balanced bridge spans

huangyhg

balanced bridge spans
the term "balanced spans" on multiple span bridges is common, but not well defined.
what is your "rule of thumb" to obtain balanced spans on bridges? where does this come from? do you know of anywhere that i can read up on this?
rule of thumb is that the exterior spans are 75% of the interior span(s).
i have a chart (somewhere)with various configurations but i can't find it. when i do, i'll post it.
number of spans             ratio of spans
      2                       1.0:1.0
      3                       0.75:1.0:0.75
      4                       0.80:1.0:1.0:0.80
      5                       0.60:0.8:1.0:0.8:0.60
bridgebuster gave very good information, i concur with all but the 5 span arrangement.
the stated 5-span arrangement likely results in a "least weight" solution (or very close to it), but it may not be the most economical for today's cost structure.
for a five span structure, you should evaluate the above arrangement as well as the following:
0.8, 1.0, 1.0, 1.0, 0.8
this will have a slightly higher girder weight but the redundancy of details can save significant cost (at least in steel bridges).
the term "balanced spans" or "balanced span arrangement" refers to the proportions of end span to interior span that generally result in consistent max. negative moment (+-5-10% at piers) and positive moments (+-5-10%).  it is just a vague term that might also be stated "in normal circumstances" or something else to that effect.
you may take it for granted that this would occur universally, but because of utilities or other obstacles, severly unbalanced span arrangements are more and more common.  i even recall a project where an interior span never experienced positive moment.