Original subtitles

where 16 people were killed, 61 people were injured,

you can have a severe explosion.

narrator: in 2007, this aluminum factory in manchester, georgia,

was partly demolished when water seeped into the casting process.

that moisture got, got in. man: it』s very, very apparent

man: heard just a loud boom, and we had no idea what it was.

man: i mean, i』ve never heard anything

like it in my life and hope i never hear it again.

narrator: explosions like these are far from isolated.

simensen: if you are a metallurgist

who is dealing with aluminum,

you know that they are taking place

250 explosions in aluminum casthouse since 1980.

the worst explosion is probably the one in shandong in 2007,

coming together with liquid aluminum in your furnace,

and there was nothing left of the factory.

narrator: pound for pound,

molten aluminum is three times more powerful than tnt.

each one of these ingots

contains the same potential explosive energy

as 50 pounds of dynamite.

frank and christian believe

that the explosive properties of molten aluminum

are directly responsible

for the rapid collapse of the twin towers.

simensen: the weight of the aircrafts

were about hundred tons.

30 tons of that were of aluminum,

and workers at this plant know the dangers first-hand.

narrator: under normal conditions,

this incredibly flexible and versatile metal

is entirely safe, but in extreme circumstances,

aluminum can become incredibly volatile.

christian is on his way

to an aluminum recycling plant near oslo.

the aluminum plants have long been aware

of the extreme dangers of the metal.

simensen: after the early 1970s,

with a few accidents with aluminum, people became aware

of how dangerous aluminum can be under certain circumstances.

narrator: one of those circumstances

is when the metal is molten,

and those were situated all over the aircraft.

terje lofthus: we have had one explosion 15 years ago,

which was quite severe.

luckily nobody was hurt or injured.

narrator: molten aluminum is safe until it comes into contact

with one of most common molecules on the planet,

causing a highly explosive exothermic reaction.

that abundant substance is water.

when water comes into contact with molten aluminum,

the water molecules recombine,

producing aluminum oxide and hydrogen,

releasing an explosive amount of energy.

lofthus: if you have a closed container containing water

greening: so i started to question,

sufficiently to weaken them with or without fireproofing.

the fires in the building

were certainly hot enough to melt the aluminum.

according to the official report, 15 minutes after impact,

around 40 percent of the fire

had reached temperatures of over 700 degrees.

gratton: the main alloy used in the boeing 767 was 2024.

around 550 degrees centigrade, it』s got a melting point

slightly lower than pure aluminum.

narrator: but the showering metal in the footage

is glowing bright yellow,

and aluminum only reaches that color

over 1,000 degrees centigrade. if it』s heated to well

it would provide enough force to damage the steel supports

"well, could it get hot enough to glow yellow?"

we need really hot aluminum

to be convinced that this really is aluminum.

narrator: the question is critical.

unless the temperature inside the towers were high enough,

frank』s theory could fall apart.

metallurgist christian believes

that the temperature inside the tower

could have reached those inside this smelter.

simensen: my experience with aluminum smelters,

with molten metal and burners, that that is very close

in the twin towers. to what』s happened

narrator: inside this smelter,

narrator: if they』re right,

they were on the outside. they were inside the wings.

they were everywhere.

you had some parts which were made of aluminum.

narrator: their new theory states that after the impacts,

anything up to 30 tons of aluminum

lay in the middle of the burning floors of the towers,

and once it became molten,

it had the potential explosive force of 90 tons of tnt.

greening: behind that wall there

is the fuselage of a large boeing 767

which contains about 30 tons of aluminum,

and i believe it started to melt,

out of the building. and it』s pouring

and the basic load of all of this structure that』s in here.

the molten aluminum inside the building

provided the explosive force that brought down the towers.

greening: there was a bomb in the building,

but it was not manmade.

it was made by the combination

of all the ingredients that were being cooked up,

and it was just a matter of time

before the whole thing blew up and down come the towers.

narrator: the theory could explain several of the questions

unanswered by the official report,

including the explosions heard by 118 firemen.

if enough aluminum reacted in a single event,

across several floors,

and how the building overheated, and that it started to crumple.

for one minute i didn』t believe that

because one hour is not long enough for the steel to heat up.

it take much longer time.

narrator: working independently,

both scientists arrive at the same conclusion.

simensen: the official report was concentrating

on the steel bars, but the missing element is the aircraft.

greening: the missing evidence quite simply

is there was a huge aircraft sitting inside the building.

narrator: the official report

assumes the aircraft disintegrated on impact

and the debris scattered harmlessly

it only had, uh, theory about the building,

so the aircraft wreckage was not included in any fire tests.

this omission is also questioned

by experts from the aviation industry.

guy gratton: i think it is a little surprising

that they did not include anything from the aircraft

in their estimates of the fire behavior.

narrator: dr. guy gratton is an aeronautical engineer.

having studied the official report,

he rejects the idea that the planes disintegrated.

gratton: when the airplane flew into the tower,

it would have hit

the metal and concrete structure of the tower,

would have broken up into smaller pieces

to the initial collapse of the buildings?

and a metal extensively used

in the manufacture of aircraft--

aluminum.

and to frank』s disbelief,

he finds that aluminum aircraft parts formed no part

extensive fire tests. of the official investigation』s

greening: they did great tests for an office fire,

in their work, and that』s a huge omission

and i feel until those tests are repeated with an aircraft

included in the office,

uh, their results are essentially meaningless.

narrator: could the presence of aluminum

have provided the trigger that led

which would have come in to rest.

frank shares his concerns in a research paper online.

meanwhile, across the atlantic, another scientist

spots the same oversight in the official report.

christian simensen is a senior metallurgist at sintef,

one of europe』s

largest independent research laboratories

specializing in materials science.

foremost experts in metals he』s one of the world』s

and also published a paper in the journal of engineering

over the short time that mirrored frank』s concern

it took for the steel structures to give way.

christian simensen: when i read that 300 pages,

wood just wasn』t up to it,

simensen: when i read about the aircraft,

it was made by 30-ton aluminum,

something of the same order with steel,

and then some carbonite and so on,

all kinds of materials,

and the most important part of these materials were aluminum.

narrator: aluminum is the most abundant metal on the planet.

softer, lighter, and more flexible than iron and steel,

it is the ideal material for building aircraft.

gratton: back in the early days of commercial aviation,

aircraft were manufactured from wood.

by world war ii, they started to pressurize aircraft

and also carry a lot more payload.

heard explosions prior to the collapse.

and aluminum and steel, particularly aluminum alloys,

became the main materials that we used.

for the larger aircraft structures,

the fuselage, the wings,

what you need is the ability

to make very large, very lightweight complex shapes,

are primarily used so that』s why aluminum alloys

for those parts of an airplane.

this is a typical aluminum aircraft structure,

a few millimeters thick. so you』ve got this skin

that takes the pressure difference at altitude.

it also takes the bending

of the most explosive substances in the world.

narrator: gratton believes that during the impact,

as the planes sliced through the steel columns,

substantial chunks of metal would have come to rest

in the very center of the building.

by excluding the aircraft from its calculations,

the official report has overlooked

potentially crucial evidence.

if the temperature inside the burning buildings

got hot enough to melt the wreckage,

the results would almost certainly

have been catastrophic,

under the right conditions, and that』s because,

molten aluminum can be one

it takes 30 minutes to melt 50 tons of aluminum.

working on either side of the atlantic,

two renowned scientists,

frank greening and christian simensen,

are challenging the official 9/11 report

into the collapse of the twin towers.

they believe that by ignoring

the presence of aircraft wreckage

in the fires, the report is incomplete.

the presence of aircraft debris in the fires

could provide a solution

to the many outstanding questions over the disaster...

like why so many people

but the authority refused access to any material from the site.

you had iron oxide. you had gypsum.

you had crushed concrete.

those are the key ingredients for a terrible explosion.

narrator: proof for their claims can only lie

in the remaining rubble of the collapsed towers,

but the majority of the debris was put into landfill.

what remains is now in the possession

of the new york port authority.

if there was an aluminum explosion,

then the debris will contain

the tell-tale droplets of exploded aluminum oxide.

"missing evidence" approached the port authority

with a request to search for the alloy,

you had water, and then you had the catalysts.

someday frank and christian hope to fully test their theory,

but until the port authority

gives access to the remaining debris,

the truth may never be known.

"missing evidence" also approached nist,

the body responsible for the official report,

to comment on frank and christian』s theory.

nist stood by their claims

that the aircraft had disintegrated on impact.

any molten aluminum, they said,

would have flowed downward and quickly re-solidified.

at the time of the collapse, no liquid aluminum

would have been available to react with water.

they meet for the first time.

at that point, the collapse is inevitable.

narrator: did the firefighters actually hear a series

of aluminum explosions inside the towers?

were those explosions large enough

to cause the building to collapse so unexpectedly?

frank and christian need physical evidence

if they are to prove their theory,

and that is lying in the remaining rubble

of the twin towers.

up until now,

frank and christian have collaborated long distance.

now at a memorial

for the firefighters who lost their lives on 9/11,

frank is disappointed with nist』s response.

simensen: the view is beautiful.

greening: it sure is.

who would be about my age. that』s someone

i was born in 1947.

simensen: marianne simone.

jennifer lynn.

stephen gerard siller.

greening: to me, the amazing thing about what happened

in the collapse of the twin towers

was you had all these ingredients

that were thrown together, that had a terrible consequence.

you had aluminum that was-- became molten.

i think it would be great,

narrator: until samples of the aircraft

are identified and analyzed, frank and christian believe

that the conspiracy theorists will never go away.

greening: there was 60 tons of aircraft missing.

without that, you』re never gonna,

uh, reach, uh, a definitive answer

as to what caused those towers to collapse.

narrator: those who lost friends and family

deserve to know what really happened.

mcguire: out of the 343 firefighters we lost,

over 40 of them were personal friends, and one was my nephew.

i think if they did have an inquiry, um,

regarding what really happened and what caused the collapse,

we couldn』t get to him.

especially if it would shut up these imbeciles

that believe in a conspiracy theory.

greening: in science, there』s a very useful principle

called occam』s razor, which says

that in developing a theory, keep it simple.

i believe that in the case of the world trade center,

we had molten aluminum, we had a fire,

we had everything you need to create a terrible explosion

and bring down the buildings.

that』s the simplest solution,

and i believe it』s the correct solution.

you will never sort out what happened

until you include molten aluminum in your analysis.

that this is most probably molten aluminum.

video evidence clearly shows a molten metal

pouring from the south tower moments before the collapse.

greening: just behind this stream in the building

is, is the aircraft,

fire, which is extremely hot, and it』s sitting right in the

heat to melt the aluminum. and there』s certainly enough

contradicts the visual evidence narrator: nist』s response

and the conclusions of their own report.

in a crucial paragraph,

they state that the likely source of the molten metal

seen in the seven minutes before the collapse

was aluminum from the aircraft.

greening: i agree with nist

greening: like a bomb going off.

just say, "there it is," what』s interesting is that nist

and see no further significance.

i say this is extremely significant.

narrator: frank believes that the failure to investigate

the role of molten aluminum in the collapse of both towers

is a critical failure of the report,

one which has opened the door to outlandish conspiracy theories.

only the presence of aluminum explains

the multiple explosions heard before the towers collapsed.

grillo: there was a heavy-duty explosion.

johnston: the third explosion, the whole lobby collapsed on us.

grillo: this guy』s still trapped.

believe they have found the answer to some of the questions

heating the aluminum sufficiently

to create the bright yellow molten metal

seen pouring down the building.

and if the molten aluminum

was cascading down the side of the building,

it would also be seeping through the fractures

in the damaged floor below.

the only ingredient needed now was water.

but was there enough water in the twin towers

to react with the molten aluminum

to cause two catastrophic explosions?

nuclear chemist frank greening

and metallurgist christian simensen

like the insulating walls in the industrial smelter,

left unexplained by the official report

into the collapse of the twin towers.

they suspect nist may have missed

the true cause of the collapse--

one of the most potentially explosive metals on earth,

aluminum.

but molten aluminum won』t explode without water,

and christian believes that in the floors below the fires,

there was water in abundance.

simensen: they had sprinkler device in every room,

so the sprinkler device will come into action

because there has been nothing wrong in those rooms.

narrator: spread throughout each tower

and around it

that』s because the furnace

is perfectly engineered to intensify the temperature.

simensen: in this furnace here is one burner

which is heating up the aluminum,

by insulation material. and it』s surrounded

narrator: christian believes

that the building rubble inside the towers

acted as insulation,

recreating similar conditions to this furnace,

accelerating temperatures to 1,000 degrees.

to the situation in new york. simensen: it』s very similar

you have the burners in the center

with all the aluminum from the aircraft,

were 30,000 gallons of water.

is about ten centimeters insulation material,

which is the building material.

narrator: the official report appears to support his theory,

showing that in specific areas,

the temperature reached a surprising 900 degrees.

after crashing into the tower, christian』s theory is that

the aircraft wreckage came to rest

in the center of the building.

on the way, the plane collected building rubble,

in particular the gypsum plasterboard

that formed the internal walls and paneling.

the plasterboard rubble acted

on impact, the steel frame of the towers

on the steel beams. there』s iron oxide

then you have concrete in abundance.

[siren]

narrator: both towers were packed with multiple reactants,

all or any one of which could potentially trigger

the largest molten aluminum explosion in history.

christian and frank believe this answers

why and how the towers collapsed.

their theory could explain

the molten metal pouring from the building,

as well as the explosions heard by witnesses

in the moments before the towers collapsed.

the theory goes like this--

that was all over the building.

cut the aircraft into large sections.

inside the burning floors,

the building debris collects to form an insulating wall

around the aircraft wreckage, creating a furnace.

inside, the aluminum reaches 1,000 degrees.

at 700 degrees above its melting point,

the aluminum now flows like water.

finding cracks and holes in the damaged floor,

the molten metal lands in pools of water

from the sprinkler system on the floor below.

simensen: molten aluminum will react with the water underneath,

and you get a huge explosion.

there』s water bottles.

in the event of fire, the sprinkler system was designed

to pump hundreds of gallons onto the burning floors.

could it be that the system designed to prevent disaster

was the very thing that destroyed the towers?

according to the official report,

the sprinkler systems on the crashed floors

were damaged by the aircraft impacts.

however, eyewitness reports confirm

that on the floors below,

the system was pumping out gallons of water,

and as frank points out, the sprinkler system

source of water. wasn』t the only available

to washrooms, kitchenettes. greening: there』s the water

a metal present in huge quantities on the burning floor

coke machines all being smashed. there』s, uh,

there was plenty of water.

narrator: but water isn』t the only danger.

there are other substances that contain the components of water,

oxygen and hydrogen, that will also react

with molten aluminum.

among them are lime, iron oxide, and gypsum.

all were present in substantial quantities

inside the towers.

greening: those are precisely the elements

that are present in the world trade center.

the gypsum is gyprock, wallboard, plasterboard.

completely destroying the structural steel frame

that before the airplanes were flown into the towers,

bombs were planted inside, guaranteeing their collapse.

gage: so many architects and engineers,

technical and building professionals,

have found evidence

for explosive, controlled demolition.

narrator: for these conspiracy theorists,

only controlled demolitions

could have brought down the towers in the manner we see.

gage: what we have in the world trade centers

is massive explosions,

recorded by first responders,

traveling down the sides of the building,

he claims that the explosions are evidence

in 12 seconds.

are highly implausible. narrator: gage』s claims

after the terrorist attack in 1993,

the world trade center towers

were among the most secure in new york.

if gage was right,

explosives would have had to be smuggled into both towers

and their detonation precisely coordinated.

for many of those directly affected by the disaster,

claims of government involvement are deeply offensive.

the conspiracy theories, mcguire: we』ve all heard

and to be honest with you,

of one single firefighter i don』t know

tyrone johnston: the third explosion,

its founder is richard gage.

richard gage: we now have 2,200 architects and engineers

demanding a new investigation

of what happened at the world trade center complex

on 9/11.

narrator: being offensive to many, despite gage』s opinions

to gain mainstream acceptability he』s attempting

with a combination of pseudoscience and conjecture

to exploit what many see

as a major shortcoming in the official report.

gage: the official report does not account

for the hundreds of eyewitness testimony of explosions.

jimmy grillo: there was a heavy-duty explosion.

that puts any, you know, heed into that.

the whole lobby collapsed on us.

narrator: in all, over 118 new york firefighters

claim they heard explosions

just before the collapse of both towers,

yet their testimony is not addressed

in the official report.

gage: this explosive testimony

that was not released until august of 2005,

it was being held by the city,

but the new york court state of appeals forced the overturning.

narrator: for gage, the withholding of this testimony

is no oversight, it』s part of a cover-up.

from explosives, as some people were suggesting.

and then, of course, the south tower collapses,

the top section just crushes the lower section.

a little later the north tower does the same thing,

and i』m just shocked, horrified,

and i say to myself, "how could this happen?"

narrator: greening felt that until the mechanics

of the collapse were properly understood,

the fanatical rumors would simply not go away.

he decided to calculate whether the towers would collapse

under their own weight, without the need for explosives.

greening : the first thing i felt needed to be cleared up

was could the building collapse the way that we saw it,

or did it actually need help

unfold like everybody did,

i really threw myself into 9/11 research,

and i decided i wanted to apply science, physics,

to studying this problem.

i set up a computer program, based on momentum transfer,

to look at each impact of each floor.

momentum transfer program narrator: greening』s

calculated the weight of the buildings

above the impact floors.

he discovered that if one floor failed,

the weight of the higher floors

would cause each floor below to give way exactly as seen.

as the impacts were on different floors,

the simulation would need to account

it not only answered

i kinda think anyone

that thinks our own government would do that

is kind of out there somewhere.

i certainly don』t believe it.

i think we all know who did this and why.

narrator: for dr. frank greening,

a nuclear research chemist of over 20 years,

the question of government conspiracy was never an option,

but there were still many aspects of the official inquiry

that troubled him,

and until they were explained completely,

there would always be fertile ground for conspiracy theories.

what greening discovered was so remarkable,

is architects & engineers for 9/11 truth.

the outlandish claims of the theorists,

it would change our understanding

of the disaster forever.

the official government report into the twin towers tragedy

only tackled events on the initial impact floors,

not the physics behind the subsequent collapse.

it left the field wide open for conspiracy theorists

who point to the unexpectedly rapid collapse,

amid reports of explosions.

nuclear chemist dr. frank greening

was also surprised that the towers collapsed so quickly.

frank greening: i watched this whole thing

constructed from 200,000 tons of reinforced steel,

an explanation found not in the fabric of the towers,

but in the airplanes themselves.

man: there was a bomb in the building,

and it was just a matter of time

before the whole thing blew up and down come the towers.

narrator: built in the early 1970s,

at the time, the twin towers

were the tallest buildings in the world.

brash, bold, and iconic,

they were a marvel of design and architecture,

a symbol of america』s global power.

but the revolutionary structures

were about more than just their looks.

that could signal the end to the conspiracy theories,

they were built to withstand hurricanes,

and even a collision from a 707 jet.

yet within two hours of the terrorist attack,

both had collapsed.

today all that remains of the twin towers,

the most famous buildings that once stood in new york,

are two giant footprints.

of the 2,700 victims named here,

most died not when the planes hit the building,

but as the towers collapsed.

for many new yorkers,

the freedom tower acts as a symbol of resilience

for a city that lost so much,

to collapse?

a day remembered for the worst terrorist atrocity

in living memory.

hijackers fly planes into the world trade center towers.

before the day is over,

two questions ring out around the world--

who was responsible,

and why did these two huge buildings

collapse the way they did?

the first question has long been solved,

but the second refuses to go away.

built to survive a direct impact by an airliner,

could office fires really cause

most sophisticated buildings two of the world』s

and for bobby mcguire, a firefighter from long island,

horrified. man: i』m just shocked,

i say to myself, "how could this happen?"

narrator: the official report into the collapse

leaves many unsatisfied.

the question has spawned a thousand conspiracy theories.

man: many architects and engineers,

technical and building professionals,

have found evidence

for explosive, controlled demolition.

narrator: now a new theory developed independently

by two scientists on either side of the atlantic

discovers missing evidence

under the effects of the airplane impact and the fuel,

in an attempt to explain

the structural failures of the towers.

they examine the debris from the disaster,

run hundreds of computer simulations,

and conduct extensive lab tests.

in october of 2005, they publish their conclusions.

according to the report,

the buildings collapsed because when the aircrafts hit,

the impact dislodged the fireproofing

structural steel skeleton. around the building』s

unprotected from the fires,

the steel frame weakened, and the towers collapsed.

man: we say here the buildings would not have collapsed

from the national institute of standards and technology

jet fuel ignited multi-floor fires.

the reason it collapsed

is because the fireproofing was dislodged.

narrator: but the nist report

is restricted to finding an explanation

for the collapse of the steel supports

on the impact floors alone.

it does not deal with

the subsequent disintegration of the towers,

and the dramatic nature of their collapse

becomes an obsession for online conspiracy websites.

one of the most prominent and controversial conspiracy sites

not really knowing where to go.

it has a very special significance.

bobby mcguire: you know, out of the 343 firefighters we lost,

i lost many, many friends, and i lost my nephew.

he was a proby, which means less than a year on the job,

and it was his very first fire.

his unit was one of the first to respond,

and he was in the north tower when it collapsed.

i gotta think he was pretty high up.

narrator: bobby arrived on the scene

just as the second tower was coming down.

mcguire: it was complete chaos,

and then you had thousands and thousands of people

running for their lives in every direction,

for the differing mass above each

reports of more planes coming in added to the confusion.

collapse like that before, no. i』ve never seen a building

it was like a pancake effect.

any type of building, i don』t remember having

certainly nothing that size, collapse.

narrator: the 110-story world trade center towers

were designed to withstand

the impact of a commercial jetliner,

yet within two hours of the attacks, both collapse,

so why did the twin towers fail so disastrously and so quickly?

under public pressure,

the u.s. government commissions a $16 million investigation

where it loses a lot of its strength,

there is on the steel.

narrator: the important steel girders inside the towers

were treated with a spray-on heat-retardant concrete

that would certainly have protected the steel

from the fires.

nist, the organization carrying out the official investigation,

recreated the conditions inside the burning buildings

to check what impact the fires would have had

on the protected steel.

the results were surprising.

agrawal: as part of the official investigation,

a number of experiments were done to understand

how quickly steel gets to that critical temperature,

that time lag depends on how much protection

and this was done when the steel was protected

with this sprayed-on insulation, and it took over ten hours.

narrator: yet within two hours,

both towers had collapsed, long before they should have.

to explain the discrepancy,

nist removed the fire protection and repeated the tests.

agrawal: when the same experiment was done

without any protection on the steel at all,

it took a mere 13 minutes.

narrator: the conclusion drawn by the nist report

lies at the heart of the debate.

they concluded that because the steel girders gave way,

substantial numbers must have lost their fireproofing.

to significantly weaken the steel

yet most of the fuel burned off within seconds

and all within a few minutes.

after that, the fires that spread over several floors

were sustained by the contents of the offices.

agrawal: there was a lot

of what engineers call "fire load" in the building,

so materials like wood,

which would-- may have been in the flooring,

and the furniture and so on,

provide a lot of fuel to these fires,

so even after the jet fuel may have burnt out,

that can cause the fire to burn. there』s lots of other stuff

narrator: but were the fires hot enough

but how?

in just the short time it took the towers to collapse?

the question is one of complex thermodynamics.

agrawal: the official report explains

that once the steel gets to

above about 600 degrees centigrade,

which is over 1,000 fahrenheit,

that it starts to lose

a significant proportion of its strength.

the key question is how long does it take,

so the fires might get there very, very quickly,

but the steel might take a bit longer,

from one to the other. and there』s a bit of a time lag

pouring from one of the windows.

when one passage catches his attention.

according to the report,

just seven minutes before the south tower fell,

there were signs of explosive activity within the tower

and a glowing liquid

began to pour from one of the impact floors.

the liquid can clearly be seen on this video footage.

greening: one of the most interesting clips

that people discuss at great length

concerning the world trade center collapse

is this footage of the south tower,

and just before the collapse,

we see a golden stream of what looks like a molten metal

looking for clues that might suggest an alternative answer,

narrator: the official report concluded the molten metal

played no part in the collapse, but to frank greening,

the cascade was extremely significant.

it was the missing evidence he』d been searching for.

greening: i came with a chemist』s perspective.

i see the chemistry as the key,

what is, is the probable reason and i think that』s

why nist failed to consider the role of chemistry.

basically they』re not chemists.

narrator: frank suspects that the metal

seen pouring from the window

is a material with a far lower melting point than steel,

including the structural fire group

the report assumed

that fireproofing had been knocked off

by the initial impact from the aircraft.

the conclusion of the entire official report

hinges on this critical assumption.

without it, there is simply no way of explaining

how the steel structures gave way.

agrawal: the findings of the official report

really rest on the question

of whether the steel was protected or not

and whether that fire protection

had been dislodged or not as a result of the impacts.

narrator: there are many respected industry experts,

were at a maximum, around 10,000 gallons,

engineering firms in one of the world』s largest

and professor of fire protection engineering

at the university of maryland,

who do not believe that the fireproofing

could have been dislodged,

throwing the conclusions of the report into serious doubt.

if the fireproofing was not dislodged,

the nist report offers no other explanation

for the event that triggered the collapse

and caused the deaths of so many people.

nuclear chemist dr. frank greening

scours the 10,000-page report

super-tall skyscrapers dominate our skyline.

then the fires subsided somewhat,

and then suddenly out of the blue,

the north tower and the south tower just collapsed.

the question is why. what started that?

narrator: although he has dispelled

about controlled demolition, the conspiracy theorists』 ideas

there is still the nagging question about the explosions

heard by firefighters before the collapse,

but central to his concern

was the conclusion that the office fires

were capable of weakening sufficient numbers

of the structural steel girders to trigger collapse.

around the world,

greening: towers were burning away furiously.

the pinnacle of design and engineering,

these towers are built to last, and just like the twin towers,

the key to their strength

is that they are constructed around a strong steel skeleton.

structural engineer roma agrawal

helped build europe』s tallest skyscraper, london』s shard.

roma agrawal: the twin towers had two tubes,

one inside the other, to form the structure.

the inner tube was called the core,

and it had vertical steel columns in it.

the outer tube also had vertical steel columns in it,

and the horizontal beams going around the building

formed a lattice with those columns, to keep it stable.

but with a greater number of floors above the impact site

there were 15 floors above the impact point in the north tower

and 22 floors above the south tower.

would also need to prove frank』s calculations

that the floors below each impact point

would disintegrate in a manner

that looks similar to a controlled demolition.

greening: i ran the program,

and it showed that the, uh, the buildings

would indeed come down.

even more interesting was,

the, the computer program predicted the collapse times.

narrator: the north tower

falls in 13 seconds,

narrator: this tube lattice design

of the south tower, it takes only 11 seconds.

greening: once the building started to collapse,

it was no stopping it.

it was a juggernaut.

the mass of the upper block in the north tower

was equivalent to the mass of the titanic.

it was about 38,000 tons.

narrator: in 2008, frank』s work

is published in the journal of engineering mechanics.

it dismisses the idea of controlled explosions

but his investigation leads him to question

fundamental aspects of the official report.

without it, high-rise buildings

47 columns, 52 inches by 22 inches.

it』s solid steel.

there』s no evidence

for temperatures reaching the structural steel

that could even cause the weakening of that steel.

greening: for the skeptics,

it was the fires were not that intense.

they were kind of patchy at best,

and they argued that insufficient heat

was being supplied to the columns,

and therefore there was no way a fire could weaken the steel.

narrator: steel has been used

in the construction industry for over a century.

of these towers was massive, as was the core--

that dominate our cities would not have been possible.

a manmade alloy of iron and carbon,

steel offers unrivaled strength and flexibility.

manufactured to be fire-resistant,

steel retains its strength

in high temperatures for extended periods of time,

and around the world,

high-rise steel buildings have remained standing

despite the most intense fire damage,

so why did the steel frame of the twin towers fail?

the 9/11 hijackers chose long-distance flights

because they knew the fuel loads inside the jets

something caused the surviving structural steel columns

meant the towers initially stood firm.

agrawal: when the building was impacted by the plane,

a number of the columns and the beams were severed,

but because of this tube lattice system,

the weight of the building could actually find a different way

to go around the hole,

so the impact by itself

would not necessarily have caused the tower to collapse.

narrator: by redistributing the loads of the higher floors

around the damaged structures,

the buildings survived the plane』s initial impacts,

but they do collapse,

one after 56 minutes, the other after 103 minutes.

narrator: september 11, 2001,

to fail.

agrawal: the official report explained

that the temperatures of the fires were high enough

to cause the steel to lose its strength

and to compromise its integrity,

and this is what ultimately brought the buildings down.

gage: no skyscraper has ever collapsed

due to fire prior to 9/11.

narrator: 9/11 truth movement founder richard gage

doesn』t believe the fires

could have weakened the steel structure.

gage: the exterior perimeter structure