Question

High Rise Construction

1.) Please discuss the importance of developing building technologies to resist "earth movement, high winds and other lateral forces" in high-rise construction.

2.) Examine relevant techniques and other factors such as "building load and height" issues in structural design and support your points with some examples. I am particularly looking for relevant technologies that are in use today to help compensate for these factors in construction.

Answer 1

Q.1

- In the twentieth century, strength was the center of focus in the construction of most tall buildings. Partitions and heavy masonry coating of that time greatly contributed to the stiffness and damping of the buildings (Ali & Moon, 2007). Therefore, accelerations and drifts in the buildings were relatively small. In mid twentieth century, high-strength steels, light-weight curtain systems and board partitions were introduced. Designs made of different framing systems, building proportions, and occupancies challenged the performance of wind thus reducing damages caused by vibrations. A new generation of high-rise buildings later emerged in the late twentieth century. The new structural system comprised super-columns, high strength concrete, core or bracing walls for stiffness, and better testing and analytical tools.

- High rise buildings or skyscrapers were first developed in the 19th century constituting an American building style which means that most of them were developed in the United States of America (Ali & Moon, 2007). Today, their development has rapidly increased across the world. Factors such as increase in population and increase in the value of land also lead to increase in the construction of skyscrapers. Human passionate desire to create tall structures also contributed to the rise in construction of these buildings. Though first generation skyscrapers were built with heavy masonry coating to increase the stiffness of the structures, they were still vulnerable to vibrations induced by earthquake and wind. Reduction in dumping of these buildings due to increase in height and excessive accelerations due to vibrations induced by the wind can lead to damages or discomfort for humans. Therefore, there was need for other solutions to be engineered to reduce the excessive accelerations caused by the vibrations.

Q.2.

The effect of wind forces on tall buildings is twofold. A tall building may be thought of as a cantilever beam with its fixed end at the ground; the pressure of the wind on the building causes it to bend with the maximum deflection at the top. In addition, the flow of wind past the building produces vortices near the corners on the leeward side; these vortices are unstable and every minute or so they break away downwind, alternating from one side to another. The change of pressure as a vortex breaks away imparts a sway, or periodic motion, to the building perpendicular to the direction of the wind. Thus, under wind forces there are several performance criteria that a high-rise structure must meet. The first is stability—the building must not topple over; second, the deflection, or sidesway at the top, must not exceed a maximum value (usually taken as 1/500 of the height) to avoid damage to brittle building elements such as partitions; and, third, the swaying motion due to vortex shedding must not be readily perceptible to the building occupants in the form of acceleration, usually stated as a fraction of gravity, or g. The threshold of perception of lateral motion varies considerably with individuals; a small proportion of the population can sense 0.003 g or 0.004 g. The recommendation for motion perception is to limit acceleration to 0.010 g for wind forces that would recur in 10-year intervals. The fourth criterioninvolves the natural period of the building structure. This is the vibration period at which the swaying cantilever motions of the building naturally reinforce and enhance each other and could become large enough to damage the building or even cause it to collapse. The natural period of the building should be less than one minute, which is the period of vibration due to the shedding of wind vortexes.

- Earthquake or seismic forces, unlike wind forces, are generally confined to relatively small areas, primarily along the edges of the slowly moving continental plates that form the Earth’s crust. When abrupt movements of the edges of these plates occur, the energy released propagates waves through the crust; this wave motion of the Earth is imparted to buildings resting on it. Timber frame buildings are light and flexible and are usually little damaged by earthquakes; masonrybuildings are heavy and brittle and are susceptible to severe damage. Continuous frames of steel or reinforced concrete fall between these extremes in their seismic response, and they can be designed to survive with relatively little damage.