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Modification factor for slab deflection check

 
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Vignesh_Aravinthan
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PostPosted: Fri Jun 04, 2021 7:08 am    Post subject: Modification factor for slab deflection check Reply with quote

Hi, I am designing a two way slab and while checking for the deflection I need to calculate the modification factor to take in to account the effect of tension reinforcement on L/d ratio. For which I am getting following values,
fs = 117
Pt = 0.36%
But I am confused on how much should be the modification factore used as shown in Fig 4 of IS 456:2000.
Should I take it as 2 or 0?
What is the meaning of the curve which ends much near to 0.8% tension reinforcement?
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vikram.jeet
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PostPosted: Fri Jun 04, 2021 8:04 am    Post subject: Reply with quote

Dear Er

For Deflection check , the load factor is 1.0.

However Tension steel is provided in the slab based on Load Factor of 1.5.

Therefore equivalent tension steel at load factor 1.0 needs to be worked  based on M/bd2 using working moment , thereby taking Load factorFas  1.0. The corresponding steel % pt to be seen from table . This steel % to be used in working out deflection.
Otherwise % steel provided in slab can be directly divided by 1.5 , to arrive at  steel % for Deflection calcs.under Load Factor 1.0.

This is my personal opinion and to be ignored if not directly related to yr querryquery.


Best wishes
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vikram.jeet
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PostPosted: Fri Jun 04, 2021 2:31 pm    Post subject: Reply with quote

Further in continuation of previous post:

The modification factor as per figure 3 of IS 456 is higher if % tension steel is less.

If at load factor 1.5 reinf. worked is   0.6% say to be actually provided in slab.
For modifications factor , the reinf % of (0.6/1.5*1.0)= o.4 % to be used to see the modification factor  . Load factor = 1.0 is required for Deflection check.

Alternatively it can also be said that at load factor = 1.0, the steel provided in design will have a stress = (0.87 Fy)/1.5
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