design of cantilever slab as per is 456

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design of cantilever slab as per is 456, maximum cantilever slab length, cantilever slab reinforcement details, cantilever slab without beam


In the earlier post i.e. Design Steps of RCC Building  And Preparing Prerequisite of Design Various Elements of Design  we have already discussed what are the design steps and pre- requisites of RCC Building Design.
Now we are talking about Design of cantilever slab.

DATA FOR BUILDING

Type of building: Single story RC Frame structure.
Use of building: Public Building
Floor to floor height : 3.2 m
Loads : Dead load-calculate as per self weight of elements
             Live load-1.5 KN/m2
             Floor finish-1.0 KN/m2
Material specification: Concrete Grade-M25
                                        Steel Grade-Fe-415
Wall thickness-230 mm (Internal,external and parapet)

Assumptions for design: Slab simply supported over beams.
                                          Beam simply supported over columns.
Design Based on: Limit state design as per IS:456-2000
Design of concrete: 25 KN/m3
Density of Concrete:20 KN/m3

Slab Size (Assumed)

Design of Cantilever Slab













S1-CANTILEVER SLAB

lx =900 mm
ly= 1200 mm
L.L.=1.5 KN/m2
Roof Finish =1 KN/m2
Fck = M25 =25 KN/m2
Fy = 415 KN/m2
Take D= 125 mm , Effective cover = 25 mm
Effective depth (d) =100 mm
  • Effective Span = Clear Span + d/2
                                  = 900 + 100/2
                                  = 950 mm
  • Design Loads :
Dead load = Self weight = 0.125 x 25 = 3.125 KN/m
Live load = 1.5 x 1 =1.5 KN/m
Roof finish = 1 x 1 = 1 KN/m

Total load = 3.125 + 1.5 + 1 =5.625 KN/m
Factored Load (Wu) = 5.625 x 1.5 = 8.4375 KN/m

                         Wu x (leff)^2
   Factored  = ------------------  = 3.80 KN-m or 3.80 x 10^6 N-mm
    moment               2

  • Check for depth:
Mu = 0.138 Fck b d^2                                                                    [for M-25]
3.80 x 10^6 N-mm = 0.138 x 25 x 950 x d^2
da = 34.08 mm
(d)assumed > da  , Hence provided 100 mm depth is OK.............

  • Main Reinforcement :
Data available -     Mu = 3.80 KN-m , b = 950 mm , d = 100 mm , Fck =25 , Fy = 415


            50Fck                  4.6x Mu
Pt =     ---------  [1-{1-   ------------  }^1/2]
              Fy                     fck b d^2


Put the all values and find Pt.
Pt = 0.13 %

(Ast) required =  Pt/100 *bd

                    0.13                                                                          {Let 1 bar 8 mm dia.
(Ast) req =  ------ X 950 X 100  =  123.5 mm2                              @st = 50.26 mm2}
                    100
  • Spacing = @st/Ast   X 1000   =  406.96 mm
Rule for spacing as per IS : 456-2000
(i) 3d
(ii) 300 mm                                              {provide lesser of these values}
(iii) Spacing required

-  300 mm is lesser value
-  Provide 8 mm #  @300 mm C/C

Ast provided =  @st/ Spacing X 1000

=50.26/300  X 1000  =167.53 mm2  > Ast req.

  • Distribution Reinforcement :
- Provide Minimum reinforcement
- Provide Pt = 0.12 %   ( As per IS: 456-2000)

Ast  = Pt min/100  X b X D
Ast  = 150 mm2
Provide 8 mm# @300 mm C/C [Ast prov. =167.5 mm2]

Design of Cantilever Slab
  • Check for development length:
             # (0.87 fy)
Ld  =  ---------------
               4 x Tbd
Ld  =  376 mm
  • Check for deflection :
(l/d)prov  <   (l/d)max
(l/d)max = 7 X M.F.

M.F. will be calculated with the help of the value of fs and  and From FIG. 4  (IS : 456-2000)

Design of Cantilever Slab

fs = 0.58fy X Ast req. /Ast prov.

fs = 177.47 mm2

Take M.F. = 1.3

(l/d)max = 7 X 1.7 =11.9

(l/d)prov  = 950/100 = 9.5

Hence Proved (l/d)prov  <   (l/d)max

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