AS 3600 in Clause 6.9.4, states that insitu concrete can be assumed to act as horizontal diaphragms
. Unfortunately, there is no guidance in either Standard on the loads, the design of the diaphragm or the transfer of actions from diaphragms into the vertical elements.
In the 2018 IBC, he adds, the design methodology will be mandatory for precast concrete used as horizontal diaphragms
in buildings assigned to Seismic Design Category (SDC) C, D, E or F, and optional in SDC B according to ASCE 7-16.