Many buildings or other types of constructions remain unfinished at various stages, put on hold, or suffer serios time delays from the schedule. Before resuming the works, these constructions may require extensive structural assessment to determine the condition of the structure, identify the damages, and provide recommendations for intervention. This paper summarizes the intervention works for the retrofitting of an existing 11 story building made with a hybrid system of reinforced concrete walls, steel frames, and composite floor decking. The building is located in a marine environment and moderate seismicity area. The constant exposure to aggressive conditions caused different extents of damage, and most affected were the composite floor steel decking. Experimental tests were carried out to validate a strengthening solution for the composite floor slabs based on adding a new concrete layer and post-installed shear studs. Numerical models have been calibrated against test data. Additional requirements were considered, i.e., reducing the potential for progressive collapse and improving global behavior for seismic response (torsional stiffness, floor diaphragm effect).