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World Trade Center Collapse
Verification and ValidationSpring 2010
Tim Handy
On September 11th, 2001 four jetliners were hijacked by al-Qaeda
One plane was crashed outside of Shanksville, PA after passengers/crew attempted to retake it – 40 people were killed
A second plane was flown into the Pentagon in Arlington, VA – 184 people were killed
The final two planes struck both of the World Trade Center towers – 2752 people were killed, 6000+ injured
9/11
NIST estimated 17,400 civilians were in both Towers at the time of the attack
Over 90% of those two died in the Towers were above the points of impact
411 of the dead were emergency workers attempting to rescue those inside the buildings
Multiple other surrounding buildings were either destroyed or damaged beyond repair
9/11
WTC Buildings Innovative building design (Architect – Minoru
Yamasaki)◦ Frame tube structure – essentially a pipe sticking out of
the ground, with minor support in the center (Vienrendeel trusses – rectangular openings)
◦ Express elevators + Local elevators + Frame = Open office space
◦ Used wind tunnel experiments in the process◦ Prefabricated components
Followed a draft of New York City building codes, even though it required none, as it was being overseen by the Port Authority (reduced number of exit stairwells from 6 to 3)
WTC Buildings Tube frame design required 40% less steel The perimeter tubes were assembled in 3
story sections off-site Staggered perimeter tubes allowed
transmission of lateral loads
Spanning from the central core to the outer trusses were prefab floor trusses
4” thick concrete on a steel deck Grid of lightweight bridging and main
trusses supported the floors Trusses connected to the outer columns by
bolts through seats welded to the perimeter Connected to the inside core via welding
WTC Buildings – Floors
WTC Buildings
In the designing of the towers, consideration was given to the possibility of aircraft impact◦ Empire State Building had been hit, as well as
other near misses Original report lost when Port Authority
offices were destroyed In 1993, an engineer remembered
performing the analysis and had remembered his conclusion was that the building would remain standing
WTC Buildings – Aircraft Considerations
Sprayed on fire resistant material was applied to all floor trusses and beams, as well as some structural elements
Core columns protected primarily by gypsum wall boards
All floors in WTC1 directly affected by the plane had proper SFRM applications
Only 3 floors of WTC2 directly affected by its plane had proper SFRM applications
WTC Buildings – Fireproofing
In 2002, Congress requested the National Institute of Standards and Technology (NIST) investigate the collapse of the Twin Towers.
This study was headed by civil engineer S. Shyam Sunder
The report was finished in 2005 and published at wtc.nist.gov
NIST Report
The NIST investigative team used a combination of experimental tests and multiple computational models to forensically investigate the collapse
Impact, Fire, Thermal Penetration, Collapse Analysis
NIST Report – Techniques
Performed with LS-DYNA◦ General purpose dynamic transient finite element
program Used to do component impact studies, as
well as complete aircraft impact Simulated fuel dispersion via Smoothed
Particle Hydrodynamics (SPH) to determine the spread of initial fires
NIST Impact Analysis
“FDS is a CFD model of fire-driven fluid flow.”
Solves Navier-Stokes for thermally driven flows via finite difference on 3-D rectilinear meshes
Radiation terms are computed with a finite volume technique
Lagrangian particles are used for smoke and sprinkler output
NIST Fire Dynamics Simulator
“FDS is a CFD model of fire-driven fluid flow.”
Solves Navier-Stokes for thermally driven flows via finite difference on 3-D rectilinear meshes
Radiation terms are computed with a finite volume technique
Lagrangian particles are used for smoke and sprinkler output
NIST Fire Dynamics Simulator
Explicit predictor-corrector scheme, second order in space and time
Turbulence is treated with the Large Eddy Simulation (LES) Smagorinsky Model
NIST FDS – Hydrodynamic Model
Mixture fraction combustion model◦ Mixture fraction = (% of gas at a point that
started as fuel)◦ Conserved quantity
NIST FDS – Combustion Model
Solution of radiation transport equation for non-scattering gray gas
Solved using a technique similar to the convective finite
NIST FDS – Radiation Transport
Used output of FDS coupled with ANSYS to study long term fire effects on the steel structure
ANSYS is a commercial finite element program originally designed for structural simulations
NIST Fire-Structure Interaction
In brief, the weak link was the spray adhesive fireproofing.
The impact of the plane blew away the coating on trusses, leaving them exposed to potential (in this case, inevitable) fires.
So Why Did They Collapse?
The NIST report concludes that catastrophic failure was produced due to a combination of impact damage from the aircraft and structural weakening due to prolonged fire exposure.
Neither of the two alone could have caused failure
So Why Did They Collapse?
The bare steel exposed to high temperatures for prolonged periods of time caused it to lose strength and transformed it into Play-doh ©
This failure caused the floors to sag, in turn pulling on the exterior walls which were also weak
So Why Did They Collapse?
Image from PBS NOVA.
The exterior eventually reached a critical point and broke
The newly freed floor then collapsed down to the floor below, which was also compromised
Remember that main support columns in the core were also heavily damaged or severed completely
This convergence of circumstances turned into a domino effect, destroying the tower
So Why Did They Collapse?
No one involved with the buildings.
Who’s to Blame?
The NIST determined that any discrepancies in building codes would not have prevented this disaster.
The building was designed to hold up under fire conditions according to plan
Unfortunately, the building was not designed to hold up to both a large jetliner strike and a conflagration
Who’s to Blame?
The NIST recommended 30 improvements to building standards.
Shyam Sunder’s top five recommendations:1. Better fireproofing materials and practices2. Active fire protection systems (redundancy)3. Better communication for emergency
responders4. Better evacuation routes from large buildings5. Redundant structural systems to prevent
progressive collapse
Recommendations
1. Wtc.nist.gov – Official site of the NIST WTC investigation
2. http://www.pbs.org/wgbh/nova/wtc/ - Interview with lead investigator Shyam Sunder
References