Is this reasonable damage?

[hunbahubba]

Assuming you are confident that the work has been carried out to spec (correct steels specified and installed etc) then the most likely cause is the structure not having been adequately propped. Some damage of this type is not unusual, but the extent of the cracking in your case is excessive. The good news is now the steels are in that should stabilise everything. If you want some additional assurance have the works and damage inspected by a structural engineer, ask them to specify the repairs required, and then hand that to the builder and ask him to put things right.