Expert: Arshad Khan - 1/27/2011

Question
Salaam,
I have one serious problem in my project of the new construction of an old market after demolition the whole one.I have completed the 3/4 part of the building but  In third part of building one corner of the proposed building work,one preoccupied shop owner is not allowing to erect column in his area. I have to cover 18 feet wide & 25 ft in length slab above the said shop, without touching the slab of the said shop. We have provision of erecting columns on east side and north side of the said shop and there is a road on south side.Since the shop owner not allowing to erect column, we have no option other than cantilever slab  of 18x25 feet. Above the said shop  we have to construct two stories above  the shop. Can you help us advising us the best  solution  of RCC designing. I have RCC designing by experts but the said shop owner challenging cantilever on his shop  with 2 stories is dangerous. Is there danger to his shop. Is it dangerous? How can you help us?

Answer
Hi Juzar,

Your designers are right to be concerned, because for that sort of construction, you will probably need cantilevering beams of between 1.4 to 1.6m depth, so that you can support the slab and also 2 more floors. You will also need very large columns and foundations for the columns supporting the cantilevering beams, and in addition require back-spans for the cantilevering beams to cater for the out-of-balance moments.

You will also have problems supporting the temporary formwork, if the shop owner does not allow even temporary props outside his shop.

An alternative solution, which will be more economical, is to introduce bracing ties (at say 45 degrees), to support the cantilevers, but then you will need to work around these to build the upper story walls.