I have used STAADPRO to model composite steel girder and
concrete deck bridges. I use member incidences to model the
girders and element incidences(plates) to model the center
of the concrete deck; both are modeled with the individual
centroid as the node joints coordinates. I then use beam
members to connect the plate centroids to the girder centroids. With the release of a new module under STAADPRO named BEAVA this can be done with very few commands.
I tried the same method using sap2000. I also used shell element to model slab and used beam element to model the beam under the slab. Stiff element was then used to connect the beam element placed at its cetroidal axia with the shell element at the nodal points. I comparred the result by modeling the same T beam section using the beam element. Results were very different.
Instead of using a stiff element to connect deck to beam
try using a beam with cross sectional properties equal to
width of beam flange and spacing of node to connect deck
to beam. Use concrete properties for this beam. Also, do not overconstrain model at supports. This works very well
for me using STAADPRO.
