Los Angeles Fire Department
Historical Archive

    September 11, 2001
    World Trade Center
    New York, New York

World Trade Center September 11, 2001 Photo by: Steve Spak

    Why the World Trade Center Buildings Collapsed
A Fire 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 seconds.    You 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

In 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. 

Steel Framing

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 degrees F.

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

A 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 lower floors.

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.

Concrete removal

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 twisted steel.

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:

  1. Failure 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

  2. Poor or uneven application of the spray-on fire retarding was discovered during post fire investigations

  3. Variation of spray-on material during manufacture makes it ineffective

  4. Lack of thoroughness in covering the steel during application is a problem

  5. Failure to replace spray-on material dislodged by other trades people performing work around the steel during the construction of the building.

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.

Recommendations for constructing the new high rise buildings on ground zero

  • The 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.

  • Lightweight 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 exposure.

  •  For life safety in high-rise buildings bring back the smoke proof tower. This allows people to escape fire using smoke free stairways.

  •  Stairs and elevator shaft ways should be enclosed in masonry to prevent smoke spread.

  • Heating 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.

  • The 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.

  • Increase 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.

  • Automatic 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.

  • Federal, 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.

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