reply to post by P1DrummerBoy
It is not necessary for every part of the structure to fail in order to bring
building down - collapse of WTC 7 is what known as progressive collapse
where one section of structure fails. Stress is transmitted to adjacent
sections which in turn fail, which in turn cause additional sections to fail
until entire structure collapses.
Can see this in WTC 7 - kink forms in building as column fails, rooftop
penthouse falls into building as that part of structure fails. Remainder
of structure begns to fail until entire building is pulled down as section
after section collapses.
Think of it as row of dominoes - one domino topples knocking down
adjacent domino which knocks over another until entire row falls down
WTC 7 had unusual structural configuration - was built over Con Ed
substation. Originally was designed for much smaller building, some 25 stories. Building was expanded to 47 stories - to handle extra dimensions
required a cantilever truss system to transfer loads into building supports
Each column was then required to carry very heavy loads to support the building
The building was constructed above a Con Edison substation that had been on the site since 1967.[3] The substation had a caisson foundation designed
to carry the weight of a future building of 25 stories containing 600,000 sq ft (55,700 m²).[4] The final design for 7 World Trade Center was for a
much larger building covering a larger footprint than originally planned when the substation was built.[5]
The structural design of 7 World Trade Center included features to allow a larger building than originally planned to be constructed. A system of
gravity column transfer trusses and girders was located between floors 5 and 7 to transfer loads to the smaller foundation.[3] Existing caissons
installed in 1967 were used, along with new ones, to accommodate the building. The fifth floor functioned as a structural diaphragm, providing lateral
stability and distribution of loads between the new and old caissons. Above the seventh floor, the building's structure was a typical tube-frame
design, with columns in the core and on the perimeter, and lateral loads resisted by perimeter moment frames.[4]
its progress report, NIST released a video and still-photo analysis of 7 World Trade Center before its collapse that appears to indicate a greater
degree of structural damage from falling debris than originally assumed by FEMA. Specifically, NIST's interim report on 7 World Trade Center displays
photographs of the southwest facade of the building that show it to have significant damage. The report also highlights a 10-story gash in the center
of the south facade, toward the bottom, extending approximately a quarter of the way into the interior.[3][40] A unique aspect of the design of 7
World Trade Center was that each outer structural column was responsible for supporting 2,000 sq ft (186 m²) of floor space, suggesting that the
simultaneous removal of a number of columns severely compromised the structure's integrity.[41] Consistent with this theory, news footage shows
cracking and bowing of the building's east wall immediately before the collapse, which began at the penthouse floors.[3] In video of the collapse,
taken from the north by CBS News and other news media, the first visible sign of collapse is movement in the east penthouse 8.2 seconds before the
north wall began to collapse, which took at least another 7 seconds.[3][42]
The working hypothesis, released in the June 2004 progress report and reiterated in a June 2007 status update, was that an initial failure in a
critical column occurred below the 13th floor, caused by damage from fire and/or debris from the collapse of the two main towers. The collapse
progressed vertically up to the east mechanical penthouse. The interior structure was unable to handle the redistributed load, resulting in horizontal
progression of the failure across lower floors, particularly the 5th to 7th floors. This resulted in "a disproportionate collapse of the entire
structure."[3][39][43]
