want to be sure I got it right. A structural engineer could answer a lot of questions.

Good luck.

Steve Z


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## willworkforbeer (Jan 14, 2007)

Yes, they will mount on the top bar, and will be 18-24" deep (tall), no bottom chord extension I think.
Should have clearance from debris in this area - unless a tree floats by - and then it would not matter what type of construction we used
Still working on the abutments - will have to make some measurements on the actual joist ends to finalize - I think bare minimum is something like a railroad tie as a base, secured by driving rebar into the banks, and/or a couple of 4x4 posts set in concrete to keep it in place.
Will definitely need X bracing to keep them in place and prevent independent movement, not a big deal though.


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## RidingMyTrail (Oct 7, 2012)

My plan was to use 3 of them on 16" centers. X bracing and welding to create a box structure. The biggest concern I had was the 24" depth and constructing the approaches. My situation is a little different as the creek I'm crossing becomes a raging torrent several times a year. 

I've since procured some 40' telephone poles and am going to use them for my first crossing. Depending on how that goes, will dictate the other three designs.


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## RidingMyTrail (Oct 7, 2012)

Moved to my own thread to prevent a hijack. 

http://forums.mtbr.com/trail-building-advocacy/building-pole-bridge-824879.html


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## radair (Dec 19, 2002)

We have built a couple of bridges using steel bar joists. I am a structural engineer and love building with steel. For long spans it's very economical.

Both of these bridges are 3' wide, with one using 3 stringers and having a 5' section in the middle using cantilevered decking. The other we spaced the stringers at 2' and overhung the decking 6" each side. This one has a railing on only one side. We also had to lengthen one as our donated joists were not long enough, which was a pain but came out well.

In both cases we welded horizontal braces and burned holes to attach 4x4 posts for the rails. We also used a bunch of diagonal bracing, alternating from top chord to top chord (horizontal plane) and from top chord to bottom chord of other joist (vert plane). They are both very stiff and pretty bombproof.

We used concrete abutments, in one case pre-casting them in my garage and bringing them in with a small tractor (double-track access), the other we used a portable mixer & generator and poured on site. We pre-fabbed the steel superstructures, put them on temporary wheels and rolled them in - each were wheeled in and decked on the same day.

Great projects. Let me know if you have questions.


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## radair (Dec 19, 2002)

swampboy62 said:


> The first thing I'd wonder about is the bearing condition - are they top chord bearing joists? Do they have the extended bottom chord to tie in?
> 
> How deep are they? Having flood-borne debris come downstream against the joists would be a bad way to load them.
> 
> ...


Steve's points are right on the money and are important considerations. Top chord bearing joists are not going to be the right choice for some sites.


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## RidingMyTrail (Oct 7, 2012)

radair said:


> We have built a couple of bridges using steel bar joists. I am a structural engineer and love building with steel. For long spans it's very economical.
> 
> Both of these bridges are 3' wide, with one using 3 stringers and having a 5' section in the middle using cantilevered decking. The other we spaced the stringers at 2' and overhung the decking 6" each side. This one has a railing on only one side. We also had to lengthen one as our donated joists were not long enough, which was a pain but came out well.


Those are some nice looking bridges. :thumbsup:

Thanks for the guidance.


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## willworkforbeer (Jan 14, 2007)

Nice bridges!

I think ours will be similar in construction, but the cross bracing will have to be bolted on in some fashion (without drilling the top or bottom chord) unless I can get an engine driven welder to the site.


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## RidingMyTrail (Oct 7, 2012)

willworkforbeer said:


> Nice bridges!
> 
> I think ours will be similar in construction, but the cross bracing will have to be bolted on in some fashion (without drilling the top or bottom chord) unless I can get an engine driven welder to the site.


Nick did get that skid steer back there, so we should be able to carry a welder/generator back there in the bucket.


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## willworkforbeer (Jan 14, 2007)

True, but he has already contributed the bar joists, I hate to pester further.
For every problem there is a work-around.........


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## Larry_K (Jul 10, 2010)

Here's what we got done on it today. 4x4's in 24" deep holes (using concrete) bolted to 6x6's. The outer joists are 4' apart with one centered between them.

Still quite a bit of work left but the "hard" part is more or less complete now. If the welder can get there tomorrow to get the cross bracing in we may be able to get it completed by the end of the weekend but it may be next weekend before we're able to finish up. This section of trail isn't heavily used but it'll be great to have it complete once again. The portion of a "bridge" under the joists is what folks have been using to get across that ditch.


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## radair (Dec 19, 2002)

Looking good! It's nice to have an old 'bridge' underneath to help get the new stringers across, that can be the hardest part of a longer span project. Excavating space for the bottom chords is a pain, eh? Be sure to leave enough room for air to get in there. If leaves and debris pile up they will trap moisture and shorten the life of that steel.

Nice work, cheers!


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## willworkforbeer (Jan 14, 2007)

The latest word - welder for cross bracing may or may not show.
Will be bolting in some bracing tomorrow to handle it. More is better, but ............
Will also secure the bar joists to the 6x6 on both ends, and add the 2x4 strips to the tops to screw the decks to.
Bought all the deck material today, but need the bracing first.
Should be rideable next weekend, God willing and the creek don't rise.


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## RidingMyTrail (Oct 7, 2012)

Looks great. Yesterday was a great day for being out there. Today is looking good too.


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## willworkforbeer (Jan 14, 2007)

Bridge complete - got no camera on my phone to take pics - somebody take some tomorrow when you ride it.
Bar joists for bridges are great, I don't think I would do a bridge with wood from here on out.
(unless you can get a couple of phone poles to use, and somebody to deliver and place them .......)


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## Larry_K (Jul 10, 2010)

Thanks for all the hard work over the weekend.

Looks like we still need to do a bit of work on the transitions to the decking but here's the completed bridge.. Do you still have some 2x6's left in case we need to add anything to the ends? I'll try to make it out with Matt again by Saturday/Sunday to see what we can do to help the ends, we might be alright with just moving some soil. Hopefully we'll make our way to the bridge on our Thursday night ride but I won't be carrying any tools, but will finally be able to cross it in person.

The bridge looks great :thumbsup:, and looks much smaller in pictures, doesn't look like a 30'x5' span at all.


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## willworkforbeer (Jan 14, 2007)

Yeah,
The entrances on both ends need a little work, but we ran out of light saturday, and I took sunday off from bridge building. I think the west side will need a board, the other end maybe just some dirt. Might bring a little sakrete down there to mix in with the dirt at the ends to make that clay a little harder, and we need to look at the drainage on the west side to see if we can channel the majority of the water away from the bridge.


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## texaspaperboy (Mar 3, 2013)

I saw how Radar used bar joists to build a bridge, which is exactly what I am wanting to do. I currently have a pole bridge that has been in place for over 50 years and need to be replaced. In the thread I have seen interesting tops for the bridge, both wood and something that looks like fiberboard squares,...I am interested in how Radar extended the joists as I need to go 60 to 65 feet clear spand


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