Difference between cracked moment of inertia and effective moment of inertia
Answers
Cracked section :- now due to such loading the compressive stresses will develop above neutral axis nd tensile stresses will setup below neutral axis..! now we know concrete is very weak in tension nd its tensile strength = compressive strength/10 generally..! so as d load is being increased d magnitude of stresses will also increase..! nd a time will come when the tensile stresses developed in concrete below N.A will be more than the tensile strength of concrete..so basically concrete is not able to take up such tensile stresses nd so it gets cracked and the whole tensile stresses is now borne up by steel..! so now below N.A the concrete area is found to be ineffective as it already got cracked and is unable to withstand d tensile stresses anymore ..! generally the first tensile crack occurs at a stress value of 0.7root(fck) and this is also called modulus of rupture or flexural tensile strength of concrete(fcr)..! In other way we can also say that if the applied external moment M is more than d cracking moment Mcr..! then also the section is said to be a cracked section..!
ii) Un-cracked section :- As u already got an idea about cracked section, so uncracked section is such a section where the applied external moment(M) is less than cracking moment (Mcr)..or u can say if the load is of such value that the magnitude of tensile stresses developed below N.A is less than d permissible tensile strength of concrete then easily concrete can take up such tensile stresses nd the concrete area is also found to be effective below N.A..so such a sections is referred to as uncracked section..!
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Mass is nothing but a resistance to motion similarly mass moment of Inertia is resistance to rotational motion. Area is a resistance to stress developed similarly polar moment of inertia is resistance to torsional stresses developed.