Sea Land & Air in New York

Sea, Land & Air in New York

Contest Procedure

"AT THE EDGE"



 Topics 




   "Any enclosure is defined by a boundary...  The boundaries of a built

   space are known as floor, wall and ceiling.  " (Norberg.Schultz)



   "....the wall - the point of change becomes an architectural event.

   Architecture occurs at the meeting of the interior and exterior"

   (Venturi)



   "An edge may be more than simply a dominant barrier if some visual or

   motion penetration is allowed..  It then becomes a seam rather than a

   barrier, a line of exchange...  " (Lynch)



   "Boundaries are the precarious products of opposing forces.  " (Arnheim)



   "...space has the property of setting frontiers or limits to bodies

   in it ...  space is therefore not some pure extension...  but is

   rather a kind of primordial atmosphere...  endowed with pressure

   ...  tension and bounded by the infinite void ..." (Jammer) 



   "A boundary is not that at which something stops but, as the Greeks

   recognised, the boundary is that from which something begins its

   presenting." (Heidegger)



 

Information and booking forms from :



  Dr George Cairns

  University of Strathclyde

  Department of Architecture and Building Science 

  131 Rottenrow, Glasgow G4 ONG, Scotland

  Tel (041) 552 4411 ext 3173, Fax (041) 552 3997



 Call for participation 



The Department of Architecture & Building Science at the University

of Strathclyde in Glasgow, Scotland, invites EAAE affiliated Schools

of Architecture to participate in Workshop 28 "at the edge" which

will be, held on September 4-7, 1994.



At a moment in Europe's history when, on the one hand, many of the

old frontiers which separated the continent's people on political and

military grounds have been moved and yet, on the other hand, new

barriers are being thrown up for economic or ethnic advantage, it may

be ironically appropriate to invoke the architectural significance of

the boundary.



Well aware of its peripherical position, on the margin of Europe's

culture space - a position which borders on the void of disregard but

one which inevitably intensifies identity - the Department of

Architecture proposes that in preparation for, and during the 28th

EAAE Workshop, some architectural games might be played with the

metaphor of the edge.



On the assumption that it is indeed "at the edge ", at the boundary,

at a wall, that "architecture occurs"or "begins its presency", the

workshop will focus on the varying approaches brought to bear on such

fundamental issues by the participating schools, not only in the

creative act, i.e in the evaluation of these submissions which the

workshop will elicit.



Student competition and nature of the workshop



Schools of architecture across Europe, but particularly those who

feel themselves to be on an edge - perhaps on the still sensitive

fault-line that separated East from West for over 40 years, perhaps

at some seam of cultural fusion or divide, perhaps on the perimeter

marches or margins of the continent - are invited to submit for

presentation and discussion at the EAAE workshop two Al panels in

which the concept of the edge is presented as a polemic manifestation

of potent architectural imagery.



Submissions should reach the School of Architecture in Glasgow by

Friday 15 July 1994.



A selection of 8 projects will be made by a jury consisting of a

small number of internationally renowned architects specially invited

also to participate at the workshop and members of the EAAE Council.



The students and tutor teams of the selected schools are invited to

present their projects at the workshop explaining the political,

social, cultural priorities, the values, approaches, methodologies

that have led to their choice of the specific EDGE situation and to

their particular response to the theme.



With the projects and their presentation certain cultural,

philosophical, theoretical, pragmatic and methodological differences

in architectural and urban design as well as education are expected

to come to the foreground.  These differences will provoke discussions

held after each session of team presentations which will reveal the

values approaches and methodologies we share and will suggest what we

tan learn from one another.  The workshop will end with the award of

prizes by the jury for the three best projects.



COMITE D'ORGANISATION :



Professeur Frank A. WALKER

Professeur Per KARTVEDT

Dr George CAIRNS

Dr Hildebrand W. FREY

project : "At the Edge": spring 1994

d e s i g n d e s c r i p t i o n

of Sea Land and Air in New York

the highest building in Manhattan

written & designed by Rob Robbers

bwaurr@urc.tue.nl :: lava@urc.tue.nl

L A V A r e s e a r c h e r

"...space has the property of setting frontiers or limits to bodies in it... space is therefore not some pure extension... but is rather a kind of primordial atmosphere, endowed with pressure and tension and bounded by the infinite void." (Jammer)

I n t r o d u c t i o n

@@@ Several definitions and descriptions were used, in the present contest, to depict an edge. The use of popular quotes from well known authors in order to define edges relay a complex and abstract way of viewing edges which seems rather unnecessary. "The Edge"; the project and its underlying concept brought forth but another challenge, that is, to architecturally define a basic edge in the simplest possible manner. Thus, an edge as a transition; a line created between two objects which then becomes trans formed into an edge. These objects can be both physical or psychological, an example of a physical edge may be the line formed between land and sea, psychological edges such as cultural boundaries may exist and create limitations, in order to anticipate and conquer such confrontations edges must be attenuated to their maximum capacity. Thus, it can be assumed that an edge is not necessarily always perceived as an edge by all observers, personal background history and perception all play an important role in how an edge is viewed. Individual differences between observers create difficulties which exerts an extra challenge for a designer when it comes to creating an edge that must be recognized by everyone.

Since an edge is relative as a concept, a designer must take special care not to create any unnecessary aversion towards the design because of ruff or unesthetical edges. This point was especially taken into account concerning this project, considerations were made so that the edges would not be perceived as being psychologically aversive. New York city (NYC) is a perfect example of an environment were edges are continuously fading, the edges referred to are primarily cultural ones created between different groups of people. NYC the "melting pot" contains many different nationalities, people with different religious and cultural backgrounds all trying to live under the same roof, the term implying that boundaries and limitations between people are melting away into one harmonious pot.

d e s i g n

@@@ A term such as the melting pot projects a vivid image and creates an adequate description for the basis of an architectural concept particularly for this project. In order to design a building that both represented the fading of edges and symbolized the concept behind the term melting pot, it was of primary importance to predict what effects the edges would create in this project. One of the main goals in this project was to insure the understanding of the underlying concept of the design, concurrently creating a human-friendly environment enabling people to fully explore themselfs and reach the edge of their capacities. Defining the concept, one can derive that it is important to make a human-friendly environment on the inside and that a symbolic aspect of a concept may be exhibited on the outside. Since individual differences do play a major role in interpretations and in order to avoid the use of text to describe the exterior of a building an environment should be created in which people are forced to think or question it. For exa- mple; Why is there no shadow present underneath a certain building ? or, Why can the sky be seen when standing directly underneath a certain building ?

e d g e s

@@@ The edges in this project are derived from elementary sources :
S E A - L A N D - A I R

Edges between natural elements are constantly changing. For example; in The Netherlands land is gained from the sea, in order to accommodate more housing facilities for the city of Amsterdam. Moreover "land" can be gained from air by designing and building high-rises. Combining these three elements together can produce a powerful environment. In this particular high-rise design "land gain" is one of the key terms. The building is situated at the lowest point in Manhattan next to Battery Park. The park was a location for a battery of cannons which protected the island hundreds of years ago. The building "SLANY" : SEA LAND & AIR in NY is located next to the park and supported by two columns. Each support represents a different element. The west support represents sea and therefore has its place in the water. There is no visual connection from the land to this point. Connections are made only in above laying sectors and underneath the building, thereby creating a semi- monolith. The east support represents land. Main entrances are situated in this part of the building, these are the only entrances available entering on foot from sealevel. There is however, one exception; the elevator, which extends services from sealevel to the airtower, it is located between the land and sea supports. The elevator appears to float in space and is connected to the air towers by a steel bar. It enters the tower through a space where there is a large light hole. This light hole is created due to open spaces (atria) in the building which are joined at this point.

An arch connects both supports and is the roof construction for a platform. The platform is used for arrivals and departures, it may be seen as a symbolic space which constitutes a transitional area of the building. In order to avoid loosing total connection between sea and land, exactly in the middle of the building, on the floor level, the sound of the sea can be experienced in an unique amplified way. The platform's roofconstruction contains curved soundblasters, that are placed in a specific manner allowing for different angles between soundblasters which results in amplified sounds of the sea. This creates an interactive sea-and-land surface environment for the immediate surroundings. Visually the appearance of the building is dominated by this interactive surface. The wall on the north-side (facing Manhattan) depicts rows of windows which contain individually transformed virtual blind systems. The blinds also serve a second purpose, since they are not curved, but instead have an angle in the middle which divides each blind into four parts. This provides four different color labels allowing for a sort of camouflage, enabling the possibility of the outer appearance to blend in with the sky.

This creates an illusion: the fading away of edges and the blending in with the sky. Once this illusion comes to life, one can but say "the sky IS the limit"!

New York

i n t r o d u c t i o n

@@@ In New York every language is spoken, every cuisine is tasted, every country is represented, every dream is dreamt and every crime is committed. If an idea, object, hope, taste, scent or sin can not be found in NYC then... it does not exist.

i n h a b i t a n t s

@@@ NYC is one of the biggest cities in the world, it contains an area of 776 square km, and a population of over seven million. The largest ethnical minority is most likely the Hispanic population, but there is certainly no ethnical majority. Many different nations exist, the first however, to immigrate were the Dutch, Germans and other North-Europeans.

c i t y o f v i l l a g e s

@@@ NYC is surrounded by water and consists of five different parts, called the boroughs, one of which is called the Bronx situated at the North-American mainland. The island of Manhattan most centrally situated part. Brooklyn (consisting of the highest population) and Queens (which contains the biggest area) are situated at the west-point of Long Island, which is 200 km long and extends into the Atlantic Ocean. Staten Island situated south of Long Island, is connected to New Jersey. From the east end continuing through to the industrial area NYC's famous skyline can be experienced. In the south, the financial district is located and dominated by the twin towers (World Trade Center designed by the Japanese architect Minuro Yamasaki & Assoc). More towards the northside, the decline of the skyline continues until its next major landmark, the Empire State building designed by William Lamb. Finally, the skyline reaches its end and meets the ground, this point is known as the ink black tide stream which bares the name: "De Spuyten Duyvil". The sound of traffic is captured between the skyscrapers and experienced on the streets. Traffic migrates in batches and is dominated by yellow cabs, steam can be seen creeping out from multiple holes along the streets, every corner has its own portable wagon vendor selling anything from diamond watches to hot bagels and mustard. The stimuli and perceptual inputs make you dizzy, but.... there are also quite and serene places. The most important is Central Park situated in the center of Manhattan. Monaco could fit twice in this park and there would still be space left over. Here is were NYC comes to "get away" from it all.

h i s t o r y

@@@ NYC's harbour was discovered by Giovanni da Verrazno in 1524. It took more than a century before the first colony settled on a permanent base. In 1626 colonists of the Dutch Westindian Company founded Fort Amsterdam and the entire area was named New Amsterdam. In 1664, the British took over and New Amsterdam became New York. Resentment against the British resulted in the War of Independence in 1775. During this era there were approximately thirty thousand inhabitants. Due to the incredible immigration in the 19th century the amount of people in New York raised enormously. During the 40 years that followed the Civil War (1861-1865) Central Park was completed and the first skyscrapers were constructed. In 1898 four boroughs where created in NYC resulting in the biggest city of the world. Over three million inhabitants resided within the city. Despite the crash of Wall Street in 1929 NYC completed the construction of the Empire State Building by 1931. Only twenty years later were there over eight million residents. A decision on behalf of the united nations to choose NYC as the location for their headquarters increased the city's popularity all the more, helping it become the capital city of the world.

notes taken from :

h i g h r i s e b u i l d i n g s t r u c t u r e s



      by Wolfgang Schueller

      school of Architecture

      Syracuse University



      A Wiley-Interscience

      Publication

      John Wiley & Sons

      New York-London-Sydney-Toronto

INTRODUCTION

@@@ High-rise buildings are closely related to the city; they are a natural response to dense population concentration, scarcity of land, and high land costs. The massing of the high-rise building evolves out of the designer's interpretation of the environmental context and his response to the purpose of the building. A high rise building may be free standing that is, vertical and slender or horizontal and bulky or, it may be placed directly adjacent to other tall buildings, thus forming a solid building block. ln both approaches the building is basically an isolated object. However the tall building of the future may very well be an integral part of one large building organism, the city, where the buildings or activity cells are interconnected by multilevel movement systems. High-rise buildings range in height from below 10 to more than 100 stories. A rather complex planning process is necessary to determine the height or the massing of a building. Some of the factors to be considered are the client's needs versus the land available and the location of the land as related to facets of the environmental context, for example necessary services to support the building and its inhabitants or the ecological impact of the building or the scenic character of the landscape.

THE TALL BUILDING IN THE URBAN CONTEXT

@@@ The development of the high-rise building follows closely the growth of the city. The process of urbanization, which started with the age of industrialization, is still in progress in many parts of the world. In the United States this process began in the nineteenth century; people migrated from rural to urban areas, thereby forcing an increase in the density of cities. Technology responded to this pressure with the lightweight steel cage structure, the elevator, and the energy supply systems necessitated by the high density vertical city. At the beginning of this century building blocks about 20 stories high were set opposite each other, separated only by dark narrow streets, forming urban canyons. Primary concern was the placement of a maximum number of people on a minimum area of land. The resulting congestion and its impact on people and the city as an organic interaction system was hardly a design consideration. The needs for light, air, and open ground level for public activity spaces led to the evolution of the free standing skyscraper. It is much taller, since it must provide a density at least equivalent to the building block it is replacing. Present technology is far enough advanced to allow construction of the single skyscraper at an economically feasible cost. From a technological or material space point of view, the design of tall buildings is relatively well understood, however consideration of the behavioral space, that is, identification of human needs and space adaptability, is still in an early developmental stage. The isolation and lack of contact between people within the building, and the loss of contact with street life are some of the problems designers are trying to overcome. Although to some degree the density of tall buildings in cities is now controlled by zoning regulations, this design is not based on the context of the total, dynamic urban fabrics. The consequences to the urban environment of close grouping of tall buildings are of utmost importance. The impact of scale of some of the super skyscrapers on the city, such as the 109 story Sears Tower in Chicago, more than a quarter mile high, is apparent. The building's electrical system can serve a city of 147,000 people and its air conditioning complex can cool 6000 one-family houses. A total of 102 elevators are needed to distribute about 16.500 daily users to the different parts of the building Visualize the many elevators as equivalent to a dead-end street system and the sky lobbies as plazas where people pass from one part of the building to another either by nonstop, doubledeck, express elevators to the next sky lobby or by local, low speed shuttle elevators. Since the building contains all necessary services and amenities, theoretically the people have never to leave it. The support facilities, such as shopping, entertainment, recreation, health, education, security, transportation, parking, utilities, waste, and sewage services, are equivalent to the services needed for a small city. A building of this scale forms a city within a city. The design of such an intricate interaction system requires systematic programming of social, ecological, economical, and political implications exerted not just on the surrounding urban context but also on its own environment. For many metropolitan areas, the tall building is the only answer to continuous growth of population concentration. It should not be rejected because of its dehumanizing effects or put aside as a symbol of technological achievement. To the contrary, educational and other research institutions should take much more initiative to systematically investigate the high rise building environment and its context to improve its living conditions.

THE TALL BUlLDlNG AND lTS SUPPORT STRUCTURE

@@@ The design of a tall building, whether it be for such single uses as apartments, offices, schools, and hospitals or for the larger scale multiple uses just outlined, requires a team approach between the various disciplines of design, material fabrication, and building- construction. The architect coordinates the team effort so that the different material, service, and activity components act as a whole. No longer can the architect speak of freedom of design. Not only is he limited by the generally closed form of the skyscraper and the necessity for efficient usage of materials, he must also observe many more specifications related to complex security, fire, and health requirements. The architect must approach the design of a building as a total system in which the physical support structure as an Organic part grows with the design of the building; structural it cannot be considered separately as an unrelated addition to be plugged into the functional space later by the engineer. Though this total design approach should apply to the design of any architectural building, it is essential with respect to the scale of a high-rise, which requires rather complex structural support systems where the physical, environmental forces are among the primary design determinants. The building must cope with vertical forces of gravity and horizontal forces of wind above ground and seismic forces below ground. The building envelope has to accommodate the differences in temperature, air pressure, and humidity between the exterior and interior environments. The structural elements of the building must respond to all these forces. The members must be arranged and connected to one another in such a manner as to absorb the forces and guide them safely with a minimum effort to the ground. The architect who is sensitive to these forces and their sources, and is aware of the nature of structural order, can respond with a reasonable layout in the early design stage. He can communicate with the structural engineer because he talks his language that is, an architect having a basic understanding of engineering principles can truly collaborate with the structural specialist to achieve the optimum solution. The structural elements are the necessary bones for the building body, and it is the architect who can manipulate these structural elements and expose them to express clearly the spirit of the building, thus identify and reflect its purpose as an enclosure for the interaction of different activity systems.

LOAD ACTION ON HIGH-RISE BUILDINGS

@@@ Loads acting on a structure are generated either directly by the forces of nature or by man himself; that is, there are two basic sources for building loads; geophysical and man made. The geophysical forces, being the result of continuous changes in nature, may be further subdivided into gravitational, meteorological, and seismological forces. As a result of gravity, the weight of a building itself produces on the structure forces called dead load, and this load remains constant throughout the building's life span. The ever changing occupancy of a building is also subject to gravitational effects producing a variation of loads over a period of time. Meteorological loads vary with time and location and appear in the form of wind, temperature, humidity, rain, snow, and ice. Seismological forces result from the erratic motion of the ground (i.e., earthquakes). The man made sources of loading may be the variations of shocks generated by cars, elevators, machines, and so on, or they may be the movement of people and equipment or the result of blast and impact. Furthermore, forces may be locked into the structure during the manufacturing and construction processes. The stability of the building may require prestressing, which induces forces. Geophysical and man made sources for building loads are often mutually dependent. The mass, size, shape, and materials of a building influence the geophysical force action. For instance, if building elements are restrained from responding to temperature and humidity changes, forces are induced into the building. Careful studies of the building's theoretical response to load actions must be made, to ensure that future problems are eliminated and structural efficiency is achieved. The designer must understand forces and their respective load actions so that the building will be safe and serviceable.

DEAD LOADS

@@@ Relative to the gravitational forces to which a building is subjected, loads can be classified into two distinct categories; static and dynamic. Static loads are always a permanent part of the structure. Dynamic loads are all temporary; they change as time and season change, or as a function of spaces within or on a structure. Dead loads may be defined as the static forces caused by the weight of every element within the structure. The forces resulting in dead load consist of the weights of the load-bearing elements of the building, floor, and ceiling finishes, permanent partitioning walls, facade cladding, storage tanks, mechanical distribution systems, and so on. The combined weights of all these elements make up the dead load of a building. It appears to be a simple matter to determine the weights of materials, thus the dead load of a structure. However the estimate of dead loads may be in error by 15 to 20% or more because of various problems in making an accurate analysis of the loads. At an early design stage it is impossible for the structural analyst to predict accurately the weight of building materials not yet selected. Specific nonstructural materials to be chosen include prefabricated facade panels, light fixtures, ceiling systems, pipes, ducts, electrical lines, and components of special interior requirements. The weight of stiffening elements and joinery systems for steel structures is estimated only on a percentage basis. The unit weights of materials given by the producers or codes are not always consistent with those of the manufactured product. The nominal sizes of building elements may differ from the actual sizes ; the formwork for poured-in-place concrete may have inaccuracies of 1/2 in. These few examples indicate that in the absence of precise information, dead loads cannot be accurately predicted.

LIVE LOADS

@@@ Live loads differ from dead loads in their character; they are variable and unpredictable. Change in live loads occurs not only over time but also as a function of location. The change may be a short or long-term one, thus making it almost impossible to predict live loads in statical terms. Loads caused by the contents of objects within or on a building are called occupancy loads. These loads include allowance for the weights of people, furniture, movable partitions, safes, books, filing cabinets, fixtures, mechanical equipment (e.g., computers, business machines), automobiles, industrial equipment, and all other semipermanent or temporary loads that act on a building system but are not part of the structure and are not considered under dead loads. Given the potential versatility of high-rise structures, it is nearly impossible to predict the possible live load conditions to which a structure will be subjected. Through experience, survey analysis and practice, however, recommended load values for various occupancies have been developed. The results are in the form of load table listings given in building codes and featuring built-in empirical safety factors to account for maximum possible loading conditions. The load values take the form of equivalent uniform loads and prescribed concentration loads. Concentrated loads indicate possible single load action at critical locations such as on stair treads, accessible ceilings, parking garages (e,g., jack for changing a tire), and other vulnerable areas that are subject to high concentrated stresses. Although it may appear that the regulations are too conservative, there is always the unpredictable element to consider, the minimum regulated safety factors are warranted by such uncontrollable, extraordinary situations as people crowding because of ceremonies, parties, and fire drills, or overloading of parts of the building due to change of occupancy or furniture and wall rearrangements that will exert more load on a specific area; the likelihood of having a full occupancy load simultaneously on every square foot of every floor supported by a column is very slim. The actual loading consists of different areas with different loading conditions. Generally, the smaller the area, the larger the potential load intensity. The occupancy loads on floors are never uniform. Building codes take this into account by allowing the use of live load reduction factors. For example, the New York State Building Construction Code, excerpted below allows an 80% occupancy load on the top three floors of a building and a 5 % decrease per floor to at least 50% of assumed load. Notice that the 0.08% check permits an increase in the percentage of reduction with a corresponding increase in the amount of contributory area. Codes do not take into account that live load action on a building element is reduced because of the ability of the continuous building structure to redistribute loading as it deforms. On the other hand, the load capacity of buildings is reduced, since they are subject to fatigue brought about by years of combating wind loads, vibrations, temperature changes, settlement, and the continuous change of environmental forces. How ever the concrete materials have the advantage of gaining strength with age, therefore increasing their loading capacity. From a structural standpoint, the choice of an appropriate structural system depends on the knowledge of three factors:


        * The loads to be carried.   

        * The property of the construction materials.  

        * The structural action by which the load fores are 

           transferred through the members into the ground.

Bearing in mind these three elements, the structural designer uses realistic models to predict material and structural behaviour; however empirical code values are used to predict loading intensities, this seems to be contradictory, since the economy of construction and materials is considered in one case and neglected in the other. Future research will make possible more accurate prediction of actual loading conditions.

CONSTRUCTION LOADS

@@@ Structural members are generally designed for dead and live loads; however a member may be subject to loads larger by far than the design loads during erection of a building. These loads, called construction loads, constitute an important consideration in the design of structural elements. Every contractor has developed a construction process proved to him to be economical. Although an architect may design a building to suit a particular construction system, he may not know the individual practices of the contractor. Contractors commonly stockpile heavy equipment and materials on a small area of the structure, this causes concentrated loads that are much larger than the assumed live loads for which the structure was designed. Structural failures have resulted from such conditions. A major problem in concrete construction results when the contractor fails to allow sufficient curing time before removal of shoring and formwork. Concrete increases its strength with time ; but since time is money to the contractor, he may remove the forms before the concrete has reached its minimum design strength, whereupon the structural element may be subject to loads it is unable to support, and failure may result. Construction loads also must be considered for a beam designed to act compositely with a concrete slab, assuming that no temporary shoring is used during the construction process. In this case, the beam has to be checked with respect to carrying construction loads in noncomposite action. For precast concrete, the most critical period is at the lifting of a heavy panel element from its form. The number of lifting points and their placement must be known, also since the element has to be designed for any possible position it may encounter during handling and erection, impact and stress at that time must be considered.

SNOW, RAIN, AND ICE LOADS

@@@ Observation of the depth and density of snowfalls over years has resulted in a reasonable prediction of maxim loads. Snow loads need be considered only for roofs and other areas of a building that may gather snow, such as elevated courtyards, balconies, and sun decks. The snow loads, as established by codes, are based on the maximum snow on the ground. ln general these loads tend to be higher than the snow loads acting on a roof, since the wind blows the loose snow off the roof or the snow melts and evaporates because of heat loss through the roof skin. Codes generally allow a percentage reduction of the load value on pitched roofs, since the snow can easily slide off the roof. However certain roof conditions may influence the behaviour of the wind, resulting in high snow load accumulations locally, water, though not often thought of when calculating live load, should certainly be kept in mind when designing. Rain loads in general are less than snow loads, but it should be remembered that the accumulation of water, weighing 62.4 lb/ft3, will result in appreciable loads. Heavy loads can occur On flat roofs because of clogged drains, As water accumulates, the roof deflects, allowing more water to collect, and resulting in more deflection. This process is called ponding and may cause the eventual collapse of the roof. Ice will collect on protruding elements, especially on exterior ornamental elements that otherwise receive no load other than their own. It is therefore necessary to design and secure such elements to withstand heavy loads of icicles. Furthermore, the ice formation on open trussed structures will increase the area as well as the weight, resulting in larger wind pressure.

WIND LOADS

@@@ The first skyscrapers were not vulnerable to the complex consequences of lateral action caused by wind. The enormous weight of the masonry bearing wall building was such that wind action could not overcome the locked-in-gravity forces. Even when the bearing wall system was replaced by the rigid frame structure in the late 1800s, gravity remained the prime determining factor. Heavy stone facades with small openings, closely spaced columns, massive built-up frame members, and heavy partition walls still generated so much weight that wind action was not a major problem. The glass-walled skyscraper of the l950s with its optimum interior open space and relatively small weight was first to respond to the complexity of wind forces. With the introduction of the lightweight steel frame, weight was no longer a factor limiting the potential height of buildings. The era of the high-rise building, however, has brought with it new problems. To reduce dead weight and create larger, more flexible spaces, longer spanning beams, movable non load bearing interior partitions, and non-load-carrying curtain walls have been developed. All these innovations have taken away from the overall rigidity of the structure ; now the lateral stiffness (i.e. lateral sway) of a building may be a more important consideration than its strength wind action has become a major problem for the designer of high-rise buildings. To understand the wind and predict its behaviour in precise scientific terms may be impossible. Wind action on a building is dynamic and is influenced by such environmental factors as large scale roughness and form of terrain, the shape, slenderness and facade texture of the structure itself, and the arrangement of adjacent buildings. How do all these elements influence the speed, direction, and behaviour of the wind as it acts on a building?

WIND VELOCITY

@@@ Wind velocity readings were recorded at a specific height on a building, indicating two phenomen: a generally constant mean wind velocity and a varying gust velocity. Hence the wind has two components, one static and one dynamic. The mean wind, velocity in general increases with height. The rate of increase in the mean velocity is a function of the ground roughness, however, since wind is retarded near the ground by friction, the greater the interference by surrounding objects (i,e,, trees, land forms, buildings), the higher the altitude at which maximum velocity Vmax will occur.

WIND LOADING AS RELATED TO BUILDING CODES

@@@ Extensive research into the prediction of wind action on high-rise buildings is now being conducted. Building codes, however, still reflect a static approach to the dynamic action of the wind. Wind pressure values are given as functions of maximum annual mean wind velocities, in miles per hour, 30 ft above ground, for a 50 year recurrence interval. The maximum regional wind values are published by the U.S. Weather Bureau. The pressure generated by the wind on a building is calculated from the formula :


p = 0.002558(CD)(V)2, 



where :

 p  = pressure (psf) on a building face 

 CD = shape coefficient 

 V  = maximum mean velocity (mph)

The shape coefficient CD depends on the form of the building and the roof slope. For rectangular buildings CD = 1.3, which includes the pressure effect acting on tbe windward face (0.8) and the sucction effect present on the leeward side (0.5) of the structure. The New York State Building Construction Code gives minimum required wind loads as a function of the height of the building. The code values are for rectangular buildings and are based on tbe mean wind velocity of 75mph at 30ft above the ground. The preceding formula gives the wind pressure on a rectangular building 30 ft above ground with a wind velocity V=75mph as p=0.002558(l.3)(75)2 = 18.7psf This pressure value compares with the code requirements given above. For buildings hexagonal or octagonal in plan, tabular values may be reduced by 20%. For buildings that are round or elliptical in plan, values may be reduced by 40 %. The code approach is insufficient in predicting the true complexity of wind action because it fails to consider the dynamic nature of gust effects or the impact of the physical context on wind behavior. Designers must develop a better conceptual understanding of the dynamic character of wind behavior. The main types of wind action affecting high-rise buildings are discussed in the following sections.

TOPOGRAPHY AS WIND PRESSURE DETERMENT

@@@ A study conducted on the Earth Sciences Building at the Massachusetts Institute of Technology demonstrates several types of wind action and provides special insight into topographical elements affecting air movement. The M.I.T. Center is located in the center of a large courtyard north of the Charles River. To the east and west are rows of lower four or five-story buildings. lt was observed that high pressure airflow repeatedly blew off the river and moved northward through the courtyard even before the tower was constructed. Since construction, the M.I.T. Center has experienced unusually large wind velocities around and through the building. Especially critical is the wind action prevalent in the 21 ft high arcade at the base of the structure. At times wind velocities reach such proportions that pedestrians find it difficult to walk by the building or open doors to it. In an attempt to explain these occurrences, wind tunnel studies were made, using scale models. As a positive high pressure air mass moved from the Charles River, across the courtyard, it encountered the M.l.T. Center, creating a high pressure zone on the windward face.
Wind tunnel studies demonstrated that wind pressure was highest at the center of the windward face where wind motion almost stopped, lessening as the wind velocity increased toward the edge of the surface. The location of the arcade was significant in that the opening was placed on the windward surface at a point where maximum wind pressure was normally observed. In addition, the opening created an exit for a high pressure air mass to an area normally characterized by low pressure because of its location on the leeward side of the building. In combining these findings it is easy to see why wind velocities recorded in and around the building's arcade were at times twice the normal wind velocity of the area. One may conclude that wind velocity that is, wind pressure does not necessarily increase with height as is assumed by building codes. Pressure is greatest at midheight of the building with the arcade, or at the base if there is no opening.

WIND DIRECTION

@@@ All building movement is in response to wind direction. When an air mass moving in a given direction contacts a building surface, an overturning force is created. This overturning force is wind pressure, and it can become greater either by an increase in wind velocity or by an increase in the area of the obstructing surface. Substantial wind action on more than one building face may cause double flexure in the building. The primary wind direction can be separated into two components showing the resulting wind action on each building face. Double flexure may have either positive or negative effects on building motion, the multidirectional displacement may be less than it would have been if the same airflow had encountered the building on only one face. The aerodynamic design of the building may also help to alleviate building displacement in double flexure. Wind pressure is always greatest when the wind direction is perpendicular to the building face. Hence when the airflow strikes the building surface at other than 90degrees to the face, much of the wind force is naturally dissipated. Wind loads induced by double flexure, however, also place additional shear and torsional stresses on the structural members that do not develop in unidirectional displacement.

WIND PRESSURE

@@@ The wind pressure originates from two components, mean velocity and gust velocity. Since static mean velocities are averaged over longer periods of time, the resulting wind pressures are also average pressures and exert a steady deflection on the building. The dynamic gust velocities produce correspondingly dynamic wind pressures that create additional displacement possibly equal to the steady deflection of the building; for slender buildings they may become dominant! Such dynamic movement is called gust buffeting. The random forces created by gust action induce building oscillation generally parallel to the wind direction.

TURBULENCE

@@@ When any moving air mass meets an obstruction, such as a building, it responds like any fluids by moving to each side, then rejoining the major airflow, wind velocity is stabilized. ln any turbulent airflow positive air pressures are recorded as long as the air is in contact with the building's surface. When the building face is too sharply convex or the airflow is too rapid, the air mass will leave the surface of the building, creating dead air zones of negative pressure. Vortices and eddies are circular air currents generated by turbulent winds in these low pressure areas. Vortices are high velocity air currents that create circular updrafts and suction streams adjacent to the building. As the periodic shedding of vortices about the building approaches the natural frequency of the structure, oscillation occurs. The resulting motion is generally transverse to the direction of the wind. The shedding frequency is a function of the shape and size of the building and often can be reduced by the use of rough textured walls and/or irregular building shapes. Eddies, though formed much the same as vortices, are slow moving circular air currents creating little perceivable building motion.

TOLERANCE TO WIND ACTION

@@@ Human tolerance to wind action both inside and outside buildings has become an increasingly important factor in the design of high-rise buildings. Excessive lateral sway that a building's structural system may be able to withstand still must be reduced to the acceptable limits for human use. Some inhabitants in existing buildings have experienced motion sicknesses caused by building sway; people feel the movement and sense the twisting of the building. ln some restaurants on top of tall buildings wines are not clear when served because wind action has caused sediment to be stirred up. At times minor damage to furniture and equipment has occurred, strange creaking sounds from shaking elevator shafts and air leakage around windows were noticed, and unpleasant whistling of wind around the sides of the building itself was heard.
ln several buildings in the 40 to 5O story range in New York City, excessive lateral sway and noise have made it impossible for people to work at their desks. Employees are regularly excused from work during high wind storms. Strange occurrences observed outside high-rise buildings also cause discomfort and annoyance to both inhabitants and neighbours. Changes in the local wind character such as vortex currents formed in the wake of tall buildings have torn wash from clothes lines, damaged gardens, wrenched opened doors off automobiles, and scattered debris through the air. Some building occupants find it impossible to use balconies except on totally calm days because of constantly turbulent winds on the building face, worse jet, windows can be smashed or sucked from buildings, causing serious injury or death to people walking below. The list of examples can go on and on. What is important, however, is the need to recognize that a concern for human tolerance and the activities to be performed in and around the building must be a major factor in the design of today's high-rise buildings.

CONCLUSION

@@@ The true complexity of wind action on tall buildings is just beginning to be investigated. To find acceptable answers to the problems that are now apparent, designers must attempt to overcome the present limitations through the following avenues of investigation:


  * Wind tunnel studies using general models

     to establish a data bank of information for

     wind behaviour and wind loading.  

  * The derivation of scientific formulas and

     theory models tested against the wind

     tunnel data.  

  * Modification of existing building concepts

    through structural damping, flexural

    control, facade treatment and building form.

SEISMIC LOADING

@@@ The earth's crust is not static; it is subject to constant motion. According to the geological theory the surface of the earth consists of several thick rock plates that float on the earth's molten mantle. New tectonic plates are continuously formed along the deep rift valleys of the ocean floor, where molten material from the earth's interior is pushed upward, thus building up the edges of the oceanic plates to cause the so called continental drift; that is, the ocean plates are pushed against the continental plates. Where the plates collide they may be locked in place, thus being temporarily prevented from moving by the frictional resistance along the plate bound. aries. Stresses are built up along the plate edges until sudden slippage due to elastic rebound or fracture of the rock occurs, resulting in a sudden release of strain energy that may cause the upper crust of the earth to fracture along a certain direction and form a fault. Some of that energy is propagated in the form of shock waves in all directions. lt is this wave motion that is known as earthquake. lt is apparent that a fault which has suffered from earthquakes in the past is most likely subject of future disturbances.

BUILDING BEHAVIOUR DURING EARTHQUAKES

@@@ Since the foundation is the point of contact between the building and the earth, seismic motion acts on the building by shaking the foundation back and forth. The mass of the building resists this motion, setting up inertia forces throughout the structure; the action is similar to the lateral inertia experienced by a person in a car that stops suddenly. This is obviously an over simplification, since seismic motion also acts to distort the foundation and to shake it back and forth. Vertical inertia forces are ignored, thus we consider only horizontal forces that may exceed the wind forces acting on a structure. The practice of neglecting vertical forces, especially at a building location close to the surface rupture of a fault, is currently being reexamined. The application of the response spectrum to actual building structures depends on how closely the behaviour of a simple oscillator simulates the complex action of a building. Because of lack of more information, present codes use the response spectrum as a simple way to predict the maximum building response as caused by earthquake motion. The Uniform Building Code makes use of the seismic coefficient, which responds roughly in shape to the response spectra of some recorded earthquakes and reflects the fact that taller buildings, like taller pendulums, have a longer period of vibration, thus are subject to smaller inertia forces than stiff, short buildings having short periods. The bundled tube system can be visualized as an assemblage of individual tubes resulting in a multiple cell tube. The increase in stiffness is apparent. The system allows for the greatest height and the most floor area.

GENERAL ECONOMIC CONSIDERATIONS

@@@ The architect is usually obliged to respond to the purpose of many building types, in order to make money. As he forms a better understanding of the economic aspects of the design process, he may improve his chance of creating better architecture. The important point to realize is that a building system should not just be a preconceived preference; rather, it should incorporate careful consideration of economic factors. Thus two or more different methods of construction may hold up a particular building and may even look very similar, but one system usually is more economical to build. A designer must think not only about how much the project costs to build but also about how much the finished project costs to operate (e.g., expenses associated with utilities, maintenance, insurance, taxes, interest on borrowed money; he has to deal with the building economy. As the height of the building increases, more and more space is needed for structure, mechanical systems, and elevators, leaving less rental space.
In addition, the costs of elevators and mechanical systems increase with height. The same reasoning applies to contractor costs, since more sophisticated construction equipment is necessary as buildings get taller. However all these cost increases may be offset by the high land costs and the need for the building at a specific location. As the building height increases, the land costs per square foot of floor area obviously decrease. Similarly, management costs are reduced, since it costs less per square foot to operate one large building than several small structures. Accurate evaluation of all the complex economic considerations for high-rise buildings has come to depend on the computer. It is beyond human calculation to decipher all the factors along with all the ramifications of each factor concerned with the skyscrapers of today. The coordination of architect, engineer and contractor during a project's planning and drawing stage will improve the potential of achieving an economical solution. Such team efforts may allow building construction to start before all final drawings are completed. When construction begins earlier, buildings save money on inflating construction prices and earn profits sooner.

COMMON HIGH-RISE BUILDING STRUCTURES AND THEIR BEHAVIOUR UNDER LOAD

@@@ As building heights increase, the importance of lateral force action rises at an accelerating rate. At a certain height the lateral sway of a building becomes so great that considerations of stiffness, rather than strength of structural material, control the design. The degree of stiffness depends primarily on the type of structural system. Furthermore, the efficiency of a particular system is directly related to the quantity of materials used. Thus optimization of a structure for certain spatial requirements should yield the maximum stiffness with the least weight; this results in innovative structural systems applicable to certain height ranges. Some of the factors responsible for the development of these new systems are as follows:


  * High strength structural materials   

  * Composite action between structural elements.  

  * New fastening techniques (e.g., welding

     and bolting).  

  * Prediction of complex structural behaviour

     by computer.   

  * Use of lighter construction materials.

  * New construction techniques.

TUBULAR SYSTEMS

@@@ A recent development in structural design is the concept of tubular behaviour introduced by Fazlur Khan of S.O.M. At present, four of the world's five tallest buildings are tubular systems. They are the Hancock Building, the Sears Building, and the Standard Oil Building in Chicago, and the World Trade Center in New York. Tubular systems are so efficient that in most cases the amount of structural material used per square foot of floor space is comparable to that used in conventionally framed buildings half the size. Tubular design assumes that the facade structure responds to lateral loads as a closed hollow box beam cantilevering out of the ground. Since the exterior walls resist all or most of the wind load, costly interior diagonal bracing or shear walls are eliminated. The tube's walls consist of closely spaced columns around the perimeter of the building tied together by deep spandrel beams. This facade structure looks like a perforated wall. The stiffness of the facade wall may be further increased by adding diagonal braces, causing trusslike action. The rigidity of the tube is so high that it responds to lateral loading similar to a cantilever beam. As we see later, the exterior tube alone can resist all lateral loads entirely, or it can be further stiffened by adding interior bracing of some kind. The earliest application of the tubular concept, was first used in the 43-story Dewitt Chestnut Apartment Building in Chicago (S.O.M., 1961). In this Vierendeel tube system the exterior walls of the building, consisting of a closely spaced rectangular grid of beams and columns rigidly connected together, resist lateral loads through cantilever tube action without using interior bracing. The interior columns are assumed to carry gravity loads only and do not contribute to the exterior tube's stiffness. The stiff floors act as diaphragms with respect to distributing the lateral forces to the perimeter walls. Other examples of hollow framed tube buildings are the 83-story Standard Oil Building in Chicago and the 110-story World Trade Center in New York. Although these buildings have interior cores, they act as hollow tubes because the cores are not designed to resist lateral loads.

TUBE IN TUBE

@@@ The tube in tube approach bas been used in the 38-story Brunswick Building in Chicago, and the 52-story One Shell Plaza Building in Houston . Taking the tube-in-tube concept one step further, the designers of a 60-story office building in Tokyo used a triple tube. In this system the exterior tube alone resists wind loads, but all three tubes, connected by the floor systems interact in resisting earthquake loads, a significant design factor in Japan.

MODIFIED TUBE

@@@ Tubular action is most efficient in round and nearly square buildings. Buildings deviating from these forms present special structural considerations when tubular action is desired.

FRAMED TUBE WITH RIGID FRAMES

@@@ The hexagonal shape of a 40-story office building in Charlotte, N.C. forced the designers to modify the tubular principle. The pointed ends of this hexagonal building exhibited excessive shear lag, making it impossible to get effective tubular response. Adding rigid frames in the transverse direction served to tie the exterior walls together. Thus the end walls in triangular arrangement were reinforced by the rigid frames. By tying together the perimeter walls, effective tubular action was achieved. The irregular plan of the 32-story Western Pennsylvania National Bank in Pittsburgh gave rise to still another special solution of tubular design. In most tubular buildings, the tubular effect is generated by the exterior walls. In this building, however, the two intersecting octagons form a structural tube in the central part of the building. The two end portions of the building are stiffened by channel-like wall frame systems. The wind is resisted by the combination of interior tube and the huge exterior end-wall channels.

MODULAR TUBES

@@@ The latest development in tubular design is the modular or bundled tube principle. This system has been used for the Sears Building in Chicago currently the tallest office building in the world. The exterior framed tube is stiffened by interior cross diaphragms in both directions; an assemblage of cell tubes is formed. These individual tubes are independently strong, therefore may be bundled in any configuration and discontinued at any level. A further advantage of this bundled tube system lies in the extremely large floor areas that may be enclosed. The interior diaphragms act as webs of a huge cantilever beam in resisting shear forces, thus minimizing shear lag. In addition, they contribute strength against bending. The diaphragms parallel to the wind absorb shear, thereby generating points of peak stress at points of intersection with perpendicular walls. Indicating the individual action of each tube. Note the differents in axial stress distribution if there are no internal stiffeners that is, a single tube. The vertical diaphragms tend to distribute the axial stresses equally, although shear lag still occurs to some extent. However the deviation from ideal tubular behaviour, indicated by broken lines, does not seem to be very significant.

COMPOSITE BUILDINGS

@@@ ln the hybrid structure, a recent development aimed at increasing the lateral stiffness of framed skyscrapers, concrete and steel act together as a structural unit. This concept has been applied for several years to individual structural members such as floors and columns. However designing the entire building by composite construction is a completely new approach.

TUBULAR COMPOSITE BUILDINGS

@@@ In a system developed by Skidmore, Owings, and Merrill, the exterior steel frame is stiffened against lateral deformation by a tast-in-place concrete, perforated perimeter wall. The resulting sway of the building resembles that of a rigid tube cantilevering out of the ground. This approach combines the fast erection and high strength (thus the interior space flexibility) of steel construction, with the fireproofing, insulating, lateral rigidity, and mouldability of the concrete curtain wall. It has been used for the 36-story Gateway III in Chicago, the 5O-story One Shell Square Tower in New Orleans, and the 24-story CDC Building in Houston, where precast concrete panels became the formwork for the cast-in-place concrete. The construction process consists of first erecting the steel frame for 8 to 10 stories. Since the exterior columns only carry construction loads, they are lighter than the interior ones. The outer frame is temporarily table braced for lateral stability. Next cellular steel decking is laid and the concrete floor is poured, to stabilize the skeleton and to allow interior work to begin. After reinforcing cages and formwork are placed around the columns and for the spandrels, concrete is cast to form a continuous perforated wall. This process is repeated in increments of 8 to 10 stories.

However the difference in movement between the exterior steel-concrete columns and the interior steel columns causes a problem. An adjustment in the placement of girders must be made to overcome the unequal-shortening of the columns due to elastic behaviour, shrinkage, and creep. This system the tubular skin resists all lateral forces, and girders framing the utility core can be lighter because they carry gravitational loads only. Also, the net usable upper stories is increased where the core area is reduced. Reid and Tarics Associates of San Francisco developed another tubular composite building system. As facade structure they use steel spandrel girders and tubular steel columns filled with concrete. Again the envelope provides enough stiffness to carry all lateral loading. The prefabricated tree consists of a one tubular column two stories high and two cantilevering steel girders. The trees are bolted to each other at midspan of the beams and midheight of the columns; these connection points are the least stressed with respect to lateral loading. The natural continuity of the girders is not interrupted at the columns where stresses are highest; they pierce the columns with only the web connected to the tube. Thus the number of highly stressed building connections is greatly reduced.

COMPARISON OF HIGH-RISE STRUCTURAL SYSTEMS

@@@ The 102-story Empire State Building is characterized by a rigid frame-shear wall interaction system, indicated as applying to buildings less than 40 stories high. The chart is organized according to structural efficiency (i.e., optimization) as measured by the weight per square foot; that is, the weight of the total building structure divided by the total square footage of gross floor area.
Low- to medium-rise buildings are normally designed for gravity loads, then checked for their ability to resist lateral loads. However high-rise buildings are much more susceptible to lateral force action. With respect to gravity loads, the weight of the structure increases almost linearly with the number of stories. However the amount of material needed for resistance of lateral forces increases at a drastically accelerating rate. The most efficient structure is one in which the wind stresses superimposed on the gravity amount to an increase of less than 33% over the gravity stresses; the codes sanction an increase of 33% in allowable stresses if gravity and wind or earthquake act together. The Chase Manhattan Building is a long-span rigid frame that needs huge girders to resist wind forces. The IDS Building owes its efficiency to the belt truss system.
Disproportionate amounts of steel do not necessarily indicate that the structural design of the building is poor. For instance, the Civic Center in Chicago uses about twice as much steel as other buildings in its height range. However it has to satisfy the functional requirements of size and location of courtrooms. Thus the girder spans are 87ft and the floor heights are much larger than usual (i.e., 30 stories for 640 ft). Buildings that solely house office space have high loads and high story heights (12-14 ft); those containing some apartment units have lower loads and their story heights are less (9- 11 ft.) In recent years wind and earthquake loading have become determining factors in high-rise building design. The skyscrapers of today sway and oscillate, in contrast to the heavyweight high rise of the past. For instance, the Empire State Building (1931) deflects only 6.5 in., then vibrates 7.2 in. resulting in a maximum deflection of 10.1 in. at an 80 mph wind. The control of the dynamic response of a high-rise building can be achieved in the following ways:




  * By increasing stiffness through the use of

     an efficient structural system.   

  * By increasing the weight of the structure

     (not feasible).  

  * By increasing the building density through

     the addition of more structural and fill-in

     materials (not feasible).  

  * By selecting an efficient building shape.   

  * By generating additional forces in the

     building to counteract the external lateral

     action.

EFFICIENT BUILDING FORMS

@@@ Generally, architects are limiting high-rise building forms to rectangular prisms, which from a geometrical point of view are rather susceptible to lateral drift. Other building shapes are not as responsive to lateral force action. The rigidity of a building can be greatly increased by sloping the exterior columns, resulting in a truncated pyramid, a rather stiff, closed form (John Hancock Center in Chicago). The reductions in lateral drift range from 10 to 50%, with the greatest influence in the taller and narrower building structures. A variation of the John Hancock Building's truncated pyramid is the full pyramid of the 5O-story Transamerica Building in San Francisco. This 853 ft high building consists of a moment resisting facade frame having only the four corner columns joined at the spire, achieving A-frame action. The vertical interior columns do not intersect with the sloping exterior columns; they are discontinued 15 ft before intersection and support only the floors. Reduction of lateral building displacement can also be achieved by tapering the exterior frame, as in the 60-story First National Bank of Chicago. The structural benefits are greatest when the taper extends the full height of the building. In the First National Bank the taper of the exterior steel columns starts at one-third the way down from the top of the building. A cylindrical building form provides true tubular geometry and true-three-dimensional response to lateral loading. The Marina City Towers in Chicago are prime examples of that type of form. A typical tower consists of a ring of columns at the perimeter and around the corridor adjacent to the central concrete core. These columns reduce the required size of the radial beams and distribute the loads to the caisson footings. Approximately 70% of the lateral loads will be carried by the core. To preserve the lateral stiffness of the core, the openings in the core were staggered from floor to floor. In addition to the structural advantage of three-dimensional action, the cylindrical building offers less surface area perpendicular to the wind direction, thus the magnitude of the wind pressure is greatly reduced, compared to what a prismatic building experiences. Building codes permit a reduction of the wind pressure design loads for circular buildings by 20 to 40% of the usual values for comparably sized rectangular buildings.

The elliptical building offers advantages similar to the circular one.
The architect of the Le France Building in Paris claims a 27% reduction of wind loading attributable to the elliptical shape. The building's lateral loads are resisted by a central core and interior and exterior shear walls. Only shallow foundations were required, since the shear wall system distributes lateral forces across a wide area. Again building codes offer a reduction of the wind load requirements by 20 to 40% of the values required for a rectangular building. The triangular prism is another structurally efficient building form. The U.S. Steel Building in Pittsburgh uses an equilateral prism with notched corners to aid in reducing the building's lateral response to wind. The American Broadcasting Company Building in Los Angeles also has a triangular, prismatic shape. The tower rises 576 ft, and the exterior walls act as Vierendeel trusses. The trussed facade girders transmit gravity loads to the corner columns, and lateral loads are transferred by the floor diaphragm to the central core. A building can be a crescent or serpentine shape to increase its stiffness with respect to lateral forces. Its action is analogous to corrugated steel flooring and folded or undulating roof shells efficiently resisting gravity loading. The Toronto City Hall consisting of two crescent-shaped towers rising 20 stories (261 ft) and 27 stories (327 ft) above a two-story podium, uses this approach. The structure of a typical tower consists of an exterior, windowless vertical shell wall from which radial beams extend to a central row of columns and beyond to cantilever 6.5 ft to the curtain wall. The floor slab spans between the radiating beams. The gravity loads are carned by the column-shell-frame. Lateral loads are resisted by the vertical shell, which is stiffened by the ribbing action of the floor structure. The crescent shell form is efficient in resisting lateral forces acting symmetrically on it.
However it is rather inefficient when considering asymmetrical loading. This produces torsional stresses, which in the Toronto City Hall were counteracted by thick, vertical edge beams at the ends of the towers. The curved shape of the shell combined with the close proximity of the towers, greatly amplifies the wind pressure. Wind tunnel tests showed suction pressures at the sides of the building nearly four times higher than specified by codes.

THE COUNTERACTING FORCE OR DYNAMIC RESPONSE

@@@ There have been several examples of controlling building sway and oscillation in nonconventional ways. Each of these allows the building to respond dynamically rather than statically in withstanding external lateral action. Eugene Freyssinet of France and Lev Zetlin of the United States proposed to control lateral deflection of a building by introducing stressed tendons within the structure to generate an opposing deformation. This eliminates the need to achieve the lateral stiffness of a building by additional material, most of which may be used only once in 100 years to absorb maximum wind velocities. Cables near the exterior facade are attached to jacks at their base. A sensor unit measures the wind velocity and direction. This information is transmitted to a control unit at the base, which causes the jacks to tension the cables. This off-center tensioning induces a bending moment opposing the wind moment.
Thus the moments are neutralized and the lateral deformation is greatly reduced. The amounts of tension in the cables and in the building side being stressed vary according to the magnitude and direction of the wind pressure. The concept is analogous to the tension felt in the muscles of an outstretched arm when the hand accepts a heavy object and the arm attempts to maintain its position. Damping is another approach toward the reduction of the gusty wind effects on a high-rise structure. Like the dampers used for slowing down the closing of doors, nonstructural energy absorbers can be employed to decrease the swaying and oscillating motions of a building. In New York's World Trade Center viscoelastic dampers were attached to the bottom chord ends of the open web steel joints and to the adjacent columns. As the name suggests, viscoelastic material is both elastic (returns to its original position like a rubber band) and viscous (tends to flow under pressure like a liquid). Viscoelastic material resists forces in shear; it does not store energy like a spring, however, but converts it into heat that is diffused to the surrounding environment.
Hence after the forces are released, the material does not snap back and forth like a spring but slowly returns to its unstressed position. The building does not oscillate with damping; instead, gust winds emit heat inside the building because of the response of the dampers. The principle can also be applied to a building whose primary structure supports a secondary structural frame, as in the U.S. Steel Building in Pittsburgh. The secondary floor structure system, which is rather susceptible to lateral sway, can be attached to the columns through viscoelastic dampers, thus making it possible for the dampers to convert energy of oscillation into heat energy as the building laterally sways. In another proposal expansion joints with viscoelastic dampers would be inserted between floor structure and building core. Again, the relative movement of floor structure and rigid core causes energy of oscillation to be dissipated in the dampers. Fazlur Khan of Skidmore, Owings, and Merrill and Mark Fintel of the Portland Cement Association developed a concept that permits the control of damage caused by earthquake. ln their "Shock Absorbing Soft Story Concept," the first story is allowed to deform with the earthquake while the upper portion of the building remains almost unaffected, thus in the elastic range. The bottom story consists of a series of stability walls that are predesigned for forces greater than earthquake action, thus controlling excessive displacement of the upper story. Other ideas have been considered, but neither technology nor the precise understanding of the phenomenon has been developed to apply them as solutions. For instance, since the positive pressure on the windward building face generates suction on the leeward side, the pressure difference could be overcome through discharge of air on the leeward side. Thus not only is the magnitude of the lateral action reduced, there is also a diminution of the oscillations that form after a change in wind velocity allows air to rush into the negative pressure region and push the building in the opposite direction. It has also been proposed to use the wind pressure as a source of energy for building maintenance by allowing the wind to enter the building at certain locations. The principle of resisting wind pressure in any manner other than static is still in its infancy. The future will bring many new developments in this area.