The effect of openings on the shear strength as well as flexure action (in case of large opening) of slab must be investigate carefully. The effect becomes severe when the openings are within the column strip areas of slabs or within middle strip areas when the opening are closer than 10 times the slab thickness(10h) from a column, measured radial.
To account for reduction of shear strength, it is considered that a portion of the critical section b0 is ineffective which is enclosed by straight lines projecting from the column centroid to the edges of the opening. If two much resisting perimeter is lost, the designer must be sure adequate tow-way bending is really present; otherwise the lower shear permitted in beams become the limiting values for the slab.
For slabs with shear reinforcement, the ineffective portion of the perimeter b0 is one-half of that without shear reinforcement. One-half factor is interpreted to apply equally to shearhead reinforcement and bar or wire reinforcement.
Code Recommendation (ACI-318)
22.214.171.124 Where opening in slabs are located at a distance less than 10 times the slab thickness from a concentrated load or reaction area or when opening in slabs are located within column strip, the critical sections shall be modified as follows:
1) For slabs without shearhead, that part of the perimeter of the critical section that is enclosed by straight lines projecting from the centroid of the column, concentrated load or reaction area and tangent to the boundaries of the opening shall be considered ineffective.
2) for slabs with shearheads, the ineffective portion of the perimeter shall be one-half that defined in (1) above.