Viaduct expansion pier location/distances

[Jean_Wong]

Good Day!
I am designing a 21 span viaduct (with aashto girders). Where should I put the expansions? I'm assuming that that there should be expansions when the girder type changes. My basis is AASHTO LRFD 2012 but I can't seem to find expansion piers in the code.
Thanks!

 

[bridgebuster]

Is this the railroad viaduct you're working on? Usually, railroad bridges are all simple spans. If you have a highway bridge the spans are usually continuous. There's no span limit in AASHTO on continuous girders. However, the benefits of continuity - reduced section - diminish after 5 spans. Usually with a continuous span the fixed pier is in the middle or near the middle and everything else is expansion.