I am building a bridge consisting of 2 wf14x26 steel beams spaced 6
feet apart with a 36 foot unsupported span. The bridge decking will be
3"x8" oak planks spaced 1/4" apart, 8'-0" long. What is the maximum
safe load this structure can take without excessive deflection...for
example, a 4000# automobile?

I would be glad to take a look at your design for $50. Would you
really want to drive your $30,000 car over a bridge designed for $5.

redhoss-ga
Good comment!

Where is this bridge?  I want to avoid it!

What's a wf14x26 steel beam?  I couldn't find it.  An I-beam?  14" by 26"?

A W14x26 is 14" deep (nominal) and weighs 26# per foot.

Thanks, learn something new every day, which is saying that I really
don?t know much.
I checked out the dimensions for the crane bridge beams on a couple of
sites, looking at 36 ft spans.
Here is one that does have 14? beams.  You will have two, so I expect
that 1 ½ ton each is adequate max. load weight.  You can see what is
recommended for this load and span, and also see what load is
considered max. for 14? beams, whereby they all seemed to be of
greater weight per foot.
As far as I can see, WF14x26 seems pretty light for the job. 
Admittedly, there is probably a very high safety factor in the
recommendations, something an expert could tell us or someone might
find somewhere.  An expert might tell us that there is a risk that a
free beam with a top load could twist, maybe that with a top load and
no bracing to avoid this an even stronger beam is required.  With the
crane hanging from the bottom of the ?I? this is not a problem.
Hmmm?

http://www.cranemart.com/library/public/cranemart/Manuals/CraneSource/Single%20Girder%20Underhung%20Crane%20Bridge%20Kits%20-%20113533-71.pdf

Good luck, Myoarin