I recommend too Christian Lalanne(auth.) - Mechanical Shock_ Mechanical Vibration and Shock Analysis, Volume 2, Second Edition (2009, Wiley-ISTE)
In this book also explayning responce spectrum analysis for shock. The problem is precisely in getting the load. To do this, you need to use another...
Hi. Response spectrum is conservative analysis, which calculate quickly. You use damping for get the level of load. It's analysis not simple how look.
You need to read this article of mr. Irvine:
https://www.vibrationdata.com/tutorials2/srs_intr.pdf
You also can use transient analysis for shock.