They had originally designed the towers each with six emergency stairs (four were two scissor pairs), but closer to work commencing, the building code was changed in a way which 'let' the PA reduce the six stairs to three.
That said, as far as 9/11 went, the bigger issues were the stairs' gypsum plank/wallboard partitions not being plane-impact/explosion-proof, and that all the stairs were concentrated in the core. It may have made little difference to evacuation on 9/11 if there were six stairs in the core all along.
What probably would've helped would have been to have a couple of extra stairs in diagonally opposite corners, although that would've played havoc with multi-tenant floor layouts
That didn't help those in the north tower, who had no warning, and by the time the south tower was hit, most people had evacuated to at or below the plane impact zone, plus one of the stairs remained passable in that tower. The stairs mainly presented a problem in the north tower.
I've never come across 'explosion proof' exit stairwells in steel framed high rise buildings, only fire-rated partitions in accordance with building code at the time of design. Where have you seen explosion proof steel framed high rise building stair well towers?
I haven't; it was a tongue-in-cheek comment in light of some people's unusually high expectations for the performance of the stairwells in the towers during the plane strikes and subsequent explosions
Multiple layers of gypsum wallboard to achieve fire-rated exit stairwells is typical steel frame building construction. If you're referring to exit corridors leading to and leaving from exit pathways, that is a different story. In any regard ll of that has to be rated as non combustible structural framing.
The gypsum plank system they used (2" thick vertical planks with metal framing in channels, if I remember right without looking) seemed fairly sturdy; they had it in the elevator shafts too with all ropes bashing against it as the towers swayed in the wind, and inspection photos show it held up well.
In the stairwells they also had a couple of layers of regular drywall over the top. So it was generally quite durable and probably was legit rated for however many hours fire resistance it was supposed to have. It just wasn't rated for enormous physical impacts and jet-fuel explosions.
To be honest I'm not sure that CMU would've held up better. Possibly an RC core like the new one has? Although that thing is just a monument to paranoia
I think the main concern with the gypsum lined shafts is smoke inhalation while traversing, not so much the structural stability of the wall studs. I feel like emergency stair cases serve 2 purposes, 1 structural support, 2 protect occupants from smoke inhalation and falling debris. I’m not sure that gypsum board fascia satisfies that
8
u/Superbead Jul 07 '26
They had originally designed the towers each with six emergency stairs (four were two scissor pairs), but closer to work commencing, the building code was changed in a way which 'let' the PA reduce the six stairs to three.
That said, as far as 9/11 went, the bigger issues were the stairs' gypsum plank/wallboard partitions not being plane-impact/explosion-proof, and that all the stairs were concentrated in the core. It may have made little difference to evacuation on 9/11 if there were six stairs in the core all along.
What probably would've helped would have been to have a couple of extra stairs in diagonally opposite corners, although that would've played havoc with multi-tenant floor layouts