Why the World Trade
Center Buildings Collapsed
Chief ’s Assessment
By: Deputy Chief Vincent Dunn ret.
After the 767 jet liner crashed into the world trade center
building creating the worst terror attack in history, a fire burned for
56 minutes inside the World Trade Center building number two. The top 20
floors of the building collapsed on the 90 floors below. The entire one
hundred and ten-story building collapsed in
8 seconds... After a fire burned inside WTC tower number one for
102 minutes, the top 30 floors collapsed on the lower 80 floors. And the
entire one hundred and ten stories of this building collapsed in 10
can say the reason they collapsed was they were struck with a 185 ton
jet airliner and the 24,000 gallons of jet fuel caused a fire of 1500 to
2000 degrees F which weakened the steel and cause the collapse.
Or you can take a closer look at the buildings construction of
the WTC buildings. And ask yourself why did these structures collapse so
fast and so completely. The
answer can be found by examining high-rise construction in New York City
over the past 50 years
World Trade Center tower construction
terms of structural system the twin towers departed completely from
other high-rise buildings. Conventional skyscrapers since the 19th
century have been built with a skeleton of interior supporting columns
that supports the structure. Exterior walls of glass steel or synthetic
material do not carry any load. The
Twin towers are radically different in structural design as the exterior
wall is used as the load-bearing wall. (A load bearing wall supports the
weight of the floors.) The only interior columns are located in the core
area, which contains the elevators. The outer wall carries the building
vertical loads and provides the entire resistance to wind. The wall
consists of closely spaced vertical columns (21 columns 10 feet apart)
tied together by horizontal spandrel beams that girdle the tower at
every floor. On the inside of the structure the floor sections consist
of trusses spanning from the core to the outer wall.
Bearing walls and Open floor design
When the jet liners
crashed into the towers based upon knowledge of the tower construction
and high-rise firefighting experience the following happened: First the
plane broke through the tubular steel-bearing wall. This started the
building failure. Next the exploding, disintegrating, 185-ton jet plane
slid across an open office floor area and severed many of the steel
interior columns in the center core area. Plane parts also crashed
through the plasterboard-enclosed stairways, cutting off the exits from
the upper floors. The jet collapsed the ceilings and scraped most of the
spray-on fire retarding asbestos from the steel trusses.
The steel truss floor supports probably started to fail quickly
from the flames and the center steel supporting columns severed by plane
parts heated by the flames began to buckle, sag, warp and fail. Then the
top part of the tower crashed down on the lower portion of the
structure. This pancake collapse triggered the entire cascading collapse
of the 110-story structure.
The most noticeable change in the modern high-rise
construction is a trend to using more steel and shaping lightweight
steel into tubes, curves, and angles to increase its load bearing
capability. The WTC has tubular steel bearing walls, fluted corrugated
steel flooring and bent bar steel truss floor supports.
To a modern high rise building designer steel framing is
economical and concrete is a costly material. For a high-rise structural
frame: columns, girders, floors and walls, steel provides greater
strength per pound than concrete. Concrete
is heavy. Concrete creates excessive weight in the structure of a
building. Architects, designers , and builders all know if you remove
concrete from a structure you have a building that weights less. So if
you create a lighter building you can use columns, girders and beams of
smaller dimensions, or better yet you can use the same size steel
framing and build a taller structure. In News York City where space is
limited you must build high. The trend over the past half-century is to
create lightweight high buildings. To do this you use thin steel bent
bar truss construction instead of solid steel beams.
To do this you use hollow tube steel bearing walls, and curved
sheet steel (corrugated) under floors. To do this you eliminate as much
concrete from the structure as you can and replace it with steel.
Lightweight construction means economy. It means building more
with less. If you reduce the structure’s mass you can build cheaper
and builder higher. Unfortunately unprotected steel warps, melts, sags
and collapses when heated to normal fire temperatures about 1100 to 1200
The fire service believes there is a direct relation of fire
resistance to mass of structure.
The more mass the more fire resistance. The best fire resistive building
in America is a concrete structure. The structures that limit and
confine fires best, and suffer fewer collapses are reinforced concrete
pre WWII buildings such as housing projects and older high rise
buildings like the empire state building, The more concrete, the more
fire resistance; and the more concrete the less probability of total
collapse. The evolution of high- rise construction can be seen, by
comparing the empire state building to the WTC.
My estimate is the ratio of concrete to steel in the empire state
building is 60/40. The ratio of concrete to steel in the WTC is 40/60.
The tallest building in the world, the Petronas Towers, in Kula
Lumpur, Malaysia, is more like the concrete to steel ratio of the empire
state building than concrete to steel ratio of the WTC.
Donald Trump in New York City has constructed the tallest
reinforced concrete high-rise residence building.
The computer designed high rise building
The computer has allowed engineers to reduce the mass of a
structure by its ability to more accurately determine the load bearing
capability of structural framework. Years ago before the computer,
builders were not sure of a structural elements load bearing capability,
so they over built by using a so called “safety factor”. This built
in safety factor could result in a structure with twice the required
load bearing strength. Because
of computer calculation this no longer occurs. The older buildings use
to have built in a so called “safety factor” of two-to-one.
Not so today, if the building code requires a load bearing factor
of 40 pounds per square foot that is exactly what you get. There is no
margin for error.
Effects of jet crash and fire on a
skeleton steel high rise
plane that only weighted 10 tons struck the Empire State Building and
the high-octane gasoline fire quickly flamed out after 35 minutes. When
the firefighters walked up to the 79 floor most of the fire had
dissipated. The Empire State Building in my opinion, and most fire
chiefs in New York City, is the most fire safe building in America. I
believe it would have not collapsed like the WTC towers. I believe the
Empire State Building, and for that matter any other skeleton steel
building in New York City, would have withstood the impact and fire of
the terrorist’s jet plane better than the WTC towers. If the jet
liners struck any other skeleton steel high rise, the people on the
upper floors and where the jet crashed may not have survived; there
might have been local floor and exterior wall collapse. However, I
believe a skeleton steel frame high rise would not suffer a cascading
total pancake collapse of the lower floors in 8 and 10 seconds.
Hopefully some engineer using computer calculations, can reconstruct the
effects of a 767 jetliner crashing into another New York City high
building. In any other high rise in New York City, I say, the floors
below the crash and fire, would not collapse in such a total a cascading
pancake cave-in. Most of
the occupants and rescuers killed in the WTC tower collapse were on the
The Empire State Building
Perhaps builders should take a second look at the Empire
State Buildings construction. There might be something to learn when
they rebuild on ground zero. The empire state building has exterior
Indiana limestone exterior wall, 8 inches thick. The floors are also 8
inches thick consisting of one-inch cement over 7 inches of cinder and
concrete. All columns, girders and floor beams are solid steel covered
with 1 to 2 inches of brick
terracotta and concrete. There is virtually no opening in the floors.
And there are no air ducts of a HVAC heating cooling and venting system
penetrating fire partitions, floor, and ceilings. Each floor has its own
HVAC unit. The elevators and utility shafts are masonry enclosed. And
for life safety there is a 4-inch brick enclosed so-called “smoke
proof stairway”. This stairway is designed to allow people to leave a
floor without smoke following them and filing up the stairway. This is
accomplished because this smoke proof stairway has an intermediate
vestibule, which contains a vent shaft. Any smoke that seeps out the
occupancy is sucked up a vent shaft.
Since the end of WWII builders designed most of the concrete from
the modern high-rise constriction. First concrete they eliminated was
the stone exterior wall. They replace them with the “curtain walls of
glass, sheet steel, or plastics. This curtain wall acted as a
lightweight skin to enclose the structure from the outside elements.
Next the 8-inch thick concrete floors went. They were replaced with a
combination of 2 or 3 inches of concrete on top of thin corrugated steel
sheets. Next the masonry enclosure for stairs and elevators were
replaced with several layers of sheet rock.
Then the masonry smoke proof tower was eliminated in the 1968
building code. It contained too much concrete weight and took up
valuable floor space. Then
the solid steel beam was replace by the steel truss. And finally the
concrete and brick encasement of steel columns girders and floor
supports was eliminated. A
lightweight spray-on coating of asbestos or mineral fiber was sprayed
over the steel. This coating provided fireproofing. After asbestos was
discovered hazardous vermiculite or volcanic rock ash substance was used
as a spray-on coating for steel. Outside of the foundation walls and a
thin 2 or 3 inches of floors surface, concrete has almost been
eliminated from high-rise office building construction. If you look at
the WTC rubble at ground zero you see very little concrete and lots of
The performance building code
How did lightweight high-rise construction evolve since WWII?
It evolved with the help of the so-called performance code. After
WWII the builders complained about
building codes. They said they were too restrictive and specified
every detail of construction. They called the old building codes
“specification codes”. They
complained the codes specified the size and type and some times even the
make of a product used in construction. They decried the specification
code as old fashion. They wanted the building codes changed to what they
called “performance codes.” They wanted the building codes to
specify the performance requirements only; and, not specify the size and
type of building material to use. For example, with fire resistive
requirements they wanted the code to state just the hours of fire
resistance (one, two, three or four hours) required by law; and not to
state the specific type and material used to protect structural steel
and enclosures for stairways and elevators shafts. For example, a
performance building code states: the steel has to be protected against
heat of flames for one, two, three or four hours during a fire. It does
not state what to use as a fire resisting material. This performance
code signaled the end to concrete encasement fire protection and allowed
a spray on fire protection for steel and plasterboard enclosed stairs
and elevator shafts. Builders
hailed the New York City building code of 1968 as a good performance
code. However, some fire chiefs decried it as a law that substituted
frills for real construction safety. The asbestos spray on coating of
steel trusses used in the WTC towers was considered by Chief of the New
York City Fire Department, at the time, John T. O’ Hagan to be
inferior to concrete encasement of steel. Writing in his book, High
Rise Fire and Life Safety. l976, he
listed the following problems of spray-on fire protection of steel:
to prepare the steel for spray-on coating adhesion. Rust and dirt
allowed spray-on fire retarding coating to scale and fall away from
steel during construction
or uneven application of the spray-on fire retarding was discovered
during post fire investigations
of spray-on material during manufacture makes it ineffective
of thoroughness in covering the steel during application is a
to replace spray-on material dislodged by other trades people
performing work around the steel during the construction of the
The WTC started construction in the 1970s. And the WTC towers
built by the Port Authority of New York did not have to comply with the
minimum requirements of the new1968 performance building code.
constructing the new high rise buildings on ground zero
steel columns, girders and floor beams should be encased in masonry
or other more effective
fire retarding material. Spray-on fire retarding is ineffective.
Post fire investigations reveals the spray on fire retardant has
scaled off and steel beams and concrete and steel floor slabs crack
and allow flame spread.
bar joists should not be used to support floors in high-rise
buildings. The National Fire Protection Association has shown
unprotected steel bar joist fail after five or ten minutes of fire
life safety in high-rise buildings bring back the smoke proof tower.
This allows people to escape fire using smoke free stairways.
and elevator shaft ways should be enclosed in masonry to prevent
ventilation and air condition HVAC systems should be provided by
unit system serving only one or two floors. Central air system
serving 10 or 20 floors creates shaft ways and duct systems that
penetrate fire rated floors walls partitions and ceilings. Smoke
spreads throughout ducts of central HVAC systems.
high rise building framework should be skeleton steel framing not
center core steel column framing. There should be no bearing wall
high rise construction. Reduce the size of open floor design.
the thickness of concrete in floor construction. The two or three
inches of concrete over corrugated steel fails during most serious
high rise fires and must be replaced.
sprinklers should protect all high rise buildings. Firefighters can
extinguish approximately 2,500 square foot of fire with one hose
line. Two hose steams may quench 5,000 square feet of fire. The
World Trade Center floor areas were 40,000 square feet in area.
State and Port Authority buildings should comply with New York City
building codes and actually in some cases should exceed them.
Remember building codes are only minimum standards.