Design of shear walls for the earthquake resistant building structures are quite common. In general vertical load carrying members of these structures are shear walls and the horizontal components are floor plates. Shear walls are pierced, wide and box shaped structural components. Similarly storey floors include some openings because of existence of stairs, elevator, ventilation holes etc. Therefore analysing the structure by equivalent frame models does not represent the actual behaviour of the structure. On the other side the special purpose, pseudo three dimensional computer programs are based on some assumptions in the formulation. In the present study a three dimensional finite element computer program is developed and used to obtain accurate solutions to determine the bounds of the simplifying assumptions commonly made for the analysis of multi-storey building structures made of pierced shear walls of open and/or closed cross-section and flat plates. The developed computer program is presented in [1-5]' Last version aof the program is presented in the previous study ("A Method For the 3D Computer Analysis of Multistorey Shear Wall Type Building Structures"). Effects of the infinitely rigid floor and foundation assumptions on the shear walls are investigated and the results are compared with those of a slab with appropriate rigidity. Practical bounds are established for the above mentioned assumptions.