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