The Twin Towers were extraordinary for their time. Built over a period of years in the late 1960s, they were part of the “new wave” of high-rise construction that used an all-steel frame with lightweight steel truss floors and a central core. They may not have been the first high-rise buildings constructed in this fashion, but they were, undisputedly, the tallest and the biggest. The towers rose 110 stories above the plaza (mezzanine) level, and there were seven stories below ground. The North Tower was 1,368 feet; the South Tower, six inches shorter. The footprint of each tower was one acre.

The basic structural construction of the towers was as follows:

Huge “box”columns, approximately 14 square inches, three stories high, and spaced about three feet on center, tied together horizontally with thick spandrels and welded together above and below, formed the exterior walls. There were 59 columns on each side of the 207-square-foot buildings.

Floors were supported by a series of lightweight steel trusses that spanned the exterior walls to the central core. The trusses spanned 60 feet to the sides and 35 feet to the ends of the central core. They were bolted into the exterior wall and the center core with 5/8-inch bolts and gusset plates. The floors consisted of lightweight concrete, four inches thick, on a 11/2-inch noncomposite steel deck.


The center core, which contained the buildings’ stair towers, elevators, and other services, was comprised of a series of box and flange-shaped columns, about 47 per floor. Some of the columns were very large—as large as 14 2 36 inches. The floors in the core also were concrete fill on metal deck, and they were supported by a variety of horizontal steel support configurations, which were in turn supported by the box and flanged columns.

There were numerous construction elements/configurations in the towers that added to the buildings’ strength or increased load redundancy, including the following:

•In general, horizontal splices between the three-story vertical structural modules were staggered up and down so that not more than about a third of the columns were spliced on any one floor.

•The spandrel plates were thick and short (three columns wide). This allowed for lateral and vertical stiffness. The overall effect of the column-and-truss construction was said to be a “tubular system” that allowed the buildings to stand up to heavy winds. Wind resistance was heavily figured into the design of the towers.

•Truss diagonals were extended above the top chord to allow the truss to act more in “composite behavior” with the floor slab.

•Trusses were placed in pairs and then doubled up in key locations.

•The metal deck was supported by transverse bridging trusses and other elements. (The metal deck supports helped transfer loads to the columns.)

•The top five floors of each building were constructed with additional truss framing (“outrigger” trusses). These helped overall in wind resistance. They also helped, in the case of the North Tower, to support the transmission tower.

Of course, neither the structural engineering nor its enhancements compensated for the combination of the planes’ impact or the ensuing structural weakening by fire. Although the structural behavior will and must be studied in the near future, bringing all scientific methodology to bear, it is presumed here, given much supporting anecdotal but documented evidence, that the progressive failure of stressed lightweight trusses under fire conditions conforms to the general pattern of trusses in general under such conditions; the weakest link in the truss network—in this case, the 5/8-inch bolts connecting the exterior wall to the center core columns—is a major collapse factor, as similar connections have been in many fires past. (As was coined by one fire service expert and repeated by many others, “Beware the truss!”)


FEMA’s World Trade Center Building Performance Study (BPS) states that between 31 and 36 columns on the north side of the North Tower, spanning floors 94 to 98, were destroyed by the impact of the first plane. Evidence suggests that local collapse of the floor assemblies occurred on these floors. The plane extensively damaged the steel members in the center core. It appears that there was collapse in that area as well.

As with the case in the North Tower, the second plane’s impact on the South Tower took out many columns—at least 32 columns spanning floors 78 to 84—and produced localized floor collapses across a horizontal width of about 70 feet, though, according to the BPS, other areas of the floors remained intact. The core on the South Tower was also extensively damaged, although it appears that one stair tower remained open, from which 18 people escaped from the impact floors and above.

Yet, with extensive damage to the upper floors, the North Tower remained intact for 1 hour and 43 minutes. The BPS states, “The building’s structural system, composed of the exterior loadbearing frame, the gravity loadbearing frame at the central core, the system of deep outrigger trusses in upper stories, was highly redundant. This permitted the building to limit the immediate zone of collapse to the area where several stories of exterior columns were destroyed by the initial impact and, perhaps, to portions of the central core.” The building compensated by shifting its load paths—for almost two hours. The BPS continues, “Neglecting the potential loss of lateral support resulting from collapsed floor slabs and any loss of strength due to elevated temperatures from fires, the most heavily loaded columns were probably near, but not over, their ultimate capacities.” [emphasis added]

FEMA’s BPS continues, “Although the structure may have been able to remain standing in this weakened condition for an indefinite period, it had limited ability to resist additional loading and could potentially have collapsed as a result of any severe loading event, such as that produced by high winds or earthquakes. WTC 1 probably experienced some additional loading and damage due to the collapse of the adjacent WTC 2. The extent of such damage is not known but likely included broken window and façade elements along the south face. This additional damage was not sufficient to cause collapse. The first event of sufficient severity to cause collapse was the fires that followed the aircraft impact.” [emphasis added]

Likewise, damage to the South Tower was severe, though as the BPS states, “Damage to the exterior walls was not severe except at the zone of impact. The exact extent of this damage will likely never be known with certainty. It is evident that the structure retained sufficient integrity and strength to remain globally stable for a period of approximately 56 minutes.”

Obviously, the planes hit the buildings at different points and different locations. Much has been made about the fact that the second plane that hit the South Tower was “much closer to the corner of the building.” This, it has been said, is a significant difference in considering time to collapse between the two towers. And, no doubt, location and angles of entry must be considered in future analysis.

However, as it has been shown in the BPS, albeit a preliminary document, the impact of the second plane into the South Tower was possibly only three columns closer to the corner of the building than the impact of the first plane into the North Tower. The second plane turned more radically to the left the moment before impact, and therefore the plane directly impacted two to three floors more than the plane that crashed into the North Tower. Furthermore, it would appear that plane’s forces into the South Tower were more concentrated closer to the corner of the building. But it appears, subject to further study and corroboration, that there are other more significant collapse factors that figured into the rapid collapse of the South Tower, 44 minutes quicker than the North Tower—most notably, the height at which the planes struck (lower strike equals greater stress on structural members to compensate for a heavier load); and the speed of the planes—the second plane was traveling at least 120 mph faster than the first plane.


The speed of the jet that crashed into the South Tower is estimated to have been 590 mph (as compared with the first plane flying at 470 mph). Therefore, as the BPS states, “The additional speed would have given the aircraft greater ability to destroy portions of the structure …. The orientation of the core in WTC 2 was such that the aircraft debris would only have to travel 35 feet across the floor before it began to impact and damage elements of the core structure …. [T]he zone of impact in WTC 2 was nearly 20 stories lower than that in WTC 1, so columns in this area were carrying substantially larger loads. It is possible, therefore, that structural damage to WTC 2 was more severe than that to WTC 1 ….

“Review of video footage of the WTC 2 collapse suggests that it probably initiated with a partial collapse of the floor in the southeast corner of the building at approximately the 80th level. This appears to have been followed rapidly by collapse of the entire floor level along the east side, as evidenced by a line of dust blowing out of the side of the building. As this floor collapse occurred, columns along the east face of the building appeared to buckle in the region of the collapsed floor, beginning at the south side and progressing to the north, causing the top of the building to rotate toward the east and south and to begin to collapse downward.”

In the North Tower, in addition to the stress on the columns and damage to the floors, the degradation of the building’s strength to withstand additional loads after the impact was compounded by the fact that, according to the BPS, “The force of the impact and the resulting debris field and fireballs probably compromised spray-applied fire protection of some steel members in the immediate area impact. The exact extent of the damage may never be known, but this likely resulted in greater susceptibility of the structure to fire-related failure ….

“As fire spread and raised the temperature of structural members, the structure was further stressed and weakened until it eventually was unable to support its immense weight.” The BPS identifies these factors as probable in the chain of events in the collapse of the North Tower:

  1. The trusses are heated by the fire and so expand, producing greater stresses on other structural elements and possibly stress the assemblies beyond the capacities of their connections.
  2. The increasingly high temperatures cause the floor slabs and lightweight steel assemblies to sag, which produces an outward deflection of the columns and a stress overload from above, in addition to the loss of floor connections. Debris on the floors exacerbates this situation, and, as stated in the BPS: “In addition to overloading floors below, and potentially resulting in a pancake-type collapse of successive floors, local floor collapse would also immediately increase the laterally unsupported length of columns, permitting buckling to begin …. [T]he propensity of exterior columns to buckle would have been governed by the relatively weak bolted column splices between the vertically stacked prefabricated exterior wall units. This effect would be even more likely to occur in a fire that involves several adjacent floor levels simultaneously, because the columns could effectively lose lateral support over several stories.”
  3. Increased temperature of the steel reduces the strength and elasticity of the columns, decreasing buckling strength and possibly making the columns, in fact, buckle, even with lateral support. According to the BPS, this would be a significant possibility with the failure of core columns.

The FEMA study indicates that the transmission tower on the top of the North Tower began to move downward before there was any movement in the exterior wall. This would indicate that collapses in the North Tower began with structural failures in the center core of the building.

Both towers experienced rapid and catastrophic collapse. Regarding collapse progression, the BPS states, with regard to the collapse of the South Tower: “As in WTC 1, a very large quantity of potential energy was stored in the building during its construction. Once collapse initiated, much of this energy was rapidly released and converted into kinetic energy, in the form of the rapidly accelerating mass of the structure above the aircraft impact zone. The impact of this rapidly moving mass on the lower structure caused a wide range of structural failures in the floors directly at and below the aircraft impact zone, in turn causing failure of these floors. As additional floor plates failed, the mass associated with each of these floors joined that of the tower above the impact area, increasing the destructive energy on the floors immediately below. This initiated a chain of progressive failures that resulted in the total collapse of the building.”

Fire Engineering eagerly awaits more in-depth and detailed analyses of the collapse from the upcoming study by the National Institute of Standards and Technology.

Click here to view pdf Construction and Collapse Factors


  1. Primary technical source: World Trade Center Building Performance Study, Federal Emergency Management Agency, 2002.

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