In article ,
Adam H. Kerman wrote:

But what about the way spans are designed to flex? There are several
trigonometric formulas that apply (that I never learned). Aren't there
instances in which the same live load on various wheelbases can positively
or negatively impact the span's flexibility by creating different kinds
of deflection?

Things get complicated, and messy, in the real world. wry grin

Longer distances between axles can serve to spread the load over a larger
part of the spam, without increasing peak loading.

Additional axles, on the same overall wheelbase, can reduce the 'rate of
change' of the load at a particular point, BUT they can also _increase_ the
peak load at a particular point. This can be significant, at/near the weakest
point -- mid-span.

'Drive' wheels apply different forces on a span than "non-drive" wheels do.
Powered axles, in addition to the 'down' force of the load, provide a 'push'
towards the rear of the vehicle. Un-powered ones have the effect of adding
a push towards the -front- of the vehicle. This adds increased compression
effects _between_ the drive and non-drive axles, an a reduction before the
drive axles, as well as after the non-drive ones. These forces aren't large,
but can make for some significant changes as they pass over the mid-span
'divide'.