Cantilever slab design sheet
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Author: king12
Last Post: concreteok
Replies 8
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Cantilever slab design sheet
#1
Simple cantilever slab

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This is a simple cantilever slab design sheet based on IS code. Kindly review and suggest.

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#2
king12 wrote:-
...This is a simple cantilever slab design sheet based on IS code. Kindly review and suggest....
Comment:-
Your slab is too shallow for a cantilever slab of the span that you specified. Use span/effective depth ratio = 7. This implies that effective depth = span/7. Since the span = 1.75m (=1750mm), then effective depth = 1750/7=250mm. If you are to have a clear cover of 20mm and you are using 12mm diameter steel, then the overall depth of your slab should not be less than 250+20+12=282mm (say 280mm at worse).
Again, I think that the loading of 2.56kN/m^2 for the floor finishing is too much. It should be about 1kN/m^2. The miscellaneous load should also not be more than 1.5KN/M^2. Try not to over load your structure as this could lead to over design and indicate the wrong forces and stresses at the wrong places. You did not indicate on how you arrived at your design moment of 34.3kNm (it would be helpful to indicate that). As to the other parts of your design, I did not take a look at them as I felt that that is easy enough as not to warrant any comments.
Regards
Teddy
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#3
minimum slab thickness, cantilever = L/10
thus; h = 1750/10 = 175mm (min.), but needs to check deflection
reference, table 5.1 minimum slab thickness "h" page 206,
design of concrete structures by, george winter, arthur nilson 9th ed 1980 "very old book"

sample loading from a 4 storey bldg i designed (uniformly distributed load only)
SECOND/FOURTH FLOOR SLAB:
DL: RC SLAB = 0.115m(24kN/m.m.m) = 2.76kN/m/m
FLR. FIN. = 0.05m(24kN/m.m.m) = 1.20 kN/m/m
MOVABLE PARTITION (LIGHT WALL) = 1.00 kN/m/m
ACOUSTIC BOARD = 0.12kN/m.m = 0.12 kN/m/m
-------------------------------------------------------------- Wdl = 5.08 kN/m/m
Engineering is the professional art of applying science to the optimum conversion of natural resources to the benefit of man
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#4
1) According to ACI table 9.C minimum depth for cantilever slabs is span (face of support to edge of cantilever) divided by 10..

2)Floor finishes of 3"(75mm) = 3/12 * 144 = 36psf = 1.75kPa
3)Parition wall load must be per actual plan otherwise for light part. walls its 20psf = 1kPa.
4)For ceiliing and services general load is 15psf = 0.75kPa
WS
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#5
RedHorse wrote:-
....minimum slab thickness, cantilever = L/10
thus; h = 1750/10 = 175mm (min.), but needs to check deflection....

Comment:-
I think that you are using a code other than BS or the EUROCODE. For BS or EUROCODE, span/effective depth ratio =7 (not 10). Well, that is a code issue as such we may not have to deliberate on that.
RedHorse wrote:-
…sample loading from a 4 storey bldg i designed (uniformly distributed load only)
SECOND/FOURTH FLOOR SLAB:
DL: RC SLAB = 0.115m(24kN/m.m.m) = 2.76kN/m/m

Comment:
Could you please indicate the span of the cantilever slab that you are using a thickness of 115mm to design? I believe that that should be a very short cantilever indeed as such will have nothing to do with the issue in discuss.
Regards
Teddy
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#6
Dear All,

I think we are discussing in wrong topic. "king12" share us spreadsheet conforming to IS Codes. It is calculation tool for us. Numbers to be put in spreadsheet cells are based on user/designer judgement. What we should discuss here is how formula in spreadsheet is correct/not correct, good/no good, useful/not useful... etc.

I don't see any thank given to "kig12" from all of you who is discussing here. At least, give him "THANK".

Regards,
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#7
Dear friends,
kowheng is right i have not attached the spreadsheet to ask about cantilever slab it is the sheet which I thought i'l share with you all if I had to ask anything related to it I would have asked in problem section or elsewhere not here. I asked suggestion regarding the sheet not about the values indicated in the sheet :dash2:
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#8
kowheng wrote:-
...What we should discuss here is how formula in spreadsheet is correct/not correct, good/no good, useful/not useful... etc. ...
Comment:-
But I wonder if what we have done before you posted is not just that. Where did we go wrong or said something that did not conform to your suggestion?
king12 wrote:-
....I thought i'l share with you all if I had to ask anything related to it I would have asked in problem section or elsewhere not here. I asked suggestion regarding the sheet not about the values indicated in the sheet....
Comment:-
I think that we are misunderstanding things here. If you did not want us to make comments about it, why did you say and I quote “…Kindly review and suggest…. Later changed to…..I asked suggestion regarding the sheet not about the values indicated in the sheet…”. How do we make our suggestions if we do not make comments or point out things that we feel do not fit properly?
Please gentlemen, let’s be conscious of what we say and try to put information across in such a way that we will not come back later to reject what we said and try to justify our change in position, using all sorts of interpretations. We are gentlemen. So should we be. Again, I do not think that that electronic sheet is more important than the information that it contains. People read information coming out of this site as such we should not give them the impression that we are not capable to do simple things right.
Regards
Teddy

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#9
chigozie,

thats a just a simple calc done by king12. i am not trying to side the super mod here but i when king12 posted a spreadsheet on cantilever slab, he does not expect comments on the load used in the sheet. he will definately titled his thread here as 'loads used for cantilever slab' if he wants that. and why do't you try to reason out whether he has done anything wrong in the section design? whether the lever arm was computed correctly etc?

and instead u try to dictate the loads he keyed in? and your replied kowheng with almost half a screen when he try to 'guide' you of your post.

from your posts here you must be an experienced engineer, why do you want to create a debate out of a simple cantilever slab calc.

you said should not overload the structure. for your arguement sake, why if the architect wanna a 4" thick marble floor at this cantilever slab? and what clause in BS that we need to go to 1:7 ratio for cantilever slab? the modification factors for tension and compression reinforcement, what are they for? they are always more than 1.0. imagine a window hood with virtually no imposed load, you still follow 1:7 ratio for it? you are instead misleading with your post. and can you design to IS? do you know what's in there?

i presume you will write another half page to reply and i advise you, go to the problem section, take on those heavy topics with others experienced engineer and leave alone our fellow king12. thank you.
concrete solution advocate
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