Displacement Amplification Factor.

                             

If you look at it the right way, that's the whole of Seismic Analysis as per ASCE 7-10. 10 means the year of publication, the latest is 7-22.

There are three important factors here, R = Response Reduction Factor, Omega = Overstrength Factor and Cd = Displacement Amplification Factor. Today I will be writing about the later one.
Now it has to be seen as per Linear and Non-Linear Analysis. After a certain level of strain, the structure goes into a state of Non-Linearity, where there is more strain/deformation in structures over the increase in stress. In a perfectly elastic state, the change in uniform. If one wants to do linear elastic analysis of a structure under full seismic loading, the resultant forces will be significantly large, as seen in the graph, and the design of the structure will be uneconomical. We are not taking advantage of the of ductility and inelastic energy dissipation in such approach.

What American code suggests is, tone down the resultant stress due to elastic to the inelastic range. This is done by the factor R. But toning it down would surely effect the strain/displacement of the structure and hence we use the Displacement Amplification factor to amplify the displacement response to the initial elastic response. In this way, we get the reactions that would be similar during an earthquake.

So why did I choose this topic, it's because of a linked in post I encountered.

The Base Shear Scaling of the Response Spectrum Analysis is done to ensure that the minimum strength of a structure designed by RSA is similar to the strength required by the Equivalent Static Method. As per 7-22, we should scale it to 100%. 
My project being done in ASCE, I wondered why didn't we amplify the displacement as per IS 1893. We did the base shear scaling alright. 

The point is, IS 1893 does not have any provision for the displacement amplification factor. So we have to create two models for seismic analysis if we design it using IS 1893, one scaled and the other. Whereas, in using ASCE 7-22, Cd takes care of it.

Conclusion : Codes vary. Read and implement as per requirement. The basics are almost similar for all codes. 

Always open for discussion.

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