Wooden Bridge

[jgratton]

Good afternoon! I live in Northern NY (6 hours North of NYC, 3 miles from the Canadian border). I have a gulley that I would like to install a bridge over. The gulley is approximately 18' wide and 6' deep that drains the surrounding woods and an overgrown hay meadow. I have a nice wide wood road built on both sides of the gulley. I own 130 acres of land, of which approximately 105 acres are a mix of cedar, hemlock, pine, cherry, beech and tons of gray birch. I would like to build a bridge over the gulley that will handle the weight of a 68 HP International 2944 tractor with an old manure spreader attached to bring the wood out. I want to construct it solely with the materials I have on my property. I am going with hemlock stringers, 2' apart for a total of a 10' wide decking. I plan on the stringers being a minimum of 12" in circumference. The decking for the bridge, I want to be 3"x8"x10' hemlock.. Lastly, I want to install pillars in the center of the stringers to a concrete footer approximately 3' of concrete in each pillar with approximately 8" of concrete exposed so the pillar is not constantly in contact with a damp surface. The pillar size will also be 12" in diameter. I am far from an engineer! I would appreciate any comments on this whether I am over building or under building it. Thank you!

[sprucebunny]

Welcome to the Forestry Forum !

I don't really understand your description. I keep picturing the cement support being in the stream to connect to the 'center' of the stringers. I would think a cement pad on either end of the stringers would do more to protect them from moisture.
In any case, rain etc will be wetting the stringers and eventually degrade them. By themselves, 4 12" x 12" stringers should be enough for the weight ( by my guess)

I made a bridge over a similar stream by placing an old homemade trailer set on some stones at either end across the gap and decking it with pressure treated 2x6s. ( It had a steel deck where the tires would travel on it.)

[ChugiakTinkerer]

...
I would appreciate any comments on this whether I am over building or under building it. Thank you!

Anything worth doing is worth overdoing!  So from that perspective you can't overbuild this bridge.

I've played with the beam and column calculators on the FF Toolbox page, but they are built around the assumption of using squared off timbers.  I'll jump in using those and run some calculations based on the largest square timber you can cut out of your hemlock logs.  if it works for the timbers it sure will work for the logs. 

First off we need to get some weights for the load calculations.  I'm not familiar with your tractor, but I notice that a IH 664 has a 68 hp engine and weighs 6,000 lbs.  There is a 25-bushel spreader that weighs about 500 lbs empty, and about 2,500 lbs full.  Let's throw in some extra odds and ends and say the total load is 10,000 lbs.

You'll occasionally have snow to contend with, although probably not when you're spreading manure.  Still, let's figure out the snow load.  It looks like portions of northern New York go as high as 80 pounds per square foot (psf) for the ground snow load, with 50 psf at the low end.  I'll use 80 psf for this exercise, but you would want to adjust that based on where you live.  Whoops, just realized you're talking about hauling lumber instead of manure, so the 10,000 lbs load is probably more accurate.  For the snow you multiply the load by the area.  Let's look at a 10' wide bridge.  Your area is 180 sf, and the snow load is 9,000 lbs.  In a heavy snow scenario you would have a bridge supporting a load of 19,000 lbs, which if evenly distributed is 106 pounds per square foot.

Each stringer is supporting an area 2' wide by 18' long, which amounts to 36 sf. Thus each stringer is carrying a load up to 3816 lbs.  With a support pillar in the middle, the shortest unsupported span will be about 9'.  We can take these values and plug them into the FF Toolbox beam calculator at https://forestryforum.com/members/donp/beamclc06b.htm.  You can also access the toolbox by clicking on the bottom-most red icon at the left side of this page, below the sponsor links.  Using an 8x8 timber of #2 Eastern Hemlock, the stringers pass the load calculation with no problems.

There is a log beam calculator too, and it is located at https://forestryforum.com/members/donp/logbeamcalc.htm.  All you need to do is input the load, the span, and the species and grade.  With a load of 1906 lbs, a span of 108 inches, and selecting Hem/Fir #3, the minimum diameter log is 7.1".  According to the this same calculator, your logs will work even if you don't have a pillar in the middle.  Using a load of 3816 lbs and a span of 216 inches, you can get by with 12" minimum diameter in almost every Unsawn species.  In your shoes I would probably still build with a center pillar, but it would give me great comfort to know that the logs could support the bridge even if the pillar failed.

Looking at the pillar strength, there is a column load calculator too.  It assumes a square post, so I am using an 8x8 which is the size of a cant from a 12" log.  A single log is capable of supporting the entire load of tractor and snow.  You would of course want to have at least two posts, but you could use that and a cross-beam rather than putting a post under each stringer.

The last thing to look at is the footing for your columns.  Soil strengths vary considerably, but a conservative value is 1,500 psf for clayey sand, etc.  The pillars will potentially be bearing half of the total load, so you want a footing capable of supporting 9,500 lbs.  That means a footing with an area of at least 6.33 square feet.  A 12" diameter pillar of concrete has an area of 0.785 sf, so you would need 9 pillars to meet that area requirement.  That doesn't sound like what you are aiming for.  If you increase the footprint of each concrete pillar by using a Bigfoot form, then you could have a single pillar that has an area much larger.  A 28" bigfoot pier has an area of 5.6 sf, so it would only take two of them to support your bridge adequately.

Bottom line, I think your bridge plan is great.  The stringers and columns are more than adequate, and I wouldn't go so far as to say overbuilt.  Your column footings though could use some beefing up, especially if your soils are poor.  I would suggest a center support structure formed of two upright logs topped with a cross-beam and cross braced.  Use two concrete pillars for your footing but have a much larger base than you typically get from a sonotube dropped in an auger hole.  Or find another way to increase the footprint of the footings.  All of this is caveated by the fact that I am not an engineer, merely an amateur who enjoys puzzling these things out.  On the off chance you trust my advice, you should verify before putting yourself or your equipment at risk.

[sandsawmill14]

we used to build bridge that long or a little longer on our place with 4 12-16" white oak trees layed across the ditch with a 2 1/2 " thick top with NO center supports and would drive loaded grain trucks across it ( 9-10 tons) with no problems it would last about 10 years and we would have to redo it  the problem with the pillars in the creek is catching stuff washing down the creek and backing up the water and washing the bridge out  : but as i said we used white oak and i have no idea how hemlock compares as far as strength or rot resistance ???
we got lucky and bought a overhead crane out of the factory and cut enough I beam off of it and used 2 of them the last time about 15 years ago now it needs a new top but it still held a 285 MF and round hay baler this year but hope to retop it before next season the rest of the crane is in my shop there are some pics of it in my gallery

[Magicman]

I am going with hemlock stringers, 2' apart for a total of a 10' wide decking. I plan on the stringers being a minimum of 12" in circumference. The decking for the bridge, I want to be 3"x8"x10' hemlock..

I am assuming that you meant diameter, not circumference?

My bridges have all been built with treated discarded utility poles for stringers, but the principal is the same:  Building a new bridge LINK and Rebuilding another bridge: LINK

I prefer to lay the stringers ~4"-6" apart and use 2" thick decking.  I am easily spanning the same distance and I have never used or even considered the need for a center support.

[DDW_OR]

I agree with the utility poles.

another option to a wood bridge is to use "flat pack shipping containers". the type i am talking about is just the floor.

[Ox]

The International 2944 stood out to me as I never heard of it.  Tried to search and nothing came up.  Is 2944 correct?  It should have come up in TractorData.com.

[GAB]

Make sure the hemlock doesn't have the shake, because if it does I would not trust the above mentioned calculations.
Gerald

[MSU_Keith]

http://www.dot.state.mn.us/bridge/pdf/insp/USFS-TimberBridgeManual/
http://www.woodcenter.org/docs/em7700_8--entire-publication.pdf

I built a 12' span bridge strong enough for a 30hp tractor following the guidelines in the link above.  No issues for the first decade.  Make sure you calculate loads/deflection/shear and choose the size/species of your deck beams to have a nice safety factor for the weight of your tractor and you should have no issues.