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Alteration of an existing slab

This blog is from a doubt posed by one of the students in our structural discussion forum.

We at civilera,do engage with students in a forum to discuss their doubts and engage with us on structural design training and on various technical aspects.

This is also an area where we are able to convince young engineers that etabs tutorials and staadpro courses can be effective only if they have sound basics. So if you want to join our forum,

read this blog and then see the video below and then find the link in the video below. However first read the blog full as you need to understand the question first.

The question posed by the student was as in the figure.

He wanted to know if the slab can be cut open to accomodate a stair.Please note that the requirement is after the construction on an existing structure.

There are a few structural aspects that needs to be looked in to that related to seismic and horizontal irregularity.However,I will discuss the possibility interms of gravity load alone.There can be 2 situations in this.

Situation - 01 Slab is one way

If the slab is considered one way in the shorter direction in the initial design,then the slab design point of view it is perfect. No design issues will arise. All you need to ensure is the opening is made perfectly with our causing any issues to the existing structure in a controlled manner.

What if the opening was needed along the longer direction?

Even then it might work if the minimum long direction bars provided is adequate to span in the longer direction.You need to do a check.See the video to understand.

Situation - 02 Slab is Two way

If the slab was considered two way in the initial design,then the slab design point of view it is needs a quick check.Here too in many cases, it may be possible because the slab rebars provided might be many times adequate to span it in one direction. If not then it is not possible.

See the video to understand what it really means.

In all these cases,You also need to check if the beams and columns and footings are adequate. However in most cases,the conservative assumptions we have taken and the rounding off in loads, and steel will allow us to make minor alterations with out compromise. See the video where I have explained it in detail.

Other Points to check

We also need to ensure all openings are smaller and not affecting the diaphragm action of the building that was considered. Also that this is not creating horizontal irregularity in the building

You can read another blog I have written related to this topic in this link here.


All minor alterations in a building also need a careful study to understand its possibility. However a right first design and adoption of all code provisions will allow us to incorporate minor changes with out any design compromise.This is the reason,adhering to all code provisions even pertaining to minimum steel requirement is so important.

Join our structural forum HERE Ensure you write a message in the form that you want to join structural discussion forum.

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