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Background Attack Aftermath Evidence Misinformation Analysis Memorial

The Truss Failure Theory

Fanciful Theory Doesn't Begin to Explain Total Collapse

FEMA's cartoon truss failure theory
Figure 2-20 from FEMA's Building Performance Study gives the impression that floors spanned the entire width of the Towers. The fine print indicates that the illustration depicts only a section of floors spanning the perimeter (left) and core (right).

The truss failure theory, a key ingredient of the better known floor pancake theory, was endorsed by FEMA in its 2002 World Trade Center Building Performance Study . It invites us to imagine the floors assemblies detaching from their connections to the columns of the core and perimeter walls, precipitating a chain reaction of floors falling on one another. Without the lateral support of the floors, the columns, FEMA tells us, buckled and precipitated total building collapse.

The truss-failure/pancake theory offered a way around the obvious problem with the column failure theory: the need for all the columns to be heated to 800 C. It offered instead prerequisite conditions that were far less implausible: that trusses holding up the floor slabs were heated to that temperature, and began to experience some combination of expansion and sagging. Floor trusses are much easier to heat because, unlike the columns, they are not well thermally coupled to the rest of the steel structure.

The Truss Failure Theory was was abandoned by NIST's investigation in 2004 because NIST was unable to get floor assemblies to fail as required by the theory. Documentaries that had promoted the truss failure theory became obsolete, and were quietly replaced with updated versions.


The Missing Steel

Some critics of FEMA's theory attacked the truss failure theory for the wrong reasons. One assumption of the theory is that the floor sections that spanned the Towers' cores and perimeter walls were undergirded only by the light web trusses. Although many structural details remain mysterious thanks to the unavailability of detailed engineering drawings, this assumption appears to be mostly true, modulo the observation that some floors appeared to be framed entirely with solid I-beams.

However, the anonymous Guardian author suggested that the idea that so many of the floors rested only on web trusses was a lie concocted to sell the pancake theory, arguing in a 2002 article that:

  • FEMA's building description leaves 32,000 tons of steel unaccounted for in each tower, given that the towers were known to each use 96,000 tons of steel.
  • A truss-only-based floor construction system would leave the floors too weak to transfer loads between the core and perimeter walls.

Guardian's conclusion about the extent of web trusses in the Towers appears to be mistaken: Between construction photographs and 60s-era articles in the Engineering News Record, there appears to be sufficient evidence to establish that floors outside of the cores, with the exceptions of top-most, bottom-most, and mechanical equipment floors, were supported entirely by web trusses. However, Guardian's calculations about the quantities of steel accounted for by FEMA's building description underline the failure of the official reports to provide a truthful and complete picture of the Towers' construction.


Deceptive Propaganda

Since the failure of a few trusses on a floor wouldn't automatically lead to a whole floor falling and starting the pancake syndrome, some fine tuning in the theory was needed. Dr. Thomas Eagar provided us with the zipper theory to explain how the failure of one truss could cause adjacent ones to fail. A horizontal domino effect of unzipping would precede the vertical one of pancaking. NOVA created a website to feature Eagar's promotion of the pancake theory which included a misleading animation of falling trusses, which failed to show either the transverse trusses or the steel floor pans.


From Sagging Trusses to Leveled Building

The unverified assumptions of the truss theory listed above are the least of its problems. It pretends that a few truss failures would automatically lead to the entire steel building crushing itself. What would be the likely chain of events following a floor failure envisioned by the truss theory?

Let's accept Dr. Eagar's zipper scenario (despite the clear evidence that fires did not cover a whole floor in either tower) and imagine that all the trusses of a floor failed in rapid succession and the whole floor fell. Then what? It would fall down about ten feet, then come to rest on the floor below, which was designed to support at least five times the weight of both floors, the fall cushioned by the folding of the trusses beneath the upper floor. But let's imagine that the lower floor suddenly gave up the ghost, and the two floors fell onto the next, and that failed, and floors kept falling. Then what? The floor diaphragms would have slid down around the core like records on a spindle, leaving both the core and perimeter wall standing.

Truss theory proponents hold that the core and perimeter wall lacked structural integrity without mutual bracing provided by the floor diaphragms. That may have been true in the event of a 140 mph wind, but not on a calm day. Note that the core had abundant cross-bracing, and would have been perfectly capable of standing in a hurricane by itself. And even if one imagines the outer wall buckling without that support, it does not begin to explain how it shattered into thousands of pieces, many of the column sections ripped from the spandrel plates at the welds, and how it shattered so quickly that no part of the wall remained standing above the falling dust cloud.

page last modified: 2013-03-20