Summary of NIST’s Estimate of Interior Damage to South Tower [View All]

[Kevin Fenton]

Taken from: http://wtc.nist.gov/pubs/NISTNCSTAR1Draft.pdf

“33 exterior columns severed, 1 heavily damaged.
10 core columns severed, 1 heavily damaged.
39 of 47 core columns stripped of insulation on one or more floors.
Insulation stripped from trusses covering 80,000 ft2 of floor area.”
Page 41/95

Although only 10 core columns were severed, most of them were severed on more than one floor.

What does NIST think severed the core columns? Did any elements of the aircraft retain the ability to sever core columns after smashing through the perimeter wall?

Wingtips
As can be seen from Figure 3-2 on page 39/93 South Face Damage of WTC 2 the wingtips were unable to even sever the perimeter columns. Having been diced on the perimeter, they could hardly sever or cause heavy damage to core columns.

Engines
Regarding a simulation performed on an engine, NIST said, “Moving at 500 mph, an engine broke any exterior column it hit. If the engine missed the floor slab, the majority of the engine core remained intact and had enough residual momentum to sever a core column upon direct impact.
Page 105/159
Note the use of the word “if” in the second sentence. This means that if an engine hit a floor slab, it would not have enough residual energy to sever a core column, even upon direct impact. Note the use of the word “a” in the last clause; “a” means one, not nine.

Inner Wing
Regarding simulations performed on wings, NIST said, “The impact of the inner half of an empty wing significantly damaged exterior columns but did not result in their complete failure. Impact of the same wing section, but filled with fuel, did result in failure of the exterior columns.”
Page 105/159
However, once the wings hit the perimeter, the fuel would escape, greatly reducing the wing’s ability to sever core columns. This ability would be further reduced by the loss of speed and mechanical damage caused by the impact against the perimeter columns.

Fuselage
The heavier, lower part of the fuselage may also have retained the ability to sever core columns after breaking through the perimeter.

Let’s turn to the conclusions of the NIST’s computer model of the overall damage to the interior of the South Tower and see how they match up with the component-level simulations.

“The middle of the left wing hit the 78th floor, severing nine perimeter columns and breaking 19 windows on the south face. The SFRM was stripped from the floor trusses over the same width as the building core. The stripping of insulation from the trusses continued inward across the tenant space and about two thirds of the way into the core. There was no direct core column damage from the debris on this floor. However, the southeast corner core column was so damaged on the 80th floor that it broke at its splices on the 77th and 83rd floors.” Page 40/94
The insulation was “stripped”, not damaged. Stripped appears to mean removed in its entirety, at least for the purposes of the computer simulation of the ensuing fire damage. It is unusual that the “SFRM was stripped from the floor trusses over the same width as the building core” because the heavy damage to the building begins several feet inside the western end of the core. It is also unusual that the lightweight wingtip debris were able to strip insulation from around two-thirds of the core. NIST appears to have something against the large southeast corner column, which it has broken in four places.

“There was heavier column damage to the 79th floor. The left engine and the inboard section of the left wing shattered a 25ft section of the center of the floor slab all the way to the core of the building and severed 15 perimeter columns. Reaching the building core, the debris severed nine columns, heavily damaged another, and abraded the SFRM from the eastern two thirds of the columns and trusses all the way to the north end of the core.”
If you look at Figure 3-2 on page 39/93, you can see that the left engine hit the floor slab smack in the middle. Given that, according to the component-level analyses, a wing could not sever a column without being full of fuel, which it was not when it hit the core, and that an engine could only sever a core column it hit directly provided it did not previously hit a floor slab, I am puzzled as to why NIST thinks 9 core columns were severed on this floor. The component-level analyses indicate zero would be a more reasonable figure.

“The damage was most severe on the 80th and 81st floors, hit directly be the fuselage. On the lower floor, a chunk of the floor slab was broken, just above the affected piece of the 79th floor. In addition, a 70ft deep strip along the east side of the core floor was crushed. The north side floor slab sagged along its eastern end. Ten of the perimeter columns severed on the 79th floor were displaced here also. Within the building core, ten columns were severed, including many that were severed on the 79th floor. The SFRM was stripped not only from the eastern two thirds of the core structural elements, nearly to the north wall, but also from most of the trusses of the east tenant space, all the way to the north façade.”
You can see which ten columns were severed by looking at Figure 3-3 Aircraft Impact Damage on page 40/94. It seems that some columns may have been heavily damaged or severed by the plane’s fuselage (the heavier lower portion of which would have hit this floor), but 10 seems to be an overestimate, especially when we consider that 4-6 of the columns which were supposedly severed were not in the path of the fuselage and that some columns were protected by others. Again, the amount of insulation stripped from members is remarkable.

“On the 81st floor, the fuselage pulverized a section of the floor 40ft wide that extended into the southeast corner of the core. The SFRM and gypsum fire protection on the full depth of the east side of the core and in the entire east side of the tenant space was stripped. The structural damage to the core columns was limited to near the southeast corner, but as mentioned above, the impulses felt here caused damage to the key corner column all the way down to the 78th floor. The right engine passed all the way through the 81st floor, exited from the northeast corner, and damaged the roof of a building on Church Street, before coming to rest some 1,500ft northeast of WTC 2 near the corner of Murray and Church Streets. The right landing gear assembly passed through the 81st floor at the east side of the north face and landed near the engine on the roof of a building in Park Place.
Note that the engine passed through this floor and the extensive damage to fireproofing that NIST thinks occurred.

“The right engine caused truss damage up to the southeast corner of the core and severed five columns. As on the 81st floor, the fire protection on the east side of the tenant space and the east side of the core was dislodged.
This is somewhat confusing. To which floor does it refer? Given that it’s between the paragraphs about floors 81 and 83, it should be about floor 82. However, what is the right engine, which we last encountered “some 1,500 ft northeast of WTC 2 near the corner of Murray and Church Streets”, doing here? I checked, the plane had only one right engine. Figure 3-2 South Face Damage on page 39/93 shows that the engine hit the floor 82 floor slab, but it seems it only caught it a glancing blow and proceeded across floor 81 to the corner of Murray and Church Streets. Perhaps a fragment of it was severed by the floor impact and ended up on the 82nd floor. The five columns appear to be perimeter, not core columns. Again, NIST got rid of lots of fireproofing on this floor.

“The 83rd floor caught the middle of the starboard wing. The east side floor slab appeared to be dislodged and sagged at least half of the way into the building.

Interesting, eh?

By the way, NIST claims that the collapse of WTC 2 was a direct result of the impact damage to the core, not the fires, which played a secondary role here, unlike in WTC 1. NIST says, “Several factors led to the collapse of WTC 2: Direct structural damage from the aircraft impact, which included more severe damage to the core columns than in WTC 1;” and “That the aircraft impact damage to the core was more severe in WTC 2 than in WTC 1 contributed to the shorter time of collapse.” Page 45/99

One question keeps nagging at me: if NIST ran their simulation with a more reasonable estimate of damage to the core columns and fireproofing, how much longer than 56 minutes would the building stand?