I appreciate the responses.  You guys have pretty much confirmed what we expected....it's not strong enough.  Now I just need to know if it's completely useless or if there is some way to make it work. 

Cross bracing between the beams is not a problem.  That should alleviate the problem of tires in the middle of the span of the plate.  

How much span would be possible with the structure as presented, with adequate cross bracing? 

Is there any way to get to the 15' span using more of the spec'd beams, without thickening the vertical section of the platform?  Alternatively, how tall of a beam section is necessary to make the span, assuming it is added between the existing beams so that the traffic is above the taller beam.... 8", 13", and 21" H are available.  To the best of my knowledge all are the same width and height,  but I will have to find out the specs on them. 

Further information...most trucks will have tandem axles with axles 54" center to center.  Maximum axle load is 20,000 legally on a road truck, but for the purpose of weighing machinery 30k is probably a reasonable design goal.  

BTW, 17,500 is what the platform has been calculated to weigh, so I think that we are on the right page here in regard to material.