Expert: Arshad Khan - 12/12/2008

Question
QUESTION: 1. if a flat slab system is analyzed three dimensionally on software-etabs then to calculate punching shear is it that only the axial load in the column is  sufficient or whether the stresses due unbalanced moment needs to be calculated and then added to stresses due to axial load.
2. what is meant by ductile detailing of flat slabs.
  is it by providing shear reinforcement even when not required or any detailing procedures in providing steel

ANSWER: Hi Ravindra,

Yes, you need to add the stresses from both axial as well as unbalanced bending, which could also be caused by lateral laoding if the frames are not braced.

Ductility in RC structures is achieved by not over-reinforcing and by confinement of main tensile/compression bars. So, for flat slabs this can be achieved by having closely spaced links of small diameter, mainly near the supports and for the rebar within the slab "column strips". Also, avoid having rebar laps in zones of maximum moments.


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

QUESTION: thank you for your prompt early reply.
1.As you said that confinement of bars in slabs can induce ductility. I would like to know whether there is any guideline for providing confinement.If the punching shear stresses are well within permissible limits then is confinement required.Upto what value of permissible punching stresses confinement is not required.Are there any detailing procedures for tension and compression bars for ductile detailing in flat slabs.
2. my second question is whether a punching shear check is required for a flat slab supported over a very long shear wall.Upto what length of shear wall punching shear check required.

Answer
Hi Ravindra,

There are several references to ductile detailing, such as the ones below:



http://books.google.co.uk/books?id=-jnlb-oJxcEC&pg=PA274&lpg=PA274&dq=ductility+

www.nicee.org/conferences/eC_ISC113.pdf

http://www.diva-portal.org/kth/theses/abstract.xsql?dbid=203

Puching shear is generally circular around a column support so does not apply to a line support like a shear wall. Here you just need to check for normal shear within the slab effective depth.