Every engineer is familiar with the openings in a slab. We all know that opening in slabs cannot be avoided in site construction as it has a purpose. In fact, in the early stages of planning, this is given in considerations. Mostly opening in slabs is provided due to MEP considerations, some are for architectural purposes and so on. We must admit that some engineers are not worrying about and neglect to design the slab openings as it is included in the typical details. For site construction, it is sometimes confusing on what diameter and how many reinforcements we will go to provide along slab openings, but did you know that the structural design code particularly the ACI code tells something about the openings in a slab? Whatever is the purpose of slab openings in our project; this should not be taken for granted as somehow, it has an impact in our slab design.
In this article, we will dig deeper into what does the code says about the slab openings. What are the considerations in providing trimmer bars along with slab opening and how to design it? Openings in the slab system can be found on Section 8.5.4 of ACI 318M-14. This states that “Openings of any size shall be permitted in the slab system is shown by analysis that all strength and serviceability requirements, including the limits on deflections, are satisfied”.
How to Calculate the Extra Bars along Slab Openings?
A self-explanatory excerpt from Section 8.5.4.2 states that, the opening shall be permitted in slab systems without beams in accordance with the following:
- Opening in any size shall be permitted in the area common to intersecting middle strips, but the total quantity of reinforcement in the panel shall be at least that required for the panel without opening.
- At two intersecting column strips, not more than 1/8th the width of column strip in either span shall be interrupted by openings. A quantity of reinforcement at least equal to the interrupted by an opening shall be added on the sides of the opening,
- At the intersection of one column strip and one middle strip, not more than 1/4th of the reinforcement in either strip shall be interrupted by openings. A quantity of reinforcement at least equal to the interrupted by an opening shall be added on the sides of the opening
In Summary
As mentioned in the code above, the top and bottom extra bars that usually provided along slab openings should be at least equal to the area of the openings which supposed to have a steel reinforcement or a mesh reinforcement. The total area of these rebars should be distributed along the side of the slab openings.
Refer to the figure below for the recommended details of reinforcements in slab openings (click the image for a clearer view):
Opening in slabs for Punching Design Considerations
If an opening is located within a column strip or closer than 10h from the concentrated load or reaction area the below figure R22.6.4.3 taken from ACI 318M-14 shall be satisfied.
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Sorry . I find something a bit hard to understand in Figure R22.6.4.3
Does it indicate that if we have an openings in case of figure (a) ,it will be regarded as figure (c) ?
And if we have an openings in case of figure (b) , it will be regarded as figure (d)?