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It should be dubbed as building frame system. The relative stiffness of vertical members of lateral load system and their strength and ductility decides the category of structural system.

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.

membrane is capable of taking tensile stresses only like our skin or tarpal (urdu word). Word slab used in structural engineering cant be membrane. shall is capable of taking shear and moment. Any thing that can take shear or moment can also take direct tension or compression. plate is shell/slab loaded in its plane like shear wall.

@RANA when we say that uncracked section has more inertia and more stiffness and more moment taking but cracked section has less inertia and less stiffness and less moment taking,inertia is the geometric property moment taking is related to strength, uncracked concrete section has less strength than cracked section so sir Rana what do you say about that?also in case of concrete when section is cracked it means it has started taking loads otherwise its strength is not fully generated so what about the stiffness of cracked and uncracked section?

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.