Can you imagine two BA men trying to run out two lengths of 45mm hose and charging it in a fire-fighting lobby that can be as small as 5 sq m. The thought has always tickled me.
For years firefighters have been using the floor below as their bridgehead, more recently two floors below, though with satisfactory ventilation some are sticking to one floor.
Yes, this results in doors being held open, that's what the ventilation in the lobby and staircase is for. With modern smoke shafts it is an advantage to have certain of the doors open - the idea being that buoyant smoke rises up the shaft (opened by fire fighters) drawing replacement air from the staircase - note that the direction of air movement will prevent, to a large extent, smoke from entering the stairs.
Having the fire service using a staircase for fire fighting operations will make that staircase unavailable for evacuation. The latest iteration of ADB discusses this briefly for tall buildings but, depending on the anticipated attendance time of the fire service, this may need to be considered for smaller buildings that have fire fighting shafts (i.e. you may have to lose a staircase when calculating vertical means of escape).
Stu