Why stiffness modifier value is higher for column than beam?

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The pattern and extent of cracks will vary substantially from structure to structures and members to members even in a similar structure. It is very difficult to assign the unique values of the stiffness modifiers for different members. The values which are suggested in the code are based on the numerous experiments and might have been referred from different international standards. The stiffness modifier value for column is higher than the beam since the columns will have higher axial compression than the beam. Hence, the extent of cracks would be lesser in the columns as compared to the beams. Hence, the stiffness modifier value for column is higher than the beams.

What impact stiffness modifiers will make on overall analysis?

There will be two major impacts in the structural analysis results as compared to the model using 100% of gross moment of inertia:

  1. Since we are considering the reduced moment of inertia, the overall stiffness of the structure will reduce. Due to the reduced stiffness, the structure will be relatively flexible and hence would attract the lower seismic forces.
  2. Drift of the structure will increase because it is relatively flexible

Are stiffness modifiers required only for analysis or for design as well?

The stiffness modifiers should be considered only for the structural analysis. The analysis results to be derived considering the stiffness modifiers. The structural design to be done with the conventional procedure considering the analysis results.

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