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here is an excerpt from on of my fav books on structural design, I read it some 3 years back, and it always reminds me how we structural engineers can exploit material properties to achieve robust designs.

To sum it here, design your members for augmented loads(with load factors) and check them for serviceability with cracked section.. its a conservative way, super conservative to get good design. All members are good, even if they crack and meet serviceability requirements (which may be different upon their intended use).Update: Members should also be checked for strength using cracked sections. Its more safe to apply to both serviceability and strength conditions.

flexural stiffness of section is ability to attract moments. so cracked section will attract lesser moment as only part of section is available to resist rotation. Do not confuse moment attracting ability( stiffness) of section with its strength. Section with lesser stiffness can have greater strength as it depends upon material strengths, reinforcement and size and shape crossection.

for simplicity, remember this if it has out of plane stiffness its shell, if not its membrane.

Umar is right. Use section modifiers for serviceability checks. This set of modifiers are mentioned in section 10.11.1 of 05 addition which is dedicated to computation of lateral deflections of frame. Remember that code specifications are based on worst case scenario, and these values are worst case scenario for lateral deflections of frame.For elastic analysis of frame it is OK to use gross properties based on rectangular section as it is done in ETABS . We provide rectangular beam section properties in ETABS, but cast in place beam has T section in positive region while rectangular section in negative region, so using rectangular section along entire length compensates for that. Moreover if bottom reinforcement of beam is developed in column, as it is normally done, it increases stiffness of beam in negative region. Amount of reinforcement provided in section also plays its role and we dont know how much reinforcement will be required before starting analysis. Bottom line: it is complex topic and one have to use assumptions. Even if one is using 0.35 and 0.7 factors to size the xsection of member, structure should still stand provided assumptions are uniform through out the analysis, as concrete has this ability to distribute moments according to provided reinforcement.