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Building to Extremes

Professor John E. Fernandez

John E. Fernandez has been a Professor of Design and Building Technology in the School of Architecture and Planning at MIT since the fall of 1999. His research work addresses the field of architectural materials through both technical and design investigations intent on revealing productive opportunities for both performance improvements and design. Currently he is a principal in the Boston and New York City firm of Lampietti-Fernandez Architects.

Why does the Shanghai World Financial Center have a huge hole at the top of the tower?

The circular opening at the top of the tower was conceived as a grand architectural response to the ending of the super-tall volume of the building. However, there are what I would call "collateral" effects that benefit the structural engineering of the form. Because the floors become quite small at the top of the tower, and wind loads on any tower increase with height, it made both structural and economic sense to void the top and reduce the stress imposed on the building. Because towers are essentially acting as cantilevered beams, loaded with the distributed load of the wind, eliminating loads at the extreme of the length of the beam (at the highest points of the tower) reduces wind load on the form. In designing super-tall buildings, aesthetic decisions often have significant technical ramifications. Correlating the architectural language with the technical performance yields both economic and structural benefits.

In addition, there are important urban consequences that arise from the shaping of the tops of tall towers. Obviously, the tops of these towers impose their profiles on the urban landscape of the skyline. It has been persuasively argued that the box forms of the 1950s and 1960s imposed an unnecessarily banal skyline on many American cities, especially those of the Midwest United States. Because tall buildings are essentially large boxes of volume with repetitive floor plates, the bottom and top of these kinds of buildings offer opportunities for architectural design.

What does the counter balance in Taipei 101 look like?

The counterbalance, or to use the more precise term, "tuned mass damper," is a large sphere, composed of a number of circular steel plates hung on cables form the superstructure of the building. A tuned mass damper works as an inertial counterbalance to the swaying of the mass of the tower's superstructure. The damper in Taipei 101 will be left open to be viewed by visitors to the building.

Would the Taipei 101 or Shanghai Financial Center buildings be able to withstand the power of an airplane crashing into their towers, or would they collapse like the World Trade Center?

This is a sensitive question that is best answered by the engineers of the building themselves. However, the issue is one that all responsible engineers working on super-tall buildings are now considering. Suffice it to say that buildings built today are safer, more robust, than those built in the 1950s and 1960s. The primary reasons for this are better knowledge of the effects of fire on the interior structural frames of buildings and the use of hybrid structures in the making of the superstructural frames. Hybrid structures refer to building frames composed of more than one primary material. For super-tall buildings, this usually means frames of both concrete and steel. The use of concrete gives the building more mass, and therefore more energy-absorbing potential.

The World Trade Center used concrete as a topping on the precast open-web steel joist floor constructions. While technically, this concrete was structural (transmitting loads from the perimeter tube to the interior core), it was a relatively thin layer and could not absorb much of the kinetic energy of the incoming planes. Many super-tall buildings today use concrete to encase structural steel columns and sometimes beams. Buildings like the Petronas Towers use steel and concrete to make the large columns that carry much of the load of the building. These very large columns are able to withstand very large impacts. Therefore, super-tall building built today have a much greater capacity to withstand the impact and absorb the energy from catastrophic events.

From an engineer's perspective, do any of the world's tallest buildings stand out as being better designed than the others? Which buildings were the most expensive to build (land cost excluded)?

It is not reasonable, in my opinion, to compare towers strictly in terms of their engineering. The various forces that converge to produce these buildings are diverse: economic, owner preferences, aesthetic judgment, space requirements and the particularities of the engineering constraints. All of these factors necessarily place demands on the engineering of these buildings and act to influence the structural frames in localized ways. Comparing one set of engineering decisions to another is really comparing one set of very particular responses to unique contextual conditions with another distinct, and often quite different, set of engineering decisions.

However, it is possible to highlight some particularly outstanding advancement in the structural engineering of tall and super-tall buildings. Buildings that have been the leading edge of structural engineering often are the physical manifestation of a dramatically different approach to addressing vertical static loads and dynamic lateral loads. The Sears Tower in Chicago is one good example. In that building the engineers decided to "bundle" nine square tubes together to provide the lateral bracing needed to resist the high wind loads. Essentially, nine super-tall columns were tied together horizontally providing significant lateral support to the overall building. The World Trade Towers were also a milestone in structural engineering with their lightweight perimeter tube and open-web steel truss design.

Recently, the use of supercolumns of steel and concrete that form very large scale rigid frames are being used for the largest buildings in Asia. The Petronas Tower uses such a structure. With improvements in the strength and durability of concrete, these buildings are said to have a more robust structure for counteracting catastrophic events such as earthquakes, fires, and blasts. Also, the use of computers for the design and analysis of building superstructures has completely changed the way in which these buildings are designed. Sophisticated computational tools can model the flow of wind into and around a building, the spread of a fire through the building, the behavior of the superstructure under a variety of loads, even the behavior of large numbers of people as they evacuate the building.

I am a fire inspector in New Jersey, amazed at how advanced these buildings in Asia are. How will the evacuation time compare between the Petronas Towers and the new World Trade Center?

It is true that Asian buildings have been more intensely focused on the issues that arise from catastrophic events. Part of the reason behind this is the fact that many Asian buildings need to be engineered for seismic considerations.

The Freedom Tower, the tallest tower designated in Daniel Libeskind's master plan, is intended to rise to a total height of 1,776 feet. I refer you to an article in the Sunday NEW YORK TIMES, March 14, 2004, in which the various strategies employed in the tower evacuation are listed. Currently the tower is still under design and travel times cannot be precisely determined. However, the building will most likely adopt a set of guidelines that will make it one of the most easily evacuated super-tall buildings in the world. However, as the INNOVATION episode on tall buildings clearly documented, travel time during evacuation is highly dependent on the training and attentiveness of the authorities charged with directing people to the most efficient egress routes. Designing a building for efficient evacuation is only the first step, training individuals who occupy the buildings is just as critical, if not more so.

Is it true that the recipe for Roman cement was lost after the fall of the Empire? If so, what did they use to build large structures before Portland cement was invented?

Cement, the primary binding component of concrete, was used in wall plasters in Babylonia and Egypt, but the first structural concrete was developed by the Romans around the 3rd century, BC. It is well known that the material technology and many of the techniques of construction for concrete were lost for centuries and partially recovered in the mid-18th century, when Joseph Aspedin received a patent for Portland cement in 1824. The introduction of steel reinforcing by J.L Lambot in 1848 heralded the beginning of 150 years of development, application, global proliferation, and design using modern reinforced concrete.

Modern concrete is a composite consisting of cement, water, aggregates small and large, various admixtures from superplasticizers, other polymers, pozzolans, and other materials. Reinforced concrete has the added component of some kind of tensile reinforcing that increases its flexural, shear, tensile, and other mechanical properties. Reinforcement is typically accomplished using steel bars anchored together and placed in the volume of the concrete formwork. It may also be achieved using very small fibers called 'whiskers' made of glass, synthetic polymers, or steel.

But before the widespread adoption of modern reinforced concrete, many large buildings and civil structures used masonry as the primary load transfer material. Gothic cathedrals, the next great structural typology after the Roman arches, vaults, and domes, are marvels of compressive architecture -- based as they are on the use of low stress, tensile weak, stone masonry. These buildings required the precise delineation of compressive load paths, such that the stone would not be placed in tension or significant bending. Under these strict constraints, medieval builders collectively created one of the most purely rational -- and beautiful -- architectural languages of all time. Eventually though, the use of metals started to make an appearance in large architectural constructs. First, cast iron was used for large buildings and civic structures. Large bridges and towers were built using cast iron. Soon afterward, though, modern reinforced concrete started to dominate the construction industry globally.

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