🔥 NO PLANES WERE USED ON 9/11! NO PLANE WOULD BE ABLE TO FLY COMPLETELY THROUGH AND "HOT KNIFE THROUGH BUTTER," SPEED AND HAVE THE NOSE EXTEND THROUGH THE OTHER SIDE!
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🔥 The photo is from original World Trade Center construction around 1970. The finished-looking tower on the right is the North Tower with cladding going on; the open steel cage in the foreground is the South Tower rising. Almost everything you can see in the unfinished structure is steel. Concrete is present, but most of it is either below grade or not yet poured on these floors.
🔥 Steel — the load-bearing system you see.
🔥 The towers used a framed-tube design rather than a conventional interior grid of columns. Gravity load was shared roughly equally between a stiff perimeter “tube” and a central steel core. Wind load was carried almost entirely by the perimeter.
🔥 Perimeter tube:** From about the 10th floor up, each face had 59 closely spaced built-up box columns (236 total around the square plan). Adjacent columns were tied at every floor by deep steel spandrel plates (typically about 52 in. deep). Three columns plus their connecting spandrels were shop-fabricated as three-story wall panels, then stacked and welded on site. That is the dense vertical grid visible in the photo. Column plate thickness and steel grade varied with height and location (yield strengths from about 42 ksi to 100 ksi) so the tube could take both gravity and wind without wasting material.
Core:** A rectangular service core of large steel box columns and wide-flange columns carried elevators, stairs, and mechanical shafts. The heaviest base sections were multi-story pieces set on steel grillage and concrete on Manhattan schist.
Floors:** Prefabricated steel trusses (roughly 36 ft or 60 ft long) spanned from core columns to the perimeter wall. They supported corrugated metal deck. Those trusses also helped stabilize the columns. A hat truss at the roof later tied core and perimeter together.
🔥 The complex used on the order of 200,000 tons of structural steel. Prefabrication of wall panels and floor units is why the skeleton could rise several floors a week once steel erection was underway.
🔥 Concrete — foundation, floors, and the “bathtub”
🔥 Concrete is less obvious in this shot because the South Tower is still a bare frame.
🔥 Slurry-wall foundation (“bathtub”):** Because the site was landfill next to the Hudson, a continuous reinforced-concrete retaining wall was built first. Trenches were filled with bentonite slurry, steel cages were lowered in, then concrete was tremied from the bottom, displacing the slurry. That wall held back groundwater so the site could be excavated about 70 ft to bedrock.
Foundations:** Core and perimeter loads went to steel grillage on concrete footings bearing on schist.
Floor slabs:** After the steel trusses and metal deck were in place, workers poured lightweight concrete — typically about 4 in. on 1½-in. deck outside the core and about 5 in. in the core. The slab acted compositely with the steel. Those pours had not yet happened on the open floors in the photo. The neighboring North Tower already has cladding, so its lower floors would already have slabs.
🔥 The whole World Trade Center complex used roughly 425,000 cubic yards of concrete. In this particular image, though, the dramatic visual is the steel tube and core; the concrete work is either already in the ground or waiting for the next cycle of decking and pours.
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