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In building safety and construction, asmokeproof enclosure is a type ofexit stairwell that has been designed to keep out smoke (and othercombustion products) in the event of a fire, so that building occupants may moresafely 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-calledopen 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 ventilatedopen 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] TheLife Safety Code states that such stairwells be "approved systems with a design pressure difference across the barrier of not less than 0.05in. water column (12.5Pa) insprinkled 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 ispositively 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] TheLife 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 30lbf (133N) at the door knob or handle.[4] These 'pressurization' problems are, of course, non-existent with naturally ventilated smokeproof enclosures.
The fans andair 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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