Ned,

You raise a very good point, one that is often overlooked even in
industrial applications.

For unbraced lengths of beam the maximum bending moment must be reduced
so as not to encroach on the buckling resistance of the compression
flange. I had to do a heck of a lot of digging (for a client) to get a
handle on this since textbooks don't address this very well.

I found what I needed in a copy of the Crane Manufacturers Association
of America, Inc. "Specification for Electric Overhead Travelling
Cranes" , spec # 70. (Spec # 74 is more appropriate for manual cranes,
but # 70 is all I got).

It states that the factor of safety of all hoists is 5, BASED ON THE
PUBLISHED AVERAGE ULTIMATE STRENGTH OF THE MATERIAL.

The booklet contains much useful info, and it gives the following limit
on compressive stress for unbraced, single web-beam, crane runways:

Max. compressive stress = 12000 x A / Ld ; with a maximum of .6 x
yield strength

where A = area of the compression flange
L = unsupported / unbraced length of span
d = depth of the beam, in inches

Maximum shear stress = .35 x yield strength

Maximum deflection of beam not to exceed .001125 inches per inch of
span, based on trolley weight and rated load.

Trust this enlightens a little.

Wolfgang