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I mentioned awhile back that we are doing preliminary design for an airport
taxiway bridge.

This bridge will have to handle what I call the B.A.A. ("Big-Ass Airplane").
FAA parlance, it is called a "Super Group VI", that is, a little bigger than
a Group VI but not quite so big as the anticipated Group VII (none of which
exists yet).

In this situation, we have a very definite notion of how big/how heavy this
monster is. Contrast this with highway bridge design, which uses so-called
"notional loading" to try to represent the preponderance of heavy vehicles
operating on the bridge during its lifespan.

In the past, it has not been uncommon to use AASHTO standard bridge design
load factors for such structures.  However, the loading factors for live
load under AASHTO are really big.  Under AASHTO LRFD, for example, the
smallest factor would be 1.75 in a certain combination, and the largest
about 2.3.  And to that we must add 30% impact.

My inclination at this point is to go with the more reasonable (to me,
anyway) ASCE 7/ACI 318 Appendix C load and resistance factors.  My rationale
is that the live loading (in particular) is well-known, and to subject such
a large (and quantifiable) live load to such extraordinary load factors
would be wasteful.

Cost is a factor as this structure promises to represent a big outlay in

We structural engineers tend to be conservative--I usually am--but when are
we being TOO conservative?

I'd like some feedback.