Calculating loads on oak beam for ditch crossing

[VanBurenSoilGuy]

I am proposing to construct a temporary crossing across a wide ditch using 12" x 12" oak timbers. Four 20' long timbers are bolted together, similar to a crane mat. I am proposing to place a set of them from top of bank to top of bank to form a 8' wide deck. The load will be a 35,000 lbs 3 axle earth moving vehicle. How would I calculate if the timbers would hold up to this type of loading? Also, what are some important factors I would need to consider?
Thank you in advance.

 

[CANPRO]

you need to determine the load distribution for your vehicle and then apply the loads to the supporting timbers. Determine the resulting stresses within your timbers and see if they are within an allowable range. Not sure where you are located, but I'm sure there will be governing standard/code for this temporary structure. You haven't specified the span of the opening.

On another note, 20' long 12"x12" Oak sounds like a nice chunk of timber...seems like a waste to just drive over them.

 

[VanBurenSoilGuy]

CANPRO,
The span of the opening is approximately 12', with 4' of both ends supported by earth. These supporting areas will need analysis as well, but I'm not concerned about those at the moment. Could I just apply a point load mid-span to be conservative? Also, do you have any suggestions on where I could look to see allowable stresses on timbers?
Thanks CANPRO.

 

[SlideRuleEra]

Could I just apply a point load mid-span to be conservative?

Yes, for bending. No, for shear - apply the point load adjacent to one of the (assumed) supports. Be certain to carefully select both the value of the point load and the number of timbers that it is applied to. This approach is probably too conservative.

...any suggestions on where I could look to see allowable stresses on timbers?

http://www.forestryforum.com/members/donp/Fblist.htm

Without details on the load (wheel base, tire width, weight distribution, etc.) its not possible to provide more than general (somewhat obscure) answers, such as those above.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[jayrod12]

On another note, 20' long 12"x12" Oak sounds like a nice chunk of timber...seems like a waste to just drive over them.

Amen

 

[ornerynorsk]

Machinery's Handbook has some data for wood timbers.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[JStructsteel]

Why not hire an structural engineer for $500 and loose the liability, or worry.

 

[sekwahrovert]

You might consider something like the temporary crossing structure shown here:

Bridge Video

 

[VanBurenSoilGuy]

Thanks to everybody for all your assistance.

 

[thaidavid40]

Depending on where you are located, 12x12x20' timbers could be pretty expensive. Would you be able to use some salvaged wide-flanged beams instead? You could then store them for reuse, sell them to someone else, or scrap value them. Just a thought ...

Thaidavid

 

[ornerynorsk]

Found the source I was looking for in addition to Machinery's. 15 MB download, but chock full . . .

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.