Verification and Validation Spring 2010 Tim Handy

 On September 11 th, 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, injured

 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

 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)

 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

 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

 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

 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

 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

 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

 “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

 “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

 Explicit predictor-corrector scheme, second order in space and time  Turbulence is treated with the Large Eddy Simulation (LES) Smagorinsky Model

 Mixture fraction combustion model ◦ Mixture fraction = (% of gas at a point that started as fuel) ◦ Conserved quantity

 Solution of radiation transport equation for non-scattering gray gas  Solved using a technique similar to the convective finite

 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

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.

 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

 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

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

No one involved with the buildings.

 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

 The NIST recommended 30 improvements to building standards.  Shyam Sunder’s top five recommendations: 1. Better fireproofing materials and practices 2. Active fire protection systems (redundancy) 3. Better communication for emergency responders 4. Better evacuation routes from large buildings 5. Redundant structural systems to prevent progressive collapse

1. Wtc.nist.gov – Official site of the NIST WTC investigation Interview with lead investigator Shyam Sunder