A structural grid support system adapted for transferring seismic loads to the vertical structural supports of a modular structure. The system includes larger and longer high load capacity main runners with mechanically fastened cross tees, which transfer seismic forces without failing. The structural grid can provide a diaphragm using mechanically fastened connections in a single horizontal plane without requiring additional overhead support from the roof deck, or requiring fewer such supports. The main runners may include additional connecting elements disposed within the usable space beneath the runner (e.g., ventral threaded receivers) for attaching deployment-specific equipment or hardware, such as network cable trays.