IntroductionAlthough designers and engineers work diligently toensure the viability and safety of their designs beforethey are built, sometimes, unforeseen circumstancescan occur. Whether they are the result of mechanicalfailure, natural phenomena or acts of sabotage, eachyear these incidents cause billions of dollars worth ofdamage and significant loss of human life to even thebest engineered designs, if not their utter destruction.

When these catastrophes occur, the United StatesFederal Emergency Management Agency (FEMA)and other government agencies often enlist the servic-es of engineers charged to ascertain precisely whatwent wrong in an attempt to prevent a future recur-rence. More often in these cases, when engineers needto utilize simulation software as a “forensic” investi-gation tool, the solution they most often turn to comesfrom ANSYS Inc.

This case study briefly examines the application ofANSYS software in the postmortem examinations ofthree prominent catastrophic events. While there maynot have been a way to predict or prevent these tragicincidents in the first place, it is worthy to note the sig-nificant role which ANSYS software plays in helpingengineers comprehend the unique mechanics of designfailure under extreme circumstances.

The Destruction of the Space ShuttleChallenger – January 28, 1986Seventy-three seconds after launching from theKennedy Space Center in Florida, an ignition of mixedliquid oxygen and hydrogen fuel, brought about as theresult of a faulty engine sealant, destroyed the shuttleorbiter Challenger.

Then-president Ronald Reagan appointed a specialcommission to investigate the cause of the accidentand develop corrective measures. Among the membersof the investigation team were engineers from ThiokolSpace Operations of Brigham City, Utah (U.S.A.)–theoriginal designers of the solid rocket boosters (SRB),where the failure was believed to have occurred.

Utilizing ANSYS’non-linear analysis capabilities, theThiokol team was able to identify that unusually coldweather that day had caused the rubber O-rings–which seal the components of the SRB together–tostiffen. Subsequently, the SRB lost cohesion causinga chain reaction that resulted in the shuttle’s destruc-tion.

“At the time,” observes TroyStratten, principle structural analystat Thiokol, “this was probably thelargest ANSYS non-linear modelever run.” Indeed, the representa-tion of the tang and clevis jointregion (left), where additionalhigh stress concentrations weredetected, contained 30,000 ele-ments and 100,000 degrees of

freedom.

Once these design flaws were iden-tified, NASA required the SRB to bere-certified for operation. Theresults generated by the ANSYS

simulation allowed Thiokol’s engineersto redesign the SRB’s joint system to minimize gap-ping and characterize stress concentrations. ANSYSproved to be the key in ensuring the safety and suc-cess of future shuttle missions.

The Crash of TWA Flight 800 – July 19, 1996Fourteen minutes after taking off from New York City’sJohn F. Kennedy International Airport, a Boeing 747-100—identified as Trans World Airlines Flight 800—exploded and crashed into the sea nine miles off LongIsland, N.Y. (U.S.A.).

To this day, the precise cause of the crash has not beenidentified conclusively. However, a joint investigationspearheaded by the Boeing Company, in cooperationwith the National Transportation Safety Board(NTSB) and the Federal Aviation Administration

EXECUTIVE SUMMARY

Challenge:Study various catastrophic events

to develop means of preventing

their recurrence

Solution:Various investigations using

ANSYS’ non-linear structural,

mechanical and CFD capabilities

Benefits:Investigations identify positive—

or, at the very least, most like-

ly—causes of design failure

Design adjustments improve

overall safety and performance

of structures

ANSYS “Forensic” Engineering StudiesT H I O K O L , F A A , W E I D L I N G E R A S S O C I A T E S

When catastrophes occur, government agencies often enlist the services of

engineers charged to ascertain precisely what went wrong in an attempt to

prevent a future recurrence.

CASE STUDY

www.ansys.com

Imag

e co

urte

sy T

roy

Stra

tten,

Thi

okol

Spa

ce O

pera

tions

(FAA), yielded a “most likely” scenario to explainthe events of that fateful night.

A thorough inspection of the recovered wreckage–confirmed via simulations performed in ANSYS–led investigators to hypothesize that the fuelvapor/air mixture inside the nearly empty centerwing tank (CWT) was ignited by elevated tempera-tures. The ignition of this combination of gases rup-tured the fuel tank which, in turn, caused the planeto violently break up. While fuel tank fires or explo-sions of this type are rare, two other previously doc-umented occurrences confirm that they fall com-pletely within the realm of possibility.

Patrick Safarian, then senior specialist engineer forBoeing, recalls the key to satisfactorily resolvingthis important issue was found in the robust struc-tural and Computational Fluid Dynamics (CFD)capabilities in ANSYS. “Performing failure analysisat this level was, until then, probably unheard of,”he enthuses. “It may still be unsurpassed.”

Safarian’s ANSYS rendering of Flight 800 (above)was based upon an ANSYS-modified finite elementmodel of a 747-400 freighter, consisting of 120,000shell and beam elements with 750,000 degrees offreedom. He observes that while the initial source ofignition was unknown, “We moved (it) around,remodeled, re-analyzed and took careful note of theresults. This gave us a good degree of confidence inthe failure sequences.”

As a result of Safarian’s efforts, the NTSB advisedthe FAA to take numerous steps to prevent a recur-

inherent tenacity in the moment-connected steelframe lattice. To gain a better understanding of thebuilding’s response to the impact of debris from thefalling WTC structures and identify specific designfeatures that contributed to this performance,Smilowitz’s team utilized simulation software fromANSYS Inc.

In order to rep-resent the struc-tural behavior in thedamaged state, theteam had to developnon-linear spring rep-resentations of thegirder /columnmoment con-nections (right).Detailed plate modelsof the connectionswere developed and ana-lyzed parametrically to determine the appropriatenon-linear spring characteristics. These propertieswere then specified at the corresponding connec-tions in the ANSYS model of the building.

Thanks in large part to ANSYS’ static non-linearanalysis capabilities, the team was able to deter-mine the diminished capacity of the connectionsresulted from the “out-of-plane” bending associatedwith the damaged state of the structure. This par-tially explains the damage pattern, which was con-tained in the northeast face of the building, extend-ing from the initial impact area on the 22nd floordown to the eighth floor.

“It is difficult to draw conclusions,” observesSmilowitz in his report to FEMA. “More detailedstudy is required to understand how the collapsewas halted.” He believes that a complete FEAanalysis on the Bankers Trust Building–conductedin ANSYS–will aid current and future builders inconstructing buildings better able to avert cata-strophic collapses in the event of abnormal loadingconditions.

rence. These include thorough examinations of thephysical condition of fuel tanks and all related com-ponents on more than 850 aircraft currently in usethroughout the world. Other recommended precau-tions–such as pumping inert gases into fuel tanks,refueling from ground tanks (which store fuel at alower temperature level), and carrying an “appro-priate” amount of fuel in tanks at all times (as fulltanks are less likely to explode than empty ones)–are also under consideration.

The Collapse of the World TradeCenter – September 11, 2001While catastrophes that occur via natural or mechan-ical means are difficult enough to foresee, it is near-ly impossible to predict those that result from adeliberate attack by an unknown enemy.

Such was the case the morning a pair of Boeing 767aircraft was maliciously crashed into the twin 110-story towers of the World Trade Center (WTC) inNew York City.

Following these events, FEMA formed a coalitionwith the Structural Engineering Institute of theAmerican Society of Civil Engineers(SEI/ASCE)–as well as the city of New York andseveral other Federal agencies and professionalorganizations–to document the performance ofbuildings at ground zero. Their goal was to docu-ment the sequence of events, likely root causes, andmethods or technologies that may improve or miti-gate the observed building performance.

Although WTC buildings 1, 2 and 7 collapsed com-pletely, other structures such as the Bankers TrustBuilding (located at 130 Liberty Street) remainedstanding, sustaining only moderate localized dam-age. Robert Smilowitz, consulting engineer withWeidlinger Associates Applied Science division inNew York City, led the team studying the BankersTrust Building.

While not originally designed to sustain the loss ofa column over a significant portion of its height, thisstructure’s ability to arrest collapse demonstrated an

Southpointe; 275 Technology DriveCanonsburg, PA 15317; U.S.A.

ansysinfo@ansys.com

ANSYS is registered in the U.S. Patent and Trademark

Office. ©2002 SAS IP, Inc., a wholly owned subsidiary

of ANSYS Inc. All Rights Reserved.

MCS0089-JUL02

www.ansys.com Toll-Free:1.866.ANSYS.AI (1.866.267.9784)

Toll-Free Mexico: 001.866.ANSYS.AI

CASE STUDY

Following the events of September 11, 2001, FEMA formed a coalition with

the SEI/ASCE—as well as the city of New York and several other Federal

agencies and professional organizations—to document the performance of

buildings at ground zero.

Image courtesy PatrickSafarian, FAA – Airframe Branch

Imag

e co

urte

sy R

ober

t Sm

ilow

itz, W

eidl

inge

r Ass

ocia

tes