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In building safety and construction, a smokeproof enclosure is a type of exit stairwell that has been designed to keep out smoke (and other combustion products) in the event of a fire, so that building occupants may more safely exit the building. [1] [2] [3] [4] [5]
Rather than entering the stairwell directly from the building interior, one enters a smokeproof enclosure by means of an open-air balcony or alternatively, a so-called open vestibule, and proceeds thence, to the stairwell itself. [1] The way that the balcony or vestibule is ventilated divides smokeproof enclosures into two basic types: mechanically ventilated which is not actually a smokeproof enclosures, where the vestibule and stairwell are ventilated by mechanical equipment, and naturally ventilated open air smokeproof enclosures where the open balcony or vestibule [or the stairwell] has openings directly to the outside of the building. This is the only type of the original concept of a smokeproof tower (i.e.: smokeproof enclosure). [6] [3]
Under United States building codes, the stairwell of a smokeproof enclosure must have walls with a 2-hour fire resistance rating and vestibule doors (if provided) with a 1.5 hour fire resistance rating. [7] The Life Safety Code states that such stairwells be "approved systems with a design pressure difference across the barrier of not less than 0.05 in. water column (12.5 Pa) in sprinkled buildings and 0.10 in. water column (25 Pa) in non-sprinkled buildings". [4] Approved, in the Life Safety Code, means the committee didn't want to 'decide' so, left it up to someone else to decide a particular issue.
In the so-called mechanically ventilated smokeproof enclosures, the stairwell is positively pressurized relative to the rest of the building. [6] [8] This ensures that even when access doors are opened, smoke will not enter the stairwell. [8] [4] Of course, a higher pressure within a stairwell makes it harder to open doors from the building interior to the enclosed vestibules and to the enclosed stairwell. [9] The Life Safety Code requires that the pressure differential across the barrier not be so great as to prevent the door from opening with a force of 30 lbf (133 N) at the door knob or handle. [4] These 'pressurization' problems are, of course, non-existent with naturally ventilated smokeproof enclosures.
The fans and air ducts used to pressurize the stairwell are life-critical systems, and are required to be enclosed in non-combustible, likewise two hour rated, construction. [4] (For fully sprinkled buildings, the required rating is just one hour.) [4] The fans must be connected to an emergency power supply, and capable of both automatic activation by various fire and smoke detectors, and of manual activation by a central command post or by the actuation of a general fire alarm. [4]
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