The Core of WTC 1 Failed
Below the Third Floor

World Trade Center inner core construction photo:

The World Trade Center cores were designed to support the entire weight of the buildings several times over. Far more than a mere "service core", it comprised of 47 steel box columns tied together at each floor by steel plates, similar to the 52" deep spandrel plates that tied the perimeter columns together. The largest of these core columns were 18"x36", with steel walls 4" thick near the base and tapering in thickness toward the top, and was anchored directly to the bedrock.

The lower part of the core was the most solid piece of WTC 1, but the following video shows it comes down several seconds after the building's collapse:.

This part of the core was not hit by a plane. It was solid, and its scale was huge (click image below for full size).

The core which is visible following the collapse should have remained standing. I didn't... failed beneath the third floor:

Firefighter in the stairwell of WTC 1 during the collapse: "As I turned the corner on the third floor landing the stairwell erupted into a hurricane. This huge incredible force of wind and debris actually came up the stairs, knocked my helmet off, knocked me to the ground.
It was pitch black, large amounts of dust in the stairwell. So as I was crawling around to start down the next set of stairs the landings collapsed underneath me so I dropped pretty much a full floor."

WMV video download (959kB)

The core of WTC 2 also briefly stood after the building's collapse:

The collapsing cores are consistent with a thermite reaction pooling molten iron into the central area of the WTC basement, this subsequently melted the core columns and caused the structural failures several seconds later.

See also: The 9/11 WTC Collapses: An Audio-Video Analysis

What Really Happened