For explaining the answer, we have divided the concrete member in three types as follows;
In this case , If we increase the load ‘w’, at some point the strain ‘εc’ & stress ‘σcbc’ in the given beam will reach to 0.002 & 0.446fck respectively.
Now, if we further increase the load ‘w’, then further increment in ‘εc’ will occure without any increase in ‘σcbc’ .(please refer Stress-Strain curve for concrete for more details). And the extra stress will be taken up by the subsequent underlying fibres. Hence, at a certain depth, the underlying fibres would have reached the strain of 0.002 & the stress of 0.446fck.
On further increase in the load, the ‘εc’ will ultimately reach to the value of 0.0035. Beyond this strain the concrete is believed to be fail. Subsequently once the extreme fibres fails, the cracks will develop in beam, further weakening the concrete & leading to the failure of structure.
Therefore, the IS code restricts the εc at 0.0035.
Now in this case, As we increase the axial load ‘w’, at some point the strain throughout the cross-section of column will reach the value 0.002. Now any further increase in load ‘w’ will lead to the sudden increase in strain beyond the value 0.002 (without any increase in thr stress). (refer stress-strain curve of the concrete given in IS 456) and the concrete will eventually fail. Hence, the code restricts the strain in the concrete as 0.002 and not 0.0035.
In this case, the tode restricts the strain at the highly compressed extreme fibre in concrete ‘εc1’ at
Where, εc2 is the strain at least compressed extreme fibre.
If we put value of εc2 = 0,
then we get εc1 = 0.0035.
This is similar to strain distribution in the concrete in flexure member.
If we put the value of εc2 = 0.002, then we get εc1 = 0.002. This is similar to the strain distribution in the concrete in axially loaded compression member.
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