Questions about the WTC North Tower collapse

The Strange Collapse of the "Spire"

An odd remnant of the core of the north tower stands briefly after the collapse, then appears to drop straight down in a column of dust

A fragment of the core of WTC-1 towers above dust clouds billowing from the collapse

The jagged triangular spire of steel columns seen above is a portion of the service core that remained standing for almost 20 seconds after the main body of the dust cloud from the north tower collapse reached the ground .  It can be seen in several still pictures and at least one video, and appears to be about 3/4 the height of the building, say 75-80 floors, narrowing down to just a single column at the top.  Although it appears to represent only a fraction of the entire cross-section of the core, for some reason this group of columns has not fallen with the rest.  Despite the turbulent pyroclastic slurry of building debris swirling around it the spire remained unperturbed and almost motionless as the dust clouds roiled away, swaying slightly as can be seen in the videos below.

The fact that the upper portions of these particular columns, and all of many others, have shattered and fallen away at this point is difficult to explain.  If the only trauma they have suffered was from the floors sliding down around the service core damage to the core columns should have been minimal.  Fewer than half the core columns were along the edges of the rectangle, and only those carried the hanger brackets that supported the trusses. As the floors slid down they would have sheared bolts and welds, but it is hard to see how this interaction could have completely destroyed most of the core columns. The core was extremely robust, designed to be able to support the entire weight of the buildings several times over.  Far more than  mere "service cores," they comprised 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 were anchored directly to the bedrock.  The smallest of the core columns appear to have been bigger than the 14"x14" columns that made up the outside walls. 

(note: The outer walls' primary function was  to brace against wind loads,  and they were not counted upon for weight bearing, though they did of course carry some weight in the resting state.  By placing the support against wind loading right at the surface where the loads occur the whole structure could be lighter with the same amount of lateral stability.)

 This construction picture gives a sense of how massive the core was

Pictures of the Disintegration

Beyond the mysterious shattering and disappearance of the upper ~30% of the core, there is a much more serious anomaly.  The "spire" stood for about 20 seconds after the debris cloud fell away from around it, swaying slightly without tipping over despite its height and lack of support.   It then began to drop straight down in place, but after dropping only a little way it abruptly  turned  into dust in place, very much as the towers themselves did.

Still upright, top column leaning a bit

Before and after views of the core remnant just as it turns to dust.  In the first picture WTC-7 is directly in front of the spire, and the camera is above ground level.  The second picture is shot from street level, and we have moved to the right relative to WTC-7. 

Four Views of the WTC-1 Collapse: note how dust cloud drifts to left, away from WTC-7, leaving spire behind

Note position of the midline of the tower relative to the edge of WTC-7.  The framing of the picture changes and the view pulls back to a wider angle , making WTC-7  get smaller and move a little to the right.  The top of the spire that can be seen in the last picture just matches the midline of the intact tower in the first frame.  But yet something else strange is happening here.  If the spire corresponds with part of the core, i.e. the midline of the building, why does the towering dust cloud of the main collapse move off to the left, leaving the spire completely outside the main cloud.  In fact the spire has a smaller dust cloud of its own that can be seen in the second and third frames, a remnant left behind as the axis of the collapse shifts.

Video Clips of the Disintegration

Here is a downloadable MPEG (11MB) of the spire as it appears and falls. 

 For those with less bandwidth this is a cropped 2MB version in animated GIF format.

The broken columns of the core can be seen emerging from the dust cloud as it reaches the ground, and incidentally at that moment the scene to the left of the dust cloud gets much brighter.  The spire can be seen for about 20 seconds, swaying a little from side to side.  It then starts to drop in an apparent free fall, almost perfectly straight down on itself.  But after falling for about a quarter of its height it suddenly turns to dust:  heavy dust that continues to drop straight down.  There is very little dispersion or wind drift, suggesting that the particles are heavy.  More or less what one would expect to see if the steel of the columns had turned all at once into a coarse powder.

Some Thoughts in the Face of the Impossible

What could possibly be going on here?  I really wish this were an artifact, that the resolution of the images wasn't good enough to be sure.  But looking at the still picture of a narrow vertical column of dust confirms the impression of the videos that the core remnant has turned to dust more or less in place.  Perhaps the shockwaves of those "dust ejections" (whatever name you may apply, they look a lot like explosions) that we saw during the collapses has shattered the thinner upper portions of the columns.  Might that same battering from multiple high-energy shockwaves have changed the microcrystalline grain structure of the remaining steel in such a way  that they became extremely brittle?  Something very unusual has happened to these particular steel beams, something that caused them to not merely topple or crumple in segments but to disintegrate in place.

The selectivity of the initial collapse that left behind this portion of the core while removing the rest is also difficult to explain.  Consider how cleanly the cross-braces have been sliced off along that single column at the top, short pieces of the vertical plates welded to each of the core columns sheared off without so much as generating enough force to pull down this single unsupported column.  Something very energetic has removed the rest of the building from around this slender structure with almost surgical precision, something able to release amounts of energy orders of magnitude greater than the gravitational energy of the building.