>Stairwell = chimney.

Not so.

Please don't scare people away from using the stairs. The best course of action is to get out immediately, and the stairs are the safest course.

Smoke control is a basic requirement for today's buildings. No system is perfect, but there are some very intelligent folks that have put a lot of thought into these systems. If you Google using key words like fire, alarm, pressurized, and stairwell, you should come up with links like this:

http://www.esmagazine.com/CDA/ArticleInformation/coverstory/BNPCoverStoryItem/0,2500,65962,00.html

(the following information is from this link)

Engineered smoke control systems are incorporated into a building's design to prevent the loss of life and property. These systems consist of dedicated mechanical fans for the purpose of controlling smoke migration within a building. Alternatively, fans that normally serve the building's hvac systems can use building automation system (bas) controls to modify operation to provide building smoke control. Smoke control systems are incorporated in building design to allow safe egress during smoke and fire incidents, provide firefighter staging areas, and provide areas of "safe haven" for those who may have difficulty evacuating.

Past disasters have shaped current codes and standards (Sidebar). Operating parameters for these systems have been formulated by studying fires and the physical conditions encountered in spaces engulfed in thermal smoke. Because no value can be assigned to a human life, systems must be designed, constructed, and maintained such that past failures are not repeated and additional human life and property is not lost.

Some of the most common smoke control system requirements and recommendations for high-rise buildings by local and model building codes, the NFPA, and ASHRAE include:


A smokeproof enclosure (fire-rated stairwell), with vented vestibules at each floor;
Positively pressurized egress stairwells;
Floor pressure sandwich schemes to contain smoke within the zone of incidence;
Zoned smoke control compartments on each floor;
Air-handling unit (AHU) shutdown and smoke purge;
Firefighters' hands-off auto override controls;
Sprinkler system waterflow switches and manual pull station system activation; and
Elevator hoistway pressurization.

...