Why property modifier are important in Etabs?

Property modifier are very important parameter on defining section process,they are used to model correct behavior of concrete section.we all know that the concrete is weak in tension and strong in compression. property modifiers are only applied to concrete member because of cracking, so if we are considering the rectangular section, the moment of inertia is bd^3/12.we all know that but when we make this member of concrete it will experience cracking when loading after sometime so, this cracking will occur when concrete reaches its tensile capacity which is about 7 to 10% of its compressive strength.

Lets take an example, 25 mpa will have only 2.5 mpa of tensile strength, if we consider 10% after some cracking concrete will no longer able to carry tension so the steel starts taking tension. so now if concrete cracks after 2.5 mpa the  moment of inertia will reduce because of cracking, if the moment of inertia has reduced the capacity of section to resist the bending will reduced,its stiffness is reduced,the concrete will take less moments,its deflection will increase due to less stiffness.so this moment of inertia which the correct thing is not taking anymore will be redistributed to the other structure member based on there stiffness that is why ACI code suggest different value for property modifiers for beam,column,slab and wall sections.capacity for beam section is reduced to 35% and capacity for column is reduced to 70%.

In Clause 22.3,of IS 456:2000 does talk about stiffness,but it didn't provide the modifier value so we can refer ACI 318 for modifier values.