I thought someone might misunderstand that, I said the total acceleration. Yes, because the collapse was initiated at the 12 floor and not the top, these impact forces are approximately overwhelmed from the start. If you actually do I thought experiment and represent the forces accordingly you will come to the same result. The force of the impact of the 12+1 floors into the 27th floor(singular) is practically negligible compared to gravity.

If the collapsed was initiated from the top of the building then you might see a non negligible influence of this relatively small force.

Just to give you a nice example. Draw the for diagram for a stationary cup of water weighing 3kg. Now, stratify the cup of water into three layers each weighing 1kg. What is the force diagram for each of the layers? And, what is the sum of these forces?

In reply to this comment by Squidley:

I'm not sure what they're teaching in calculus, physics, statics/dynamics...