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The Core Structure Of The World Trade Center Towers 

Was A Steel Reinforced, Cast Concrete, Tubular Core.

Some in the UK still think the WTC tower core was built as shown below. Basically a pre-stressed concrete design. Yamasaki had reviewed the design, and found no contractor that could build a 1,300 foot column of that design. We all know the towers had their stairwells and elevators inside the core. There is no room for that in the core below.

design steel tower core wind

The design was a "tube in a tube" construction where the steel reinforced, cast concrete interior tube, was surrounded with a structural steel framework configured as another tube with the load bearing capacity bias towards the perimeter wall with the core acting to reduce deformation of the steel structure maximizing its load bearing capacity. All steel structures with the proportions of the WTC towers have inherent problems with flex and torsion. Distribution of gravity loads was; perimeter walls 50%, interior core columns 30% core 20%.

Steel, no matter what temper, no matter what bracing is used, ends up with an overall flexation that exceeds design parameters for deformations and failures occur. These were fact I learned from a documentary in 1990 about the construction of the north tower. Yamsaki's decision making process was outlined and rejected core designs identified.

Both the WTC 1 & WTC 2 towers had a rectangular cast concrete core structure formed into rectangular cells that had elevators and stairways in them.

I've met numerous people that remember the early and very technical 2 hour show. One was a civil engineer who remembers the documentary aired on another cable educational channel in 1995 and remembers the concrete core as they are shown below and labeled "ACTUAL CORE OF WTC TOWERS", but as yet, is not willing to provide a declaration certifying such. The documentary showed us the true concrete core structure of the trade center towers. It aired in 2 segments on consecutive nights. I missed the first 20 minutes of the second one, otherwise I watched it all. Yamasaki's design for a torsion resistant core structure made from non flexible material, steel reinforced cast concrete, won a competition in strength with several others. All steel towers failed high winds because the steel perimeter columns could take the weight but were prone to flexing and the twisting.

The documentary focused on the concrete core because the construction of the core was a big slowdown and substantial challenge.

The Concrete Core And Its Hallways

Below on the left is WTC 1 at sunrise. The view is not looking down the hallways, we look nearly along the long axis of the towers core. The vertical line of light in the lower segment is created by sunlight reflecting off the inner core walls then shining out the core hallway.

The North Tower had a core oriented east and west. The camera perspective is not aligned with the hallway as can be seen by the orientation of the south towers roof indicating an oblique view. The light is reflected off the inner south shear wall at a hallway level where there is no doorway interrupting. Notice the very slight interruptions, dark streaks, whole dark floors. The nature of light under these conditions is to blend, blur and obscure solid areas between the light.

We see no light on the left side because the doorways on the north face do not align with the doors on the east. Above that, or the top sky lobby floor, the top floors had a different scheme with some halls on one floor crossing both directions.


The core of the south tower above is oriented north south with its long axis and had 2 halls crossing the short axis. We see no light through it because the doors on our face only reveal a shadowed inner concrete wall corner. See the 2 vertical, central lines in the image below.

Below: Tower on right, the north tower. The interior box columns followed the slight taper of the concrete core to a point then had to continue vertically plumb to the roof as the interior wall of the outside tube of the steel framework. The purpose of this section and photo is to show the space between the interior box columns and the tapering core face at the upper floors. The north tower had hallways crossing perpendicularly every other floor (diagram below). This picture of the towers is looking due south through the towers.


Above: The north tower core was oriented east west, so we are looking at the wide side. On the right tower fr then project that dddistance down to a cross section. We see, from right to left; a light space from the out side to a dark column which represents the floor space to the interior box column, then there is another narrow light space left of that. That is the space between the interior box column and the concrete core face. Going leftward we see the facing concrete shear wall, then the hallway crossing the narrow axis, then the core face again, then the space between the east core face, then interior box column, then floor space to the east side of the building.

Below I've crudely altered an earlier FEMA core diagram to show how the concrete core, interior walls and hallways were configured through the entire height of the north towers "tube in a tube" construction.

Interior walls of the core were not continuos vertically, they were interrupted by hallways perpendicularly opposed with each floor. Doorways appeared on each floor on every face every other floor. The tops of the interior walls of the concrete core served as the support for the steel interior concrete forms that had to be disassembled and lifted 40 feet to be set for the next pour. Exterior forms were plywood.

The hallway/door scheme was changed higher up. Also, the south tower was different, 2 hallways crossed the short axis of the core, perhaps with one perpendicular.

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North Tower Core

WTC concrete core structure


What is often referred to as the "cores" in raw photo evidence, and what the photos really show.

Regarding helicopter photos referred to as pictures of core columns at upper phases:

What you see in the photos of tower construction within the official story are the kangaroo cranes used to move material, the interior forms, and elevator guide rails, or to position the rebar hanging into the concrete pour. The steel framework was built up to 7 floors over the top of the concrete core being constructed inside of the steel frame obscuring the core construction from view. Other photos when the construction is lower show elevator guide rails. These are being mis identified as "core columns" on some web sites.

Photos and the tower construction.

Before another core tier could be formed, the elevator guide rails had to be lowered and set in place to a level 2 floors lower than the top of the present concrete pour. They are what is shown in the diagram at the top of this page, the FEMA core. The guide rails are presented as multiple, narrow rectangular tubes that supposedly ran full length for the tower.

rebar steel WTC concrete core structure

Photos at ground zero.

There is a photo showing the bottom basement foundation level where various columns, cut off, protrude from the concrete bottom. There is a workman near center in the photo wearing brown coveralls, firefighters are in the foreground. These columns are often referred to as "core columns". The columns that rise up from inside the concrete channels forming the interior base of the concrete core are mostly elevator guide rails. To the right of the workman in the background is an interior box column that has a fresh torch cut, at an angle, with slag hanging from the cut. Its dimensions, proportions and thickness are different. The photo is looking north through the line of the north perimeter wall of WTC 1 from just west or perhaps still inside the tower core footprint.


If there were the full length steel core columns shown in the FEMA diagram, where are they in the images below that show the concrete tubular core of WTC 2 standing momentarily, half fallen, without the outer steel framework? Where are they in the photos following that?




The core shown on the FEMA site will not leave a spire standing like we see below and no visible multiple, heavy steel core columns are seen in photos. No part of those cores could have this appearance as it is halfway to the ground Above.

I've notated available images to show what the core I saw being built in the documentary was.



Below is an image of the top portion of WTC 2 as it falls and is about to hit WTC 3. I would expect to see doorways, but do not, and assume that the top 2 floors of the concrete core were different structurally than those below where a number of floors had hallways crossing in both direction.


The pink arrow points to the core. The brownish gray well lit roof and above wall face left are the core. A basic cube shape inside the perimeter walls descending with it.

Below I'll attempt to explain and identify the building elements as the are partially seen inverted and being ripped apart by the free fall descent of a portion of the steel reinforced tubular cast concrete core.

The fine red line approximates the top edge of the tower faces and what was the horizontal plane of the top of the tower. The fine circular blue line shows the counter clockwise rotation of a group of panels of perimeter box columns that are fastened by floor beams near the face corner that was below the top by the length of the face corner.
WTC core tube collapse
The fine green line shows the approximate exterior face corner of the building and the heavy green arrow points to the edge of the perimeter box column panel that was aligned with the fine green line.


Below is image of a spire that stood for 14 seconds comprised of one corner of the interior box columns that were fastened to the outside of the concrete core. This connection between interior columns and the concrete made the core a load bearing and anti torsion element for the steel framework configured as a tube around the concrete tube comprised of four smaller vertically interrupted tubular elements formed by the interior core walls.

WTC construction photo

Below is a photo of the spire from the opposite side. The concrete shear wall of the core and interior walls interrupted by hallways can just be seen.


Below is a zoomed image of the spire with arrows indicating the structural elements.

core wall image

Following is a photo that shows the face of the concrete shear wall behind interior box columns.


The spire, a composite of an interior box column and a sliver of intact steel reinforced concrete shear wall of the core, fell all at once.

WTC core dust spire

And left the rebar of the core, delaminated from the concrete, standing for a few short seconds as seen below.



The below photo has a comb like shape of the remnant of the reinforcing bars of the core, briefly standing. The FEMA core cannot leave this shape. There are too many elements and they are too small. The box columns in the spire image above are about the same size as the steel columns the core was supposedly made from. There were 47 and they were hand fabricated in 40 foot sections then 100% welded in place. The below photo was taken at approximately the same distance.

Below are a long row of 3" rebar on 4 foot centers seen at approximately 7500 feet.

The slope to the top of the rebar was mentioned in the documentary. The engineers specified that the concrete pours not terminate with level opposing joints across the tower to maximize the strength of the tubular concrete tower core as an anti torsion element. Also the rebar was to be welded in series of butt joints in a slope across the walls of the core. Slopes ran in opposite directions on opposite sides maximizing torsion resistance of the tube. The slope of the tops of the rebar in the photo below show this.

WTC core dust spire

What you have seen in the photo near the top of the page and again below, is the tops of 3" rebar, special high tensile steel, that has broken at a series of welds. The slope of the tops of the steel bars is created because engineers did not want a series of weld joints on the vertical bars horizontally creating a possible line of fracture across the building.

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The below image shows the interior box columns and a stair well sandwiching the thick base of the core. High tensile steel rebar protrudes from the top of the cast concrete.

WTC core dust spire


This is a link to a very large image, same as below, where huge quantities of sand and gravel which all has the color of cement is seen. The official tower core design would produce HALF this much sand with NO gravel at all.

WTC core chris brown image


History And The Twin Towers Concrete Core

Oxford University in 1992 published this on the WTC concrete cores.

World Trade image oxford arch core collapse


What follows are the statements of various architects and engineers regarding the concrete core.

Leslie Robertson, Architect Of The World Trade Center Towers

Still, Robertson, whose firm is responsible for three of the six tallest buildings in the world, feels a sense of pride that the massive towers, supported by a steel-tube exoskeleton and a reinforced concrete core, held up as well as they did—managing to stand for over an hour despite direct hits from two massive commercial jetliners.

Says engineer Robertson, “If they had fallen down immediately, the death counts would have been unimaginable,” he says. “The World Trade Center has performed admirably, and everyone involved in the project should be proud.” The buildings were designed specifically to withstand the impact of a Boeing 707, the largest plane flying in 1966, the year they broke ground on the project.


A Description of the World Trade Center

The twin towers of the World Trade Center were essentially two tubes, with the north tower (1,368 feet) six feet taller than the south tower (1,362 feet), and each were 110 stories tall. Each tube contained a concrete core, which supported only the load of the central bank of elevators and stairwells (Snoonian and Czarnecki 23).


NOTE: This page has some confusion about the construction sequence of steel and concrete.

Each of the towers, in other words, was held up by its reinforced concrete core and the world's strongest curtain walls. Without the usual steel skeleton, the open floors allowed unprecedented space and flexibility. Between them, the two 1,350-foot-high towers provided 7.9 million square feet of rentable floor space, roughly the equivalent of fifty city blocks.


This Page Has A Concise, Accurate Structural Description


At the heart of the structure was a vertical steel and concrete core, housing lift shafts and stairwells. Steel beams radiate outwards and connect with steel
uprights, forming the building's outer wall.

August Domel, Jr., Ph.D., S.E., P.E. November 2001

Groundbreaking for construction of the World Trade Center took place on August 5, 1966.Tower One, standing 1368 feet high, was completed in 1970, and Tower Two, at 1362 feet high, was completed in 1972. The structural design for the World Trade Center Towers was done by Skilling, Helle, Christiansen and Robertson. It was designed as a tube building that included a perimeter moment-resisting frame consisting of steel columns spaced on 39-inch centers. The load carrying system was designed so that the steel facade would resist lateral and gravity forces and the interior concrete core would carry only gravity loads.

Dr. Domel received a Ph.D. from the University of Illinois at Chicago in 1988 and a Law Degree from Loyola University in 1992. He is a licensed Structural Engineer and Attorney at Law in the .State of Illinois and a Professional Engineer in twelve states, including the State of New York. Dr. Domel is authorized by the Department of Labor (OSHA) as a 10 and 30 hour construction safety trainer.


NOTE: The link for the following does not respond. 2/06/06

Building Design

The World Trade Center towers were an unusual design, at least at the time they were built. Their support structure is called a 'bundled tube', or in engineering terms, a glass curtain wall structure. What this means is that the buildings are tubes, made rigid by a lattice of steel beams on the outside walls. These vertical columns are strengthened by horizontal beams, and this design is what helps support the building, and keep it stable in high winds. An inner concrete core houses the elevators, and provides additional vertical load support


The usenet has been searched and messages by people found that describe the concrete core 

who saw it being constructed or knew for other reasons, the true tower core design.

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