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Q.1):- When we need to consider modifiers in design?
Ans. :- In all analysis pertaining to RCC design, we shall consider modifiers.
Q.2):- Is it to be followed while we design building for seismic conditions or for the conditions where we have to design structure for cracked sections.
Ans.:- As far as RCC design is concerned, you need to consider section modifiers in all load cases. Also note that for RCC building, all members are designed as cracked members.
The reply consists of your exact query.
I am sure that you are not satisfied with this reply. So, more discussion is necessary.
In analysis, may be Hardy-cross moment distribution or even finite element method, we consider dimensions of member and assume it to be prismatic. If a steel beam – column frame, this is o.k., but when it is an RCC beam-column frame this is not o.k., because right from lower loads, the concrete cracks and reduces its moment of inertia that depends on moment existing on the section. Therefore for a length of the beam the moment of inertia is variable since moment is variable. IS: 456, clause 22.3 gives three ways to assume the moment of inertia of the section for analysis and consider the member as prismatic.
- Gross section
- Transformed section
- Cracked section.
The section modifier is nothing but ratio of assumed moment of inertia by any of the above three methods (or any other) and the MI of gross section. Thus section modifier in case 1 is 1.0 and in other two cases, it is less than 1.
We have picked up the easiest “Gross section” up till now. As a result, we remained far from realistic analysis. It should be clear that section modifier is 1.0 with no loads and reduces with the loads. Also for columns and walls, it reduces slowly than beams and slabs because of the compression present in columns and walls.
To simplify the procedure, as per IS: 16700, two types of section modifiers are suggested.
- For unfactored loads (service condition)
- For factored loads (Ultimate condition)
For software analysis it is necessary to deal with two separate models. One with unfactored loads and the other with factored loads. The first model should be used for limit state of serviceability (for checking deflection and cracking. In multi-storeyed buildings global deflection checking means checking for drift)