If any nonconforming building shall be destroyed by reason of windstorm, fire, explosion, or other Act of God, or the public enemy to an extent of more than 50% of the building floor area, then such destruction shall be deemed complete destruction and the structure may not be rebuilt, restored or repaired except in conformity with the regulations of this chapter. Nothing in this chapter shall prevent the strengthening or restoring to a safe condition any wall, floor or roof which has been declared unsafe by the Zoning Officer.