Please see the attached STAAD model as a reference.
I am modeling a bridge with four 72" deep plate girders + 10" thick concrete deck on top. I have modeled everything using plate elements and I'm in the process of calculating the midspan moments due to dead load by summing (Sx * Plate Area * Distance to Neutral Axis) for each flange plate and web plate at midspan.
The plate girders are supported in the model at the bottom flange node to mimic the actual bridge seat condition.
1) For my moment calculation, should I consider the neutral axis to be located at the bottom flange node since this is where the support is modeled? Or should I consider the neutral axis to be located at the centroid of the plate girder even though the support is at the bottom flange?
2) Does it matter which node is assigned the master and which node is assigned the slave? In my model, I set the deck node as the master and the corresponding girder node as the slave. (Please visit the site to view this file)