  The Simple Collapse Model of Ba\v{z}ant and Zhou is evaluated for a
progressive floor collapse of a tall building. A sequence of energy estimates
indexed by the collapsing floors is derived. Each of the estimates gives a
sufficient condition to arrest the collapse at a given floor. The first
estimate of this sequence has been stated by Ba\v{z}ant and Zhou and has been
repeatedly cited later on. However, this estimate is not optimal in the sense
that the following estimates give a weaker condition to arrest the collapse.
