HighRise Building Construction Profile

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High-rise buildings in the course of history Technology of high-rise buildings Risk potential Insurance

High-Rise Buildings

Münchener Rück Munich Re Group

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4.7.1 Conversions 122 4.7.2 Rehabilitation 122 4.7.3 Demolition 123 4.7.4 Disposal 126 4.8 Other risks 126 4.8.1 Terrorism 126 4.8.2 Impact 127 4.8.3 Collapse 132 4.8.4 Wear 132 4.9 Loss of profit 132 5 Insurance 138 5.1 Property insurance 139

5.1.1 Contractors’ and erection all risks

insurance 139

5.1.2 Advance loss of profit insurance 141 5.1.3 Insurance of contractors’ plant

and machinery 142

5.1.4 Decennial liability insurance 144 5.1.5 Insurance of buildings, fire insurance 144 5.1.6 Loss of profit insurance 145

5.1.6.1 Loss of rent 145

5.1.6.2 Additional costs 145

5.1.6.3 Contingency planning 145 5.1.6.4 Prevention of access 145 5.2 Third-party liability insurance 147 5.2.1 Insurance of the designer’s risk 147 5.2.2 Insurance of the construction risk 147 5.2.3 Insurance of the operational risk 148

5.3 Problem of maximum loss 149

5.3.1 Construction phase 149

5.3.2 Decennial liability insurance 149

5.3.3 Operating phase 149

5.3.4 Accumulation control 152 5.4 Underwriting considerations 153 5.4.1 Contractors’ and erection all risks

insurance 153

5.4.2 Contractors’ plant and machinery 153 5.4.3 Decennial liability insurance 153 5.4.4 Insurance of buildings, fire insurance 153

5.5 Reinsurance 155

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1 Introduction

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Page 5 1 Introduction

1 Introduction

From their beginning in the middle of the last century and right up to the present day, high-rise buildings have always been a dominant landmark in the townscape, visible from far and wide, like the towers of Antiquity and the Middle Ages. At the same time, this sky-scraping con-struction method has always been an ideal means of dis-playing power and influence in the community. In the light of this goal, reasonable economic considerations often recede into the background during the erection and sub-sequent use of these high-rise buildings.

A prestige object for the builder, these edifices not only have an effect on their immediate neighbours, but also influence many areas of urban life in very different ways. These aspects will also be taken up in this publication. In the early years, the builders’ urge to rise to dizzying heights was limited by unsolved technical problems. In recent years, however, a real competition has developed among the builders of skyscrapers to be world champion at least for a few months before being outdone by a rival with an even higher building. Even seemingly Utopian projects now stand a good chance of becoming reality. This rapid development has only become possible be-cause the technical conditions and methods used in con-structing high-rise buildings have improved decisively and in some cases changed fundamentally in the last few years.

Up until the end of the last century, high-rise buildings were still made of solid brick masonry, which ultimately required foundation walls up to 1.8 m thick. When steel frames adapted from steel bridge construction were intro-duced, with their increased strength and lower weight, builders and architects were able to soar to greater heights. With this steel skeleton, the net weight of the structure was considerably lower than that of a solid masonry building; it thus not only cut the costs of con-struction, but also gave wings to the architects’ imagin-ation. By the turn of the century, they were designing build-ings that also looked light and delicate as even at that time the skeleton structure permitted a large proportion of windows on the outer facade.

Since then, the construction of high-rise buildings has continued to change with the requirements imposed by air-conditioning and particularly office communications. The high-rise office buildings of the nineties have little in common with their predecessors. Instead of compact walls and ceilings, we now have a high-tech structure made up of largely prefabricated elements which are welded and bonded together on site. The building comprises a skeleton of steel or reinforced concrete which is rounded off by suspended ceilings and false floors creating the space required for installations. The originally

High-rise buildings have always triggered major

debates and aroused emotion. That is hardly

surprising, considering that

this type of building

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1 Introduction Page 6

load-bearing outer wall has been replaced by a pre-fabricated facade.

However, this complex method of construction promotes the spread of fire and fumes, and therefore, in conjunction with the considerable concentration of values involved, represents an extremely sensitive risk both during con-struction and throughout the service life of the building. The major fires which broke out in a number of high-rise office buildings shortly before their completion in the early nineties show how correct the appraisal of the fire risk in high-rise buildings is. The losses incurred through these fires are several times higher than the amounts of indem-nity known to date.

This is consequently one of the main reasons why high-rise buildings constitute a new dimension of risk for the insurance industry, one which has made it necessary to draw up new concepts for underwriting, loss assessment and PML determination throughout every phase of con-struction and subsequent use.

We therefore believe that this publication on high-rise buildings is an appropriate addition to our comprehensive series of special publications, particularly those on under-ground railways and bridges.

We are fully aware of the fact that many of the aspects considered with regard to the construction, use and insur-ance of high-rise buildings naturally apply in the case of lower buildings too. Nevertheless, we do not wish to limit ourselves to aspects which only apply specifically to high-rise buildings. After a brief historical overview, we will therefore consider in detail all the risks and problems

associated with high-rise buildings and the techniques that are applied in order to illuminate possible solutions from the point of view of both construction technology and insurance.

Moreover, the more broadly based general information available will make it easier not only to assess the risk of high-rise building projects but also to arrive at a price for insuring such projects.

The definition of a high-rise building differs from one country to the next. For our purposes, we will proceed on the basis of a minimum height of 30 m and will restrict ourselves to buildings used for residential or office pur-poses.

Despite the various critical voices raised, the construction of high-rise buildings has by no means reached its zenith. The problem of high-rise buildings is one which we – as insurers and reinsurers – will also have to consider in the future.

This special publication is also intended, last but not least, as a means of passing on to others our experience from the major losses that have occurred in the recent past.

02 SAN GIMIGNANO

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2 High-rise buildings in the course of history, technology and the environment

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2.1 Historical development 2.3 Financing models 2.5 Economic aspects

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04 THE TOWER OF BABEL

Page 9 2 High-rise buildings in the course of history, technology and the environment

2.1 Historical development

What could be a more appropriate point to begin our con-sideration of high-rise buildings than with the Tower of Babel and then to trace their historical development over the centuries. However, a distinction must be made between “high buildings” and “high-rise buildings”: “high buildings” have only a few floors and not uncommonly only one, albeit very high floor. They are crowned by a high roof and turrets (in the manner typical of medieval and Gothic cathedrals). “High-rise buildings”, on the other hand, have many, usually identical floors of normal height one above the other. Seen in this light, high-rise buildings have their origins in the towers of San Gimignano rather than in the Tower of Babel or ecclesiastical structures. The first high-rise office building according to this defin-ition was built in Chicago in 1885: the Home Insurance Building. It still stands on the corner of La Salle and Adams Street, a witness of its times. It has twelve floors – there were originally ten, but two were subsequently added – and was built in roughly eighteen months. The architect W. L. B. Jenney used an uncommon new method for the construction of his building: the weight of the walls was borne by a framework of cast-iron columns and rolled I-sections which were bolted together via L-bars and the entire “skeleton” embedded in the masonry.

The early Equitable Life Building in New York, which was completed in 1872, also contributed towards the develop-ment of high-rise buildings, for it was the first tall building to have an elevator. Although it only had six floors, the

edge of the roof was no less than 130 feet (roughly 38 m) above the road surface. Due to its elevator, the upper floors were in greater demand than the lower floors. Fol-lowing completion of the “Equitable” building, it was the done thing to reside on one of the “top” floors.

Burnham and Roof’s Monadnock building, which was completed in Chicago in 1891, must also be mentioned as one of the last witnesses of a whole generation of solid masonry high-rise buildings. Sixteen floors of robust brick masonry rise skywards in stern, clear lines: an astonishing sight to eyes accustomed to the frills and fancies of the late 19th century. Standing on an oblong base measuring 59 m ҂20 m, the building is reminiscent of a thin slice and not only recalls the industrial brick buildings of the late 19th century, but also anticipates the formal simplifi-cation of the later 1920s.

The buildings rose higher and higher with the spread of pioneering construction methods – such as the steel skeleton or reliable deep foundation methods – as well as the invention and development of the elevator. The highly spectacular skylines of North American cities, particularly Chicago and New York, originated in the early years of the 20th century.

Glancing over Manhattan’s stony profile, the silhouettes dotting the first 12 km of the 22-km-long island bear vociferous testimony to this dynamic development: – the World Trade Center, currently the tallest building in

New York, 417 m high,

– the legendary Empire State Building, built in 1931, 381 m,

According to the Bible, the Tower of Babel was to

“reach unto heaven” (Genesis 11).

But when the Lord saw what the people had done, He confused their language and scattered them abroad over the face of all the earth so that they left off building the city.

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Top left: 05 EQUITABLE LIFE BUILDING Bottom left: 06 HOME INSURANCE BUILDING Right: 07 NEW YORK PANORAMA

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2 High-rise buildings in the course of history, technology and the environment Page 12

– the United Nations building erected in 1953, 215 m, – the Chrysler Building dated 1930, 320 m,

– the former Pan Am Building completed in 1963, 246 m, – the Rockefeller Center (1931–1940), a complex of

19 buildings,

– the Citicorp Center built in 1978, 279 m, and – the AT&T Building opened in 1984, a pioneering

building by the post-modern architect Philip Johnson, with an overall height of 197 m.

It is only recently that attention has also turned to interest-ing high-rise buildinterest-ings outside North America: Norman Foster’s Hong Kong and Shanghai Bank, Ieoh Ming Pei’s Bank of China in Hong Kong and the twin tops of the Petronas Towers in Kuala Lumpur, currently the tallest building in the world at 452 m.

High-rise buildings in Germany are a modern develop-ment and are concentrated particularly in Frankfurt am Main: today, Frankfurt is the only German city with a skyline dominated by skyscrapers. One of the tallest build-ings in the city is the Messeturm built in 1991 with a height of 259 m, which is not much more than half the height of the Sears Tower in Chicago, currently the tallest office and business tower in North America with a total height of 443 m.

It was the rapid growth in population that originally pro-moted the construction of high-rise buildings. New York once again provides a striking example: land became scarce well over a hundred years ago as more and more European immigrants streamed into the city. From roughly half a million in 1850, the city’s population grew to 1.4 million by 1899.

More and more skyscrapers rose higher and higher on the solid ground in Manhattan, as buildings could only be erected with great difficulty on the boggy land to the right and left of the Hudson River and East River. In this way, New York demonstrated what was meant by “urban densification” despite the considerable doubts originally voiced by experts in conjunction with this development. The first area development code to come into force in New York was the so-called “zoning law” of 1916,

accord-ing to which the height of a buildaccord-ing must not exceed two-and-a-half times the width of the road running alongside the building. The building mass was further limited by the requirement that the floor space index must not exceed twelve times the area of the site. Among other things, the zoning law stipulated that only the first twelve floors of a building were allowed to occupy the full area of the site and that all subsequent floors must then recede in zoned terraces – a requirement of major aesthetic significance, for this terraced form still dominates the silhouette of American skyscrapers today.

All doubts as to the profitability of high-rise buildings were set aside with completion of the Empire State Build-ing, the Chrysler Building and other skyscrapers in the 1930s, for they would never have been built if they could not have turned a profit. Although rentals proceeded slow-ly at first when the Empire State Building was completed in the heart of the recession in the 1930s and it was there-fore known as the “Empty State Building” for many years, it subsequently generated satisfactory revenues once all the premises had been let.

Cities in Europe and Asia grew horizontally and it was only when production and services acquired greater eco-nomic significance throughout the world and the price of land rose higher and higher in economic centres after the Second World War that they also began to grow vertically. Modern Hong Kong is a striking case in point: it encom-passes an area of 1,037 km2(Victoria, Kowloon and the New Territories), of which only one-quarter has been de-veloped, but with maximum density and impressive effi-ciency. Almost all the new buildings, office towers and particularly residential towers in the New Territories have more than thirty floors.

2.2 Architectural aspects and urban development today As the historical development of high-rise buildings has already shown, the construction of edifices reaching higher

08 HONGKONGBANK 09 MESSETURM, FRANKFURT AM MAIN HEADQUARTERS BUILDING, HONG KONG

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and higher into the sky was – and to a certain extent still is – an expression of power and strength. This is equally true of both ecclesiastical and secular buildings: the power, strength and influence of entire families – i.e. their standing in society – is mirrored in the erection of ever taller buildings culminating in a battle to build. The towers of San Gimignano are one of the best preserved examples of this development. In many North African cit-ies, too, this attitude has moulded the townscape for many centuries and will no doubt continue to do so in the future.

The names of the builders and architects have only been known since the high Middle Ages around 1000 AD. They created new stylistic elements and added their “signature” to entire periods. Looking back, this makes it difficult for us today to decide whether these master craftsmen shaped the various stylistic developments or whether a number of master builders only became so well known because their work reflected the contemporary fashion trends most accurately. That still holds true today, the only difference being that tastes change very much more rapid-ly and “degenerate” into short-lived fashions. A building that reflects the spirit of the times when it is finished can appear “old” within only a few years. The brevity of the various stylistic trends is one of the reasons for the in-homogeneous appearance of modern towns and cities. Since architects must expect that later buildings will have their own, completely different formal identity, they do not see any reason why they should base their own designs on existing standards, particularly as this would merely cause them to be considered “unimaginative”.

Three points become clear if we take a closer look at mod-ern trends in high-rise construction:

– The dictate of tastes mentioned above is expressive of the egotism prevalent in modern society with its desire for status symbols and designer brands. Unfortunately, the public not uncommonly bows to this dictate, as when town councillors set aside major urban develop-ment considerations and with seeming generosity set up public areas in the form of lobbies and plazas in high-rise buildings.

– The sheer magnitude of the projects forces all planners to adopt a scale totally out of proportion to all natural dimensions and particularly to the people concerned when planning their buildings. In the past, urban devel-opment plans were easily drawn up on a scale of 1:100 or at most 1:200, a scale which could still be directly related to the size of a human being. With today’s high-rise buildings, however, a scale of at least 1:1000 is required simply in order to depict the building on paper. This is illustrated by the example of the Sears Tower in Chicago: completed in 1974, the Tower measures 443 m in height. Drawn to a scale of 1:2000, a human being is represented by a minute dot measuring barely 0.9 mm.

– In the past, it was the master builder and architect who defined the construction and consequently the appear-ance of a building; today, on the other hand, technical developments determine what can and cannot be done;

the appropriate and basically essential symbiosis between engineering designer and artist has been aban-doned.

This critical discourse on the architectural, urban develop-ment and economic background is not basically to cast doubt on high-rise buildings as such, but it does illuminate some of the facets that are central to considering the risk potential inherent in high-rise buildings.

This almost inevitably raises the question why high-rise buildings should have to be built in today’s dimensions. – One reason is indisputably the need for a “landmark”. In

other words, to express economic and corporate power and domination in impressive visual terms. Nothing has changed in this respect since the very first high-rise buildings were erected.

– The steadily rising price of land in prime locations and an increasingly scarce supply have made it essential to make optimum use of the air space. Prices in excess of DM 50,000 per square metre are not uncommon for land in conurbations and economic centres. Despite their height, however, high-rise buildings still occupy areas of truly gigantic proportions: the ratio of height-to-base width of the cubes in the 417-m-high World Trade Cen-ter, for example, is 6:1.

– Connections to the infrastructure are improved by con-centrating so many people in such a small area. The World Trade Center alone provides jobs for over 50,000 people – that is the equivalent of a medium-sized town. All institutions of public life are united under a single roof and the distances between them have been min-imized.

However, high-rise buildings do little to prevent land being sealed on a large scale. The suburbs of modern American cities are a prime example: as far as the eye can see, the landscape is covered with single-family homes, swimming pools and artificially designed gardens simply to provide sufficient private residential land for all the people work-ing in a high-rise buildwork-ing occupywork-ing only a few thousand square metres.

– Many of the techniques and materials which are also used for “normal” buildings today would never have been invented and would never have become estab-lished if high-rise construction had not presented a challenge in terms of technical feasibility. Rationalized, automated sequences are beneficial to high-rise build-ings; at no time in the past were such huge buildings erected in such a short space of time. Short construction periods also mean shorter financing periods and conse-quently profits which partly compensate for the add-itional costs incurred in the construction and finishing of the building.

2.3 Financing models

The construction costs for high-rise buildings often run into hundreds of millions of dollars. The owner of the building will rarely be willing or able to bear these costs

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without outside assistance. On the other hand, however, debt service and exhausted credit lines will then constrict his operative freedom. Alternative financing models are therefore frequently sought; the best known models are briefly outlined below.

LEASING

Leasing of buildings, particularly high-rise buildings, can to a large extent be compared with rentals. This alternative is commonly chosen when a company finds itself in finan-cial straits and needs cash. Selling the building – often a prestige object in a prime location – to a leasing company is of two-fold advantage to the company: firstly, it acquires the urgently needed capital, and secondly, it can continue to use the building in return for a monthly leasing fee which, however, amounts to no more than a fraction of the purchase price received.

The composition of corporate assets is changed by such a transaction. This can be a disadvantage when new loans are needed, for the building is then no longer shown on the assets side as a property secured by entry in the land register.

BOT

BOT stands for “build, operate and transfer” (there are other variations but these will not be discussed in further detail here). In the case of this financing model, the owner of the land places his land at the disposal of a contractor who then erects a building on it, such as an office tower. The owner of the land can exert a certain influence on the planning and intended use, but does not share in the con-struction costs. The contractor must organize the project’s financing himself, be it with own funds or with the aid of loans (“build”).

In return, the owner of the land waives all or some of the income from occupancy of the building for a certain period of time, usually 25 years. During this time, the builder must obtain rents that are calculated to cover his debt service and draw a profit from the invested capital (“operate”). The builder’s risk with regard to rents and debt interest is often considerable. At the end of the agreed occupancy period, both the land and the office tower become the property of the landowner (“transfer”). There are differences between these financing models: al-though the BOT model grants the landowner the right to ownership, he is for a long time excluded from occupancy of the property. With the leasing model, the high capital investment required is transferred to the lessor and the financing costs are replaced by monthly payments akin to rent by the lessee.

DEVELOPER

The developer is a new profession born out of the explo-sive rise in construction costs which has been intensified by increasingly large buildings and structures. This was triggered by urban renewal programmes and changes in tax regulations for large construction projects for which new financing models were developed in the USA in the sixties and seventies.

The developer usually draws up what is known as a mas-ter plan for complete districts and then retains (usually prominent) architects to design the various components of the master plan independently of one another. The devel-oper then seeks to find tenants or lessees for the building which at this stage only exists on paper. Construction work begins when tenants or lessees have been found. La Défense in the Paris Basin is a typical example of such a development.

This suburb was created on the drawing board in the 1950s. A dilapidated district was demolished and com-pletely redesigned. The traffic systems, such as Metro, urban railway, motorway and access roads were moved below ground level and covered by a concrete slab 1.2 km long.

Mostly office towers were erected on this slab with open squares and green areas in between. The ensemble is rounded off by the Grande Arche de la Défense designed by the Danish architect Johann Otto von Spreckelsen and completed in 1989. The Grande Arche is a huge cube which is open on two sides with 37 office floors and a height of 110 m equal to its ground lengths. All the capital invested on the site came from private sources and was controlled by a public-law community of interests. In times of sluggish investment activity, however, it is not uncommon to find that only certain parts of the master plan are actually realized. Originally planned as a homoge-neous townscape, the result is then nothing more than an unrelated fragment and areas that should have been filled with life appear to be deserted and uninhabited instead. In the mid-nineties London’s Docklands provided a dramatic example of such a development: the transformation of the West India Docks built between 1802 and 1806 resulted in what was for a while the highest mountain of debt in the world with the high-rise obelisk on Canary Wharf. After having consumed roughly US$ 3bn, the half-finished pro-ject was temporarily abandoned before finally being com-pleted and let following a variety of financial transactions.

2.4 Infrastructural aspects

The different fates of La Défense and Canary Wharf are not (only) due to the extremely different planning periods of 30 years (La Défense) and 8 years (Canary Wharf), but above all to the manner in which the necessary infrastruc-ture for the two projects was tackled.

In the case of La Défense, the entire necessary infrastruc-ture was completed before the construction work actually started: underground railway lines and roads, service systems were all planned and built beforehand. As a re-sult, a fully functional and above all adequately dimen-sioned infrastructure was consequently available when the buildings were taken into service. This made La Défense attractive to investors and tenants alike; the new district soon pulsated with life as an economically sound basis for the entire project.

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A jungle of political, economic and investment difficulties must be overcome for such prospective planning because the owner of the high-rise complex bears no direct respon-sibility for the large majority of these far-reaching infra-structural measures. The project’s progress is consequent-ly controlled by the municipal authorities, as well as by supply and operating companies and not by the owner of the complex.

The situation of Canary Wharf in London’s Docklands is exactly the opposite and proves that the La Défense type of planning is the economically more appropriate approach, despite the associated delay in starting con-struction work and the longer preliminary financing required.

A second City of London was to be created in the heart of the Docklands within the shortest possible space of time, with thousands of square metres of tailor-made office space, hotels, shops and apartments for high-income ten-ants. A rail-bound fully automatic cabin railway known as the Docklands Light Railway was to ensure the necessary access.

However, this transport system fell far short of meeting the requirements, as its capacity was far too low and it lacked the essential connection to the London Under-ground. The road connections for private traffic and public buses were similarly inadequate. This made the Docklands unattractive to both commercial and private tenants. An Underground link was finally built after extensive planning and at the enormous cost of roughly US$ 1.7bn; the road connections were likewise improved at the cost of almost US$ 1bn. Only then did the precarious economic situation of Canary Wharf improve.

As these examples show, almost every high-rise construc-tion project is doomed to at least economic failure if the infrastructure is not considered, planned and actually in-stalled down to the very last detail.

2.5 Economic aspects

Hundreds of companies and thousands of people depend on the smooth operation of a high-rise building, from the one-man business of a newspaper vendor or shoeshiner and corporations with thousands of employees, such as banks, brokers or global players with a daily turnover in the order of several billions to radio, television and tele-communications companies which use the roofs and tops of high-rise buildings for the transmission and receiving installations. In addition, there are innumerable other busi-nesses and workers with their families whose economic situation is directly or indirectly linked with the high-rise building. These range from transport companies and catering firms to tradesmen under long-term contract in the building.

Nor should it be overlooked that even the municipal au-thorities and the service companies are also affected by the “failure” of a high-rise building and that its effects can be felt nationwide or even worldwide in the worst case. This scenario not only applies to such total failure as a

major fire or collapse of the building. Despite (or precisely because of) its size, a high-rise building is an incredibly sensitive and vulnerable system. Even a brief power fail-ure can result in operational and economic chaos. The same applies to outside disturbances in the form of strikes by public transport corporations or a malfunction in the underground or urban railway system.

2.6 Social and ecological aspects

Criticism today focuses particularly on the social and eco-logical effects of high-rise buildings.

The most commonly voiced reservations with regard to high-rise apartment blocks concern the social aspect. It is claimed – and there are probably a number of studies to prove – that cohabitation in high-rise buildings does not work as smoothly as in homogeneous, historically grown districts with numerous small, manageable dwellings. The anonymity suffered by the people in these “residential fac-tories” is criticized in particular – above all on account of the total isolation from other residents in order to avoid the stress of permanent contact.

Organic, homogeneous population structures with their positive effects on social conduct are rarely found and the charge that high-rise apartment blocks are hostile to fam-ilies and children is consequently not entirely unfounded. Two diametrically opposed ghetto situations can easily arise in high-rise apartment blocks: since the costs for con-struction and maintenance of these buildings are dispro-portionately high, correspondingly high rents must be charged, with the result that these blocks are more or less reserved for the well-off, while the socially weaker classes are excluded. Conversely, however, high-rise apartment blocks can rapidly cease to be attractive if compromises are made with regard to the building quality, maintenance or infrastructure on account of the high investment costs entailed. A building in disrepair will soon drive away the “good” tenants and become a slum.

The ghetto situation is intensified when high-rise apart-ment blocks are built in newly developed fringe areas – far away from cultural and social centres – on account of the high cost of land in inner city areas. It is not without good cause that these areas are commonly referred to as “dor-mitory towns”.

Studies have also proved beyond all doubt that criminal activity is promoted by huge apartment blocks and particu-larly high-rise buildings. According to these studies, this phenomenon is attributable to the anonymity of the resi-dents, as well as to the “pro-crime” environment with ele-vators, poorly lit corridors devoid of human beings, refuse collection rooms and bicycle garages, laundries and above all underground parking lots. It is a proven fact that con-siderably more murders, burglaries, muggings, rapes and other crimes are committed in such buildings than in resi-dential areas with smaller rented or private homes. Not only high-rise apartment blocks have a usually nega-tive effect on people’s social environment: office towers are equally disadvantageous. The vertical structure of the

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buildings simultaneously underlines the vertical hierarchy: the location of the office space becomes an indicator of a company’s “importance” and, if the company occupies several or all the floors in a high-rise building, it may also be indicative of the employee’s standing in the company. The company’s top executives reside on the uppermost floors with the best views; the floors below provide a shield and every employee can positively see the distance between himself and “them up there“. It is therefore not wrong to question whether high-rise office towers are really appropriate to modern organizational structures with their emphasis on team work and interdisciplinary cooperation.

Excessive energy consumption is a major shortcoming of high-rise buildings and one which could possibly lead to their demise one day. High-rise buildings are the farthest removed from the ideal form as regards energy efficiency – namely the sphere, or the cube in the case of houses. That applies to both heating and cooling: some skyscraper facades have to be cooled by day and heated by night in order to avoid undue stresses and the resultant damage. The World Trade Center, for example, consumes some 680,000 kWh/day electricity for air-conditioning during periods of strong solar irradiation; the Messeturm in Frankfurt burns up energy worth DM 40 per square metre of useful floor space for heating and cooling every month. A well insulated low-energy house, by comparison, uses energy worth less than DM 1 per square metre. The “energy balance” of high-rise buildings is also poor in other respects such as the water supply, which usually only operates with the aid of booster pumps, as well as in terms of the disposal systems and operation of the elevators, etc. From the point of construction economy in general, high-rise buildings will probably always be the poorest conceiv-able solution, from the particularly energy-intensive and therefore expensive construction as such to the dispropor-tionately high demolition costs. Moreover, high-rise build-ings are made almost exclusively of materials which a construction biologist would take great pains to avoid, namely concrete, steel, light metal, plastics and a wide variety of chemicals.

Although subjectively unaware of the fact, the residents are frequently exposed to constant stresses in the form of pollutant emissions and electrosmog. High-rise buildings are sometimes described as microcosms; that is no doubt meant in a positive sense, but the reality is different. The people in a high-rise building are totally cut off from the world around them, from wind and weather, from tem-perature, from smells, sounds and moods. They live in an artificial world.

At the same time, however, the high-rise buildings also have a negative effect on the world around them, for they not uncommonly generate air turbulence and downdrafts in their immediate vicinity; they can be a source of un-pleasant reflections and some adjacent areas remain per-manently in the shade. Illuminated facades and large glass fronts are a death trap for many birds.

The people outside the high-rise buildings also often have the feeling that they are being observed or threatened by

the possibility of falling objects. That fear is surely not en-tirely unfounded, for there have been cases in which parts of buildings, such as glass panes, have been torn out of their anchorage by strong winds and injured or even killed people on the street below.

Our love-hate relationship with high-rise buildings is final-ly also revealed in such recent box-office hits from Holfinal-ly- Holly-wood as “Deep Impact”, “Godzilla” or “Independence Day”. It seems that their directors simply cannot avoid the temptation of reducing one of New York’s most beautiful buildings – the Chrysler Building – to a smouldering heap of rubble with the help of floods, monsters or meteorites. As a result, these skyscrapers more or less become the real stars of the film on account of their magic attraction and immediate recognizability.

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3 Technology of high-rise construction

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Page 25 3 Technology of high-rise construction

3.1 Planning 3.1.1 Planners

The complexity of the trades to be coordinated has be-come several times greater since then. Take, for example, the new block built for Südwest-Landesbank in Stuttgart: many disciplines and different experts were involved sole-ly in the project planning:

– Architects

– Planning engineers for the supporting structures (engi-neering design and structural analyses)

– Construction and site management (resident engineer) – Planning of the technical building services (particularly

heating, ventilation, sanitation, cooling and air-conditioning)

– Interior designers

– Construction physics and construction biology – Planning and site management for data networks – Planning of the lighting and materials handling – Planning of the electrical and electronic systems – Planning of the facades

– Surveying engineers

– Geotechnology, hydrogeology and environmental protection

– Design of outdoor facilities and vegetation

– Surveying of the actual situation in surrounding build-ings

If we were to include all the contractors and specialists in-volved in the project as well, the list would probably be ten times longer. And if we then consider that bankers, construction authorities, legal advisers and even advertis-ing agencies or brokers must also be coordinated in the

course of the entire planning and construction of a sky-scraper project, it soon becomes clear that highly profes-sional management is essential for such a project. Project management companies have come to play an increasing-ly important role in recent years as they take over the en-tire organization, structurization and coordination of con-struction projects. They act as professional representatives for the client and embody the frequently voiced desire for the entire project to be coordinated by a single partner. 3.1.2 Regulations and directives

The various laws, regulations, directives and standards in force must be taken into account when planning and erect-ing a builderect-ing. The plannerect-ing engineers are also obliged to observe what are known in Germany, for instance, as the “generally accepted technical rules for construction“; in other words, generally applicable technical and trade rules must be taken into account and observed in addition to the standards and regulations.

Although each country has its own regulations and dir-ectives governing the construction of high-rise buildings, they are all basically similar in content with a few differ-ences depending on the local circumstances. It is standard practice in some countries to base the bidding and plan-ning phase for projects on foreign standards (particularly on the American ANSI Codes and UL Standards, British Standards or the German DIN standards) or to include var-ious elements of these foreign standards in the national system of standards.

As a rule, these regulations are primarily designed to en-sure personal safety and then to protect the building against damage and defects. In addition to the requiments imposed by public authorities, there are also

re-Skyscrapers are

gigantic projects

demanding

incred-ible logistics, management and strong nerves among

all concerned in their planning and construction.

As long ago as 1928, the American Colonel William

A. Starrett wrote that no peacetime activity bore

greater resemblance to a

military strategy

than the

construction of a skyscraper.

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3 Technology of high-rise construction Page 26

quirements imposed by insurance companies with the aim of ensuring greater protection for property. These require-ments can be classified in four groups:

FIRE PROTECTION AND OPERATIONAL SECURITY

Many of the construction regulations concern fire protec-tion. There can be many thousands of people in a high-rise building at any one time. If a fire breaks out, they must all be able to leave the building in the shortest possible space of time and without risk of injury. This is why regulations concerning the number and execution of escape routes and fire escapes, fire compartments and the choice of materials must be observed (see Section 4.2.5).

Operational security encompasses regulations governing the safety of elevators and escalators, the execution of stairs, railings and parapets or the installation of emer-gency lighting. Some regulations also include CO2alarm systems for underground parking lots; indeed, there are even regulations governing the non-slip nature of floor coverings in traffic areas, sanitary rooms and kitchens. STABILITY AND CONSTRUCTION PHYSICS

The regulations governing the stability of a building are usually met by the requisite structural analyses. In add-ition to demonstrating the internal structural strength of the construction and safe transfer of loads to the subsoil, the stability calculations must also include possible de-formation due to thermal expansion, wind loads and live loads or dead weight, for example. This is closely related with demonstrating the safety of the construction, for in-stance by taking steps to limit the (unavoidable) cracks in concrete elements.

PROTECTION AGAINST NATURAL HAZARDS

The regulations and directives governing protection against natural hazards are usually closely associated with the demonstration of stability. Windstorms and earth-quakes are the most serious natural hazards for high-rise buildings. As a rule, the assumed loads and design rules for the “load cases” of earthquake and windstorm will be specified by the regulations in order to ensure that the building will withstand windstorms or earthquakes up to certain load limits. At the same time, this will serve to rule out the risk of bodily injury due to falling parts of the building, especially parts of the facade.

SOCIAL ASPECTS AND PROTECTION OF THE SURROUNDINGS The regulations governing social aspects and protection of the area surrounding high-rise buildings are designed above all to prevent any indirect risk or threat to people. Such regulations may concern planning aspects, such as the minimum distance between a high-rise building and neighbouring buildings, or they may take the form of rules defining the maximum permissible influence that a build-ing can have on the microcosm surroundbuild-ing it.

Depending on the location of the high-rise building, cor-responding statutory instruments may also govern the effects on air traffic safety or the building’s influence of radio communications.

This exceedingly concise outline of applicable regulations illuminates only some of the rules to be observed when building a skyscraper. If all the regulations governing high-rise construction were to be stacked one on top of the other in printed form, they would themselves be as high as a multi-storey building.

3.1.3 Technical analyses and special questions Planning a high-rise building would be inconceivable today without the help of experts and technical consult-ants. Extensive soil analyses are required to determine the strength of the subsoil before deciding on the location for a high-rise building. In the majority of cases, cores are drilled into the load-bearing subsoil to obtain soil samples. The drilling profile of the geological strata making up the subsoil and laboratory analyses of the soil samples pro-vide the basic data for the soil report which is in turn used as the basis for planning the supporting structures and choosing a suitable foundation structure with due regard for the loads exerted by the high-rise building.

The forces acting on the high-rise structure in the event of an earthquake must be taken into account when erecting high-rise buildings in areas prone to seismic activity. The same applies to wind loads and particularly to the dynamic effects of windstorm or earthquake loads. The additional vibration loads can result in overall loads of the same order of magnitude as the load exerted by the dead weight of the structure. The situation is particularly critical if the vibrations reach the resonant frequency of the build-ing: in such a case, the vibrations can intensify until the entire building collapses. The collapse of the Tacoma Bridge in Washington State, USA, was probably the most spectacular case of destruction due to resonant vibration in a man-made structure.

In many cases, these effects cannot be determined by or-dinary computation. Even computer simulation cannot al-ways help. Sometimes a decisive element may be lacking to obtain a mathematical approximation; in other cases, the computer may be too slow or the storage capacity in-adequate.

This frequently makes it necessary to carry out model experiments in a scientific laboratory. Models of the high-rise buildings are exposed to artificial earthquakes on a vi-bratory table or subjected to a simulated hurricane in the wind tunnel. A detailed knowledge of mathematics and physics is necessary to ensure that the same physical properties and serviceable results are obtained despite the reduction in scale. For this reason, these studies can only be carried out by highly specialized test institutes. 3.1.4 Construction licensing procedure

The construction licensing procedure is normally specified in the construction laws of the country concerned. As a rule, the principal will file an application with all the requis-ite documents (description, plans, analyses, etc.) to the relevant construction supervisory authority. The involve-ment of specialists is obligatory in the case of larger and more complicated projects, such as those involving

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high-17 DETAILS FROM PLANNING DOCUMENTS Next page: 18 EXTRACT FROM A TECHNICAL REPORT

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rise buildings. Such specialists include experts from the municipal fire brigade, water authorities, trade supervisory offices, environment protection agencies or similar offices in other specific fields.

These specialists review the applications for a construction licence and specify any additional requirements to be met. The licence is then sent to the principal together with the requirements specified by the specialists; responsibility for complying with these requirements rests with the principal or owner of the building.

3.1.5 Other constraints

Even in our high-tech era, the planning and construction of a high-rise building are not dictated only by naked fac-tual constraints. Tradition, religion and even the belief in spirits and demons still play a not insignificant part in many countries.

Take, for example, the Hong Kong and Shanghai Bank building in Hong Kong: during the planning phase, a geo-mancer or expert on “fung shui” (i.e. “wind and water“) repositioned the escalators and moved executive offices and conference rooms to the other side of the building on the basis of astrological investigations and measurements in order to guarantee an optimum sense of well-being for clients and employees. However, it must be said that such intervention is limited by technical and structural require-ments.

In western countries, too, the owners are guided by similar considerations when the 13th floor is omitted from the planning or the technical installations are deliber-ately located on this floor in order to avoid the unlucky number 13.

19 OPENING IN AN APARTMENT COMPLEX ALLOWING NEGATIVE VIBES TO PASS THROUGH

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3.2 Execution 3.2.1 Foundations

Although the foundations are out of sight once the build-ing is completed, they are of immense importance for en-suring that the dead weight and live loads of the building are safely transmitted to the native subsoil.

These loads are not inconsiderable. The dead weight of a high-rise building can amount to several hundred thou-sand tonnes. This value may be exceeded several times over by the live loads which are taken as the basis for de-signing the building and include the loads from equipment and furnishings, people or moving objects, as well as wind or earthquake loads. Moreover, these loads often exert dif-ferent pressures on the subsoil, thus resulting in uneven settlement of the building. In order to avoid such develop-ments where possible, these buildings must be erected on subsoil of high load-bearing capacity, such as solid rock. Yet even if a strong native subsoil is found near the sur-face, shallow foundations will frequently be disregarded in favour a system that transfers the load to deeper layers on account of the high bending moments to be absorbed from horizontal forces.

This can be done in several ways. One is to produce round or rectangular caissons which are lowered to the required depth and bear the foundation structure. Pile foundations are probably the most widely used method, however. The piles can either be prefabricated and then inserted in the native soil or they can be produced on site in the form of concrete drilling piles. Which method is chosen will ultim-ately depend on both the structural concept and the soil

conditions prevailing on site. Drilling piles in a whole var-iety of forms can be used when working with large pile diameters and very long piles. Modern equipment can easily ram piles measuring up to 2 m in diameter to depths of well over 50 m. The piles are then combined into appropriate pile groups in accordance with the loads to be transmitted by the building.

Although the load-bearing capacity can be roughly calcu-lated on the basis of soil characteristics, the maximum permissible pile load is determined by applying test loads to the finished piles with the aid of hydraulic presses and comparing the resultant settlement with the permissible settlement.

Diaphragm walls are another means of producing deep foundations. These walls are produced directly in the ground and are between 60 and 100 cm thick. They are produced in sections with the aid of special equipment and a stabilizing bentonite slurry. The result is a continu-ous wall in the ground. This method is used in particular when subsoil of high load-bearing capacity is only found at considerable depth.

Diaphragm walls and piles are also used to safeguard the foundation pit required for construction of the under-ground part of the building. The effort entailed can be con-siderable, particularly if the neighbouring buildings are very close. Rotating drills are mostly used today to minim-ize vibrations when installing the retaining wall. Founda-tion pits can easily be produced to depths of 30 m or more using this method.

20 LARGE-BORE PILE FOUNDATION PROCESS

Bottom: VARIOUS STAGES IN THE DIAPHRAGM WALL PROCESS 21 Following page: DIAPHRAGM WALL ROTARY CUTTER

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22 RETAINING WALL TO PROTECT NEIGHBOURING BUILDINGS

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3.2.2 Supporting structure 3.2.2.1 Load-bearing parts

The steel skeleton permitted hitherto inconceivable flexi-bility in construction and layout planning. It also permitted series construction up to great heights, since the vertical dead weight was considerably lower than when using solid masonry and did not make it necessary to grade the sectional steel profiles in these areas. The tradition of steel skeleton structures predates the first high-rise building to have been erected by this method, namely the Home In-surance Building in Chicago (1885): mills and granaries, as well as engineering structures (bridges, silos) had already been built in England with an iron framework towards the end of the 18th century.

The first frame structures used for the steel skeleton were flexurally rigid frames corresponding in height to one floor. New York’s Empire State Building, which was completed in 1932, is one example which clearly shows the advantage of this new method, namely the short time required for the construction work. Moreover, the complete separation of outside wall and supporting structure permitted absolute freedom of design for the facade. Instead of requiring around 300 kg of steel per square metre of base area as in the past, modern support-ing structures only require roughly 125 kg of steel on average.

As the buildings became taller and taller, however, the main problem was no longer the vertical loads but such horizontal loads as wind and earthquake forces, as well as their transmission.

This led to the development of what was known as the core method. The individual floors with their secondary supporting structure, namely the columns, are suspended from a central core as the primary supporting element, normally in the form of a reinforced concrete or steel structure with reinforcing shear walls. The columns merely transmit vertical loads, while the core transmits both verti-cal and horizontal loads. Its primary function is to rein-force the building in horizontal direction. The cores and their surrounding walls normally accommodate vertical service installations, such as elevators, stairs, primary ser-vice shafts for electric power and HLS (heating, lighting, sanitation). A similar supporting effect is obtained with the aid of horizontal reinforcing elements in the form of shear walls, which may be considered as an “open core“. How-ever, such supporting structures are rarely found in taller buildings.

Since the middle of the 20th century, a number of im-provements in the supporting structures for skyscrapers have been introduced by the architects Skidmore, Owings & Merrill (SOM) in Chicago. One such development by SOM is the “outrigger truss”: a rigid superstructure known as the outrigger is mounted at the top of a reinforcing core with movably connected floors and columns. The outrig-ger connects the columns to the core. They are suspended from the outrigger and are therefore under tension, thus eliminating the risk of buckling that is associated with pressure elements. A supporting system in the form of

such an outrigger truss yields further advantages over a simple core construction when it comes to transmission of the horizontal loads. The bending stress applied to the core area in the lower floors is considerably reduced when using an outrigger truss. The outrigger itself usually ac-commodates such technical floors as the heating and ven-tilation systems.

The Fort Wisconsin Center built in Milwaukee in 1962 is one example of an outrigger truss structure. The produc-tion of such suspended structures gave rise to a number of innovations, such as the lift-slab process for concrete structures. The load-bearing cores are first of all erected with the outrigger on top; the individual floors are then concreted on the ground, one above the other (separated by a release spray). Finally, they are raised to their installa-tion posiinstalla-tion by means of hydraulic jacks and then con-nected to the core (see Section 3.2.2.2).

Supporting steel structures in the form of tubes are often used for extremely tall buildings. In this case, the support-ing structure is located in the outer facade, which is conse-quently designed in the form of a load-bearing facade with small openings. The result is an enclosed, intrinsically rigid tube without any unnecessary space-filling columns inside. The World Trade Center in New York is an example of such a structure. The outer walls are studded with verti-cal steel columns roughly one metre apart. A generously dimensioned development area was obtained on the ground floor by “collecting” the descending columns. America’s tallest skyscraper, the Sears Tower in Chicago (443 m high), is a further development of the conventional tube: it is a “bundled” tube. The layout of the building is subdivided into a number of tubes to relieve the columns in the corners of the building when subjected to horizontal loads; this results in more uniform distribution of the load over the facade columns. In this case, however, the inter-ior can no longer be designed with the same flexibility as when using a single tube.

The “truss tubes” perfected by Fazlar Khan (SOM) in the John Hancock Center in Chicago are another further devel-opment of the basic tube. These tubes are additionally re-inforced by diagonal struts in the facade plane and are a structural feature that has almost become a hallmark of SOM buildings.

It was only in the mid-1970s that concrete began to be more widely used in constructing skyscrapers. Until then, the length of time required for concrete construction and the associated financing problems were the main reasons for the predominant use of steel structures in the construc-tion of high-rise buildings. New developments in shutter-ing, however, resulted in dramatically shorter construction times. The octagonal concrete core of the Messeturm in Frankfurt, for example, was erected with the aid of a slip-form which was hydraulically raised one metre every day. The latest developments in supporting structures for high-rise buildings include composite structures of steel and concrete, for instance in the form of steel sections embed-ded in concrete.

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25 DEFORMATION AND BENDING MOMENTUM DUE TO WIND WITH THE CORE CONSTRUCTION METHOD 26 Background: COMMERZBANK BUILDING

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27 DEFORMATION AND BENDING MOMENTUM DUE TO WIND WITH THE OUTRIGGER TRUSS METHOD Below: 28 EXAMPLES OF CORE CONSTRUCTION METHODS (A-E) AND

BUNDLED TUBES (F-G)

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29 VARYING LOAD DISTRIBUTION WITH TUBES AND BUNDLED TUBES

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Top: 30 EXAMPLE OF THE ARRANGEMENT OF BUNDLED TUBES

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3.2.2.2 Special construction methods BMW HEADQUARTERS, MUNICH

The headquarters of BMW A.G. differs from conventional buildings to create an impressive corporate symbol in the form of a 100-m-high four-cylinder structure. The require-ments for appropriate office organization yielded a basic outline in the shape of a clover leaf. Stairways, elevators and sanitary areas are accommodated in the central core. In this way, all the offices can be reached by the shortest possible route. Trendsetting methods were also used for the construction work.

A reinforced concrete version was chosen as the most economical solution. According to the design concept, the entire building with 18 office floors and a technical floor was to be suspended from a girder cross at the top of the roughly 100-m-high core via four central king posts. This is a modification of the outrigger truss (Section 3.2.2.1). The entire load of the building is transmitted to the founda-tions via the core as the central element; it also absorbs all wind forces. A mighty girder cross with a projection of 16 m is mounted at the top of the core.

The four king posts are secured to this central girder cross, each king post comprising 105 threaded steel bars with a load-bearing capacity equal to a suspended weight of 4,600 Mp. Small outer columns are additionally located between the floors. These outer columns are designed as compression columns above the technical floor (12th floor) and as king posts below.

Time and costs were the decisive reasons for choosing this innovative construction method. All 19 floors were successively produced at the foot of the shell and core; the first floors were even produced complete with facade and

glazing during construction of the supporting cross. The finished floors were then connected to the supporting cross via the king posts and raised one floor at a time every week with the aid of hoisting gear so that another floor could be produced in the space vacated at the foot of the core and then connected to the floor above (lift-slab method). Completion of the facade, glazing, installation and interior finishing proceeded on the suspended floors, unimpeded by the structural works and lifting operations. In addition to reducing the construction time required, this method also eliminated the need for expensive tooling and assembly work.

LA GRANDE ARCHE, PARIS

This building, which has already been mentioned in Sec-tion 2, takes the form of a giant cube open on two sides with edge lengths of 110 m. It was completed at the end of 1989 on the 200th anniversary of the French Revolution and took 5 years to build (see photo on page 18).

The building has a weight of more than 300,000 Mp and is mounted on neoprene bearings, the loads being transmit-ted 30 m into the subsoil via twelve concrete pillars. The cube’s main support is in the form of four prestressed upright reinforced concrete frames 21 m apart. They are complemented by horizontal members measuring roughly 70 m at ground and roof level. Each of these members is 9 m high, the equivalent of a 3-storey building. Since the two vertical sides of the cube would be without roof-level transverse bracing during construction, the required stabil-ity for that phase of the work was produced by means of horizontal steel truss reinforcements.

A total of 37 office floors are accommodated in the two 18-m-wide wings of the cube (each with an area of 42,000 m2

).

Top: VIEW FROM THE HEADQUARTERS BUILDING Bottom: 32 HEADQUARTERS OF BMW A.G. IN MUNICH

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3.2.2.3 Facade

The skeleton construction which has increasingly been used since the turn of the century has inevitably given rise to new possibilities for the facade. The size, shape and number of windows were no longer limited by structural requirements following the introduction of curtain facades, since the loads were now primarily transmitted by posts and columns.

PLANNING

Most facade designs today are still based on empirical know-how and are not tested until the design has been established in detail. The tests are carried out on true-to-scale models of individual facade elements in order to test adequate resistance to air and water, load-bearing capacity and the possibility of excessive deformation or glass breakage when subjected to corresponding loads, e.g. with the aid of firmly anchored aircraft engines. DESIGN

Today’s modern facades are characterized by external wall elements equal to one floor in height and inserted

between the respective structural floors. Non-supporting metal facades suspended in front of the building have in-creasingly become established for economic reasons, par-ticularly in high-rise construction.

The scope for design is enlarged by coloured or mirrored window panels and linings of natural stone, ceramic tiles or brick. Almost any desired appearance can be pro-duced.

TECHNICAL PROPERTIES

Modern facades must meet complex requirements as re-gards construction technology, engineering design and construction physics. Thanks to its lightness and almost unlimited possibilities for profile design, aluminium has largely become the material of choice for the outer frame-work. The panes are made of high-grade glass filled with noble gases or with a surface coating that reflects infrared light. On the inside, modern facades are highly imperme-able to water and water vapour in order to prevent dam-age due to moisture.

Despite the large areas of glass, protection against the sun is more important than heat loss today due to good ther-mal insulation of modern facades. Even where sound-proofing and fire protection are concerned, glass and

metal facades are at least the equal of conventional con-structions.

Modern facades also require a sophisticated ventilation and cooling system. The air-conditioned or twin facade is a case in point. Here an additional facade of laminated glass is arranged in front of the conventional facade, thus creating a space through which air can circulate. More complex ventilation concepts for routing air into and out of the building may be realized by including additional vertical and horizontal bulkheads. Individually controlled ventilation flaps are capable of providing a more natural and far less complex exchange of air.

PRODUCTION AND ASSEMBLY

Due to the extensive know-how required with regard to material properties and construction physics and on ac-count of the great manufacturing depth, modern facades are only produced by specialized companies based on the architect’s design and in accordance with functional, as well as structural aspects before subsequently being as-sembled.

The degree of prefabrication in modern facades is consid-erable. The frames, glazing, parapet lining, sunshades and anti-glare finish, as well as thermal insulation and sealing are all assembled into single-storey facade elements in the manufacturer’s plant. In many cases, such technical equip-ment parts as radiators, air outlets and the ducting for electrical and electronic equipment are also already inte-grated at this stage.

In the meantime, fixing elements can be mounted on the shell of the high-rise building. These elements can usually be displaced in three planes to compensate the dimen-sional tolerances occurring in the shell. The facade elem-ents as such are fitted without the help of scaffolding, thus greatly reducing the time required for this work. The frame profiles are assembled with labyrinthine indenta-tions to compensate the deformation arising in the build-ing as a result of wind and live loads, as well as tempera-ture differences. Permanently elastic rubber profiles en-sure that the facade remains impermeable to air and water.