> For the complete documentation index, see [llms.txt](https://docs.enercalc.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.enercalc.com/sel-faq/using-the-software/calculation-modules/loads-forces/seismic-loads/seismic-base-shear-and-design.md).

# Seismic Base Shear and Design Methodology

Why the Seismic Base Shear module no longer references design methodology when calculating Cs and base shear.

## Why is there no longer an option to identify the Design Method in the Seismic Base Shear module?

The seismic base shear calculation per ASCE 7 and IBC is exactly the same regardless of the design methodology that will be used. Ground motion is ground motion, and base shear is base shear. The differences start to come in once a design methodology is identified. But even then, there is nothing in ASCE 7 or in IBC that ever tells us that we should be modifying the value of Cs based on the design methodology. ASCE 7 and IBC are both very specific in showing that we are to deal with that design methodology by factoring at the load combination stage. This is **so** important, because IBC also requires that the Construction Documents indicate certain seismic design data including the value of Cs and the Design base shear. Again, it is our interpretation that the value of Cs (in each direction) and the value of Design base shear (in each direction) are what they are, and should never be modified according to design methodology until you get to the load combinations. In fact, doing so could potentially be unconservative, because designers of performance spec items could inadvertently apply the Cs factor twice. So that’s the reasoning behind why we modified the presentation of Cs and seismic base shear to eliminate any reference to the design methodology.


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