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In working stress method we consider modular ratio m = 280/(3 σcbc). Is it correct?

 
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Dr. N. Subramanian
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PostPosted: Tue Sep 07, 2021 10:27 pm    Post subject: In working stress method we consider modular ratio m = 280/(3 σcbc). Is it correct? Reply with quote

In working stress method we consider modular ratio m = 280/(3 σcbc). Is it correct?

The short-term modulus of concrete does not take into account the effects of creep, shrinkage, and other long-term effects. Accordingly, the modular ratio m is not computed as m = Es/Ec = 200000/[5000 /sq. root(Fck)].   The value of m, as given in sec. 13.34.6 d, i.e., 280/(3σcbc), as per the code takes into account long-term effects such as creep. As per pp. 137 0f SP 24-1983, this formulae seems to have been derived based on long-term experiments. This is also mentioned in the note of cl. B-1.3 of IS 456.
The values of Es/Ec and 280/(3σcbc) are compared below:
Grade     280/(3σcbc)     Es/Ec
15          18.66          10.33
20          13.33          8.94
25          10.98          8
30          9.33               7.3

Using the expressions given in ACI and EuroCode2 for long term and short term Ec, it may be found that Ecl/Ec of concrete is not very high after 28 days and is 1.2 to 1.4 only and is dependent on the type of cement used. Thus, the modular ratio suggested by IS 456 seems to be very high.
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vikram.jeet
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PostPosted: Wed Sep 08, 2021 4:44 am    Post subject: Re: In working stress method we consider modular ratio m = 280/(3 σcbc). Is it correct? Reply with quote

Observation from Dr NS Sir is noteworthy .

It seems that for rich mixes , M25, M30 and above the RATIO OF Ecl/Ec are within the range of ACI code. However for M20 and M15 ratios are quite  high.
It is to be seen whether M20 is recommenced mix for RCC or not as per ACI. .Also Mix M15 is now not recommended for use as RCC vide IS 456 , 2000 version , though earlier constructions were in M15 also.
The ratio of Ecl/Ec
M15.   18.66/10.33 = 1.8
M20 - - - -  1.5
M25 - - - - - 1.37
M30 - - - - -1.27


Dr. N. Subramanian wrote:
In working stress method we consider modular ratio m = 280/(3 σcbc). Is it correct?

The short-term modulus of concrete does not take into account the effects of creep, shrinkage, and other long-term effects. Accordingly, the modular ratio m is not computed as m = Es/Ec = 200000/[5000 /sq. root(Fck)].   The value of m, as given in sec. 13.34.6 d, i.e., 280/(3σcbc), as per the code takes into account long-term effects such as creep. As per pp. 137 0f SP 24-1983, this formulae seems to have been derived based on long-term experiments. This is also mentioned in the note of cl. B-1.3 of IS 456.
The values of Es/Ec and 280/(3σcbc) are compared below:
Grade     280/(3σcbc)     Es/Ec
15          18.66          10.33
20          13.33          8.94
25          10.98          8
30          9.33               7.3

Using the expressions given in ACI and EuroCode2 for long term and short term Ec, it may be found that Ecl/Ec of concrete is not very high after 28 days and is 1.2 to 1.4 only and is dependent on the type of cement used. Thus, the modular ratio suggested by IS 456 seems to be very high.
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vikram.jeet
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PostPosted: Fri Sep 10, 2021 7:49 am    Post subject: Reply with quote

Continuation to previous post on this topic,

I think the conc mixes and steel strengths used in  West are richer , and feel that Minimum grade of conc mix there may be M25 and above for RCC WORKS . ACI codes , thus , covered those mixes for rcc and range of 1.2 to 1.4 is specified .
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