Tall buildings impossible without advances in technology.
The composite steel frame skeleton was nothing short of a structural revolution when it was developed in Chicago in the late nineteenth century, and it has been evolving ever since. Early tall buildings constructed with cast-iron framing were susceptible to fire and it was discovered that encasing the iron in concrete increased the material’s resistance to fire. Composite beams are structural members composed of steel and concrete joined together to act as a single unit. An example is a composite beam with a steel flange (I or W shape) attached to a concrete floor slab. There are two primary advantages: the system that results from joining the two materials is stronger, and composite action maximizes the properties of each material. The compressive strength of the concrete combined with the tensile strength of the steel yields a significantly stronger system. Initially, iron and/or steel frames replaced conventional load-bearing masonry walls to minimize the depth and width of structural members, allowing larger openings in the facades of buildings. The outer skin or curtain wall - now a non-structural element - could be thinner and filled with glass while the iron/steel skeletons were clad in materials such as terra cotta or brick. The benefits of the new construction method included reduced wall thickness, increased valuable floor space, allowed more daylight to reach into the building’s interior, and reduced weight, all of which allowed substantial increases in height. Structural steel has led the market for framing systems for commercial building construction for the past 50 years.
After the catastrophic Chicago fire in 1871, land in the business district was at a premium and the only place to go was up. The development of steel framing made this possible. The Home Insurance Building in Chicago (William Le Baron Jenney, architect-engineer), generally acknowledged as the fi rst skyscraper, was the first building constructed with a skeleton frame, although the façade still resembled masonry structures with load-bearing walls. The frame consisted of cast-iron columns supporting wrought-iron beams, with two floors of rolled-steel beams that were substituted during construction. The metal framing still had to be encased in brick or clad in terra-cotta for fire-protection.
Building on projects like the Home Insurance Building, the steel and terracotta Reliance Building in Chicago (Burnham and Company) is considered the prototype for glass-curtain wall construction due to the unprecedented use of glass in the façade. The structural system consisted of a riveted steel-frame, hollow-tile flooring on steel joists, plaster fire-proofing and steel trussed wind bracing.
In high-rise construction since 1945, advancements in reinforced concrete structural systems paralleled the development of tall steel structures. The shear wall was introduced as a means of stiffening concrete frames against lateral deflection. The shear wall acts as a narrow deep cantilever beam.
Introduction of stud shear connectors led to increased acceptance of principal of composite action, solving the inability to sustain the adhesive bond between steel and concrete in the field for long periods of time. Pushed the development of effective steel-concrete composite beams. At the same time, composite action in floor systems was recognized. Before 1950 concrete was used as a fi lling material, not as a structural contribution to the overall strength. Engineers soon realized the benefits of using steel decking over reinforced concrete slabs to create composite fl oor systems. The decking provides a sturdy, safe platform from which to work during construction stages and when it acts compositely with the concrete slab, the necessary amount of positive moment steel reinforcement can be significantly reduced.
“Cofar” composited reinforced concrete and steel deck floor system was produced by the Granco Steel Company.
In 1958, architect Milton Schwartz and engineer Henry Miller used shear walls in the design of the 39-story Executive House in Chicago.
In 1963, structural engineer, Fazlur Khan, developed the perimeter framed tube structure; a new structural system consisting of several vertical skeleton sections, or tubes, combined into one building. The system uses the entire building perimeter to resist lateral loads. Khan explained it as “a three dimensional space structure composed of three, four, or possibly more frames, braced frames, or shear walls, joined at or near their edges to form a vertical tube-like structural system capable of resisting lateral forces in any direction by cantilevering from the foundation.” (Ali Mir (2001), Art of the Skyscraper: the Genius of Fazlur Khan, Rizzoli International Publications). Framed tubes increase usable floor space because fewer interior columns are required. Khan used this system in the DeWitt-Chestnut Apartments in Chicago. Lateral stability was achieved by closely spaced columns placed around the building and connected together by deep beams.
The next step in concrete high-rise construction was the combination of the perimeter-framed tube with a largely solid-walled interior tube or shear walls to enhance lateral stability. This was employed by Eero Saarinen and Kevin Roche in the CBS Building in New York in 1965. In 1967, American Iron and Steel Institute funded research to develop a method of design for composite floor systems. Similar to composite construction for beams, a means of creating interaction between the steel and concrete beyond its adhesive bond was necessary. Manufacturers began to design new deck geometries to ensure full interaction between the decking and the concrete slab.
The Hancock Tower was designed by engineer Fazlur Kahn and architect Bruce Graham from Skidmore, Owings & Merrill in 1969. Fazlur Kahn devised a new construction method - the frame-tube system, which he also utilized in the Sears Tower. This was made possible by new technologies, such as more advancedsoftware and new developments in steel. That enabled Kahn to design constructions capable of resisting the strong wind forces more effectively by having the forces absorbed by all three dimensions of the building. The diagonals, connected to the exterior columns, allow the forces to be carried from the braces to the columns and back. The innovative construction resulted in a 50% decrease of required steel compared to skyscrapers built with interior columns.
The Sears Tower in Chicago is a variation on the tube frame called a bundled tube. The building is a bundle of nine tubes of different heights and is thought to be one of the most efficient structures designed to withstand wind. Columns and beams were moved from the core to the perimeter, creating a hollow, rigid tube. The tube weighs less than the core design but is equally strong. The primary advantage of using a bundled tubular frame is that it limits the influence of shear lag. The rigid outer walls act like the walls of a hollow tube.
Another new structural form was introduced by Fazlur Khan in the 780 Third Avenue Office Building in NewYork: a framed tube with diagonal bracing. The bracing results from filling in diagonal rows of window openings creating exterior bracing members.
The VERSA:T composite framing system is an advance in composite design. Historically, composite design was developed in reaction to construction practices, rather than with the primary intent of maximizing the composite design of the beam. Conversely, the Diversakore system begins with the composite design and produces a beam that exploits the capabilities of the composite method.
Bibliography
Alli, Mir M. Art of the Skyscraper: The Genius of Fazlur Khan. New York: Rizzoli, 2001.
Ali, Mir M.. "Evolution of Concrete Skyscrapers: From Ingalls to Jin Mao." Electronic Journal of Structural Engineering 1.1 (2001): 2-14.
Ali, Mir M., and Kyoung S. Moon. "Structural Developments in Tall Buildings: Current Trends and Future Prospects." Architectural Science Review 50.3 (2007): 205-23.
Ching, Francis D. K. A Visual Dictionary of Architecture. New York: Wiley, 1996.
Dupre, Judith. Skyscrapers. New York: Black Dog & Leventhal Publishers, 2001.
Taranath, Bungale S. Steel, Concrete, and Composite Design of Tall Buildings. New York:McGraw-Hill, 1997.
