The Freedom Tower

The National Institute of Standards and Technology conducted its own investigation of the collapse of the World Trade Center and has posted answers to frequently asked questions about the disaster. Here is a short excerpt.

Was there enough gravitational energy present in the World Trade Center Towers to cause the collapse of the intact floors below the impact floors?  Why was the collapse of WTC 1 and 2 not arrested by the intact structure below the floors where columns first began to buckle?

Yes, there was more than enough gravitational load to cause the collapse of the floors below the level of collapse initiation in both WTC Towers.  The vertical capacity of the connections supporting an intact floor below the level of collapse was adequate to carry the load of 11 additional floors if the load was applied gradually and 6 additional floors if the load was applied suddenly (as was the case).  Since the number of floors above the approximate floor of collapse initiation exceeded six in each WTC Tower (12 and 29 floors, respectively), the floors below the level of collapse initiation were unable to resist the suddenly applied gravitational load from the upper floors of the buildings.  Details of this finding are provided below:

Consider a typical floor immediately below the level of collapse initiation and conservatively assume that the floor is still supported on all columns (i.e., the columns below the intact floor did not buckle or peel-off due to the failure of the columns above).  Consider further the truss seat connections between the primary floor trusses and the exterior wall columns or core columns.  The individual connection capacities ranged from 94,000 lb to 395,000 lb, with a total vertical load capacity for the connections on a typical floor of 29,000,000 lb .
The total floor area outside the core was approximately 31,000 ft2, and the average load on a floor under service conditions on September 11, 2001 was 80 lb/ft2.   Thus, the total vertical load on a floor outside the core can be estimated by multiplying the floor area (31,000 ft2) by the gravitational load (80 lb/ft2), which yields 2,500,000 lb (this is a conservative load estimate since it ignores the weight contribution of the heavier mechanical floors at the top of each WTC Tower).  By dividing the total vertical connection capacity (29,000,000 lb) of a floor by the total vertical load applied to the connections (2,500,000 lb), the number of floors that can be supported by an intact floor is calculated to be a total of 12 floors or 11 additional floors.

This simplified and conservative analysis indicates that the floor connections could have carried only a maximum of about 11 additional floors if the load from these floors were applied statically.  Even this number is (conservatively) high, since the load from above the collapsing floor is being applied suddenly.  Since the dynamic amplification factor for a suddenly applied load is 2, an intact floor below the level of collapse initiation could not have supported more than six floors.  Since the number of floors above the level where the collapse initiated, exceeded 6 for both towers (12 for WTC 1 and 29 for WTC 2), neither tower could have arrested the progression of collapse once collapse initiated.  In reality, the highest intact floor was about three (WTC 2) to six (WTC 1) floors below the level of collapse initiation.  Thus, more than the 12 to 29 floors reported above actually loaded the intact floor suddenly.

2.  Were the basic principles of conservation of momentum and energy satisfied in NIST’s analysis of the structural response of the towers to the aircraft impact and the fires?

Yes.  The basic principles of conservation of momentum and conservation of energy were satisfied in these analyses.

In the case of the aircraft impact analyses, which involved a moving aircraft (velocity) and an initially stationary building, the analysis did, indeed, account for conservation of momentum and energy (kinetic energy, strain energy).

After each tower had finished oscillating from the aircraft impact, the subsequent degradation of the structure involved only minute (essentially zero) velocities.  Thus, a static analysis of the structural response and collapse initiation was appropriate.  Since the velocities were zero and since momentum is equal to mass times velocity, the momentum terms also equaled zero and therefore dropped out of the governing equations.  The analyses accounted for conservation of energy.

3.  How does NIST explain the absence of a timeline for the WTC Towers?

NIST developed and reported detailed timelines for various aspects of the WTC disaster beginning with the impact of the aircraft.  These timelines included the progression of fires through the buildings, the response of the structure to damage and to fire, egress of occupants from the towers, and the emergency response.  The timelines were developed based on extensive analysis of the photographic and video evidence, analysis of computer models, first person interviews, radio transmissions, and other data documenting the events of September 11, 2001.
4.  Why was physical evidence not collected immediately following the collapse of the WTC Towers?

The complete collapse of the WTC Towers destroyed virtually all physical evidence except the major pieces of steel and mechanical equipment.  In the initial days and weeks following the WTC disaster, the emphasis was on rescue and later on recovery, necessitating the removal of steel and disturbing the collapse site.  FEMA, which had launched its Building Performance Study in early October 2001, sent a team of experts to review the steel at the WTC site and the salvage yards.  These experts, including one from NIST, identified pieces of steel of potential interest to a follow-on investigation.  Beginning in February 2002, NIST, on its own initiative, began identifying additional steel pieces of potential interest at the salvage yards and transporting them to NIST to preserve and secure the evidence in anticipation of launching its own investigation, which it did in August 2002.  NIST did not receive the legal authority to collect and preserve physical evidence from a disaster/failure site until the National Construction Safety Team Act became law in October 2002.

A full report can be found at www. wtc.nist.gov/pubs/factsheets/faqs_12_2007.htm