IT WAS THE FIRE, CAUSED THE
TWIN TOWER COLLAPSE
An analysis
of the cause of the collapse of the
The following is an essay
on the possible causes of the
To enter the debate as to whether the plane crashes or the
resultant fires caused the collapse of
All of the ‘plane caused’ collapse theories depend on the destruction of numbers of core columns by the plane crash impact and the subsequent failure of the remaining core columns by the heat of the fire. The core is the interior rectangular section of the building containing the stairways, elevators air conditioning supply and return shafts, utility shafts and restrooms etc. An outer-ring containing the large open area office space surrounds the core. The outer-ring floor assembles consisted of long-span, open-web steel bar joists spanning the distance between the outer perimeter columns and the interior core columns. These bar joists supported a steel pan and concrete floor. Each façade had 59, high strength, steel box columns 40 inches on center. Apparently the exterior steel box
columns were very strong and being 36 feet long, each was backed up by at least two of the 4-inch concrete floors on edge, built at 12-foot intervals vertically.
Each box column was bolted to the column above and below and welded to spandrel girder ringing the perimeter at each story. The shiny aluminum skin covering each column would, of course be stripped by the planes and melted away by any fuel fire.
I have not, as yet,
seen the enhanced videos but, I maintain that since, in the pictures I have
seen, we can not really see the remaining columns because of the heavy black
smoke issuing from the impact area; we cannot tell how many, if any, were
severed. Using CAD simulations Tony Fitzpatric of Arup
I believe the intensity of the fire (as it relates to building collapse) was comparable to a heavy ordinary combustible fire after the explosion dissipated much of the jet fuel. According to Francis Brannigan author of Building Construction for the Fire Service, “…temperatures in excess of 2000 degrees are the rule in severe fires. The average person has no idea of the temperatures which can be reached in a quite ordinary fire.”(Brannigan 1971, p245). The heat output of an interior fire is limited, by the amount of air reaching the combustibles and the smoke produced. In the standard furnace tests used to determine the collapse-resistance of building components, authorities switched from oil fires to natural gas since; “The smoke emitted by the fire at times seriously interferes with the transfer of heat by radiant energy within the fire building. Test fires use smokeless natural gas, so radiant heat transfer is important in tests.”(Brannigan p206). A jet fuel fire would produce great quantities of smoke, which would reduce the radiant heat energy entering structural components. According to G. Charles Clifton HERA structural engineer, speaking of the fires in the Towers; “In my opinion, based on available evidence, there appears no indication that the fires were as severe as a fully developed multi-story fire in an initially undamaged building would typically be.”(Elaboration..., p5)
My point is that given the inadequate partial sprinkler system, the use of lightweight long-span steel bar joists, deficient ‘fireproofing’ on the steel, and large open areas undivided by fire walls, any uncontrolled large area fire would
have eventually produced the same total collapse. The importance of early fire control, to save lives by prevention of collapse in most ordinary constructed buildings is generally not appreciated even by engineers. Many years ago hard experience had taught the Fire Dept. that when the fire was beyond their control in the old brick and joist, first generation high-rise buildings the fire forces were withdrawn and the fire fought from the outside in anticipation of collapse and preparations begun to limit fire spread to exposures. The whole concept of the second generation fireproof buildings was that fires could be fought from the inside without the danger of collapse until all the occupants could be removed and the fire extinguished. “Heavy reliance (was placed) on the integrity of the building, its design and its systems.” (O’Hagan p 145, 149). Consideration of collapse in fire ‘proof’ resistive buildings has not been of critical importance, until now.
Instead
of the columns failing first, I believe the weakest link was the long-span,
open web, steel bar joists. The position of these joists, over the fire and the
small-diameter steel elements of these joists would allow them to heat up to
the failure temperature, (approximately 1100 degrees F.), much more rapidly
than the massive columns which would act as a heat sink and conduct some heat
away.
According
to Deputy Chief, (Ret.) Vincent Dunn, FDNY writing in his book Collapse of Burning Buildings, “A large steel I-beam can absorb heat and take a
relatively long time to reach its failure temperature, while a lightweight
steel beam, such as an open web bar joist, can be heated to its failure
temperature much faster.” (Dunn, 1988, p142)
It has been shown that, at times, at the WTC,
the fire resistance of both bar joists and columns were deficient, due to
flaking off of sprayed on coverings in certain places. (NY Times, Science Sec.
“In
fact, successive failure of trusses appears to be the rule rather than the
exception.” (Brannigan p46).
As in a truss, a fire resistive building
without built in redundancy depends on all the critical elements and their
connections retaining their fire resistance and thus their integrity during a
fire. The WTC exterior box column walls bes9ides being the bearing walls for
the floors were shear-walls transmitting lateral wind loads through the
membrane floors to each other and to the ground. These exterior walls “together
with the floors, formed a torsionally rigid framed tube fixed to the
foundations.”(
I surmise that as those floor sections, which
were intact after the plane crash and fuel explosion, were weakened by the heat
and let down their concrete loads and live loads onto the floors below, a
progressive mechanical collapse began in the floors. In conventional fire
resistive, steel frame construction the spans are shorter and beams and columns
rigidly restrained vertically, horizontally and diagonally, by strong
connections and masonry walls built between columns. A progressive collapse due
to impact loads is less likely since the masonry walls and strong connections
between columns and girders can redistribute the loads. A floor collapse in a
conventional steel-framed building would have been localized since the area
between girders would be small. On the
other hand “when huge spans are achieved by…trusses or space frames, collapse
can be sudden, general and tragic.” (Brannigan p215).
A pancake, V- shaped collapse or a lean-to
collapse of a long span bar joist floor would impart a concentrated impact load
on the floor below. I doubt weather the
long span bar joists in the floor below could sustain such an impact, as well
as steel I beams and reinforced concrete floors could. The connections of the
joist ends to the columns, at the WTC, have been shown to be a likely spot for
impact load failure. An impact causing a depression anywhere in the top chord
of a truss could also cause collapse of the truss since such top chord is in
compression and could buckle. The floors were providing lateral support to the
exterior columns and core columns and in effect were integral to the stability
of the whole structure. Removal of lateral support for enough of these columns,
(by floor collapse) would allow the weight of the building above to buckle both
the outer perimeter and interior core columns, letting down the entire upper
portion of the building.
More
likely, as the architect Mr. Malott points out in “Why the World Trade Center
Collapsed”, Nov./Dec issue of Designer Builder magazine, the bar joists
themselves pulling with them the exterior walls” and the core columns started
the collapse. “Steel (floor) members which sag due to fire will try to carry
their loads
as
suspension members. This causes large horizontal forces; if they are
transmitted to the fire wall, it can be destroyed.”(Brannigan p253) It seems
likely
that
such floor sagging in the Southeast corner of Tower 2 affected the corner core
columns and/or corner perimeter columns causing the initial list to the
Southeast just before the rapid, avalanche collapse of the 110 story structure.
In Tower 1 the “bulging ripple going down the outside of the skin in advance of the collapsing floors” (Malott p12) could have, in fact, been caused by floors collapsing ahead of the column failure. If it was a flat pancake collapse of the floors, the increasing dynamic weight of the concrete laden floors along with
their live loads, hitting each level could easily break the connections to the columns or spandrels on each floor. The tips of the joist ends sliding down the
interior face of the columns could have caused the “bulging ripple”; or this moving bulge could have been caused by the air pressure from the bellows effect as the collapsing floors compressed the air which pushed on and bowed out the windows and the aluminum skin covering the columns.
This
type of flat floor collapse reminds me of bathroom floor failures in old six
story apartment buildings. These localized, progressive collapses were so
common, in the
In Tower 1 it appears the top floor or floors began failing first possibly because the top floors were receiving most of the super heated gasses rising up the damaged elevator and stair shafts and other vertical openings such as un-fire-stopped pipe or wire runs, or air conditioning shafts. These fire gasses could have accumulated and heated the entire upper ceiling area or truss voids of one floor, or several floors, starting a softening and sagging of the joists. Or, more likely, after filling the upper floors (mushrooming) these heated gasses could have exploded, and triggered the initial floor collapse. This happens at times in unventilated void spaces at serious fires. A third possibility as to a contributing cause for the Tower 1 collapse is sprinkler system water overloading one or more floors. For instance, if the restaurant on the 107th floor were sprinklered and the heated smoke set off some of the spray heads, after a time, the water weight buildup over a large floor area could exacerbate the sequential bar-joist failure. Water would accumulate in the depression in the sagging floors caused by the bar joists softening, thus hastening the collapse. “From the video footage this collapse appeared to occur (begin) uniformly around the building (“at or near the top of the
building”) and spread
rapidly down to the floor above the impact region. That region than pancaked…”
(
The fact that the collapse began, apparently simultaneously, around the entire upper floor outer ring and possibly the inner core of Tower 1 rather suggests an explosion or rapid combustion of flammable gasses, such as carbon monoxide or vaporized jet fuel, over-pressuring the area.
Incomplete combustion, due to lack of oxygen,
in the main body of fire in addition to producing these flammable gasses, may
have been may have been another reason the fire temperatures in general not
being any greater than an average fire. According to Charles G. Clifton “The
observed fire behavior points to temperatures in the building not being
particularly severe – say no more than about 600 to 700 Deg. C. Possible reasons for this may involve the coating
of combustible material in dust from pulverized concrete and wall linings (gypsum)
and the volatility of the aviation fuel leading to large amounts of fuel being
pyrolised but not burnt in the interior of the building.”(
Air Conditioning
Central heating, ventilation
and air conditioning (HVAC) systems could have affected the building stability
in several ways. According to Former Fire Commissioner John T. O’Hagan;
“The air-conditioning system increases the
flow of air and of oxygen to the seat of the fire, thereby increasing its rate
of development and its ultimate severity. The return portion of the system
recirculates
smoke and contaminated air
on the floors above and below the fire increasing the life hazard, complicating
the evacuation and rescue problem, increasing the difficulty of locating the
seat of the fire and delaying the actual extinguishing operation. This allows
the fire to increase in severity and extent. The undivided ceiling space which
is utilized as a plenum for the collection and direction of the recirculated
air to the return shaft is also an effective medium for the transfer of heat to
the remainder of the floor area.”(p132)
In writing
about a third alarm World Trade Center fire which occurred on Feb.13, 1975, Commissioner
O’Hagan speaks of the “common ceiling plenum” as the means of fire spread
between two areas. (p37). This plenum (the 3 foot deep ceiling void formed by
the bar joist truss system) became a route for the fire gasses since it covered
the entire outer ring floor area and served as the primary avenue for return
air to the air conditioners. Even after the air conditioning fans were shut
down this truss-produced void could accumulate and spread super-heated gasses since
these gasses would continue entering through the ceiling grills. This
configuration, using the truss voids as a return space for air movement back to
the mechanical equipment fan rooms, would negate any fire protection that the
sheetrock ceilings afforded the steel trusses thereby hastening their
softening, sagging and eventual collapse.
In
tests done by Fire Commissioner O’Hagan, combustible gasses were shown to
spread to other areas of buildings through the ventilation systems. “It was
noted that locations which were somewhat remote from the fire achieved a
relatively high concentration of carbon monoxide, moreover, these did not
necessarily correspond to regions wherein either the temperature or the smoke
concentration was extremely high.”(p90). Carbon monoxide is a highly toxic gas
with a wide explosive range and is lighter than air. It could have easily
spread along with other explosive products of incomplete combustion to remote
areas within the truss voids or other spaces on other floors and upon ignition,
caused explosions which contributed to the collapse.
Large Open
Areas
Large open areas, containing
combustibles, within buildings, are a nightmare for firefighters because of the
possibility of spread of fire, throughout the space and the resultant large volume of
fire. The size
of a fire is also a major factor that affects steel failure. “A large area fire
in which flames involve much of the steel
beam
in a short period of time will heat the steel beam to its critical temperature
more quickly. A so called “flash fire,” suddenly involving a large area with
flame,
can
heat steel rapidly to its failure temperature.”(Dunn p142). Because truss construction is often
used to provide this wide-open space within buildings this additional hazard is
produced compounding the collapse problem. As pointed out these lightweight
steel trusses are affected much sooner by fire than heavy beams and since they
span such large distances, any failure becomes more serious than a short span
element.
This rapid
fire growth situation is exacerbated in high-rise structures when elevators and
standpipes must be used by responding firefighters, delaying the operation of
hose streams and rescue. The difficulty in extinguishing such large, open area
fires when extend throughout an interior space, arises because, as the fire in
one section is extinguished and the hose streams are repositioned to attack
another area, the fire re-ignites in the previous section by convected and
radiated heat from the freely burning section. The convected and radiated heat
becomes an impossible barrier to hose line advancement. This hazardous
situation occurs even in well-ventilated areas and fire-suppression in such
large open areas, within buildings, often requires the cooling of all spaces at
once, an effect, which in high-rise buildings sometimes, can only be
accomplished by sprinkler systems. The truss voids used as return plenums for
HVAC systems adds to the problem by allowing fire to spread in these concealed
voids possibly over the heads of firefighters. According to Chief Dunn “The
best kept secret in
“The main problem associated with the protection of life from fires in high-rise buildings is the limitation of the size of the fire. Public fire protection services can usually contain a fire in an elevated portion of the building if the fire area is limited to 5000 square feet of less. If the horizontal spread of the fire exceeds this limit, the heat developed will also increase the risk of vertical spread. If the building contains vertical arteries, then a tragedy can be expected.”(p245, 246)
Sprinklers
According to Chief Dunn, “The only real fire protection for a commercial or residential high-rise building is an automatic sprinkler and smoke-removal system to vent the smoke after the sprinkler extinguishes the fire.” Mr. Brannigan comes
to the same “…inescapable conclusion that full automatic sprinkler protection is vital to the safety of occupants of high-rise structures.”(Brannigan. p370).
“However, if the fire originates in or penetrates the (truss) void, the sprinklers will not be in a position to control the fire” (Brannigan.p548, 01) In my opinion, total sprinkler protection including the truss voids, if it had been installed and remained intact, would have provided enough cooling of the protected steel to slow down total collapse at the WTC fuel fire. It certainly would have reduced the smoke and heat output to a more manageable level thereby saving many more lives.
Full automatic sprinkler protection means every area and room and every void space on every floor is covered by the discharge pattern of a sprinkler head. While a partial water spray system is often recommended and necessary for certain special hazard areas, it is not generally known that partial water spray systems can sometimes cause difficult problems if they are only installed in hallways or exit-ways, or only on certain floors. If a fire starts in an unsprinklered area the fire may rage out of control in this area and the superheated gasses can flow across the ceiling or up open shafts to a sprinklered exit-way setting off the spray heads in this area. These sprinkler heads cannot control the fire since they are not over the fire, but will create expanding quantities of steam, at times, making line advancement down a hall or exit-way difficult or may even trap people if the exit-hall becomes untenable. Full coverage with a sprinkler system will solve the problem.
Note: I found this out the hard way at
a training exercise I was giving at the
The other way partial sprinkler systems can be troublesome is if the fire in the unprotected area gets out of control and cannot be cooled quickly, the heated gasses can be forced up elevator shafts or other openings such as unfire-stopped pipe and wire shafts or poke through openings, to remote floors above the fire and set off the heads there.
Since the areas above an uncontrolled fire may be dangerous to enter because of developing smoke and heat conditions, we may not be able to quickly shut off sprinkler water which consequently accumulates on the floor or in the contents, overloading the floors, leading to floor collapse from the weight of the water. If the steel bar joist floors began to sag from the heat, water would tend to accumulate in the depression in the floors and not drain off down stairways or other shafts, further hastening collapse. Again full coverage by sprinklers will mitigate this problem by reducing or eliminating the production of these super heated gasses. On-off sprinkler heads and floor drains or scuppers to drain the water may also help. In spite of all these problems, “Sprinklers are the core of fire safety for the occupants of high-rise buildings” (Brannigan 1992, p502).
Conclusion
For some arcane legal reason the Port Authority of NY
State and NJ did not have to comply with the New York City Building Code, and
Fire Codes. Since
Building and Fire Code regulations and procedures are of critical importance for life safety and property protection. According to Fire Commissioner O’Hagan, “The time and place to ensure life safety in high-rise buildings is during the period that the building is being designed.”(p243). If the Port Authority had to submit plans and get plan approval from the City before starting construction, it would have been subject to plan review by experienced Code experts and inspection during construction by experienced Building inspectors and Fire inspectors who had the power to stop the job until construction violations were corrected. The Port Authority would have had to receive a final inspection and certificate of occupancy before opening the building. In my opinion the design
and construction would have been radically changed and many more lives could have been saved.
All buildings built in the City should, at least, have to follow the City Codes. The Port authority had a ‘policy’ to comply with City Codes, but still there were serious deficiencies in sprinkler protection, steel protection from heat, exit-ways & enclosures and building design, which would have been detrimental in any serious fires in these buildings. Sprinkler systems were not even built into the original buildings, and many areas were never fully covered by subsequent retrofits.
Recommendations for
Possible Code Changes
Long
span, steel bar joists should be prohibited for floor construction in any new
public building of any size, due to their early failure under fire conditions. Since
this early failure problem is endemic in any lightweight steel joist system
such as steel C joists, all lightweight steel joists should be banned from use
in floor construction.
I believe a survey and re-assessment of all
existing buildings which use long-span, steel bar joists should be conducted in
order to consider rebuilding them, using conventional methods.
I
support Mr. Malott’s and Chief Dunn’s suggestions about encasing columns and
beams in concrete or masonry for protection rather than using current
ineffective spray-on fire retarding material.
Full-scale furnace tests for ‘long-span’, I beam floor assemblies with sprayed-on “fireproofing” should be conducted to determine their actual fire rating (endurance time), and what the effect of removal of sections of fire insulation would have on the collapse resistance of such ‘long span’ steel girders or beams.
These
tests should include tests using dropped ceilings as HVAC return plenums for
the heated gasses, to determine the effect this configuration has on I beam and
Q deck supported, concrete floor failure.
Since
long span floors (including I beams supported floors) are inherently weaker
than short span floors, impact load tests to determine their progressive
collapse potential should be productive.
The
effect of an ordinary natural gas or smoke explosion on such long span floors
should be determined.
Since
it is impossible to evacuate a high-rise building rapidly, each floor in a high
rise building should be able to support the impact weight of several floors
collapsing from above; this in order to prevent a progressive collapse.
Columns
should be designed to support their loads even with the collapse of several
floors.
Any
and every critical element and its fire protection may be important in
maintaining the integrity of the entire building at a serious fire. Since this
situation is compounded as a building’s height and weight is increased, columns,
girders and beams and walls and floors should be strengthened and redundancy of
protective systems increased accordingly with an increased factor of safety to
take care of unexpected emergencies.
Evidently
the crashing plane parts or the fuel air explosion destroyed some of the wall
enclosures of the stairways and elevator shafts and cut off escape from above
by filling the stairways with debris and heated toxic smoke. The elevators were
also disabled due to shaft destruction and flaming jet fuel, pouring down the
shafts. Tests should be developed to determine whether the impact load of a
fuel vapor air explosion alone or of a hose stream could affect the integrity
of the “shaftwall” gypsum board, enclosing the stairways and elevator shafts.
If an ordinary natural gas or smoke explosion, or the impact of an interior or
exterior hose stream could affect the integrity of stairways or elevator shafts
than then this type of “shaftwall”gypsum board construction should not be
allowed for such use
in
any public building. As building heights increase more effective protection for
exit way enclosures such as reinforced masonry or concrete should be required
throughout.
Egress
pathways leading between stairways or to the outside should also be hardened to
preserve their integrity and continuity.
Scissor
stairs should be re-evaluated because of the possibility of both stairways
being affected by a disruption of the enclosure.
The
plane impacts apparently moved the buildings several feet, wracking the walls in
the central core thereby binding some exit doors in their frames. This suggests inadequate diagonal bracing
throughout the core areas of these buildings.
The
‘shaftwall’ and other drywall gypsum were dislodged in numerous places by the impact
loads or the building shifting. This suggests the means of attachment was
possibly inadequate.
Escaping
occupants had to reverse direction and go back up stairways in several
instances due to locked exit doors from the stairwells. I understand the need
for
security
but this suggests a total lack of understanding on management’s part of the
function of and critical need for availability of fire exit stairways in high
rise buildings. Automatic fail-safe door latches should be installed and
maintained throughout the stairways to unlock all exit-way doors in the event
of fire. If a stairway suddenly fills with smoke occupants should be able to
exit these stairways immediately and find other stairs or areas of refuge.
There seems to be a natural tendency for
people to flee up the stairways if the fire is below them; this has to be
discouraged since it is a most dangerous practice. Smoke and heated gasses
expand, produce pressure and become buoyant and rise up any available open
shafts, including stairways if their enclosures are breached, however roof doors in stairways are required
to be easily open able from the inside, in the City Codes, recognizing that
some people will attempt it.
Since
elevators frequently fail to provide adequate Fire Department response to the
floors of high-rise buildings, provisions for fire and smoke resistive, impact
protected, elevator shaft enclosures should be developed for Fire Dept. access
to upper floors and handicapped rescue from upper floors. Fire proof,
ventilated vestibules as presently used in the old ‘fire tower’ stairways could
be used. Ventilated ‘areas of refuge’ as elevator landing areas on each floor, could
be used in conjunction with fire rated elevator shaft doors. If we can ring an
entire, 16-acre, foundation area with 3-foot thick reinforced concrete 7
stories high to keep the
Full
sprinkler protection should be mandatory in all buildings over 6 stories or 75
feet in height, no matter what the building occupancy. We cannot always control
the amount and type of combustibles entering the buildings.
Sprinkler
systems should be separate from Standpipe systems since with combination
systems failure of either systems piping or supply will affect water supply to
the other system possibly leaving areas deficient or devoid of extinguishment
capabilities.
Since
hose stream effective coverage is limited at large area fires and sprinklers
are sometimes inactivated, the area of open floors should be limited by fire
containment walls which extend through any ceiling plenums to the floor above, in
order to keep fires to controllable size.
The
Fire Department should not have to carry hose up the stairs. Sufficient F.D.
specification hose should be available on each floor at the standpipe hose
outlets.
Central
air conditioning systems servicing many floors have shown time and again to
accelerate the fire and spread deadly smoke through out many floors in
high-rise fires. Chief Dunn’s recommendation that air conditioning systems
should cover only one or two floors should be implemented. Presently HVAC
systems using common open ceiling voids (plenums) to return air for
reconditioning
can hamper fire control and rescue problems and aggravate collapse problems by spreading
fire, heat and smoke to remote areas and heating structural steel within the
plenum. Perhaps air return systems could be designed using separate ducts to be
able to safely exhaust fire gases directly to the outside after sprinkler
extinguishment. Supply fans feeding air to fire area should be shut down
automatically to avoid accelerating the fire, and return fans also shut and
dampers closed until extinguishment to avoid lateral and vertical fire and
smoke spread.
I
am sure there will be many additional recommendations for Building Code
improvements, which will be gleaned, from the WTC catastrophe. History has proven
that a good Fire Prevention and Building Codes, knowledgeable people and strong
enforcement capabilities are absolutely necessary to build and maintain safe
buildings. Critical code sections should be protected from special interest changes.
Code changes allowing smoke detectors to substitute for full sprinkler coverage
in high rise buildings is a good example. The actual fire is the ultimate test
of construction practices and the
Bibliography;
Brannigan,
Francis L. - Building Construction for
the Fire Service first edition 1971, second
edition 1982 & third edition 1992, National Fire Protection Association –
Towers; HERA,
Darton,
Eric, Divided We Stand, A Biography
of New York’s
Dunn,
Vincent -Collapse of Burning Buildings, a
guide to fireground safety, Fire Engineering Books and Videos, a Division of
Penn Well Publishing Company, 1988
Dunn,
V “A Fire Chief’s Assessment”, http://vincentdunn.com/
Dunn,
V, Why Can’t the Fire Service Extinguish
fires in High-rise Buildings, Fire Engineering Dec. 1995, Penn Well
Corporation, 21-00 Route 208, South Fair Lawn, NJ, 07410-2602
Dunn,
V, Backdraft (Smoke) Explosions,
“With New York Firefighters” WNYF 3rd
issue 2000, FDNY, 9 Metro Tech Center
Fitzpatric, Tony, of Arup
Hamburger, Ronald. In “Structural engineer describes
collapse of the
Malott, Jim, Why the World Trade Center Collapsed: an architects assessment, Designer Builder Magazine, Kingsley Hammett Editor, 2405 Maclovia Lane, Santa Fe, NM 87505, www.designerbuildermagazine.com/
O’Hagan
John T., High Rise/Fire and Life Safety,
Dun-Donnelley Pub. Corp. N.Y C., NY 1977
World Trade Center Locked Stairwells/ Evacuation Problems Accumulating Problems On… © 2001 Mike Barkly, http://www.mjbarkl.com/locked.htm