I-beam bridge design - my head is spinning...

I'm trying to come up with a suitable bridge design for my property to span a creek, and could use some help with all the formulas. This will be a simply-supported bridge, and the length of the i-beams that I have available is 18'. The beams measure 6" x 6" x .25" thick all over. I plan to space the beams to be directly under my compact tractor's tires (approx. 45" apart center-to-center). The bridge will be decked with lumber (2 x 8's ?). No support underneath in the middle of the span.

I'm trying to determine if this will be sufficient to support a max load of ~6000 lbs (tractor w/FEL, tires weighted with beet juice, loaded bucket, rotary cutter & driver) and what the max deflection in the center might be. I can use an additional i-beam (for a total of 3) for the bridge. I have 6 i-beams total, but would like to construct at least 2 bridges.

Any insight you might be able to offer? If it turns out to be overkill, I'll go for it. If it's only marginally adequate or completely inadequate, I'll have to come up with a better solution.

-WRM

Download Beamboy.

If you want to do the calculations yourself and have a little bit of an interest in math, youtube factor of safety calculations and you'll eventually get to the awnsers you seek. The math isn't too overly complex but the number of steps can get a little overwhelming.

Just err on the side of a larger than necessary factor of safety, like 2 or something. And remember static loads are much different than dynamic loads so overbuild accordingly.