This research is executed to investigate and examine the critical impact of making openings and holes (needed for mechanical and electrical installations) on the flat slabs’ workability and performance. Numerical analysis and simulations using the ABAQUS software were implemented. Besides, a comparative study (between nine cases) was adopted to explore the influence of openings on the workability and performance of flat slabs. The damaged zone area, maximum load, maximum deflection, and maximum displacement were recorded. Also, the effect of the opening size on these variables was analyzed. The results revealed that cases C1, C2, and C3 have approximately similar maximum displacement values ranging between 31.6 mm and 32.7 mm. The maximum load (failure load) in case C2 (50×50 cm) was 223.7 kN, 34% less than in case C1, and case C3 (100×100 cm) was 186.5 kN, 45% less than in case C1. The damaged zone area increased by 47 percent and 144 percent for C2 and C3, respectively. It is evident that when opening sizes increase, punching shear resistance decreases because the perimeter decreases. Also, different values of loads were recorded corresponding to various amounts of displacement in cases from one to nine, in which the load increases with elevating the displacement.