0Stiffness modification: Significance In IS 456,as per Cl.22.3 gross sectional properties are allowed to be used (if consistently used for all members) for force calculations. For deflections, 22.3.2 allows only the usage of cracked mom of inertia. What's wrong if we use uncracked mom of inertia? Concrete has inherent property of cracking what ever we do. So if we dont capture this, we are making the supports more stiff than real, which means support moments are more which means the span moments and deflections are lesser in the model than real. To take care of this, in tall buildings, generally cracked mom of inertia is recommended for both force calculations and deflection checks. However, it is not easy to formulate this for all members as crack widths varies. So, ACI makes it more generic based on what is seen actually and uses a flat value.
