Wah

Comp. metal deck Comp. beam

Gravity columns

Comp. metal deck Comp. beam

Gravity columns

Comp. girder

10-story high X-brace

9 ft dia. comp. pipe column

Figure 5.16. Gateway Tower; 62-story building.

in diameter by 8 in. (203 mm) long are welded to the inside face of the tube at a vertical spacing of 12 in. (305 mm) and a radial spacing of 9 in. (228 mm). The bending capacity of the pipe column is developed into the foundation by 1) welding shear studs around the outer surface of the column embedded in the foundation; and 2) by extending the mild steel reinforcement inside the column into the foundation. Figure 5.18b shows a

Gravity column Composite beam Composite girder Composite deck

Gravity column Composite beam Composite girder Composite deck

10 ft diameter composite pipe column filled with 19000 psi concrete

Bracing around core

Figure 5.17. Two Union Square; 58-story building.

Figure 5.18. Fremont Street Experience: (a) general view; (b) detail at column; (c) typical section; (d) concrete-filled composite pipe column.

schematic section through the space frame. The architectural design is by the Jerde Partnership, Inc., Venice, CA; the structural engineering of the vault support is by John A. Martin & Associates, Inc., Los Angeles; and the space frame design is by Pearce Systems International, Inc.

5.3.2. Buildings with Formed Composite Columns

5.3.2.1. InterFirst Plaza, Dallas

An example of a building that uses formed composite super columns is shown in Fig. 5.19. In this 73-story, 921-ft (281-m)-tall building, the entire weight of the building is supported on 16 composite columns located up to 20 ft (6 m) inboard from the building exterior. The lateral loads are resisted by the composite columns interconnected to a system of 7-story two-way vierendeel trusses. The composite columns vary in size from 7 ft X 7 ft (2.1 m X 2.1 m)

Figure 5.18. (Continued).

Figure 5.19. Dallas Main Center: InterFirst Plaza, 26th-43rd floor framing plan.

Figure 5.19. Dallas Main Center: InterFirst Plaza, 26th-43rd floor framing plan.

at the base to 5 ft X 5 ft (1.5 m X 1.5 m) at the top; 36-in. (0.30-m) deep steel shapes are encased in 10-ksi (69-Mpa) concrete columns, reinforced with 75 ksi (517 Mpa) mild steel reinforcement.

5.3.2.2. Bank of China Tower, Hong Kong

This prism-shaped building (shown in Fig. 5.20), designed by the architectural firm of I. M. Pei and Partners and structural engineer Leslie E. Robertson, is a 76-story building consisting of four quadrants. Each of the quadrants rises to a different height, and only one reaches the full 76 stories. The lateral bracing consists of a space truss spanning between the four corner columns. From the top quadrant down, the gravity loads are systematically transferred out to the building corner columns by truss action. Transverse trusses wrap around the building at selected levels. At the 25th floor, the center column is transferred to the corners by the space truss, providing for an uninterrupted 158-ft (48-m) clear span at the lobby. At the fourth floor, the horizontal shear forces are transferred from the space truss to the interior composite core walls through 1/2-in. (12-mm)-thick steel plate diaphragms acting compositely with the floor slab. The foundation for the building consists of caissons as large as 30 ft (9.1 m) in diameter hand-dug to bedrock.

5.3.2.3. Bank of Southwest Tower, Houston, TX

The Bank of Southwest Tower, an 82-story, 1220-ft (372-m) building proposed for, and not built in, downtown Houston, TX, uses the unique concept of composite columns with

Figure 5.20. Bank of China Tower, Hong Kong.

interior steel diagonal bracings. The diagonals transfer both the gravity and the lateral loads into eight composite super columns. The building has a base of only 165 ft (50.32 in.), giving it a height-to-width ratio of 7.4. The characteristic feature of the design consists of a system of internal braces that extend through the service core and span the entire width of the building in two directions. A typical bracing consists of an inverted K-type brace rising for nine floors, there being two such braces in each direction. Eight of these 9-story trusses are assembled one on top of another within the tower. All of the gravity loads are transferred to eight composite columns located at the building perimeter. The structural engineering is by LeMessurier Consultants, Inc., and Walter P. Moore & Associates, Inc. A schematic bracing diagram is shown in Fig. 5.21.

5.3.3. Buildings with Composite Shear Walls and Frames

5.3.3.1. First City Tower

Designed by structural engineers Walter P. Moore & Associates, Inc., this 49-story tower comprises a number of distinctly different composite elements (Figs. 5.22 and 5.23).

Figure 5.21. Bank of Southwest Tower (structural engineers: LeMessurier Associates and Walter P. Moore Associates; architects: Murphy/Jahn and Lloyd Jones and Fillpot). (a) Schematic representation of interior diagonal bracing; (b) schematic plan; (c) schematic section; (d) photograph of model. (Photo courtesy of Malcolm Stewart, Century Development Corporation.)

Figure 5.21. Bank of Southwest Tower (structural engineers: LeMessurier Associates and Walter P. Moore Associates; architects: Murphy/Jahn and Lloyd Jones and Fillpot). (a) Schematic representation of interior diagonal bracing; (b) schematic plan; (c) schematic section; (d) photograph of model. (Photo courtesy of Malcolm Stewart, Century Development Corporation.)

Figure 5.22. Composite floor-framing plan.

Shown in Figs. 5.24 and 5.25 are the details of the composite columns. Typiclly, the embedded steel columns vary from a W14 x 370 (W360 x 551) at the bottom to a W14 x 68 (W360 x 101) at the top. The vertical reinforcement in the columns varies from #18 bars (57 mm) at the bottom to #7 bars (22 mm) at the top. Open ties permitted in low seismic zones are used throughout.

Figure 5.24 shows the arrangement of reinforcement around a W10 x 72 (W250 x 107) erection column embedded in the shear walls. Ties are used where the vertical reinforcement ratio is more than 1% or where the reinforcement is required for resisting compression. This requirement has been in the ACI 318 code for the past 25 years. See Section 14.3.6 of ACI 318-02.

Figure 5.25 shows the connection detail between the concrete shear wall and a typical steel link beam. The moment capacity of the beam is developed through a shear transfer mechanism by steel studs welded to the top and bottom flanges of the beam. The stiffener plate, set flush with the wall face, has no structural purpose but helps in simplifying the form work around the beam. The construction sequence generally used in a composite section is shown in Fig. 5.26.

Figure 5.23. Composite elements.

5.3.4. Building with Composite Tube System

5.3.4.1. America Tower, Houston, TX

Shown Figs. 5.27, 5.28, and 5.29 are the details for a 42-story office building called America Tower, designed by structural engineers Walter P. Moore and Associates, Houston, TX. This building uses a hybrid tubular perimeter frame consisting of composite columns above the third floor and structural steel columns below (see Fig. 5.29). This integration of steel columns and composite columns eliminated form work for columns at nontypical lower levels.

5.4. SUPER-TALL BUILDINGS: STRUCTURAL CONCEPT

A super-tall building is generally referred to as a skyscraper when it is taller than some 80 stories or so. Its silhouette has a slender form with a height-to-width ratio well in

Figure 5.25. Arrangement of link beam in shear wall.

excess of 8. An ideal structural system for such a slim building is one that can at once resist the effect of bending, torsion, shear, and vibration in a unified manner. A perfect form is a chimney with its walls located at the farthest extremity from the horizontal center, but as an architectural form, it is less than inspiring as a building model. A practical interpretation presents itself in a skeletal structure with its lateral stiffness located at the farthest extremity from the building center. Two additional requirements need to be incorporated within this basic concept to achieve high efficiency: 1) transfer as much of the gravity load, preferably all of the gravity load, into these columns to

Derrick floor

Core frame 2 floors maximum

Steel frame 10 floors maximum

Concrete deck 4 floors maximum

Completed structure

Figure 5.26. General construction sequence in composite structures.

Core frame 2 floors maximum

Steel frame 10 floors maximum

Concrete deck 4 floors maximum

Completed structure

Figure 5.26. General construction sequence in composite structures.

Figure 5.27. Structural floor framing plan; 42-story America Tower, Houston, TX.

enhance their capacity for resisting overturning moment; and 2) connect the columns with a system capable of resisting the shear forces.

The ultimate structure for a rectangular building, then, will have just four corner columns interconnected with a shear-resisting system. Such a concept, proposed by the author for a super-tall building, is shown in Fig. 5.30. The columns are deliberately located inboard from the building corners to allow for architectural freedom in modulating the short faces of the building. The shear in the transverse direction is resisted by a system of 12-story-high braces, while in the longitudinal direction the shear resistance is provided primarily by the full-height vierendeel frames located on the long faces. The story-high longitudinal trusses located at every 12th floor permit cantilevering of the floor system. The primary function of the interior vierendeel frame is to transfer the gravity loads of the interior columns to the composite columns via chevron braces. However, because of the geometry, it also resists external shear forces in the long direction.

The scheme shown in Fig. 5.30 can be modified to fit a variety of architectural shapes. Any desired slicing and dicing of the building on the short faces may be accommodated without inflicting an undue penalty on systems efficiency.

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