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Design of slabs
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shivani soman
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PostPosted: Sun Feb 02, 2020 1:53 pm    Post subject: Design of slabs Reply with quote

Hello sefi members,
I was reading about design of slabs from book by S Ramamrutham in which  for deflection criteria it was given that span/d should be multipled by a modification factor.
So for same % of tension reinforcement mild steel has a greater modification factor as compared to HYSD bars.Which means say for cantilever slab,
Span/d = 7(modification factor)
so, d= Span/7(modification factor)
Which means for same %steel,  
Slab with mild steel will have lesser depth than slab with HYSD Steel as  modification factor of mild Steel is more.
Then what is the benefit of using HYSD bar there?
Looking forward for your help in clarification.
Thank you
Shivani Soman
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kawshik9
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PostPosted: Mon Feb 03, 2020 5:58 am    Post subject: Reply with quote

Er.Shivani soman,

Cl.23.2.1 of IS 456 states that the vertical deflection limits may generally be assumed to be satisfied provided that the span to depth ratios are not greater that some specified values for different type of beams with modification factors based on required area of reinforcement  and stress in the reinforcement.

My understanding is little tricky please try to understand

1)  By saying same  % of required reinforcement, we are admitting that the slab with HYSD bars (For which permissible stress is 230 MPa) is having more load than the slab with Mild steel bars(For which permissible stress is 140 MPa).

2)As the loading is more in HYSD bars slab, in order to maintain same deflection limit the depth required will be more . Hence the ration L/d will be less than that of the case with mild steel.

3)For better understanding, please compare the slabs with same loading using HYSD/Mild steel bars  and not same % of steel. Hope you understand the difference.

Regards,

Ramakanth
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shivani soman
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PostPosted: Mon Feb 03, 2020 6:40 am    Post subject: Reply with quote

Thanks you @kawshik9 for your time in clarifying my doubt.
Point wise explanation made it easy to understand.


Last edited by shivani soman on Tue Feb 04, 2020 4:55 pm; edited 1 time in total
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es_jayakumar
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PostPosted: Tue Feb 04, 2020 2:03 am    Post subject: Reply with quote

In this regard, here I follow an analytical approach. A simply supported slab of span 3.00m of overall depth 120mm subjected to a BM of 12 kNm is considered for illustration purpose. M20 concrete is used. Reinforcement requirement using Fe-415 steel and Mild Steel (fy = 250) is computed using the attached Excel work sheet, for the governing of flexure and deflection control criteria. It can be seen that Fe-415 steel consumes less quantity compared to MS, even though more steel than that required for flexure was necessitated to satisfy deflection control criteria for Fe-415 steel.

E S Jayakumar



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BVRAO
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PostPosted: Tue Feb 04, 2020 4:23 pm    Post subject: Reply with quote

u can check  every calculation along with deflection check  as per the  fig 4  of IS 456 -2000. And Print the details


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thirumalaichettiar
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PostPosted: Sat Feb 08, 2020 11:29 pm    Post subject: Reply with quote

BVRAO wrote:
u can check  every calculation along with deflection check  as per the  fig 4  of IS 456 -2000. And Print the details

Er.BVRAO,


Can u please inform what software  Or excel  u use to design the slab?


It seems simple with elegant design.


Thanks,


T.Rangarajan
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es_jayakumar
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PostPosted: Sun Feb 09, 2020 5:14 am    Post subject: Reply with quote

It looks like Visual Basic.
It shows a spacing of 400mm C/c for the bars in longer direction. But, I think, the maximum permissible spacing of bars in slab is restricted to 300mm c/c even for temperature steel, as per Amendment No.3 to IS 456 : 2000.

E S Jayakumar
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thirumalaichettiar
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PostPosted: Sun Feb 09, 2020 6:14 am    Post subject: Reply with quote

es_jayakumar wrote:
It looks like Visual Basic.
It shows a spacing of 400mm C/c for the bars in longer direction. But, I think, the maximum permissible spacing of bars in slab is restricted to 300mm c/c even for temperature steel, as per Amendment No.3 to IS 456 : 2000.

E S Jayakumar



As far as I am concerned use smaller dia so that the maximum spacing of Main bar to 200mm and the temperature-distributor bars at 250mm. The spacing I specify is good for control of cracks.

Views differ.

In many projects when u specify the main bar 10mm  at 200mm the client  changes to 12mm at 150mm trusting the bar bender and he does not realize that over reinforcement is not a good option.

T.Rangarjan.
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BVRAO
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PostPosted: Sun Feb 09, 2020 8:23 am    Post subject: Reply with quote



Can u please inform what software  Or excel  u use to design the slab?

T.Rangarajan[/quote]

Sir,
It is in VB .


Last edited by BVRAO on Sun Feb 09, 2020 5:24 pm; edited 1 time in total
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thirumalaichettiar
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PostPosted: Sun Feb 09, 2020 11:49 am    Post subject: Reply with quote

Er.BVRAo,
I owe debt of thanks by sending the file.

As per code as u mentioned for Continuous Span/Depth is 26 and Simply supported  it is Span/L=20.
The above r the basic but need to be modified based on % of tension and compression steel. Definitely the value will be more than the basic Span/depth.
Usually it goes from 28 to 32 . U can use 30 as an average value.

The net deflection should be within the limits of 20 mm or l/250 or l/360 depending on the deflection caused by the presence of partitions on the slab.
Whatever  the main bars can be spaced to a maximum distance of 200 mm as per Khann's HB and distributors at 250 mm so that to minimize the  cracks if occurs.

T.Rangarajan
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