Presented are methods of assembling and disassembling a modular unit so that the modular unit. The modular unit includes at least a concrete column with an embedded steel receptacle and an access port, and a concrete joist/slab with an embedded steel receptacle and an access port. The methods include inserting a first end of the rigid member into the steel receptacle of the joist/slab and a second end of the rigid member into the steel receptacle of the column to form the modular unit. The first and second ends of the rigid member are releasably coupled to the steel receptacles of the joist/slab and column, respectively. The first and second ends of the rigid member are then decoupled from the steel receptacles of the joist/slab and column, respectively. The joist/slab, column, and rigid member are separated so that at least joist/slab and column can be reused and/or recycled.