what's the safe working load of my 12" x 8" wood timbers? 1

[jackdub]

I am planning a small bridge on private property to access firewood. I have 4 douglas fir timbers 12" x 8" x 12 ft long. The span would be 11 ft. I intend to use two timbers per wheel 30" apart C/L to C/L. I'd like to know how to calculate the safe working load. My truck with a load of wood weighs 6300lbs. Could I safely drive anything heavier over it with this design? How do I figure my maximum limit?

 

[austim]

Can you also tell us all:
(a) the distance between axles;
(b) your best estimate of the proportion of full load taken
on each axle.

Armed with that info, you may confidently expect a fair number of replies. (some of them may even be in agreement with each other )

 

[jackdub]

more information.
Distance between the axles 11ft-8in.
front axle load est 40% 2520 lbs
rear axle load est 60% 3780 lbs
total 6300 lbs

 

[austim]

Hi again, jackdub,

Perhaps I should have resorted to some primitive calculations before posting my previous response.

Subject to others telling us that I have got a serious error in my calcs, the simple answers to your questions are :

Yes, you should be able to drive your 6300 lb truck over your 12"*8" timbers with complete safety.

Yes, you should be able to drive something at least twice the weight and still be safe. Three times the load is not out of the question.

As to how do you work it out, that is a different matter. The basic arithmetic is fairly simple (and can be found in any elementary text on structural design), once you know the safe bending stress and shear stress. The real problem is in assessing just how good your particular sticks are.

For example, the Australian timber design code works on a system of stress graded timbers. For North American Douglas Fir, there are 5 possible grades used for engineering structures. (There may be others considered to be too inferior for engineering use, for all that I know).

For short term loads (less than 5 minutes application) we are permitted to use stresses of 1100 to 2800 psi in bending, 85 to 175 psi in average shear stress. (For the sort of span that you are proposing, the shear stresses are non-critical).

Thus depending on the quality of your sticks, you could have a variation in safe load of more than 2.5 times.

Fortunately, even if I assume that you timber is of minimal quality with a working stress of only 1100 psi, and assume that your truck is being driven quite furiously (impact factor of 50%) I find that you could safely apply a central point load of up to about 3000 lbs on a SINGLE 12*8 (12" vertical, of course).

To be quite sure that you maximise the safe load of your bridge, I strongly recommend that you fix some longitudinal guide timbers above your deck so that your truck wheels are constrained to be fairly close to the middle of each pair of timbers. That way you could take a static axle load of at least 12000 lbs or so over your bridge.

Of course, if your timber is of reasonable quality, then a 10 ton axle would be quite reasonable.