The following is taken from the M.I.T., Rotch Visual Collections article Sixty State Street - A Case Study. This teaching resource was available on the internet for years until the 9-11 cover-up required its withdrawal from the public domain. If one desires additional evidence pointing to the group behind the September 11 disaster, all one has to do is to endeavor to find pre-9-11 technical information concerning the World Trade Center (or in fact any other similarly built skyscraper). What one will find is that the internet has been swept clean of such information. Neither Al Qaeda, nor the Arabs, or any other group of Islamics, have the capability to do this. September 11, was clearly an inside job.
3.2 THE STRUCTURAL SYSTEM
The primary function of a structural system is to help define and then support the spaces desired within a building. In general, this requires that the structure be designed by considering two basically different situations. First, the structure must be able to carry all of the loads associated with the use of the building, and its own weight, safely down through the structure, to the foundation and then into the ground. In addition to these vertical loads, the structure must also be able to resist the horizontal forces associated with the wind and with the possibility of an earthquake. Thus, the structural system must function in two different modes: the vertical mode of gravity induced forces and the lateral mode of wind (and earthquake) induced forces.
The Sixty State Street tower uses different portions of its structural system to respond to these vertical and lateral loads. The overall system is called tubular framing, since the perimeter frames of the building are designed to act as a cantilevered tube in resisting lateral forces. The interior framing of the structure acts semi-independantly of the perimeter tube and carries the gravity loads downward to the foundation. The figure below, presents a perspective of Sixty State Street in which the portions of the structural system are identified.
The structural engineering group of Skidmore, Owings and Merrill was one of the the principal developers of the tubular concept in the early 1960's. Tubular framing was originally developed to help make feasible the construction of reinforced concrete buildings taller than 40 stories, and has since been extended to steel framing and to composite concrete and steel framing.
Today four of the world's tallest buildings employ the tubular concept: the John Hancock Center in Chicago (1105 ft), the Standard Oil of Indiana Building in Chicago (1125 ft), the World Trade Center in New York (1350 ft), and the Sears Tower in Chicago (1450 ft).
The exterior frames of 60 State Street consist of closely spaced columns and deep spandrel girders (see above). These members are designed to provide a perimeter framing system capable of acting as a tubular beam cantilevered upward from the foundation. Spandrel members behave in a relatively stiff manner in order to transfer the shear forces required for tube-like behavior. As can be seen in Figure 3.11, this requirement leads to very short and deep spandrel beams. The nature of this tubular framing system lends itself to the fabrication of the perimeter framing system as a series of column trees whose branches are the spandrel beams. To erect the column trees, it is then necessary only to bolt together the web of the spandrel beams. Upon alignment of the frame, the bolts are tightened and the column connections are welded.
The tubular frame, in addition to resisting lateral forces, must also support its own weight, the curtain wall, and a portion of the interior gravity loads. The majority of the gravity loads, however, are supported by the interior beam and column framing system which does not participate in resisting lateral loads. The interior gravity frame provides the main support for the floor system (see below) which is constructed as a 5-1/4 inch thick, composite, light-weight concrete and light gauge steel deck.
The concrete acts in a composite fashion with the light gauge steel deck; in turn, through the use of shear studs, the concrete steel deck acts compositely with the floor beams (see above).
The left picture shows shear studs being welding to the steel joists. In the right picture reinforcing steel has been laid down and all is ready for the pouring of the concrete slab.
Construction workers pouring the concrete slab.
The floor beams then frame into either larger floor beams or directly into the interior gravity columns or the perimeter columns. To ensure that the perimeter framing and the interior framing act independently, all connections in the floor system allow only for shear transfer and therefore act as pinned connections.
Both the perimeter and interior columns head downward to meet the foundation mat. Baseplates provide the interface between the columns and the foundation system (see above). The baseplates are used to provide both a smooth and level bearing surface for the columns and to distribute the column load into the mat foundation. The baseplates are set over anchor bolts, previously cast into the mat, and then leveled. The leveled baseplates are then grouted into position to ensure a positive load transfer from the columns to the mat.
TRADE CENTER TOWERS.
The design concept of tubular framing (the so-called tube within a tube architecture) has been employed in the construction of many of the world's tallest buildings. These include the John Hancock Center (1105 ft), the Standard Oil of Indiana Building (1125 ft), the World Trade Center Towers (1350 ft), and the Sears Tower (1450 ft). In fact, most modern skyscrapers use this design, a design which uses a specially reinforced perimeter wall to resist all lateral loading and some of the gravity loading, and a heavily reinforced central core to resist the bulk of the gravity loading. The lateral loading (horizontal force) on the building, is mainly due to the wind while the gravity loading (downward force) is due to the weight of the building (i.e., due to gravity).
At 110 stories of the World Trade Center towers were considerably taller than the 39 story Sixty State Street. Thus the wind loading (the horizontal force due to the wind) on Sixty State Street was significantly less than that experienced by the towers. Nevertheless, these lateral forces were still sufficient to necessitate the same heavily reinforced perimeter wall and composite flooring system that was used in the construction of the towers. Without its specially designed perimeter wall and flooring system, Sixty State Street would have simply blown down in high winds.
In the tube within a tube architecture, it is of vital importance that the horizontal forces on one wall be transfered to the other walls, so that the entire structure will bend to the minimum extent possible. It is also of vital importance that these horizontal forces be transfered to the central core so that the entire structure bends uniformly as one unit.
This is achieved by the use of a composite flooring system, which is designed to act (in essence) as one super-large beam. The idea is to connect the steel joists supporting the concrete slab, to the slab, by the use of shear studs (also called shear connectors). The shear studs are usually some 3/4 inch wide and a few inches high. They are welded to the top flange of the joists and become an integral part of the slab once the concrete is poured. The combined steel joist-concrete slab, has sufficient strength to transfer the lateral loading to the core and the other walls, so that the building bends as little as possible, but when it does bend, it bends as a unit.
The composite flooring system is formed by pouring (light-weight) concrete on light-gauge steel decking that has been laid across the steel joists. Shear studs are welded to the top flange of the steel joists (but can also be bolted on, in this case, the bolt itself being the shear stud) through holes in the steel decking. Reinforcing steel is laid over the decking and the concrete slab is poured in situ. The large number of shear studs protruding into the concrete slab cause the underlying steel joists and the concrete slab to act as one unit.
The following two quotes (in mauve) are from the FEMA report into the collapse of the World Trade Center towers.
Floor construction typically consisted of 4 inches of lightweight concrete on 1-1/2-inch, 22-gauge non-composite steel deck. In the core area, slab thickness was 5 inches. Outside the central core, the floor deck was supported by a series of composite floor trusses that spanned between the central core and exterior wall. Composite behavior with the floor slab was achieved by extending the truss diagonals above the top chord so that they would act much like shear studs, as shown in Figure 2-6.
The above photo clearly shows the truss diagonals extending above the top chord. They extend a few inches above the present floor/deck level, and will be set in the concrete floor slab, once it is poured. They are the wire-like projections from the floor/deck (at the bottom of the picture). Each 60 foot span of double truss had about 15 pairs of "shear studs" with each pair of studs separated by 40 inches. Each 35 foot span of double truss had about 8 pairs of "shear studs".
In addition the transverse trusses (which ran at right angles to the double trusses) were supplied with shear studs as illustrated in the following graphic.
Pairs of flat bars extended diagonally from the exterior wall to the top chord of adjacent trusses. These diagonal flat bars, which were typically provided with shear studs, provided horizontal shear transfer between the floor slab and exterior wall, as well as out-of-plane bracing for perimeter columns not directly supporting floor trusses (Figure 2-2).
The diagonal flat bars mentioned above are the V-like features in the above photo. Strangely, the authors of the FEMA Report "forget" to mention the 24 x 18 inch metal plates that were covered with shear studs and also set in the concrete slab. These plates (together with the 6 foot long diagonal bars and the welded and bolted truss connections) provided a strong connection between the floor slab and the perimeter wall. The plates are the dark rectangular objects along the perimeter wall. The diagonal flat bars are also visible in the right-hand photo below. Note also the top chords of the trusses (yellow) and the rows of shear studs running perpendicular to the main double trusses (the rows of barely visible dots).
Both Sixty State Street and the World Trade Center towers have/had specially reinforced perimeter walls to resist lateral loading due to wind (and earthquakes). These perimeter walls consisting of closely spaced columns and deep spandrel girders as pictured below.
Both had an immensely strong central core to resist the bulk of the gravity loading. The central core consisted of a number of large columns which carried the weight of the building to the foundations. In the World Trade Center towers, the base of these steel columns were 54 inches wide by 22 inches and fabricated from 5 inch thick steel.
Sixty State Street has solid steel beams spanning the gap (of up to 40 feet) between the central core and perimeter wall (these are the beams at the back and on the left in the photo):
The World Trade Center towers had trusses for the 60 and 35 foot spans between its central core and perimeter walls.
The double trusses were pre-assembled in sets of two or three and hoisted into place.
Below are some photos of the pre-fabricated units being added to the towers. The 41st, 42nd, 75th and 76th floors housed mechanical equipment. On these, and also the first 15 floors above grade, heavy steel beams were used (rather than trusses) and composite action was provided by the more typical welded stud shear connectors. All other floors from the ninth (16th floor, once basement levels are included) to the top, had truss floor joists.
Like the mechanical floors, the top floors were also specially reinforced (in this case by outrigger trusses) to provide resistance to lateral loads. Also, as mentioned above, the composite floor slabs were vital for the towers ability to resist lateral loads. Without composite flooring the towers would have soon collapsed.
Click here for the entire M.I.T. study Sixty State Street - A Case Study.