STRUCTURAL ENGINEERING FORUM OF INDIA [SEFI]

mominak · SEFI Member Joined: 15 Sep 2012 Posts: 3

Dear

Any one an tell me about the following issue

The building is two storey and i want cut ground floor slab ( circular 2feet dia cut) for spiral staircase to install. What type of precations or any structural reinforcement or any support is required?
Or how much the structure is going to get weakened.
or it is not possible to cut the slab.
or any other suggestion to put staircase from ground floor room to directly first floor room.

B.V.Harsoda · Posted: Sat Sep 15, 2012 7:21 am Post subject:

Give details of slab:-

Length
Width
One-way OR Two-way?

Location of Cut

Near corner or away from corner ?

mominak · SEFI Member Joined: 15 Sep 2012 Posts: 3

mominak · SEFI Member Joined: 15 Sep 2012 Posts: 3

Slab is 12"' x 12' it is one way. and thickness is 5"

the 2' diameter cut to be done at one corner of room

please reply

Dr. N. Subramanian · Posted: Tue Sep 18, 2012 3:38 pm Post subject:

Dear Mr Mominak.

First of all you need to check whether the opening is OK as per design. Check with clause 31.8 and also of IS 456-though this clause is applicable to Flat slabs, it will give you some idea. Note that if the opening is near the corner, it may be critical in transferring shear. Usually if the opening is contemplated during original design we need to provide additional reinforcement around the opening!

If the design is OK, you need to support the slab and beam near the opening. The cutting should not affect the concrete in the portions away from the opening. Hence you should not use sledge hammer. Now-a-days chemicals can be used to loosen concrete (after drilling a hole, the chemicals are injected, which will loosen the concrete), and then the concrete can be removed.

I am not sure how you are going to strengthen the slab around opening.

Is the building in high EQ zone?

Best wishes,
NS

P.K.Mallick · General Sponsor Joined: 14 Dec 2008 Posts: 1109

It is not advisable to cut an opening at one corner in case of one way slab . The strip of slab having opening will have loss of support. Because of one way slab design the load transfer in other direction is not possible.

But if it is absolutely essential to cut ,then new support system has to be provided.

Dr. N. Subramanian · Posted: Tue Sep 18, 2012 5:25 pm Post subject:

Dear Er Mallick,

He has mentioned it as 12 feet by 12 feet slab!

Regards
NS

P.K.Mallick · General Sponsor Joined: 14 Dec 2008 Posts: 1109

Dr. N. Subramanian · Posted: Wed Sep 19, 2012 2:05 am Post subject:

Sorry Er Mallick! I forgot to notice that he also mentioned it as one way!! How it will act as one way? I am surprised to note that such a practice is there in India. But as far as I know, designing such a slab as a two way slab is correct as well as economical!

Warm regards,
Have a happy Ganesh Chaturthi!

Subramanian

P.K.Mallick · General Sponsor Joined: 14 Dec 2008 Posts: 1109

Respected Subramanian Sir

You are my Guru and friend of my beloved Professor Late Sri Prakash Rao. I feel very awkward when you say sorry. I know it an oversight by you.
To tell you the truth,I have seen many buildings constructed in this manner during my tenure at Hyderabad spanning from 1993 to 1998. You will find beams are provided in one direction only and slabs are designed one-way. The absence of beams in other sides will make the slab to behave one-way but what is the resistance of those buildings to lateral force?
Still worse (as far as slab design is concerned ) is the design of slab as one-way whose aspect ratio is less than 2 and slab is surrounded beams.I do not know why they do.But it is done. As you said it is uneconomical too.
Warm Regards.
Wish you a Happy Ganesh Puja too.