Overview
Buildings Home > Overview
Slightly more than half of all low-rise buildings in the United States
are constructed from concrete. Designers select concrete for one-,
two-, and three-story stores, restaurants, schools, hospitals, commercial
warehouses, terminals, and industrial buildings because of its durability
and ease of construction. In addition, concrete is often the most
economical choice: load-bearing concrete exterior walls serve not
only to enclose the buildings and keep out the elements, but also
carry roof and wind loads, eliminating the need to erect separate
cladding and structural systems.
While steel construction can be advantageous in regions of the country where local market conditions and traditions favor it, concrete is the most cost-effective choice throughout much of the South and West-regions with strong masonry traditions. Concrete often is used in low-rise construction in Florida, where the material's ability to weather hurricanes and tornadoes, and its resistance to insects, are valued. Builders in California select concrete for its fire resistance.
Four methods of concrete construction are commonly used to create
load-bearing walls for low-rise construction: tilt-up, precast, concrete
masonry, and cast-in-place. Although precast and concrete masonry
construction historically have been the standard for low-rise construction,
in recent years builders have increasingly used tilt-up construction
techniques to erect low-rise commercial buildings quickly and economically.
Tilt-Up
Tilt-up is particularly well suited to warehouse and shopping center
construction, because contractors can form the windowless, unarticulated
wall panels quickly and economically. Tilt-up can also be used for
buildings with windows and other architectural features. Using tilt-up
techniques, builders set the steel reinforcing and pour the concrete
walls in a horizontal position at the building site. Workers then
lift and tilt the walls, which average about 5.5 inches (14 cm) thick,
into place with a crane to create the building. In many cases, tilt-up
construction results in a lower first cost than alternative building
systems: ready mixed concrete is usually locally available, and special
labor skills are not required. No additional interior or exterior
finish is required. The concrete panels can be created with a wide
range of surface colors and textures, using exposed aggregate, concrete
form liners, pigments, and other techniques to create a desired appearance.
Tilt-up concrete is most commonly used for one-story buildings, but
its use in multi-story low-rise office and warehouse buildings is
growing.
[back
to top]
Precast
Precast construction is appropriate for structures in which the
concrete pattern can be repeated; the more times a concrete shape
or panel can be repeated, the greater economy can be achieved. Precast
construction also offers the advantage of factory control: concrete
strength, appearance, and quality can be tightly monitored and regulated.
Load-bearing precast wall panels-often used for low-rise schools,
hotels, hospitals, and manufacturing facilities-can either be mass-produced
in standard molds at precast plants, or can be formed in molds custom-designed
for individual projects. These panels are usually prestressed and
often contain a layer of rigid insulation. Precast concrete is commonly
used for prison construction because precast systems are economical
to construct and the material is largely impenetrable and damage-resistant.
Continued improvement in concrete strength in the past decade has been a major factor
in the development of taller buildings in the United States and throughout
the world. New structural systems—including high-strength
concrete—created either from concrete alone or with a composite system
that includes both concrete and structural steel are partly responsible.
These systems enable skyscrapers to resist the enormous wind and earthquake
loads imposed along their height and allow these structures to support
the vertical loads created by gravity, the weight of the building,
and its occupants.
A major advantage of concrete construction for high-rise buildings is the material's inherent properties of heaviness and mass, which create lateral stiffness, or resistance to horizontal movement. Occupants of concrete towers are less able to perceive building motion than occupants of comparable tall buildings with non-concrete structural systems. As a result, concrete has become the material of choice for many tall, slim towers, including many squeezed into narrow building lots in New York City in recent years. Engineers deemed concrete to be the only viable structural
option for the
structures—including City Spire on West 56th Street, with its slenderness ratio of 10 to
1—to withstand anticipated wind loading.
The first reinforced concrete high-rise was the
16-story Ingalls Building, completed in Cincinnati in 1903. Even 50
years later, concrete buildings rarely exceeded 20 stories. Concrete
high-rise buildings were not economical to lease because the massive
columns needed for their support left too little rentable floor space.
Greater building height became possible as concrete strength increased.
In the 1950s, 5000 psi (34 MPa) was considered high strength; by 1990,
two high-rise buildings were constructed in Seattle using concrete
with strengths of up to 19,000 psi (131 MPa). Ultra-high-strength
concrete is now manufactured with strengths in excess of 21,750 psi
(150 MPa).
While the United States witnessed the construction of millions of square feet of office space in high-rise buildings during the 1980s in cities such as New York and Chicago, high-rise construction fell off sharply in the 1990s. In 1985, 3.1 million tons of cement were used in U.S. high-rise construction, while in 1995
only 421,000 tons were required.
Asia Has Highest Concrete
Until recently, the world's tallest buildings were in the United States, but in 1993, the tall building construction boom shifted to Asia with the erection of the
1207 ft (368 m) Central Plaza office tower in Hong Kong.
Two major high-rises in Asia are the 1371 ft (418 m) Jin Mao Tower in Shanghai, China, and the
1378 ft (420 m), twin Petronas Towers in Kuala Lumpur, Malaysia. These monumental towers use composite structural systems, combining vertical components such as cores, columns, and shear walls of concrete that have strengths of up to 11,600 psi (80 MPa) with structural steel horizontal members to resist lateral and vertical forces.
The two tallest concrete buildings in the United States were completed in Chicago
in 1989. Both the 969 ft (295 m), 311 South Wacker Building and
the 920 ft (276 m), Two Prudential Plaza Buildings took advantage
of 12,000 psi (83 MPa) high-strength concrete in the fabrication
of cast-in-place, steel-reinforced columns and walls at the buildings'
lower levels to support the total dead and live loads of the
structures.
The middle and upper levels of the buildings, where total accumulated
forces are lower, were constructed with concrete in strengths ranging
from 4000 psi (27.6 MPa) to 10,000 psi (69 MPa).
[back
to top]
Resisting Earthquakes
The ability of any structure to withstand an earthquake—whether it is a concrete high-rise, a steel bridge, or wood-frame
house—hinges on whether the structure was properly designed, detailed, and constructed to resist the lateral or side-to-side loading created by the shaking of the earth. The design community's understanding of how to best deal with this shaking generally improves significantly in the aftermath of each major
earthquake because engineers have the opportunity to observe and learn from the way existing structures perform. This hard-won knowledge often leads to revisions in design and construction procedures that are incorporated into building codes, which govern future construction.
Contrary to popular belief, a structure's likelihood of surviving an earthquake
depends more on how well the structure is engineered than on what
type of material is used to build it. During a severe earthquake
that struck Kobe, Japan, on January 17, 1995, concrete
buildings and steel buildings in the downtown area of the city shared
comparable fates: just 4.9 % of concrete buildings and 5.3 % of steel buildings collapsed. The majority of the more than
5,000 deaths and 34,000 injuries caused by the earthquake occurred
as a result of the widespread collapse of traditional one- and two-story,
wood post-and-beam houses. These structures—with weak walls of
bamboo or thin wood and heavy ceramic tile roofs—relied on structural
connections created with interlocking pieces of wood, rather than
with nails or other positive connectors. The earthquake-induced
shaking caused these connections to fail, and the buildings collapsed,
killing and injuring occupants.
[back
to top]
Two-Pronged Approach
To successfully withstand earthquake-induced forces, structures such as bridges, elevated roadways,
and high- and low-rise buildings constructed in areas of seismic activity must be engineered with a two-pronged approach. One structural system is needed to resist gravity or downward forces, to hold the structure up under normal circumstances, and another is required to resist lateral or sideways forces generated during an earthquake. Sometimes a single structural system can satisfy both criteria: high-rise buildings can be supported by a concrete frame system detailed to resist both gravity and seismic loads. In other cases, designers use the frame to support only the gravity load, and add shearwalls—walls designed to resist sideways or in-place forces and provide lateral
rigidity—to resist earthquake-induced motion.
The choice of structural systems available for the construction of high-rise buildings in regions of high seismicity is much more limited than that available in non-seismic regions. To protect the life and safety of occupants, U.S. building codes governing seismic design reflect a strong column, weak beam philosophy. Because the vertical columns are more critical to the stability of a structure than are the horizontal beams, engineers are required to design the columns to be 120
% as strong as the beams. As a result, in the event of strong earthquake shaking motion, the beams are damaged instead of the columns, so that the building will remain standing.
One structural attribute that engineers have come to understand during the
past 30 years as being critical to effective seismic design is ductility:
the ability of a structural member, or a connection between structural
members, to bend in response to earthquake-induced forces while
simultaneously continuing to support the loads it was designed to
carry.
[back
to top]
Ductility a Key
The ductility of concrete columns can be increased by including horizontal or transverse steel reinforcing as well as vertical steel. Lack of ductility in columns, beams, and connections has been blamed for the most serious damage to major buildings and transportation structures that occurred during recent major earthquakes. Non-ductile concrete and steel columns supporting the Hanshin expressway near
Kobe—designed before 1971 when Japanese Building Standard Law was modified to require ductility in structural elements and
connections—contributed to a spectacular failure of the elevated roadway. In response to failures of non-ductile columns on bridges and roadways during the Northridge earthquake, which shook the Los Angeles area on Jan. 17, 1994, and the Loma Prieta earthquake, which struck near San Francisco on Oct. 17, 1989, the California Department of Transportation has undertaken a retrofit program of non-ductile columns. Contractors are jacketing these columns with thin sheets of steel or carbon fiber materials to confine the concrete and increase column ductility.
Detailing of connections with seismic forces in mind has also emerged as an important design consideration in recent years. After the two recent California quakes, for example, bridge engineers changed the requirements for the connection between adjacent concrete box girders that support bridges, increasing the seat width, or the area of overlap between the two sections, from about 6
in. to more than 20 in. (15 cm to 38 cm). Engineers have also begun to require much more substantial ties between separate structural members, such as box girders or beams. Structural members are now being linked by restrainers made of high-strength steel rods with steel plates at either end that are
embedded in the concrete to keep the structural members from separating during an earthquake.
|
 |
|