Case 1. Flexural members
As you increase the load ‘w’, at some point in time the compressive strain ‘εc’ and stress ‘σcbc’ in extreme fibres will reach to 0.002 and 0.446fck respectively.
If you further increase ‘w’, ‘εc’ will increase without any increase in ‘σcbc’ (refer Stress-Strain curve for concrete). Now, that extra stress which is not borne by the extreme fibres is passed on to the subsequent underlying fibres.
So, at certain depth, the fibres would have reached the strain of 0.002 and the stress of 0.446fck (as shown in following figure).
On further increasing the load, ‘εc’ will ultimately reach to the value of 0.0035. Beyond this strain the concrete is believed to fail. So, once the extreme fibres fail, the cracks will develop, further weakening the section and leading to failure of the structure.
Therefore, code restricts εc at 0.0035 for flexural members.
Case 2: Compression members: a) only Axial load
As you increase the axial load ‘w’, at some point in time the strain throughout the cross-section will reach 0.002. Any further increase in load will lead to sudden increase in strain beyond 0.002 (without any increase in stress) (refer stress-strain curve of concrete given in IS 456). Since the concrete through out the sectionwould have crossed the “limiting value of strain of 0.0035” in no time, and the excess stress is borne by nothing, the whole section crushes abruptly.
So, the code restricts the strain in concrete at 0.002 and not 0.0035 in this case.
b) Axial load along with uni-axial Bending moment
Code restricts the strain at the highly compressed extreme fibre in concrete ‘εc1’ at
εc1=0.0035−0.75εc2,εc1=0.0035−0.75εc2,where, εc2 is the strain at the least compressed extreme fibre.
If you put value of εc2 = 0, then you get εc1 = 0.0035. This case is similar to the compressive strain distribution in concrete in flexure member.
If you put value of εc2 = 0.002, then you get εc1 = 0.002. This case is similar to the strain distribution in concrete in axially loaded compression member.
Extra:
Is 456:2000
As you increase the load ‘w’, at some point in time the compressive strain ‘εc’ and stress ‘σcbc’ in extreme fibres will reach to 0.002 and 0.446fck respectively.
If you further increase ‘w’, ‘εc’ will increase without any increase in ‘σcbc’ (refer Stress-Strain curve for concrete). Now, that extra stress which is not borne by the extreme fibres is passed on to the subsequent underlying fibres.
So, at certain depth, the fibres would have reached the strain of 0.002 and the stress of 0.446fck (as shown in following figure).
On further increasing the load, ‘εc’ will ultimately reach to the value of 0.0035. Beyond this strain the concrete is believed to fail. So, once the extreme fibres fail, the cracks will develop, further weakening the section and leading to failure of the structure.
Therefore, code restricts εc at 0.0035 for flexural members.
Case 2: Compression members: a) only Axial load
As you increase the axial load ‘w’, at some point in time the strain throughout the cross-section will reach 0.002. Any further increase in load will lead to sudden increase in strain beyond 0.002 (without any increase in stress) (refer stress-strain curve of concrete given in IS 456). Since the concrete through out the sectionwould have crossed the “limiting value of strain of 0.0035” in no time, and the excess stress is borne by nothing, the whole section crushes abruptly.
So, the code restricts the strain in concrete at 0.002 and not 0.0035 in this case.
b) Axial load along with uni-axial Bending moment
Code restricts the strain at the highly compressed extreme fibre in concrete ‘εc1’ at
εc1=0.0035−0.75εc2,εc1=0.0035−0.75εc2,where, εc2 is the strain at the least compressed extreme fibre.
If you put value of εc2 = 0, then you get εc1 = 0.0035. This case is similar to the compressive strain distribution in concrete in flexure member.
If you put value of εc2 = 0.002, then you get εc1 = 0.002. This case is similar to the strain distribution in concrete in axially loaded compression member.
Extra:
- Test results show that the compressive stress in the concrete reaches its maximum possible value when the maximum compressive strain in concrete is around 0.002. Beyond 0.002, the stress falls gradually.
- Concrete fails in compression when the strain in concrete reaches at around 0.003.
- Code generalises this behaviour in stress-strain diagram for concrete and assumes that the stress remains constant between the strain values of 0.002 and 0.0035.
- Also, the code restricts maximum strain in the extreme fibres in compression at 0.0035.
Is 456:2000
