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Collapse of WTC — Its impact onskyscraper construction

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To share your opinion with our readers, you may sendin your inputs in about 1500 words via E-mail toeditor@icjonline.com

Skyscrapers are a product of the industrialrevolution, which began in England in theeighteenth century. They provided a solu-tion to the problems of overcrowding ofurban spaces � since they save space on theground. The first skyscraper was built in1884 in the city of Chicago, Illinois. It wasonly ten-storeys high. Construction of sky-scrapers was aided by the high-speed el-evator invented by Otis in 1857. In 1913,the Woolworth Building in New York daredto reach fifty-five storeys, soaring up 241m. Eighteen years later, the Empire StateBuilding, reached 102 storeys with a heightof 381 m.

The skyscraper has become the preferredsymbol of belonging to the modern, globalworld. According to the list compiled bythe Council on Tall Buildings and UrbanHabitat (CTBUH) at Lehigh University, onlyfour of the world�s tallest buildings are inthe United States. After the collapse of WTC

in New York the figure has been reduced totwo � the Sears Towers in Chicago (built inthe year 1974 with a height of 443 m) and

the Empire State Building. The world�s tall-est building is in Malaysia � the PetronasTwin Towers in Kuala Lumpur, designedby the American architect Cesar Pelli; (ofcourse, the CN Tower in Canada, built in1976 is the tallest in the world having aheight of 550 m).

Debates about the significance, efficacyand even the morality of skyscrapers havebeen raging since the building type was

N. SubramanianN. SubramanianN. SubramanianN. SubramanianN. Subramanian

A skyscraper is a symbol of a city's urban development and global outlook, besidesbeing a practical solution to the continuous demographic explosion and skyrocketingland prices. The author discusses the effect of the collapse of the World Trade Centre(WTC), New York, in the aftermath of the September 11 attack, on skyscraperconstruction and future trends while describing the design aspects, collapse anddamage suffered by the WTC.

Fig 1 View of the entrance of World Trade Centre (Note the close spacing ofcolumns)

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invented in Chicago more than a hundredyears ago. There were fear of heights(ironically, the architect of the WTC, MinoruYamasaki, was slightly acrophobic � onereason why the windows were so narrow)and fear of fire (that many peoplelegitimately experience in high structures).But the recent terrorist attacks on WTC,have renewed the debate that these typesof structures make ideal targets forterrorists � stable, highly visible and big.

Twin towers of WTCThe twin towers of WTC with heights of417 m and 415 m were owned by the PortAuthority of New York and New Jersey.With Minoru Yamasaki, of Emery Roth andSons Consulting as architect and JohnSkilling and Leslie Robertson, ofWorthington, Skilling, Helle and Jackson asengineers, construction started on August5, 1966. It was inaugurated on April 4,1973.

The architect and the engineers workedclosely and employed an innovativestructural model: a �rigid hollow tube� ofclosely spaced steel columns with floortrusses extending across to the central core.The 355 mm steel box columns, finishedwith silver-coloured aluminium alloy werespaced at 990 mm on centres, making thetowers appear windowless from a distance.At the third floor level, the columns

transitioned in an arch-like formationincreasing the spacing to 3.05 m for the lowerstoreys, Fig 1.

The twin towers were the first supertall buildings designed without masonry.Worried that the intense air pressure createdby the building's high-speed elevators mightbuckle conventional shafts, engineersdesigned a solution using a dry wall system

fixed to the reinforced steel core. For theelevators, to serve 110 storeys with atraditional configuration would haverequired half the area of the lower storeysbe used for shaft ways. Otis Elevatorsdeveloped an express and local system,whereby passengers would change at �skylobbies� on the 44th and 78th floors, halvingthe number of shaft ways.

The 63.4-m wide façade was, in effect,a prefabricated steel lattice, with thecolumns spaced at 990 mm centres, actingas wind bracing to resist all overturningforces. The central core was designed to takeonly the gravity loads of the building. Thisresulted in an economic structure, withwind bracing at the most efficient place �the outside surface of the building. Thistype of arrangement did not transfer theforce through the floor members to the core,as in most curtain-wall structures. Theoffice spaces between the outer wall andthe inner core thus did not have any internalcolumns. Each floor had 4020 m2 of officespace.

The floor construction was ofprefabricated trussed steel with a depth of838 mm, which spanned the 18.3 m betweenthe outer columns and the internal lift core,Fig 2, and acted as a diaphragm to stiffenthe outside wall against lateral bucklingresulting from wind load pressures. These

Fig 3 WTC under attack

Fig 2 Floor plan and structural arrangement of WTC

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trusses supported the 100-mm thickconcrete slab on metal deck of each floor.The construction required 200,000 t of steel,325,000 m3 of concrete and 5575 m2 ofglass window area and 239 elevators.

The collapseOn September 11, 2001, two hijackedBoeing 767 air planes, one with 92 peopleon board and the other with 65, crashedinto the WTC twin towers, disappearingwithin and triggering fire and explosions,Fig 3. The north tower, called One WTC,was hit at 8.45 a.m. at about the 90th floor,and the south tower, Two WTC, at9.03 a.m. at about the 60th floor. This re-sulted in the complete collapse of the southtower at 10 a.m. and the north tower at10.29 a.m.

Prof Frank Moscatelli, a physicsprofessor from Swarthmore College inPennsylvania, calculated that thegravitational potential energy unloadedwhen the buildings in WTC came down wasabout 6.8 × 1011 J. (Compared to this, theenergy of a severe earthquake would be1017 J and the annual energy output of areasonably large power station would be1016 J). As a matter of fact, the collapsecreated an earthquake of magnitude 2.4 onthe Richter scale. However, unlike a normalearthquake, they were richer in lowfrequency energy and poorer in highfrequency energy, the main reason for thisbeing the gravitational potential energy dueto the falling of building material. Accordingto Prof Mackin, of the University of Illinoisat Urbana-Champaign, an aircraft has theequivalent power of a small-scalecommercial power plant. The kinetic energyof a 767 jet at impact is of the order of 40mega joules as shown below:

KE = ½ mv2

where,

m = mass = 204 × 103 kg

v = velocity = 19.7 m/s

KE = kinetic energy = 39.6 × 106 J

Though this energy is considerable, it isclear that the towers withstood this impact.Though damaged, Two WTC managed toremain standing for approximately onehour, and One WTC for 90 minutes.

As such, it was not the impact, but theenergy in the fuel that affected thestructural integrity of the building. Theenergy content of fuel is approximately35 × 106 joules per litre. (Jet fuel may haveeven greater energy content). Assuming thatthe jet had 75,700 litres of fuel, (fuelcapacity of 767 jet is 90,764 litres) and itdetonated at once, the resulting energywould be 792 × 109 joules. This wouldamount to the equivalent of 2,376,000 sticksof dynamite! (3 sticks of dynamite will have1 mega joule of energy).

If we assume that the jet liner with aweight of 205 t was travelling at cruising

speed (850 kmph) and dissipated all of itsenergy in one second, then

Torque on the building = F × momentarm = 857,000 kNm

where,

F = MV = momentum per second= 4018 kN

Thus the impact force will be 4018 kNand assuming that the planes hit the 70th

floor, the torque at the base of the buildingis 857,000 kNm. The building withstoodeven this high impact.

Fig 4 The failure scenario

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Hence, it is seen that the flametemperature of fuel (1727oC) melted thesteel columns (melting temperature of steelis approximately 1570oC). Even if thetemperature was half the flametemperature, the steel would creep rapidly,losing its strength, resulting in buckling.When one floor collapsed, the weight of thefloors above created an instantaneous �pan-cake effect�, each floor collapsing on the floorbelow. The outer skin peeled back like anonion once detached from the floor slabs,Fig 4. It was as if the top of the buildingwas acting like a huge pile driver, crushingdown the floors below. Actually the impactforce is related to the failure strain in steel,the weight above the failed floor, the dropheight and the intact building height. ProfMackin calculated and found that thisimpact force is roughly 30 times the weightof the tower above (at the 70th floor level).This implies that the building should havebeen designed with a factor of safety of 30in order to withstand the forces. Based on asimplified analysis, Bezant and Zhouarrived at a similar over load ratio due tothe impact of the upper floors of WTC8.

Modern structures are designed to resistfire for a specific length of time. Safety

features such as fire retarding materials andsprinkler system help to contain fire, helpextinguish flames, or prevent steel frombeing exposed to excessively hightemperature. The WTC twin towers werethe first ones to use non-asbestosfireproofing. The fibres of the spray on fireproofing product were reportedly ceramic.Originally the north tower containedasbestos in its cementitious fire proofing asdid the first 30 storeys of the south tower.But it was removed after the 1993 bombing.This gives the occupants time to escape andallow fire fighters to extinguish blazes,before the building is catastrophicallydamaged. Naturally, in this case, the jetfuel generated fire conditions significantlymore severe than those anticipated in atypical office fire. These conditions mayhave overcome the buildings' fire defensesystem considerably faster than expected.

DamageIn addition to the collapse of the twin tow-ers, the 47-storey high, Seven WTC collapsedin the evening of September 11. The fallingdebris also caused the destruction of build-ings 4,5 and 6 at the WTC. This office com-plex was the largest in the United States.

At least 22 buildings, including a churchwere destroyed or seriously damaged in thecatastrophe. At least 3646 people died andmore than 6000 people were injured. About1.25 million m2 of commercial office spacewas destroyed in the attacks and an addi-tional 1.4 million m2 of property in nearbybuildings were damaged. Fig 5 shows athree-dimensional view of the damages tothe various buildings in New York. LarryA. Silverstein, who acquired a $3 billion,99-year lease to operate the WTC in July, isseeking $7.2 billion from insurers for thedestruction of the WTC, twice the amountinsurers say he can claim. (The two hijackedairlines that struck the 110-storey twin tow-ers were separate �occurrences� for insur-ance purposes).

The WTC disaster will cost New YorkCity�s economy as much as $90-$105 billionover the next two years, according to a report.This includes a property loss of about $34billion (which is nearly twice the damagefrom Hurricane Andrew, previously theworst disaster in American history), cleanupand rescue of $14 billion (the pulverisedash of concrete covered an area of 16 acres),other continuing costs in fiscal 2002 and2003 at a minimum of $31 billion and arough estimate of $11 billion to account forlifetime wages of the 3646 workers killed inthe attack. It may take a year or more toremove the mountain of debris. Businessinterruption costs have radiated far beyondlower Manhattan. Airlines and airfreightwere down for two days. Regan NationalAirport was closed for 23 days at a cost of$330 million to the airport and NorthernVirginia businesses and $27 billion to stateand local tax revenues. The $126 billioncommercial insurance industry is facing a$30 to $50 billion payout. In the aftermathof destruction, the hotel and tourismindustry and the US postal service havesuffered huge losses in their businesses.

Impact on skyscraperconstructionThe debate about the significance of sky-scrapers has started again, due to this col-lapse. The people who are against skyscrap-ers predict that the age of skyscrapers hascome to an end. They argue that this �ex-perimental� building topology has failedand predict that no new mega towers willbe built in the near future. It is of interest tonote that the tenants of Empire State Build-ing are planning to shift their offices to

Fig 5 Three dimensional view of the damage ( courtesy : CNN)

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smaller buildings. (In contrast to WTC, thePentagon after the air-plane attack, contin-ued to burn for two days � but the firefighters were able to tackle the blaze rightaway, even with improper equipment. Also,the Pentagon may be restored even beforeany decision is made about what to do withthe land on which the WTC once stood). Arecent poll conducted by USA Today/CNN/Gallup shows that 35 percent ofAmericans fear to enter a skyscraper.

Many felt that the impact of the failureof WTC may be short-lived. There are anumber of tall buildings under constructionthroughout the world and none of them isreported to be postponed or halted.Moreover with limited land, skyrocketingland prices and continuous demographicexplosion in big cities worldwide, there isno other possibility than to build high-risestructures. Even the WTC�s operator, LarrySilverstein has reported that his consortiumis planning to construct four 50-storeybuildings in place of the collapsed WTCtowers, with a memorial to those who diedin the WTC towers. It has to be borne inmind that over reliance on the horizontalgrowth of cities will create new problems ofurban sprawl and density. It may result inloss of valuable agricultural land. It willalso result in extra expenditure oninfrastructure facilities and added energyconsumption. Of course, due to the internetand world wide web, we now have themeans for efficient and instantaneouscommunications throughout the world,and diminished need for compactcontiguous space. Hence, it may not benecessary to build super tall skyscrapershaving more than 50 storeys.

Future trendsIt is seen that traditional steel structurescannot sustain such terrorist attacks. It isnecessary to invent better insulating mate-rials for steel sections, which will preventthe decay of steel in case of excessive tem-peratures. Even buildings constructed withconcrete cannot withstand such tempera-ture, since the concrete will undergo explo-sive thermal cracking, thermal fracture anddisintegration due to dehydration. Build-ing structures, which would look like for-tresses or nuclear reactors, with very thickmembers, is also not feasible for the public.

Non-traditional structural materialssuch as ceramics might someday provide

the framework for new fire-resistantskyscrapers. However, ceramics are muchmore brittle than steel, more susceptible tosnapping under impact. Of course, highperformance fibre-reinforced concretestructures may provide more stability andfire resistance than steel. However, they maywork out to be a costlier proposition.

Furthermore, the engineering of tallbuildings involve, much more thanstructural materials. They presentenormous problems in verticaltransportation when fire breaks out andelevators stop working. For example,during the 1993 WTC bombing, people hadto find their way down through dark andsmoke-filled stairwells, underlining theimportance of evacuation plans. It is ofinterest to note that the Petronas Towershave a sky-bridge between them � whichprovides an alternate exit from the highestfloors. Similarly the fire systems should beupdated. More emphasis should be givento life safety.

It may be prudent to strengthen theexisting tall buildings for possible attacks.The columns may be wrapped with carbonfibre reinforced sheets, as done inearthquake retrofits in California, USA andKobe, Japan. Windows may be maderelatively shatterproof. (In the Pentagon,blast-resistant windows on either side ofthe impact area remained intact above thesecond floor. In these windows, the glass isnearly 50 mm thick and each window costs$10,000!) Interior wall coverings made fromblast-resistant cloth similar to kevlar maybe stretched between beams (as done in thePentagon) to prevent debris from becomingshrapnel in the event of an externalexplosion. Such expensive measures maynot be necessary in all tall buildings, butonly in those, which are considered icons ofnational pride.

ConclusionThough the WTC twin tower failures showthe vulnerability of such super tall struc-tures for terrorist attacks, they may not puta full stop to the construction of tall sky-scrapers. Although there may be a delay inthe resumption of such projects for a fewyears, tall buildings will continue to be builtbecause of human ego and practical realestate demands. In this context, there is anurgent need to develop innovative architec-tural, structural and emergency technolo-

gies to prevent the failure of structures dueto such attacks, delay the propagation offire and to rescue people. More emphasishas to be given to life safety in such con-structions and alternative routes for safefire escapes should be provided in such tallbuildings. More research has to be con-ducted on blast-resistant building designand construction, and in the developmentof fire-resistant building materials. Use ofhigh performance fibre reinforced concretesand similar composites, though with in-creased costs, may result in better perform-ance than conventional steel or reinforcedconcrete structures.

References1. PETROSKI, H. Onward but perhaps not upward,

The Washington Post, September 16, 2001.

2. FORGEY, B. Reach for the sky � Despite theirvulnerability, towers fill a tall order, TheWashington Post, September 22, 2001.

3. POST, N.M. and WINSTON, S. Massive assaultdoomed towers, Engineering News Record,September 12, 2001 (www.enr.com)

4. DORTON, E. Divided we stand: A Biography ofNew York�s World Trade Centre, Basic Books,pp. 260, 2001.

5. http://www.Civil.usyd.edu.au/wtc.htm

6. http://www.greatbuildings.com

7. http://www.skyscraper.org

8. BEZANT, Z.P. and ZHOU, Y. Why did World TradeCentre Collapse � Simple Analysis,Communication sent to RILEM, September 2001.

9. SUBRAMANIAN, N. and VENUGOPAL, M.S. Fire safetyof multi-storey buildings, National Symposium onBuilding Services, New Delhi, March 1984.

10. SUBRAMANIAN, N. Computer analysis and designof tall structures, Civil Engineering andConstruction Review, April 1995, Vol.8, No.4,pp. 42-46.

Dr N Subramanian is thechief executive of ComputerDesign Consultants, Chennai.The highlights of his profes-sional career of 20 years in-clude: designing multi-storeyconcrete buildings, steel tow-

ers, industrial buildings and space frames.Dr Subramanian has contributed more than150 technical papers in seminars and jour-nals and published 17 books. Active inmany professional bodies, he is the pastvice president of The Indian Concrete Insti-tute and The Association of ConsultingCivil Engineers (ACCE). He is the recipientof ACCE-Nagadi Award (for his book onspace structures) and Tamil Nadu Scien-tist Award.

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