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Slab thickness
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ramu_se
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Joined: 30 Oct 2017
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PostPosted: Fri Aug 05, 2022 10:08 am    Post subject: Slab thickness Reply with quote

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
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PostPosted: Sat Aug 06, 2022 8:30 am    Post subject: Reply with quote

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
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PostPosted: Sat Aug 06, 2022 8:31 am    Post subject: Reply with quote

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
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PostPosted: Sat Aug 06, 2022 4:54 pm    Post subject: Reply with quote

Thank you sir,

If the span is 3270mm, two-way 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 IS-456, D as neither overall depth nor effective depth.

Kindly explain briefly about deflection calculation of the two-way slab.

Thanks & Regards,
Ramu.
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vikram.jeet
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PostPosted: Sun Aug 07, 2022 2:29 am    Post subject: Reply with quote

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
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PostPosted: Sun Aug 07, 2022 7:32 am    Post subject: Reply with quote

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
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PostPosted: Sun Aug 07, 2022 9:20 am    Post subject: Reply with quote

Thank you, sir

As per clause 23.2.1 in IS-456
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
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PostPosted: Sun Aug 07, 2022 11:11 am    Post subject: Reply with quote

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
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PostPosted: Mon Aug 08, 2022 5:20 am    Post subject: Reply with quote

Dear Sir,

Please check the below calculation:

l/d ratio check :

Overall depth - 115mm
Two-way 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 co-efficient - 0.045 (positive moment at mid-span)

M = 0.045 x 8 x 3.72^2 = 3.85 kN-m

Effective depth = 95 mm

M/bd^2 = 3.85 x 10^6 / 1000 x 95^2 = 0.426

Pt = 0.128% (Table-2, SP-16)

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 IS-456:2000
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
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PostPosted: Mon Aug 08, 2022 8:57 am    Post subject: Reply with quote

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 kN-m

M/bd^2 = 10.27 x 10^6 / 1000 x 105^2 = 0.931

Pt = 0.272 % (From table -2 SP-16)

Ast = 285.6 mm2

Provide T8@175mm c/c

Ast(provided) = 285.7 mm2
--------------------------
l/d = 7 x M.F (cl.23.2.1, IS-456)

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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