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ramu_se ...
Joined: 30 Oct 2017 Posts: 75

Posted: Fri Aug 05, 2022 10:08 am Post subject: Slab thickness 


Dear All,
Can we use 4.5" slab thickness for 10'9" span?
If we check the above with span to depth ratio considering the continuous slab
Span  10'9"  3270mm
span to depth ratio for continuous slab  26
span / 26 = 3270 /26 = 125.76 mm as effective depth
Overall depth will become 150 mm.
Kindly guide for 10'9" span 4.5" slab thickness is sufficient.
Thanks&Regards,
Ramu. 

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vikram.jeet General Sponsor
Joined: 26 Jan 2003 Posts: 3559

Posted: Sat Aug 06, 2022 8:30 am Post subject: 


Er Ramu
For Two way slabs see cl 23.1
L/D = 28 for simply supported edges
L/D= 32 for continuous edges
D here is overall depth
Most important in deflection check is that % reinforcement to be worked based on Load factor = 1.0 , and see deflection coefficient based on this pt , though actual reinf % is based on Load factor = 1.50.
Last edited by vikram.jeet on Sat Aug 06, 2022 8:33 am; edited 1 time in total 

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vikram.jeet General Sponsor
Joined: 26 Jan 2003 Posts: 3559

Posted: Sat Aug 06, 2022 8:31 am Post subject: 


Er Ramu
For Two way slabs see cl 23.1 IS 456 ,
L/D = 28 for simply supported edges
L/D = 32 for continuous edges
D here is overall depth
Most important in deflection check is that % reinforcement to be worked based on Load factor = 1.0 , and see deflection coefficient based on this pt , though actual reinf % is based on Load factor = 1.50. 

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ramu_se ...
Joined: 30 Oct 2017 Posts: 75

Posted: Sat Aug 06, 2022 4:54 pm Post subject: 


Thank you sir,
If the span is 3270mm, twoway slab, continuous slab
L/D = 32
3270/32 = 102.18 mm, we can provide 112.5 ie 4.5" thk slab.
Hope this is correct.
My query is in IS456, D as neither overall depth nor effective depth.
Kindly explain briefly about deflection calculation of the twoway slab.
Thanks & Regards,
Ramu. 

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vikram.jeet General Sponsor
Joined: 26 Jan 2003 Posts: 3559

Posted: Sun Aug 07, 2022 2:29 am Post subject: 


Yes , 115 mm th slab is OK as it is two way with continuity at edges AND it is in accordance with clause 23.1 of IS 456.
Only in above clause especially for solid slabs , overall depth D is taken .
Otherwise L/deff is to be taken if check is made as per clause 22,2,1 for slabs / beams.
You can make check based on both and result would be similar. 

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vikram.jeet General Sponsor
Joined: 26 Jan 2003 Posts: 3559

Posted: Sun Aug 07, 2022 7:32 am Post subject: 


Deflection ccalculation for two way slab :
Refer any text book on rcc design and chapter two way slabs you can easily get the computations from first principle.
Just to illustrate :
For ss condition
Taking a strip at mid span in both directions of two way slab
L1 short span & L2 long span AND k = L2/L1
w1 is loading along short span & w2 is along long span
Deflection at mid span for both direction has to be same as mid point is common
(5/384)* w1L1^4 /(E I ) = (5/384)*w2 L2^4 /(E I)
w1= w2 * k^4
Slab loding w = w1 + w2
Hence w1 = ( k^4 / (1+k^4) ) * w
And w2 = (1 / ( 1+ k^4) ) * w
This way deflection can be checked , for continuity a factor of 1.3 can be applied 

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ramu_se ...
Joined: 30 Oct 2017 Posts: 75

Posted: Sun Aug 07, 2022 9:20 am Post subject: 


Thank you, sir
As per clause 23.2.1 in IS456
Span to effective depth ratio for span up to 10m
Cantilever 7
Simply supported 20
Continuous 26
In this case, if we consider the span of 3270mm
L/d = 26 for continuous slab
3270/26 = 125.76 mm as effective depth
Overall depth D = 125.76 + clear cover + d/2 = 125.76 + 20 + 8/2 = 149.76mm.
Kindly check the above calculation, and correct me if I am wrong.
We cannot use the modification factor, since the span is less than 10m.
Also, for a cantilever span of 4'6" L/d ratio 7
1370/7 = 195.71mm as effective depth which looks more depth.
Kindly clarify this.
Thanks & Regards,
Ramu. 

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vikram.jeet General Sponsor
Joined: 26 Jan 2003 Posts: 3559

Posted: Sun Aug 07, 2022 11:11 am Post subject: 


Er
Pl apply modification factor ( a multiplier to value 26 ) , based on % tension reinf pt considering a slab thick Ness of 115mm and load factor =1.0 

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ramu_se ...
Joined: 30 Oct 2017 Posts: 75

Posted: Mon Aug 08, 2022 5:20 am Post subject: 


Dear Sir,
Please check the below calculation:
l/d ratio check :
Overall depth  115mm
Twoway slab
fck  20 N/mm2, fy  415 N/mm2
ly/lx  1.5
Interior panel, load factor 1
load on slab  8 kN/m2
Shorter span  3.27m
As per Table 26, the short span coefficient  0.045 (positive moment at midspan)
M = 0.045 x 8 x 3.72^2 = 3.85 kNm
Effective depth = 95 mm
M/bd^2 = 3.85 x 10^6 / 1000 x 95^2 = 0.426
Pt = 0.128% (Table2, SP16)
Ast = 122 mm2
Provide T8@ 250mm c/c
Ast provided = 200 mm2

l/d ratio for continuous slab  26
l/d = 26 x modification factor
fs = 0.58 x fy x Ast(required) / Ast(provided)
fs = 0.58 x 415 x 122 / 250 = 117.46 N/mm2
As per Fig.4 in IS456:2000
M.F = 2.0

l/d = 2 x 26 = 52
3270/52 = 62.88mm < 95mm (provided)
Hece it is safe.

Sir, can you please let me know on what basis we need consider the load factor as 1 for deflection calculation which i do not understand.
Thanks & Regards,
Ramu. 

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ramu_se ...
Joined: 30 Oct 2017 Posts: 75

Posted: Mon Aug 08, 2022 8:57 am Post subject: 


Dear Sir,
Kindly check the below calculation for the cantilever slab.
Cantilever slab
Span  1.37 m
Load on slab  7 kN/m2
Parapet wall load  2.7 kN/m
Effective depth d  105 mm
Over all depth d = 125mm
Fck  20 N/mm2, fy  415 N/mm2
M = wl^2/2 + wl = 7 x 1.37^2 + 2.7 x 1.37 = 10.27 kNm
M/bd^2 = 10.27 x 10^6 / 1000 x 105^2 = 0.931
Pt = 0.272 % (From table 2 SP16)
Ast = 285.6 mm2
Provide T8@175mm c/c
Ast(provided) = 285.7 mm2

l/d = 7 x M.F (cl.23.2.1, IS456)
fs = 0.58 x 415 x 285.6/285.7 = 240.6 N/mm2
Pt = 0.272%
M.F = 1.57
l/d = 7 x 1.57 = 10.99
1370/105 = 13.04 > 10.99
Hence need to revise the depth

Over all depth = 150 mm
Effective depth = 130 mm

1370/130 = 10.53 < 10.99
Hence this satisfies the l/d ratio. 

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