The present invention is a modular structural building system and method consisting of prefabricated, precast, composite reinforced concrete raised floor and steel beam panels with adjustable levelling connection assemblies between panels, supported by columns. The system has the ability to accommodate the use of the floor by construction personnel during the on-site assembly process. The perimeter of the raised floor slab can be provided with ducts for a field installed conventional reinforcement means to create a continuous structural diaphragm for the floor panel.