Another Bridge Thread

[ballegre]

Sorry guys. Thought this thread was dead. Just realized there was a whole 2nd page!

What happens when you decide to upgrade to a bigger tractor five or ten years from now?

The tractor I have now is far too big. I over bought so it will never be a problem.

Personally, I would not design for "should never exceed". I would go with a minimum of 2x or 3x capacity.

Unless I miscalculated. I believe I am over designing by a factor of 2.

Also, are you considering the weight of the decking material?

I am now 8'-2x8 is 30 lbs. I'll be using 18 so 540 lbs. Even if that's low there's still plenty of capacity to hold.

Does the creek flood? A couple of miles down the road, a guy put in some nice piers and set a flat bed truck trailer on it for a bridge. He built up some nice approaches with creek rock. The first good gully washer washed out the approaches and moved the deck a couple of feet The next big storm washed it out entirely. We can now see the end of the buried deck a couple of hundred feet downstream with about 5' of it sticking out. He gave up on the project.

Everything floods around here because of hurricanes. It's not really even a creek. It's mostly just a muddy mess especially when I drive my tractor through it. It's run off and some natural springs.

 

[ballegre]

Could you use 16 foot 2x12's. That would give you the two feet overhang ( each end) on your piers for the ramp.

I could. Then I would have to build support for the ramps as well.

 

[ballegre]

Oh yeah, I like that. :tractor:

The concrete support does span the entire width of bridge. It's a round column that supports the beam.

 

[ballegre]

Most of your designs are not going to fail with a point load on the center of the beam. everyone likes to point to this relatively simple equation to get their design strength. Most people forget about adding in the weight of the decking, or the dynamic loading of their equipment. usually though, the support will fail at the connection points first. The shear stress or crushing force at the supports is what mostly dicates any beam design, usually followed closely by torsional loading due to a slenderness ratio that should not be exceeded to provide a safety factor. It has been a long time since I have been in my structures class but thats vaguely what I dredged up out of my memory.

Also, an acceptable level of deflection can also be a governing equation. A beam might hold a load while deflecting 6" or more but thats not very confidence inspiring nor does it lend well to having anything else attached to it. There is a reason engineers are required to be licensed. I'm not saying that building a bridge is impossible nor does it require an engineer to design/construct a simple crossing. Knowing how to correctly design things takes more than just picking a single equation.

Thanks for the advice. If you look at the calculations on the web site I referenced (http://www.forestryforum.com/members/donp/beamclcNDS2.htm) it includes bending, deflection and shear. I used a load of 3000 lbs because I started with fully loaded tractor weight of 6000 lbs and doubled it to 12,000 lbs, then divided by 4 for each point of contact. I derived the tractor weight by using 3000 lbs as base, + 1000 for cutter, + 1000 for loader, + 1000 for payload which is 6000 lbs and then doubled to 12,000 lbs.

 

[ballegre]

Be sure you inquire about needing any permits to build this bridge. I think permits would only be required based upon the size of the structure, if it were crossing a stream & if the stream had fish etc, etc. Your county planning/building dept should be able to answer this question. I know how distasteful it is to call a gov agency and let them know what your plans are but its far better to know in the beginning then have them stop the project half way thru.

Oh yeah. I forgot about them guys. I will contact county because it is a run off into a stream that borders my property. This may complicate matters. Thanks.

 

[ballegre]

One thing to remember is that not all pressure treated wood is rated for ground contact. As dirt collects on top of the cement between the wood it will start to rot much quicker.

I'll be sure to ask. Thanks.

 

[ballegre]

You need to add a 4x safety factor.

You probably need a buildup of five 2x12s
Pre drill your bolt holes and then when you put it all together use some construction adhesive to glue the 2x12s to each other.

I should mention that this is a small crossing about 2.5 feet high at center. Most people would be content with a muddy mess and drive their tractor through it. I WANT to build a bridge if it's feasible (I don't have to). If it really takes 5 laminated 2x12's then that's beyond my scope. I do appreciate the input and concern.

If you have access to some 1/4" or 3/16 plate you can sandwich your wood around it an bolt the assembly together. It's called a Flitch plate round this part of the country.

I don't have access. If anyone has ideas about where to buy this I would like to hear.

For the approaches you could build some ramps out of 2x6. They would be semi sacrificial.
You could also get some timbers to lay in front of the bridge to form a ramp. A 8x8 then 6x6 then 4x4.

The stair step idea is a good one to consider. Thanks!

 

[medic0648]

For the flitch plates check with local fabricators or steel suppliers. If you have a plasma cutter or oxy acet outfit or know someone that does you could easily cut them yourself. Migh need a welder if you can't find long enough sheets for a decen price.

 

[Hiltz]

I could. Then I would have to build support for the ramps as well.

The ramps would be cut from your overhanging beams. Nothing extra to build. Under fill with gravel.

 

[RedNeckRacin]

Thanks for the advice. If you look at the calculations on the web site I referenced (http://www.forestryforum.com/members/donp/beamclcNDS2.htm) it includes bending, deflection and shear. I used a load of 3000 lbs because I started with fully loaded tractor weight of 6000 lbs and doubled it to 12,000 lbs, then divided by 4 for each point of contact. I derived the tractor weight by using 3000 lbs as base, + 1000 for cutter, + 1000 for loader, + 1000 for payload which is 6000 lbs and then doubled to 12,000 lbs.

So I just want to clarify that you are calculating for the 6000lbs per beam and not two single seperate 3000lb loads plus 250lbs evenly distributed for decking. Being that your beams are narrow, do the connections (i.e. nails) have any appreciable effect on the top wood fibers being in compression? Also, do you have a way to analyze the beams for torsional failure? I think some sort of bracing would be crucial for success in this instance. Some diagonal bracing would be very benefical here. I would consider very stronly going with much more than a 2 or 3x safety factor, 5 sounds much more reasonable. Wood decays outside no matter what.

Using steel plates for your transition areas from wood span to concrete support is an excellent idea and the gluing of members is also an excellent suggestion. Its not worth going cheap. Ove rdesign it in case you neglect something. Just my $.02