The Building

9 - 1 1 R e s e a r c h

an attempt to uncover the truth about September 11th 2001

mirror of “NERDCITIES/GUARDIAN” site : disclaimer

M.I.T., Rotch Visual Collections

Visual Communications in Building Technology Project

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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 a earthquake. Thus, the structural system must function in two different modes (Figure 3.9): 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. Figure 3.10 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 Et.), 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 Figure 3.11). 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.

Figure 3.11 TUBULAR FRAMING

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1. Steel framing. Placing multi-level frames.
2. View from across the street of the framing structure.

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3. Steel basement framing with foundation braces.
4. Detail of connectors.

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5. Aerial view of steel framing and floor forming.
6. Raising steel frame into place.

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7. Bolting perimeter framing.
8. Column to column detail on perimeter.
9. Detail of framing joint.

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 Figure 3.12) 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 Figure 3.12). 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 (see Figure 3.12) and therefore act as pinned connections.

Figure 3.12 INTERIOR GRAVITY LOAD FRAMING

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1. Floor framing.
2. Detail of framing joints.

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3. Placing steel studs deck to floor framing.
4. Shear stud welding.
5. Overall view of completed floor.

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6. Placement of composite concrete slab.
7. Leveling the poured concrete.

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8. Aerial view of building top during framing operation.

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9. Musicians for the topping out ceremony.
10. Raising of the pine tree on steel beam for the topping out ceremony.

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 Figure 3.13). 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.

Figure 3.13 BASEPLATES

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1. Foundation reinforcement.
2. Detail of baseplate.

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3. Column base--steel leveling plate.
4. Detail of baseplate surround.

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5. Steel baseplate--placing concrete.
6. View from above of steel beam placement onto baseplate.

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