Wednesday, April 21, 2010

911-Construction of the buildings

Collapsing Buildings

The New Phenomenon of Steel Buildings Crushing Themselves

The seven World Trade Center buildings were in the two blue regions of the map to the right. On September 11th, all 7 buildings were destroyed. 110-story WTC 1 and 2, and 47-story WTC 7 were leveled, and WTC 3, 4, 5, and 6 were severely damaged, with large portions of WTC 3, 4, and 6 being crushed.

It is interesting that, in spite of the thorough devastation of all the buildings with World Trade Center addresses, no large buildings outside the blue zones were sufficiently damaged to warrant their demolition in the months that followed the attack. ¹ The "collapse" of Building 7 was remarkably tidy. Although it was wedged between the Verizon building and the U.S. Post Office building, it barely damaged either of them.

It is even more interesting that, prior to September 11th, no steel framed building had ever undergone total collapse due to any cause or combination of causes other than controlled demolition and severe earthquakes. Such buildings have survived hurricanes, severe fires,earthquakes, and bombings, but none have fallen down of their own weight -- an event that was portrayed as inevitable on September 11th.

Of the collapses of the three huge buildings on September 11th, the collapse of Building 7 is treated separately. If you believe the official story, then Building 7 would be the only example in history of a steel framed building undergoing complete collapse as a result of fires. It and the Twin Towers would be the only permanent vertical steel structures to have fallen into themselves instead of falling over, except in cases of demolition.

The collapses of the Twin Towers are analyzed in much more detail than Building 7 because there is more evidence, and because, whereas Building 7's fall looked like a standard building demolition, the disintegrations of the Twin Towers had many features that are not easily explained.

Collapse Features

Characteristics of the Twin Towers' Destruction and What They Show

The total destructions of the two towers were almost identical. The most apparent difference is that the top of the South Tower tipped for a few seconds before falling, whereas the top of the North Tower telescoped straight down from the start. Here are some of the principal characteristics of the destructions, based on study of the surviving evidence.

The cores were obliterated. There is no gravity collapse scenario that can account for the complete leveling of the massive columns of the towers' cores.
The perimeter walls were shredded. No gravity collapse scenario can account for the ripping apart of the three-column by three-floor prefabricated column and spandrel plate units along their welds.
Nearly all the concrete was pulverized in the air, so finely that it blanketed parts of Lower Manhattan with inches of dust. In a gravity collapse, there would not have been enough energy to pulverize the concrete until it hit the ground, if then.
The towers exploded into immense clouds of dust, which were several times the original volumes of the buildings by the time their disintegration reached the ground.
Parts of the towers were thrown 500 feet laterally. The downward forces of a gravity collapse cannot account for the energetic lateral ejection of pieces.
Explosive events were visible before many floors had collapsed. Since overpressures are the only possible explanations for the explosive dust plumes emerging from the buildings, the top would have to be falling to produce them in a gravity collapse. But in the South Tower collapse, energetic dust ejections are first seen while the top is only slightly tipping, not falling.
The towers' tops mushroomed into thick dust clouds much larger than the original volumes of the buildings. Without the addition of large sources of pressure beyond the collapse itself, the falling building and its debris should have occupied about the same volume as the intact building.
Explosive ejections of dust, known as squibs, occurred well below the mushrooming region in both of the tower collapses. A gravitational collapse explanation would account for these as dust from floors pancaking well down into the tower's intact region. But if the floors -- the only major non-steel building component -- were falling well below the mushrooming cloud above, what was the source of the dense powder in the cloud?
The halting of rotation of the South Tower's top as it began its fall can only be explained by its breakup.
The curves of the perimeter wall edges of the South Tower about 2 seconds into its "collapse" show that many stories above the crash zone have been shattered.
The tops fell at near the rate of free fall. The rates of fall indicate that nearly all resistance to the downward acceleration of the tops had been eliminated ahead of them. The forms of resistance, had the collapses been gravity-driven, would include: the destruction of the structural integrity of each story; the pulverization of the concrete in the floor slabs of each story, and other non-metallic objects; and the acceleration of the remains of each story encountered either outward or downward. There would have to be enough energy to overcome all of these forms of resistance and do it rapidly enough to keep up with the near free-fall acceleration of the top

This image from the documentary Up From Zero shows the base of a core column, whose dimensions, minus the four flanges, are apparently 52 by 22 inches, with walls at least 5 inches thick.

This photograph from Ground Zero is apparently of one of the smaller core columns connected to a set of I-beams.

The Core Structures

The Structural System of the Twin Towers

Each tower was supported by a structural core extending from its bedrock foundation to its roof. The cores were rectangular pillars with numerous large columns and girders, measuring 87 feet by 133 feet. The core structures housed the elevators, stairs, and other services. The cores had their own flooring systems, which were structurally independent of the floor diaphragms that spanned the space between the cores and the perimeter walls. The core structures, like the perimeter wall structures, were 100 percent steel-framed.

The exact dimensions, arrangement, and number of the core columns remained somewhat mysterious until the publication of a leaked collection of detailed architectural drawings of the North Tower in 2007. Although the drawings show the dimensions and arrangement of core columns, they do not show other engineering details such as the core floor framing. It is clear from photographs, such as the one on the right, that the core columns were abundantly cross-braced.

Core Denial

Establishing the true nature of the core structures is of great importance given that the most widely read document on the World Trade Center attack -- the 9/11 Commission Report -- denies their very existence, claiming the towers' cores were "hollow steel shaft[s]:"

For the dimensions, see FEMA report, "World Trade Center Building Performance Study," undated. In addition, the outside of each tower was covered by a frame of 14-inch-wide steel columns; the centers of the steel columns were 40 inches apart. These exterior walls bore most of the weight of the building. The interior core of the buildings was a hollow steel shaft, in which elevators and stairwells were grouped. Ibid. For stairwells and elevators, see Port Authority response to Commission interrogatory, May 2004. ¹

The top illustration indicates what may have been typical dimensions and thickness of the smaller core columns, about half-way up the tower. The outermost rows of core columns were apparently considerably larger, measuring 54 inches wide.

Columns

The core columns were steel box-columns that were continuous for their entire height, going from their bedrock anchors in the sub-basements to near the towers' tops, where they transitioned to H-beams. Apparently the box columns, more than 1000 feet long, were built as the towers rose by welding together sections several stories tall. The sections were fabricated by mills in Japan that were uniquely equipped to produce the large pieces. 2

Some of the core columns apparently had outside dimensions of 36 inches by 16 inches. Others had larger dimensions, measuring 52 inches by 22 inches. ³ The core columns were oriented so that their longer dimensions were perpendicular to the core structures' longer, 133-foot-wide sides. Construction photographs found at the Skyscraper Museum in New York City indicate that the outermost rows of core columns on the cores' longer sides were of the larger dimensions. Both the FEMA's World Trade Center Building Performance Study and the NIST's Draft Report on the Twin Towers fail to disclose the dimensions of the core columns, and the NIST Report implies that only the four core columns on each core's corners had larger dimensions.

Like the perimeter columns -- and like steel columns in all tall buildings -- the thickness of the steel in the core columns tapered from bottom to top. Near the bottoms of the towers the steel was four inches thick, whereas near the tops it may have been as little as 1/4th inch thick. The top figure in the illustration to the right is a cross-section of one of the smaller core columns from about half-way up a tower, where the steel was about two inches thick. The bottom figure shows the base of one of the larger core columns, where the steel was five inches thick. The bases of the columns also had slabs of steel running through their centers, making them almost solid.

Column Arrangement

The exact arrangement of the columns and how they were cross-braced is not apparent from public documents such as FEMA's World Trade Center Building Performance Study. The arrangement of box columns depicted in Figure 2-10 of Chapter 2 (pictured to the right) seems plausible, even though it contradicts other illustrations in the report showing a more random arrangement. It depicts the top floors of a tower and does not indicate the widths of the columns on a typical floor.

Cross-Bracing

Construction photographs show that the core columns were connected to each other at each floor by large square girders and I-beams about two feet deep. The debris photograph below shows what appears to be one of the smaller core columns surrounded by perpendicular I-beams approximately three feet deep. In addition, the tops of core structures were further connected by the sloping beams of the hat truss structures.

The Perimeter Walls

The Structural System of the Twin Towers

The towers' perimeter walls comprised dense grids of vertical steel columns and horizontal spandrel plates. These, along with the core structures, supported the towers. In addition to supporting gravity loads, the perimeter walls stiffened the Towers against lateral loads, particularly those due to winds. The fact that these structures were on the exterior of the Towers made them particularly efficient at carrying lateral loads. Richard Roth, speaking on behalf of the architectural firm that designed the Towers, described each of the perimeter walls as essentially "a steel beam 209' deep." ¹ Regardless, it is clear that the core structures were designed to support several times the weight of each tower by themselves.

As the diagram and photograph illustrate, the perimeter wall structures were assembled from pre-fabricated units consisting of 3 column sections and 3 spandrel plate sections welded together. Adjacent units were bolted together: column sections were bolted to adjacent columns above and below, and spandrel plate sections were mated with adjacent sections on either side with numerous bolts.

There were 59 perimeter columns on each face of the towers, and one column on each corner bevel, making a total of 240 perimeter columns in each tower.

Like the core columns, the thickness of the perimeter columns tapered from the bottom to the top of the towers. The illustrated cross-sections represent columns near the top, and near the mid-section of the towers.

This diagram shows horizontal sections of the Twin Towers' perimeter columns. The leftmost figure shows a section of a column, its enveloping insulation, and the aluminum cladding with window frame conections. The left and middle figures show sections of a column near a tower's tops, where the steel was thinnest. The rightmost figure shows section of a column in the lower part of a tower, where the steel was much thicker.

References

1. City in the Sky, Times Books, Henry Hold and Company, LLC, , page 134-136

The Floors

The Structural System of the Twin Towers

This illustration from FEMA's report shows a section of the flooring system. The main double trusses, of which two are pictured, are perpendicular to the view plane.

The floors of the Twin Towers completed the structural system whose main elements were the core structures and the perimeter walls. The floor diaphragms were annular structures that spanned the distance between the core structures and the perimeter walls, providing large expanses of uninterrupted floor space. The cores had their own flooring systems, which were structurally independent of the surrounding floor diaphragms.

The floor diaphragms consisted of lightweight concrete slabs poured onto corrugated steel pans, which were supported by trusses. Primary double trusses were interwoven with transverse secondary trusses -- a fact ignored by the truss failure theory. The primary trusses were 900 mm deep, and spaced on 2.04 m centers.

The 10 cm thick concrete slabs were apparently a lightweight form of concrete typically used in high-rises. Its density and exact composition remain unknown, but such lightweight concrete is typically 60% as dense as concrete used in roads and sidewalks. The floors were the only major part of these mostly steel buildings that contained concrete.

Some Floors Had More Than Trusses

FEMA's report implies that the floor diaphragms were supported only by the webbed trusses described here. It gives no indication of other structures that may have helped transfer the substantial lateral forces due to wind loading between the perimeter walls and core structures, and it provides no detail on the flooring system in the towers' cores, which were apparently supported by heavy steel I-beams. This idea that all the floors were undergirded only by trusses is a prerequisite to the truss-failure theory, which blames a chain-reaction of failures of the trusses for the building collapses. There is evidence, however, that certain floors had solid steel-frame support structures rather than light open trusses, such as the following passage from the Engineering News-Record:

On the 41st and 42nd floors, both towers will house mechanical equipment. To accommodate the heavy loads, the floors are designed as structural steel frame slabs. All other floors from the ninth to the top (except for 75 and 76, which will also carry mechanical equipment) have typical truss floor joists and steel decking. ¹

This illustration from Multi-Storey Buildings in Steel gives a three-dimensional view of a floor section.

Truss-to-Column Connections

Most variants of the truss-failure theory blame the detachment of the truss ends from the perimeter walls for the pancaking of floors, and eventual total collapse of the buildings. It has been asserted that only one or two bolts connected the truss ends to the perimeter walls. However, according to both of the official government reports, the truss ends rested on steel plates that were both welded and bolted to the top chords of the trusses and attached via bolted damping units to their lower bottom chords.

This illustration from FEMA's report shows details of truss-to-column connections.

References

1. World's Tallest Towers Begin to Show Themselves on New York City Skyline, Engineering News-Record, 1/1/70

The Hat Trusses

The Structural System of the Twin Towers

This photograph shows the top of the hat truss of one of the towers during its construction.

The fourth primary structural subsystem in each tower was the hat truss -- a lattice of large diagonal I-beams that connected the perimeter walls to the core structure between the 107th floor and roof. This structure was also known as the "outrigger truss system."

The hat truss structure strengthened the core structure, unified the core and perimeter structures, and helped to support the large antenna mounted atop the North Tower. The hat truss, which contained both horizontal and sloping I-beams, connected core columns to each other, and connected the core to the perimeter walls. Most the beams connected core columns to each other, while a set of sixteen horizontal and sloping beams spanned the distance the core and perimeter walls. Eight of these, the outrigger trusses, connected the corners of the core to the perimeter walls, while another eight connected the centers of the core's periphery to the perimeter walls.

The hat trusses are central to the "probable collapse sequence" described by NIST's Final Report on the Twin Towers. It blames the hat truss for transferring "column instability" between the core structures and the perimeter walls. In other words, it asserts that reinforcing structures caused the Towers to self-destruct. Its section entitled "Results of Global Analysis" describes the "structural deterioration" of the North Tower as follows:

This schematic from Chapter 2 of the FEMA Report provides some detail on the geometry of the hat truss.

6.14.2 Results of Global Analysis of WTC 1

After the aircraft impact, gravity loads that were previously carried by severed columns were redistributed to other columns. The north wall lost about 7 percent of its loads after impact. Most of the load was transferred by the hat truss, and the rest was redistributed to the adjacent exterior walls by spandrels. Due to the impact damage and the tilting of the building to the north after impact, the south wall also lost gravity load, and about 7 percent was transferred by the hat truss. As a result, the east and west walls and the core gained the redistributed loads through the hat truss.

Structural steel expands when heated. In the early stages of the fire, structural temperatures in the core rose, and the resulting thermal expansion of the core was greater than the thermal expansion of the (cooler) exterior walls. About 20 min. after the aircraft impact, the difference in the thermal expansion between the core and exterior walls, which was resisted by the hat truss, caused the core column loads to increase. As the fires continued to heat the core areas without insulation, the columns were thermally weakened and shortened and began to transfer their loads to the exterior walls through the hat truss until the south wall started to bow inward. At about 100 min, approximately 20 percent of the core loads were transferred by the hat truss to the exterior walls due to thermal weakening of the core; the north and south walls each gained about 10 percent more loads, and the east and west walls each gained about 25 percent higher loads. Since the hat truss outriggers to the east and west walls were stiffer than the outriggers to the north and south walls, they transferred more loads to the east and west exterior walls.

The inward bowing of the south wall caused failure of exterior column splices and spandrels, and these columns became unstable. The instability spread horizontally across the entire south face. The south wall, now unable to bear its gravity loads, redistributed these loads to the thermally weakened core through the hat truss and to the east and west walls through the spandrels. The building section above the impact zone began tilting to the south as the columns on the east and west walls rapidly became unable to carry the increased loads. This further increased the gravity loads on the core columns. Once the upper building section began to move downwards, the weakened structure in the impact and fire zone was not able to absorb the tremendous energy of the falling building section and global collapse ensued.
¹
[emphasis added]

This illustration from Page 11 of NIST's Final Report of the National Construction Safety Team on the Collapses of the World Trade Center Towers (DRAFT) shows the hat truss.

References

1. Final Report of the National Construction Safety Team on the Collapses of the World Trade Center Towers,nist.gov, , page 144-5 (PDF pages 194-5)

The Bathtub

The Deep Basement Containing the Twin Towers' Foundations

The bathtub contained the footprints of the Twin Towers and Buildings 3 and 6.

About half of the superblock that the World Trade Center occupied contained a deep basement, the so-called bathtub. It was a skewed rectangle with sides about 980 and 520 feet, and a depth of about seven stories. ¹

The bathtub is the 9-block area of the World Trade Center site that is excavated down to bedrock and hard soils and ringed by the slurry wall. The bathtub was created to enable the building of the Twin Towers' foundations, and was ultimately filled with seven stories of basements housing the parking garage, mall, and building services. Since the ground water level at the World Trade Center site was just a few feet below the surface, while bedrock was about 70 feet below the surface, creating the bathtub required first building a 7-story dam below the water level of the adjacent Hudson River -- the slurry wall. Ground excavated to make the bathtub was deposited west of West Street to make the land on which the World Financial Center and Battery Park now stand: 1,000,000 cubic yards moved in 100,000 truck loads filled a 700- by 1400-foot area, creating $90,000,000 worth of land. ²

Four of the World Trade Center buildings -- the Towers and Buildings 3 and 6 -- rested on foundations entirely within the bathtub.

This illustration shows a cross-section of the bathtub tall. The diagonal lines represent the post-tensioned tie-backs: cables that resist pressure pushing inward on the bathtub.

Construction

Essentially, the bathtub was built underground and underwater. Construction crews would dig a section of trench three feet wide and down to bedrock level. As the trench was dug, it was filled with a bentonite slurry, which prevented the inflow of river water by filling the trench and sealing the adjacent soil. Once the section of trench was completely excavated, a pre-fabricated steel framework, weighing 22 tons, was lowered into it. Then concrete was poured through a pipe into the bottom of the trench while the lighter bentonite slurry was sucked from the top. The entire wall was built using 152 22-foot-long sections.

Each steel framework contained a set of guides for tiebacks -- cables that would pass through the walls and secure them to the ground outside the bathtub. The tiebacks were installed as the material inside of the bathtub was excavated, preventing the pressure of the soil and groundwater outside the bathtub from pushing the walls inward. The sub basement structures, once constructed, provided additional bracing of the walls.