Expert: Arshad Khan - 11/22/2009

Question
QUESTION: Dear Sir,
Is it possile to provide a cutting of 1200 mm dia in a flatslab for the purpose of spiral staircaase ?
Thanking you,

ANSWER: Hi Ravi,

You are not clear on whether this is in an existing slab or one that is being designed. In either case, it depends on the relative proportions- if the column spacings are say 3-4m, then probably the answer is no. If the spacings are 6-8m, then depending on location, it may be possible- the best location would be away from the columns, around 1/3 to 1/4 span away.

If it is an existing slab, you will probably need to incorporate some additional external reinforcement or support to make up for rebar cut by the opening.

---------- FOLLOW-UP ----------

QUESTION: thanks so much sir,actually its a new design,
the opening is abt 30 cm from the peripheral beam and 1.2m from the nearest column.Is it possible to provide an opening as I provided the beam? my flat slab thickness is 30 cm.appreciate for your help.
thanking you,

ANSWER: Hi Ravi,

You still have not given me column spacings, but in principle this should be possible- you will need to do detailed analysis to determine local stresses and rebar curtailment around the opening.



---------- FOLLOW-UP ----------

QUESTION: Dear Sir,
the spacing between the columns are 2.7m only where my circular cutting of dia.1.2m is  coming.but sir ,while I analyzed it seems to be okay .Is it accepatable if I provide the proper rebar curatailment around the opening and the proper analysis as well.thanx so much appreciate a lot.
thanking you,

Answer
Hi Ravi,

If the spacing is only 2.7m, then with a hole 1.2m diameter the slab stops behaving like a continuous flat slab and you basically have short cantilever slabs from the columns either side of the hole. So long as you detail the rebar accordingly, it should be OK.