Complex sentence: The building's main stair case extended from the front doors of the building, up to the third floor; without benefit of fire doors, the stairwell acted like a chimney, helping to spread the fire quickly.
Split into simpler sentences: The building's main stair case extended from the front doors of the building, up to the third floor; without benefit of fire doors. The stairwell acted like a chimney, helping to spread the fire quickly.