Another Bridge Thread

[Ken45101]

My Kubota L3010 with cutter and fully loaded front loader should never exceed this lb limit.

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

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

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


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.

 

[Hiltz]

Hi All,

I've been designing a bridge to hold my tractor using information I collected from here. The overall design is 12ft long with beams 5ft apart matching the wheel width on the tractor. The beams are to be 2 x laminated and bolted 12ft long, 2x12s which will give me a 12ft long 4x12 beam on each side. According to this web site http://www.forestryforum.com/members/donp/beamclcNDS2.htm, should hold at least 3250 lbs at each point for a total of 13,000 lbs using No 2 loblolly pine.

My kubota L3010 with cutter and fully loaded front loader should never exceed this lb limit. However, the devil is in the details.

My problem that I am having is due to the way I want to set the bridge beams on the concrete columns.

View attachment 320672

I'm designing to avoid having wood touch dirt to prevent rot (and yes I'm using pressure treated wood). So the colums will be at grade (grade slopes down toward the small creek) and, as you can see from the diagram, I want to set the beams on the columns and cut out a short 2 ft ramps in the beam to get on and off the bridge.

So my question, to anyone who has an opinion, is by cutting out the ramps on the beam, have I weakened the entire beam (I assume to the 7" depth that I have indicated)??

Of course I would never hold anyone liable for any information they provide and release you from any and all legal responsibilities for any opinions you offer

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

 

[Cordfed]

Get an old 40' single wide, rip off the top and use the frame as the bridge.

 

[radioman]

I would do this:

View attachment 320705

Bruce

I can one up this idea - Add ramp in concrete design.

 

[medic0648]

I second the over-building. You never know if you'll need/want to take something heavier across. Also wood will degrade and you may get a sudden failure with you on it being close to the load limit. Most road bridges are built to 2-3x the rated load to accomidate unforseen issues and degridation. I'd rather have it crack or bend rather than ride it down suddenly.

 

[bigtiller]

I can one up this idea - Add ramp in concrete design.View attachment 321305

Oh yeah, I like that. :tractor:

 

[RedNeckRacin]

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.

 

[paulwestski]

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.

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.

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.

 

[oosik]

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.

 

[crazyal]

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.