Vestibule vs open stair

[Examiner]

2012 IBC and ASHRAE 90.1-2010; The state uses ASHRAE for an energy code. A vestibule in Zone 3 is required when the space exceeds 3,000-sf that have an entrance door from the outside. Project is allowed to have a 3-story open stair due to an automatic fire suppression system. Problem is the bottom floor of the stair opens to the space needing the vestibule. However there is not enough room for a vestibule and the stair's landing area. Can the bottom of the stair be part of the required vestibule and still be an open stair? The upper floors will all be open to the stair. The design is to have a glass storefront door open into the glass enclosed vestibule. the corridor on the bottom floor that shares the same space as the stair will also have storefront doors opening into the vestibule.

In regard to the 3,000-sf; ASHRAE states when the exterior door opens into a space of 3,000-sf. The question comes up at times if the space is to include the whole corridor system since the corridor system is not separated from the vestibule. I get the application if it is a large room like a department store's main floor. If it is to include the corridor system I would assume that cross corridor doors, not held open, would meet the purpose of a vestibule's door open and closing requirements. They do mention that a minimum of 7-ft is required as well as how the doors are to operate by not opening at the same time in series. Do you include the total area of the building's corridor system or not?

 

[JBI]

Add doors at the corridors to reduce the open area below 3,000 s.f.?

 

[Examiner]

Problem with adding cross corridor doors they usually have to be double egress which the client will not allow to be closed and that defeats the purpose of the vestibule. Then the question is to how much room is allowed between the exterior doors and the vestibule doors before they cause exit issues if the exit is blocked. I have seen lobbies that have a set of interior doors opening into the lobby and then exterior doors that are the EXIT doors and I always question the potential occupant load crammed into the lobby area with only one way out via door swing in the direction of egress. Once in the lobby the door swing from the interior swings the wrong way. How much distance and room is needed to avoid egress issues seen as dead ends. I have seen cross corridor pair doors that due to them not being double egress and their location causes a dead end corridor. The energy code recommends 7-ft which is the same for door in series for Building Code and ADA. My question would be; how far can you go with door in series before it is an egress issue? The design has a small area at the stair's bottom and I was thinking of issues with an open stair if the bottom opening into the stair contained a door and glass wall. Is the stair still considered to be an open stair if on all other floors it is open?