Text 5 Trends in Tall buildings

The first reinforced concrete high-rise, the Ingalls Building was a post-and-beam system standing 15 stories. In the design of the Ingalls Building, standing at 210 ft. (64 m), every major development that had been made by then was taken into consideration. The building utilized a heavy monolithic beam-column framing system. The floor slabs contained two-way reinforcing systems. The beams were reinforced with bent bars near the supports. Hoops and continuous helixes were employed in the columns to tie the vertical reinforcement together. Ransome's square, cold-twisted reinforcing bar was used for the steel throughout the entire building for horizontal as well as vertical reinforcement. Since the construction of the Ingalls Building, rarely has any concrete building exceeded the 20-story high mark until about 1960. Many tall concrete buildings have been built all over the world since then. Only a few such representative buildings are discussed below.

The Marina City Twin Towers were built in 1962 in the center of an industrial park. Architect Bertrand Goldberg knew that whoever lived there would need 24-hour services, entertainment, parking and offices all in one structure. Marina City contains a movie theater, bowling alley, shops, offices, restaurants, meeting rooms, gym, skating rink, parking for cars and boats and, finally, apartments. Originally marketed to single adults and couples without children, the apartment complex was a success. The Towers were two of the first new mixed-use structures in downtown Chicago and were the tallest reinforced concrete buildings in the world for that year at 588 feet (179 m). Goldberg's plan in building a circular structure, which was very innovative at the time, was based on efficiency of HVAC systems and to reduce the service core of the structure. Another innovation was the 20-story parking garage directly below the 900 units of apartments. He introduced a circular core wall in hopes that it would take the entire lateral load from cantilevering floors. This was actually modified with two rows of columns so that the depth of the cantilever and beams between "petals" could be reduced. Even with these modifications, the circular core area carries 70% of the total lateral loads. The core, which acts as a circular concrete shear wall, was carefully designed with staggered openings and by minimizing their size in an effort to maintain enough stiffness.

The Water Tower Place (Fig.1.7) became yet another high-rise structure of concrete located in the downtown area of Chicago. Designed in 1975 by Loebl, Schlossman, Dart & Hackl, it stands 859 ft.(262 m) in height and serves as another mixed-use building with a mall on the interior, offices and apartments above. The strength of concrete used in this building took a dramatic jump to as high as 9,000 psi (62.1 MPa). This was, however, only one of eleven mixes placed by six cranes. Mix strengths could vary from 3,000 psi (20.7 MPa) for slabs to 9,000 psi (62.1 MPa) for columns. This building demonstrates concrete technology's ability to rival that of steel for tall buildings as it was 2/3 the height of the tallest steel building of the time. The structural system for Water Tower combines "reinforced concrete peripheral framed tube, interior steel columns, and a steel slab system with a composite concrete topping.”

 

Figure 1.7 - The Water Tower Place

 

One Magnificent Mile building in Chicago completed in 1983 was designed by SOM and is one of the last buildings engineered by Khan. One Magnificent Mile was designed with the same concepts that made the Sears Tower possible (Fig.1.8)

 

Figure 1.8 - The Sears Tower in Chicago

The bundled tube system had already proven to be successful when executed in steel, as in the Sears Tower. The designers at Skidmore, Owings and Merrill decided to translate the bundled tube system into reinforced concrete. The structural system of One Magnificent Mile consists of three hexagonal tubes bundled together (Fig.1.6). The tied tubes give the building added stiffness. As in the Sears Tower, the tubes terminate at different heights, as the gravity loads decrease.

The Onterie Center of 1985 in Chicago, another SOM building, is generally considered as the "final work" of Fazlur Khan (Fig.1.3). The structural system for Onterie Center utilizes a trussed tube system in concrete. Once again, SOM successfully translated a structural system originally intended for Hancock, a steel tower, into reinforced concrete. The result is a concrete tube structure that has visible diagonal stiffening braces on the exterior. Although the diagonal bracing of the John Hancock Center was comprised of continuous diagonal steel members, that approach was not strictly possible in concrete. Instead, the diagonal bracing is achieved by blocking out the windows along the facades by filling them in with concrete.

311 South Wacker Drive of 1990 is a supertall reinforced concrete building in Chicago (Fig.1.9). It stands at 969 feet (295 m) with 12,000 psi (82.7 m) as its highest concrete strength. The structural system is a modified tube with a reinforced concrete peripheral frame, interior steel columns, and a composite steel and concrete slab. 311 is known to be a good example of shear wall-frame interaction systems. The building is engineered in such a way that the relative stiffnesses of both internal and external elements remain the same for the entire height of the building. Two strengths of concrete were used, 10,000 and 12,000 psi (68.9 and 82.7 MPa). A self-climbing pump with a separate, mounted, placing boom pumped concrete to the top of the structure. Post-tensioned floor slabs reduced the amount of steel consumption while that for concrete was reduced with thinner elements due to the material's high strength. Two sets of flying forms were cycled every five days through the structure.

 

Figure 1.9 - 311 South Wacker Drive

 

One Peachtree Center, built in 1991 in Atlanta, Georgia, is a 62-story, 842 ft.(257 m) tall, bundled-tube design with three strengths of concrete used in its columns and shear walls - 8,500, 10,000 and 12,000 psi (58.6, 68.9 and 82.7 MPa). Designers used technology developed in the 1960s and 1970s by Material Service Corporation in Chicago. Architectural requirements dictated a column-free interior and thus a 50-ft (15.2 m) floor span was accomplished with HSC and post-tensioned steel bars. Silica fume and granite aggregates were used to achieve the necessary strengths. Of special note for the owner of the building, each floor has about 36 rentable corner offices. This building is remarkable because of its design allowing multiple desired spaces for renters, the structural design and its desired use of Chicago style high-strength concrete.

Two more recent buildings constructed around the turn of the 20th century where concrete has been primarily used as the structural material in conjunction with steel are the Petronas Towers (Fig.1.10) in Kuala Lumpur, Malaysia and the Jin Mao building (Fig.11) in Shanghai, China. These are good examples to show that concrete has greatly advanced as a material in a century since the days of Ingalls Building construction. The structural frames for the 1,483-ft.(452 m) tall Petronas Towers use columns, core and ring beams of HSC, and floor beams and decking of steel to provide cost-effectiveness, fast construction and future adaptation to the internal and external environment. The core and frame together provides adequate lateral stiffness for such a tall building.

 

Figure 1.10 - Petronas Towers

The recently built 1,380-ft. (421 m) high Jin Mao building of 1999 is a mixed system that has a number of steel outrigger trusses tying the building's concrete core to its exterior composite mega-columns.

 

Figure 1.11 - The Jin Mao building