The Construction of the WTC Towers 1 & 2
The first major structural subsystem was the exterior framing, which was a vertical square tube that consisted of 236 narrow columns, 59 on each face from the 10th floor to the 107th floor (Figure 3). There were fewer, wider-spaced columns below the 7th floor to accommodate doorways. There were also columns on alternate stories at each of the beveled corners, but these did not carry gravity loads. Each column on floor 10 to 107 was fabricated by welding four steel plates to form a tall box, nominally 0.36 m (14 in) on a side. The space between the steel columns was 0.66 m (26 in), with a framed plate glass window in each gap.
Adjacent columns were connected at each floor by steel spandrel plates, 1.3 m (52 in) high. The upper parts of the buildings had less wind load and building mass to support. Thus, on higher floors, the thickness of the steel plates making up the columns decreased, becoming as thin as 6 mm (¼ in) near the top down from as thick as 76 mm (3 in) at the lower floors. There were 10 grades of steel used for the columns and spandrels, with yield strengths ranging from 248 MPa (36 ksi) to 690 MPa (100 ksi). The grade of steel used in each location was dictated by the calculated stresses due to the gravity and wind loads. All the exterior columns and spandrels were prefabricated into welded panels, three stories tall and three columns wide. The panels, each numbered to identify its location in the tower, were then bolted to adjacent units to form the walls (Figure 4). Field panels were staggered so that every third panel was spliced at each floor level. The use of identically shaped prefabricated elements was an innovation that enabled rapid construction. (pg.7)
The second structural subsystem was a central service area, or core (Figure 3), measuring approximately 41 m by 26.5 m (135 ft by 87 ft), that extended virtually the full height of the building. The long axis of the core in WTC 1 was oriented in the east-west direction, while the long axis of the core in WTC 2 was oriented in the north-south direction. The 47 columns in this rectangular space were fabricated using primarily 248 MPa (36 ksi) and 290 MPa (42 ksi) steels and decreased in size at the higher stories. The four massive corner columns bore nearly one-fifth of the total gravity load on the core columns. The core columns were interconnected by a grid of conventional steel beams to support the core floors. (pg. 7&8)
Two types of concrete were used for the floors of the WTC towers: lightweight concrete in the tenant office areas and normal weight concrete in the core area and in the mechanical areas. Because of differences in composition and weight, the two types of concrete respond differently to elevated temperatures. While their tensile strengths degrade identically, lightweight concrete retains more of its compressive strength at higher temperatures.
The specified design strength for lightweight concrete was 20.7 MPa (3 ksi) and either 20.7 MPa (3 ksi) or 27.6 MPa (4 ksi) for normal-weight concrete, depending upon the floor location within the buildings. The actual strength of concrete at room temperature is greater than that measured from cylinders poured for testing during construction, referred to as 28-day cylinder strength, as concrete continues to strengthen with age. Methods for estimating changes in concrete strength with age are specified by the American Concrete Institute (ACI) 209 . The actual compressive strength of WTC concrete slabs was estimated to be 38 percent greater than the specified design strengths: 37.9 MPa (5.5 ksi) for 27.6 MPa (4 ksi) normal-weight concrete and 28.3 MPa (4.1 ksi) for 20.7 MPa (3 ksi ) normal-weight and lightweight concretes.
3.5. Passive Fire Protection
Building codes require that elements that support loads are to be protected to achieve a specified fire resistance rating, expressed in hours. The WTC towers were classified as Class 1B, which required the columns to have a 3 h fire endurance rating and the floor system to a have a 2 h rating when tested in accordance with ASTM E 119 . To achieve these ratings, the structural steel was protected with sprayed fire-resistive material (SFRM) or rigid fire-rated gypsum panels. Since application of SFRM to floor trusses was an innovative fire protection method in the 1960s, PANYNJ arranged for demonstrations to establish its feasibility for the World Trade Center . In 1969, The Port Authority directed that a 13 mm (0.5 in.) thick coating of SFRM be used to insulate the floor trusses. This was to achieve a Class 1A rating, even though the preponderance of evidence suggests that the towers were chosen to be Class 1B. NIST found no evidence of a technical basis for selection of the 13 mm (0.5 in.) thickness.
In 1995, The Port Authority performed a study to establish requirements for retrofit of sprayed insulation to the floor trusses during major alterations when tenants vacated spaces in the towers . Based on design information for fire ratings of a similar, but not identical, composite floor truss system contained in the Fire Resistance Directory published by Underwriters Laboratories, Inc., the study concluded that a 38 mm (1.5 in.) thickness of sprayed mineral fiber material would provide a 2 h fire rating, consistent with the Class 1B requirements.
In 1999, the removal of existing SFRM and the application of new material to this thickness became Port Authority policy for full floors undergoing new construction and renovation. In the years between 1995 and 2001, thermal protection was upgraded on 18 floors of WTC 1, including those on which the major fires occurred on September 11, 2001, and 13 floors of WTC 2 that did not include the fire floors.
Multiple approaches were used to insulate structural elements in the core. Those core
columns located in rentable and public spaces, closets, and mechanical shafts were enclosed in
boxes of gypsum wallboard. The amount of the gypsum enclosure in contact with the column
varied depending on the location of the column within the core. SFRM was applied on those
faces that were not protected by a gypsum enclosure. The thicknesses specified in the
construction documents were 35 mm (1.375 in.) for the heavier columns and 60 mm (2.375 in.)
for the lighter columns . Columns located at the elevator shafts were protected using the
same SFRM thicknesses.(pg. 18 & 19)
Possible Airplane Crashes
An additional load, not required by any building codes, but stated by PANYNJ to have been considered in the design of the towers, was the impact of a commercial airliner. Documents obtained from PANYNJ indicated that the impact of a Boeing 707 or DC 8 aircraft flying at a speed of 268 m/s (600 mph) was analyzed during the design stage of the WTC towers. The life safety considerations following such impact were also addressed. One document stated that “…Analysis indicates that such collision would result in only local damage which could not cause collapse or substantial damage to the building and would not endanger the lives and safety of occupants not in the immediate area of impact.” No other documentary evidence on the aircraft impact analysis was available to review the criteria and methods used in the analysis of the aircraft impact into the WTC towers or to provide details on the ability of the WTC towers to withstand such impacts . (pg. 6)
Twin Towers’ Designers Anticipated Jet Impacts Like September 11
Structural engineers who designed the Twin Towers carried out studies in the mid-1960s to determine how the buildings would fare if hit by large jetliners. In all cases the studies concluded that the Towers would survive the impacts and fires caused by the jetliners.
“We looked at every possible thing we could think of that could happen to the buildings, even to the extent of an airplane hitting the side… Our analysis indicated the biggest problem would be the fact that all the fuel (from the airplane) would dump into the building. There would be a horrendous fire. A lot of people would be killed. [But] the building structure would still be there.”~Lead WTC Structural Engineer John Skilling — The Seattle Times: (1993)
“The building was designed to have a fully loaded 707 crash into it. That was the largest plane at the time. I believe that the building probably could sustain multiple impacts of jetliners because this structure is like the mosquito netting on your screen door—this intense grid—and the jet plane is just a pencil puncturing that screen netting. It really does nothing to the screen netting.”~Frank A. Demartini, the on-site construction manager for the World Trade Center — (January 25, 2001)
In addition, investigators from NIST who examined the destruction of the WTC skyscrapers told The New York Times in 2007 that newly disclosed documents from the 1960s show that the new York Port Authority, the original owners of Twin Towers, also considered aircraft moving at 600 mph,slightly faster and therefore more destructive than the ones that did hit the towers.
Above a photo taken just moments after the “collapse” began. As is obviousthe huge plumes of pulverized concrete and steel beems, as well as full sections of box columns are being ejected some 70+ yards laterally from the frame of the buildings. It only took 10 seconds for the entire to collapse to the ground. This is practically ‘free fall’.
Upper stories tipping mere seconds into collapse initiation.
Cardington Steel Fire Experiments
“In the mid-1990s British Steel and the Building Research Establishment performed a series of six experiments at Cardington to investigate the behavior of steel-framed buildings. These experiments were conducted in a simulated, eight-story building. Secondary steel beams were not protected. Despite the temperature of the steel beams reaching 800-900 C (1,500-1,700 F) in three of the tests (well above the traditionally assumed critical temperature of 600 C (1,100 F), no collapse was observed in any of the six experiments).
In building fires outside of such laboratory experiments, steel beams and columns probably never exceed 500º C. In extensive fire tests of steel frame carparks conducted by Corus Construction in several countries, measured temperatures of the steel columns and beams, including in uninsulated structures, never exceeded 360ºC.”
The following information is from the most comprehensive and thorough tests yet done on the effects of fire on structural steel.
This study was cited in the NIST WTC Structure Report of 2002, in Appendix A, of that report. The same NIST report wherein the Appendix C was cited, that showed evidence of Thermite Arson on WTC7 steel samples.
The Cardington steel tests show why no multi-story steel framed building has EVER collapsed due to fire; not before 9/11, not after 9/11, and certainly not on 9/11.
THE BEHAVIOUR OF MULTI-STORY STEEL FRAMED BUILDINGS IN FIRE
© 1999 British Steel Technology Center
Cardington Steel Tests
9.1 Observed Behavior
The tests on the Cardington frame enabled the differences between the standard fire test and the actual behavior to be studied. This led to the conclusion that the cooler structure surrounding the heat effected members was extremely beneficial to the heated members. The maximum steel temperatures reached during the six tests at Cartington was in excess of 1100 °C. This occurred with no signs of structural collapse.
Using modern fire codes , which are based on the standard fire tests, failure (or structural collapse) was calculated at 680°C ; it is clear that the current level of safety is quite high.
Further work on quantifying the safety level and developing definitive design guidance, which will incorporate a more logical and economical approach to structural fire design. – pg. 64 text (pg. 73 PDF)
Every collapse theory generated by those who support the official narrative of the destruction of the WTC towers is based on the “accumulation of mass” hypothesis.
This thesis is utter nonsense, as I have pointed out time after time.
There was no accumulating solid mass.
The mass was particulates blown laterally outside of the frame of the buildings. This can be seen clearly in every single image of the towers being demolished.
FAQ #9: Were the Twin Towers Designed to Survive the Impact of the Airplanes?
When interviewed in 1993, Lead WTC Structural Engineer John Skilling told The Seattle Times:
Lead WTC Structural Engineer John Skilling was rightfully confident that neither the impact of a large passenger jet nor the ensuing office fires was capable of bringing down the Twin Towers
“We looked at every possible thing we could think of that could happen to the buildings, even to the extent of an airplane hitting the side… Our analysis indicated the biggest problem would be the fact that all the fuel (from the airplane) would dump into the building. There would be a horrendous fire. A lot of people would be killed. [But] the building structure would still be there.”
In 2001, Leslie Robertson, a WTC structural engineer who worked as a subordinate to Skilling, claimed that the Twin Towers were only able to withstand the impact of jet airplanes going no faster than 180 mph. However, not only are these statements contradicted by the design test results, they also contradict statements made by Robertson in 1984/1985, when he said that there was “little likelihood of a collapse no matter how the building was attacked.”
Critical Thinking – The 911 Conspiracy Discussion
“Since the dynamic amplification factor for a suddenly applied load is 2, an intact floor below the level of collapse initiation could not have supported more than six floors. Since the number of floors above the level where the collapse initiated exceeded six for both towers (12 for WTC 1 and 29 for WTC 2), neither tower could have arrested the progression of collapse once collapse initiated. In reality, the highest intact floor was about three (WTC 2) to six (WTC 1) floors below the level of collapse initiation. Thus, more than the 12 to 29 floors reported above actually loaded the intact floor suddenly.” – NIST FAQ 12
. . . . . .
“Thus, more than the 12 to 29 floors reported above actually loaded the intact floor suddenly.”~NIST
Nonsense, they would have impacted the ENTIRE STRUCTURE BELOW suddenly. There was not just ONE intact floor below the presumed falling mass. The entire structure below was intact — some 81 intact floors for one tower, and 98 intact floors for the other.
This is a perfect illustration of why Newton’s Third Law of Motion proves the gravity collapse hypothesis wrong.
These facts are self-evident and axiomatic.
NIST and its booster club is trying to pull the wool over your eyes with their specious rhetorical nonsense.
The ONLY way to remove the intact structure from any presumed falling mass would be to blow them apart with explosives.
Evidence of nanothermetic explosives were found in the dust of the aftermath.
There can be no doubt that the WTC towers were destroyed by explosive demolitions.
The floor connections were blown apart by explosives. Period.
. . . . .
Now when I describe what WOULD HAVE HAPPENED if the upper structure fell on the lower structure, as the meeting portions grinding and crushing each other in an equal and opposite fashion due to Newton’s 3rd Law, I am NOT saying what DID happen. I am merely pointing out that the 3rd law of motion applied to the hypothesis of a gravity collapse, proves that the collapse would be arrested long before it became total.
The theory that jet fuel poured down the elevator shafts is untenable.
The elevators in the towers were staggered, with just one going from top to bottom. They were also self sealing to prevent the chimney effect. See FEMA Building Performance Study 2002
The amount of jet fuel that poured into the buildings during the crashes was miniscule compared to the size the the structures. Most of that fuel blew up in huge fireballs outside the buildings on impact.
If the towers were made to scale as 6 ft structures, the amount of jet fuel in the plane at that scale would be less than a thimble full.
The towers were essentially three separate buildings atop one another.